Thyristors and Triacs Technical Publications Semiconductors Thyristors and Triacs Technical...

Philips Semiconductors

Thyristors and Triacs Technical Publications

Philips Semiconductors publishes many TechnicalPublications each year on various aspects of powersemiconductors. All the publications relevant to thyristorsand triacs are reproduced in this chapter. They providesome useful information on the different device types.They also give hints and ideas on how best to usethyristors and triacs.The technical information contained within these

publications always remains relevant. However due to thewide range in their issue dates, product information,where provided, will be historical and might have beensuperceded. Please bear this fact in mind when readingany selection tables. The up-to-date product informationcan be found in the selection guide at the front of this databook.

October 1997



FACTSHEET 013. UNDERSTANDING Hi-Com TRIACSIssue date August 1993 Document number 9398 510 21011

Hi-Com triacs from Philips Semiconductors arespecifically designed to give superior triac commutationperformance in the control of motors for domesticequipment and tools. These devices are suitable for usewith a wide variety of motor and inductive loads withoutthe need for a protective snubber. The use of a Hi-Comtriac greatly simplifies circuit design and gives significantcost savings to the designer.This product information sheet explains how the superiorcharacteristics and performance of Hi-Com triacsremoves design limitations of standard devices.

Triac commutation explainedA triac is an AC conduction device and may be thoughtof as two antiparallel thyristors monolithically integratedonto the same silicon chip.In phase control circuits the triac often has to be triggeredinto conduction part way into each half cycle. This meansthat at the end of each half cycle the on-state current inone direction must drop to zero and not resume in theother direction until the device is triggered again. This"commutation" turn-off capability is at the heart of triacpower control applications.If the triac were truly two separate thyristors thisrequirement would not present any problems. However,as the two are on the same piece of silicon, there is thepossibility that the "reverse recovery current" (due tounrecombined charge carriers) of one thyristor as it turnsoff may act as gate current to trigger the other thyristoras the voltage rises in the opposite direction. This isdescribed as a "commutation failure" and results in thetriac continuing to conduct in the opposite directioninstead of blocking.The probability of any device failing commutation isdependent on the rate of rise of reverse voltage (dV/dt)and the rate of decrease of conduction current (dI/dt). Thehigher the dI/dt the more unrecombined charge carriersare left at the instant of turn-off. The higher the dV/dt themore probable it is that some of these carriers will act asgate current. Thus the commutation capability of anydevice is usually specified in terms of the turn-off dI/dt andthe re-applied dV/dt it can withstand, at any particularjunction temperature.If a triac has to be operated in an inductive load circuitwith a combination of dI/dt and dV/dt that exceeds itsspecification, it is necessary to use an RC-snubbernetwork in parallel with the device to limit the dV/dt. Thisis at a penalty of extra circuit complexity and dissipationin the snubber. The "High Commutation" triacs (Hi-Comtriacs) are designed to have superior commutationcapability, so that even at a high rate of turn-off (dI/dt) anda high rate of re-applied dV/dt, they can be used withoutthe aid of a snubber network, thus greatly simplifying thecircuit. The design features of Hi-Com devices that havemade this possible are:

1. Geometric separation of the two antiparallelthyristorsCommutation failure can be avoided by physicallyseparating the two ’thyristor halves’ of a triac. However,separating them into two discrete chips would remove theadvantage of a triac being triggerable in both directionsby the same gate connection. Within the integratedstructure of a Hi-Com triac the two halves of the deviceare kept further apart by modifying the layout of the chipin order to lessen the chance of conduction in one halfaffecting the other half.

2. Emitter shorting"Emitter shorts" refer to the on-chip resistive pathsbetween emitter and base of a transistor. A higher degreeof emitter shorting means the presence of more suchpaths and lower resistance values in them. The use ofemitter shorts in a triac has two effects on commutation.Firstly it reduces the gain of the internal transistors thatmake up the triac. This means there will be fewer carriersleft to recombine when the conduction current falls to zero,and therefore a smaller probability thata sufficientnumberwill be available to re-trigger the triac. The second way inwhich emitter shorts help commutation is that anyunrecombined carriers in the conducting thyristor atturn-off will have more chance of flowing out through theemitter shorts (of the opposite thyristor) rather than actingas gate current to trigger that thyristor on.The Hi-Com triacs have a higher degree of emittershorting both around the periphery of the device and inthe central part of the active area. This both reduces thenumber of carriers available, and lessens the danger ofany available carriers acting as gate current forundesirable triggering.

Fig. 1. Standard triac triggering quadrants.

Quadrant 1Quadrant 2


G+G-

MT2+

MT2-

IG

IG

IG

IG

++

- -

+

-

+

-

October 1997



3. Modified gate structureThe gate of a triac allows conduction in both directions tobe initiated by either a positive or a negative current pulsebetween gate (G) and main terminal (MT1). The fourdifferent modes of triggering are often called 1+, 1-, 3-and 3+ (or sometimes quadrants 1, 2, 3 and 4) and areshown in Fig. 1.This triggering versatility arises from the fact that the gateconsists of some elements which conduct temporarilyduring the turn-on phase. In particular, one of thetriggering modes, 3+ (or quadrant 4), relies on the mainterminal 1 supplying electrons to trigger a thyristorelement in the gate-MT1 boundary. Conduction thenspreads to the main thyristor element from this boundary.Unfortunately the carrier distribution in this triggeringmode of operation is very similar to that existing when thetriac is commutating in the 1-to-3 direction (i.e changingfrom conduction with MT2 positive to blocking with MT1positive). The presence of the element in the gate to allow3+ triggering will therefore always also underminecommutation capability in the 1-to 3 direction. For this

reason the Hi-Com triacs have a modified gate design toremove this structure. This incurs the penalty that the 3+trigger mode cannot be used, but it greatly improves thecommutation performance of the device.

ConclusionsBy modifications to the internal design and layout of thetriac it is possible to achieve a high commutation capabilitytriac for use in inductive and motor load applications.These modifications have been implemented in theHi-Com range of devices from Philips Semiconductors.The devices can be used in all typical motor controlapplications without the need for a snubber circuit. Thecommutation capability of the devices is well in excess ofthe operating conditions in typical applications.As the loss of the fourth trigger quadrant can usually betolerated in most designs, Hi-Com triacs can be used inexisting motor control applications without the snubbernetwork required for a standard device. This gives thedesigner significant savings in design simplicity, boardspace and system cost.

October 1997



FACTSHEET 014. USING Hi-Com TRIACSIssue date September 1995 Document number 9397 750 00316

Hi-Com triacs from Philips Semiconductors arespecifically designed to give superior triac commutationperformance in the control of motors for domesticequipment and tools. These devices are suitable for usewith a wide variety of motor and inductive loads withoutthe need for a snubber. The use of a Hi-Com triac greatlysimplifies circuit design and gives significant cost savingsto the designer.This product information sheet explains how the need fora triac snubber arises and how the superior performanceof Hi-Com triacs removes design limitations of standarddevices. The Hi-Com range is summarised in Table 1.

Triac commutationFor resistive loads the device current is in phase with theline voltage. Under such conditions triac turn-off(commutation) occurs at the voltage "zero-crossover"point. This is not a very severe condition for triaccommutation: the slow rising dVCOM/dt gives time for thetriac to turn off (commutate) easily.The situation is quite different with inductive or motorloads. For these circuits, conduction current lags behindthe line voltage as shown in Fig. 1. When triaccommutation occurs, the rate of rise of voltage in theopposite direction can be very rapid and is governed bythe circuit and device characteristics. This high dVCOM/dtmeans there is a much higher probability of chargecarriers in the device re-triggering the triac and causinga commutation failure.

Fig. 1. Triac commutation waveforms (inductive load).

Hi-Com triacsHi-Com triacs are specifically designed for use with ACinductive loads such as motors. As commutationcapability is not an issue for resistive load applicationsthen standard triacs are usually the most appropriatedevices for these applications. The significant advantageof a Hi-Com triac is that it can withstand a very high rateof rise of reapplied voltage at commutation. This removesthe requirement for a snubber circuit in inductive load

circuits. An additional advantage of the Hi-Com design isthat the off-state (static) dVD/dt capability of the device isalso significantly improved.When using Hi-Com triacs in inductive load applications,the trigger circuit cannot trigger the device in the fourth(3+) quadrant (Fig. 2). Fortunately the vast majority ofcircuit designs do not require this mode of operation andso are suitable for use with Hi-Com triacs withoutmodification. The circuit of Fig. 3 is a typical example ofthe simplest type of trigger circuit. Hi-Com triacs areequally also suitable for use with microcontroller triggercircuits.

Fig. 2. Hi-Com triac triggering quadrants.

Fig. 3. Phase control circuit using Hi-Com triac.

Device limiting valuesi) Trigger current, I GTTrigger current for the Hi-Com triacs is in the range 2mAto 50mA. This means that gate currents due to noise thatare below 2mA in amplitude can be guaranteed not totrigger the devices. This gives the devices a noiseimmunity feature that is important in many applications.The trigger current delivered by the trigger circuit must begreater than 50mA under all conditions in order toguarantee triggering of the device when required. Asdiscussed above, triggering is only possible in the 1+, 1-and 3- quadrants.



G+G-

MT2+

MT2-

IG

IG

IG

++

- -

+

-

No triggering

possible

VDWM

-dI/dtdVcom/dt

Time

Time

Time

Supplyvoltage

Loadcurrent

Voltageacrosstriac

Triggerpulses

Current

Hi-Com triac

M

October 1997



ii) Rate of change of current, dI com/dtHi-Com triacs do not require a snubber network providingthat the rate of change of current prior to commutation isless than the rating specified in the device data sheet.This dIcom/dt limit is well in excess of the currents that occurin the device under normal operating condition, duringtransients such as start-up and faults such as the stalledmotor condition.As an example, for the 12A Hi-Com triacs the limitcommutating current is typically 24A/ms at 125˚C. Thiscorresponds to an RMS current of 54A at 50Hz. For the16A Hi-Com triacs the limit commutating current istypically 28A/ms at 125˚C. This corresponds to an RMScurrent of 63A at 50Hz. Typical stall currents for an 800Wdomestic appliance motor are in the range 15A to 20Aand so the commutation capability of the Hi-Com triacs is

well above the requirement for this type of application.

ConclusionsThe Hi-Com range of devices from PhilipsSemiconductors can be used in all typical motor controlapplications without the need for a snubber circuit. Thecommutation capability of the devices is well in excess ofthe operating conditions in typical applications.As the loss of the fourth trigger quadrant can usually betolerated in most designs, Hi-Com triacs can be used inexisting motor control applications. By removing thesnubber the use of a Hi-Com triac gives the designersignificant savings in design simplicity, board space andsystem cost.

VDRM IT(RMS) IGT dVD/dt dIcom/dt dIT/dt Package(V) (A) (mA) (V/µs) (A/ms) (A/µs)

BTA208-600B 600 8 2 - 50 1000 14 100 SOT78BTA208-800B 800 8 2 - 50 1000 14 100 SOT78BTA208X-600B 600 8 2 - 50 1000 14 100 SOT186ABTA208X-800B 800 8 2 - 50 1000 14 100 SOT186ABTA212-600B 600 12 2 - 50 1000 24 100 SOT78BTA212-800B 800 12 2 - 50 1000 24 100 SOT78BTA212X-600B 600 12 2 - 50 1000 24 100 SOT186ABTA212X-800B 800 12 2 - 50 1000 24 100 SOT186ABTA216-600B 600 16 2 - 50 1000 28 100 SOT78BTA216-800B 800 16 2 - 50 1000 28 100 SOT78BTA216X-600B 600 16 2 - 50 1000 28 100 SOT186ABTA216X-800B 800 16 2 - 50 1000 28 100 SOT186ABTA225-600B 600 25 2 - 50 1000 44 100 SOT78BTA225-800B 800 25 2 - 50 1000 44 100 SOT78

Table 1. Hi-Com triac range in leaded packages. (SOT78 <=> TO220.)

October 1997



FACTSHEET 067. LOGIC LEVEL AND SENSITIVE GATE TRIACSIssue date March 1996 Document number 9397 750 00759

IntroductionTriacs have been available since the early 1970s. Theycould be regarded as a mature technology. However, theyremain an extremely popular power control device for ACmains applications because of their low cost and thesimplicity of their control circuits. Nothing can better themin these two respects. There has even been an upsurgein their use in the 1990s due partly to the proliferation ofdomestic appliances with electronic controls. Commonexamples of these include air and water heaters, vacuumcleaners, refrigerators, washing machines, dishwashersand most small kitchen appliances.Despite their maturity, triacs have not "stood still" sincetheir first invention. They have evolved to meet thechanging demands of the applications. One importantchange in triac requirements is the gate sensitivityspecification IGT. Early triac trigger circuits were built usingdiscrete components which could supply high currents upto 100mA peak or more. Triacs therefore did not requiresensitive gates. This is still relevant today for phasecontrol circuits which use a diac such as the BR100/03 totrigger the triac. However, Integrated Circuit control isbecoming ever more popular for two main reasons:1. Stringent Electro Magnetic Compatibility regulationslimit the harmonic currents that can be drawn from themains and also limit the amount of Radio FrequencyInterference that can be generated by appliances. Thisrequires the use of either a dedicated zero crossing triacpower control IC such as the OM1654 or OM1682 fromPhilips Australia, or a microcontroller which has beenprogrammed to perform a similar function in which a highharmonic and RFI-generating phase control circuit isreplaced by an electrically "quieter" alternative. Anexample might be Binary Rate Modulation power control,where varying full and half mains cycles are conductedsymmetrically to ensure very low harmonic currents andzero DC component in the current waveform.2. More intelligent appliance controls are now beingdemanded such as remote control, soft start, variabletiming, automatic power ramp-up & ramp-down, andpower reduction after full power startup for energyconservation measures (applicable to refrigeratorcompressors in particular). These would be verycomplicated, expensive or even impossible to implementusing discrete components.Integrated circuits such as the dedicated triac controllerICs and microcontrollers possess a limited drive currentcapability which ranges from 10mA to 15mA typically.Moreover, since the IC’s supply is usually derived fromthe mains via a resistive/capacitive dropper and simplehalf wave rectifier, power dissipation in the resistor canvery easily become unacceptably high if the load currentis not kept to an absolute minimum. This means that theaverage current demand from the IC’s power supply mustusually be minimised. This can impose a limit on thecurrent amplitude and duration available for triggering the

triac. This is even more relevant if several triacs arecontrolled by one microcontroller. Examples where thisholds true are fridge freezers and washing machines. Thiscan mean that the drive current available for triactriggering is even less than the 10mA to 15mA suggestedabove.Figure 1 shows a simple IC-triac arrangement. The 5.6Vzener diode combined with the forward voltage drop ofthe rectifier diode produce an IC supply close to 5V. Theadvantages of connecting the zener as shown instead ofdirectly across the IC are that full wave current is drawnfrom the mains supply (no DC component), and theforward conduction of the zener means that the rectifiernever has to support full mains voltage. A cheap, lowvoltage diode can therefore be used. Attention must bepaid, however, to the additional power dissipation in theresistor due to the forward zener current.These IC-triac power control applications could not beimplemented without sensitive triacs. Philips logic level Dseries and sensitive gate E series triacs are designed tomeet fully the requirements in this burgeoning market. Fora full selection guide of available types, seeTables 1 and 2.

Fig. 1. A typical microcontroller / triac arrangement.

Gate trigger current I GT

Philips D and E series triacs are specified to trigger in allfour quadrants. However, a feature of four-quadrant triacsis that they are less sensitive and more difficult to triggerin the 4th (T2-, G+) quadrant. (For example, for the Dseries, max IGT in quadrant 1,2,3,4 is 5,5,5,10mA, and forthe E series it is 10,10,10,25mA.) The ability of triacs tosupport a high rate of rise of load current (dIT/dt) afterturn-on is also limited in the 4th quadrant. (dIT/dt is 10A/µscompared with 50A/µs in the other three quadrants). Forthese two reasons, operation in the 4th quadrant is notrecommended if it can be avoided.

uCD/E series

T1

T2

Load

Dropper

5V6

5V

0V

L

N

MAINS

SUPPLY

triac

resistor

1N4148

October 1997



Fig. 2. BT136 series D normalised IGT versus Tj.

Since the control IC operates on a single rail supply(usually+5V), its outputsare unipolar. It can be referencedto the mains circuit in order to source current (positivegate drive) or to sink current (negative gate drive). Since4th quadrant triggering should be avoided, optimumperformance will be obtained with negative gate current- i.e operation in the 2nd (T2+, G-) and 3rd (T2-, G-)quadrants (see Fig. 1).A triac’s IGT varies with junction temperature. At lowertemperature the IGT increases. It is therefore important toensure that the circuit supplies enough gate current at thelowest expected operating temperature for guaranteedtriggering. Figure 2 shows an example of normalised IGTversus Tj for the BT136 series D.

Latching current I L

If the triac is triggered by a gate current at the beginningof a mains half cycle, the load current will build upgradually from zero. The gate current must not beremoved before the triac is latched ON otherwise it willreturn to the blocking state. Latching occurs when the loadcurrent reaches IL. The gate pulse must therefore bepresent until the load current has reached IL.Just as for IGT, IL also increases at lower temperature. Thegate pulse duration must be specified at the lowestexpected operating temperature for guaranteedtriggering. Figure 3 shows an example of normalised ILversus Tj for the BT136 series D.How quickly the load current reaches the triac’s IL willdepend on the peak load current and mains frequency.Taken to the extreme case, if the load current is so lowthat its peak value is equivalent to IL, it will take one quartercycle, or 5ms for 50Hz mains, before the triac is latchedand the gate pulse can cease.It is also important to be aware that higher current triacshave a higher IL. This could compound problems or evenlead to the triac never latching ON if the load current islower than the triac’s IL. So, apart from the highercomponent cost, it would not be a good idea to use a triacwhose current rating is very much higher than the load

current when a lower current type is available.Tables 1 and 2 illustrate how IL varies with the triggeringquadrant and triac current rating.

Fig. 3. BT136 series D normalised IL versus Tj.

Minimum triac drive current calculationBecause the current demand must be minimised in manyIC applications, it is necessary to calculate the gate pulseduration to be just long enough to guarantee triactriggering while avoiding unnecessary burden on the IC’spower supply. The time to reach IL, hence the gate pulseduration, can be calculated using the equation:IL = Ipk x sin(2πft). Transposing gives:

t = 1/(2πf) x sin-1(IL/Ipk).The average gate current supplied by the IC is calculatedby multiplying its peak gate current with t/T. Hence:

IG(ave) = IG(pk) x t/T.IL = triac latching current at the lowest expected operatingtemperature,Ipk = peak load current,t = gate pulse duration,T = gate pulse cycle time.Note:- Since triac latching current is higher in the 2nd and4th quadrants, and normal operation for IC triggering isin the 2nd and 3rd quadrants, the gate current calculationsmust always be based on the worst case quadrant 2 ILcondition.If the load current is very low and the necessary gate pulseduration imposes too great a burden on the IC’s powersupply, triggering could be delayed for a few degrees toallow the supply voltage to build up a little. The time toreach IL will then be shortened by the delay time (true forresistive loads). Now that switching occurs further fromthe zero crossing, there will be a slightly increased risk ofRFI generation, even if the load current is very low as inthis case. RFI measurements will show if filtering isnecessary to meet the relevant EMC legislation.

-50 0 50 100 1500

0.5

1

1.5

2

2.5

3BT136D

Tj / C

T2+ G+T2+ G-T2- G-T2- G+

IGT(Tj)IGT(25 C)

-50 0 50 100 1500

0.5

1

1.5

2

2.5

3TRIAC

Tj / C

IL(Tj)IL(25 C)

October 1997



Holding current I H

As the load current reduces towards the end of a mainshalf cycle, a current, IH, will be reached when the triac isno longer latched. It will cease to conduct in the absenceof a gate current. IH also increases with reducingtemperature. Figure 4 shows an example of normalisedIH versus Tj for the BT136 series D.

Fig. 4. BT136 series D normalised IH versus Tj.

In some IC applications where the triac is used as a powerswitch, for example a precision electronic thermostat fora refrigerator compressor, continuous conduction mustbe maintained through the current zero crossing. (This isessential to prevent glitches and RFI generation.)Continuous conduction is achieved by monitoring the loadcurrent and applying a gate pulse before the triac’s IH is

reached, and maintaining the pulse until the current haspassed through zero and risen to the triac’s IL in thealternate quadrant. This condition must be met at thelowest expected operating temperature for continuousglitch-free conduction under worst case conditions.IH increases for the larger E series triacs. The mostsensitive D series triacs, however, are designed tomaintain a consistent, low IH of 10mA @ 25˚C even forthe higher 8A current rating.Figure 5 illustrates triac load current zero crossing andthe minimum gate pulse required for continuousconduction through the max IH and IL points. The diagramillustrates how IH remains constant in different quadrants.The point made earlier about how IL varies in differentquadrants is illustrated by the higher IL in the 2nd quadrant(T2+, G-). IG duration must meet this worst case condition.

Fig. 5. Relationship between IL, IH and IGT for BT136series D at 25˚C.

-50 0 50 100 1500

0.5

1

1.5

2

2.5

3TRIAC

Tj / C

IH(Tj)IH(25C)

I I TT

I I GG

HMax I = 10mA

LMax I = 10mA

LMax I = 15 mA

HMax I = 10 mA

GTMax I = 5 mA Max I = 5mAGT

Q2 to Q3transition

Q3 to Q2transition

Selection guide

Type number IT(RMS) VDRM IGT max IL max IH max Package(A) (V) (mA) (mA) (mA)

BT134W-...D 1 500,600 5,5,5,10 10,15,10,15 10 SOT223BT134-...D 4 SOT82BT136-...D 4 SOT78BT136F-...D 4 SOT186BT136X-...D 4 SOT186A

BT137-...D 8 500,600 5,5,5,10 15,20,15,20 10 SOT78BT137F-...D 8 SOT186BT137X-...D 8 SOT186A

Tabe 1. Philips D series logic level triacs.

October 1997



Type number IT(RMS) VDRM IGT max IL max IH max Package(A) (V) (mA) (mA) (mA)

BT134W-...E 1 500,600 10,10,10,25 15,20,15,20 15 SOT223BT134-...E 4 500,600,800 SOT82BT136-...E 4 SOT78BT136F-...E 4 SOT186BT136X-...E 4 SOT186A

BT137-...E 8 500,600,800 10,10,10,25 25,35,25,35 20 SOT78BT137F-...E 8 SOT186BT137X-...E 8 SOT186A

BT138-...E 12 500,600,800 10,10,10,25 30,40,30,40 30 SOT78BT138F-...E 12 SOT186BT138X-...E 12 SOT186ABT139-...E 16 SOT78BT139F-...E 16 SOT186BT139X-...E 16 SOT186A

Table 2. Philips E series sensitive gate triacs.

NOTE: SOT78 is equivalent to TO220AB.

October 1997



THYRISTORS AND TRIACS - TEN GOLDEN RULES FOR SUCCESS IN YOURAPPLICATIONIssue date May 1996 Document number 9397 750 00812

This Technical Publication aims to provide an interesting,descriptive and practical introduction to the golden rules

that should be followed in the successful use of thyristorsand triacs in power control applications.

ThyristorA thyristor is a controlled rectifier where the unidirectionalcurrent flow from anode to cathode is initiated by a smallsignal current from gate to cathode.

Fig. 1. Thyristor.

The thyristor’s operating characteristic is shown in Fig. 2.

Fig. 2. Thyristor V/I characteristic.

Turn-onA thyristor is turned on by making its gate positive withrespect to its cathode, thereby causing current flow intothe gate. When the gate voltage reaches the thresholdvoltage VGT and the resulting current reaches thethreshold current IGT, within a very short time known asthe gate-controlled turn-on time, tgt, the load current canflow from ’a’ to ’k’. If the gate current consists of a verynarrow pulse, say less than 1µs, its peak level will haveto increase for progressively narrower pulse widths toguarantee triggering.When the load current reaches the thyristor’s latchingcurrent IL, load current flow will be maintained even afterremoval of the gate current. As long as adequate loadcurrent continues to flow, the thyristor will continue toconduct without the gate current. It is said to be latchedON.

Note that the VGT, IGT and IL specifications given in dataare at 25 ˚C. These parameters will increase at lowertemperatures, so the drive circuit must provide adequatevoltage and current amplitude and duration for the lowestexpected operating temperature.

Rule 1. To turn a thyristor (or triac) ON, a gate current≥ IGT must be applied until the load current is≥ IL. This condition must be met at the lowestexpected operating temperature.

Sensitive gate thyristors such as the BT150 can be proneto turn-on by anode to cathode leakage current at hightemperatures. If the junction temperature Tj is increasedabove Tj max, a point will be reached where the leakagecurrent will be high enough to trigger the thyristor’ssensitive gate. It will then have lost its ability to remain inthe blocking state and conduction will commence withoutthe application of an external gate current.This method of spurious turn-on can be avoided by usingone or more of the following solutions:1. Ensure that the temperature does not exceed Tj max.2. Use a thyristor with a less sensitive gate such as theBT151, or reduce the existing thyristor’s sensitivity byincluding a gate-to-cathode resistor of 1kΩ or less.3. If it is not possible to use a less sensitive thyristor dueto circuit requirements, apply a small degree of reversebiasing to the gate during the ’off’ periods. This has theeffect of increasing IL. During negative gate current flow,particular attention should be paid to minimising the gatepower dissipation.

Turn-off (commutation)In order to turn the thyristor off, the load current must bereduced below its holding current IH for sufficient time toallow all the mobile charge carriers to vacate the junction.This is achieved by "forced commutation" in DC circuitsor at the end of the conducting half cycle in AC circuits.(Forced commutation is when the load circuit causes theload current to reduce to zero to allow the thyristor to turnoff.) At this point, the thyristor will have returned to its fullyblocking state.If the load current is not maintained below IH for longenough, the thyristor will not have returned to the fullyblocking state by the time the anode-to-cathode voltagerises again. It might then return to the conducting statewithout an externally-applied gate current.

a k

g

On-statecharacteristic

Off-statecharacteristic

Avalanchebreakdownregion

Reversecharacteristic

Reversecurrent

Forwardcurrent

Reversevoltage

Forwardvoltage

IL

IH

October 1997



Note that IH is also specified at room temperature and,like IL, it reduces at higher temperatures. The circuit musttherefore allow sufficient time for the load current to fallbelow IH at the maximum expected operating temperaturefor successful commutation.

Rule 2. To turn off (commutate) a thyristor (or triac), theload current must be < IH for sufficient time toallow a return to the blocking state. Thiscondition must be met at the highest expectedoperating temperature.

TriacA triac can be regarded as a "bidirectional thyristor"because it conducts inboth directions. Forstandard triacs,current flow in either direction between the main terminalsMT1 and MT2 is initiated by a small signal current appliedbetween MT1 and the gate terminal.

Fig. 3. Triac.

Turn-onUnlike thyristors, standard triacs can be triggered bypositive or negative current flow between the gate andMT1. (The rules for VGT, IGT and IL are the same as forthyristors. See Rule 1.) This permits triggering in four"quadrants" as summarised in Fig. 4.

Fig. 4. Triac triggering quadrants.

Fig. 5. Triac V/I characteristic.

Where the gate is to be triggered by DC or unipolar pulsesat zero-crossing of the load current, negative gate currentis to be preferred for the following reasons.The internal construction of the triac means that the gateis more remote from the main current-carrying regionwhen operating in the 3+ quadrant. This results in:1. Higher IGT -> higher peak IG required,2. Longer delay between IG and the commencement ofload current flow -> longer duration of IG required,3. Much lower dIT/dt capability -> progressive gatedegradation can occur when controlling loads with highinitial dI/dt (e.g. cold incandescent lamp filaments),4. Higher IL (also true for 1- operation) -> longer IG durationmight be needed for very small loads when conductingfrom the beginning of a mains half cycle to allow the loadcurrent to reach the higher IL.In standard AC phase control circuits such as lampdimmers and domestic motor speed controls, the gateand MT2 polarities are always the same. This means thatoperation is always in the 1+ and 3- quadrants where thetriac’s switching parameters are the same. This results insymmetrical triac switching where the gate is at its mostsensitive.Note:- The 1+, 1-, 3- and 3+ notation for the four triggeringquadrants is used for brevity instead of writing "MT2+,G+" for 1+, etc. It is derived from the graph of the triac’sV/I characteristic. Positive MT2 corresponds with positivecurrent flow into MT2, and vice versa (see Fig. 5). Hence,operation is in quadrants 1 and 3 only. The + and -superscripts refer to inward and outward gate currentrespectively.

Rule 3. When designing a triac triggering circuit, avoidtriggering in the 3+ quadrant (MT2-, G+) wherepossible.

Alternative turn-on methodsThere are undesirable ways a triac can be turned on.Some are benign, while some are potentially destructive.

On-state

Off-state

Reversecurrent

Forwardcurrent

Reversevoltage

Forwardvoltage

IL

IH

LI

HI

T2-

T2+

QUADRANT1

QUADRANT3

QUADRANT2

QUADRANT4

MT2 MT1

g

V MT2

V G

11

3 3

+-

- +

MT1

MT2+

G+

MT2+

G+

MT2- MT2-

G-

G-

MT1

MT1 MT1

October 1997



(a) Noisy gate signalIn electrically noisy environments, spurious triggering canoccur if the noise voltage on the gate exceeds VGT andenough gate current flows to initiate regenerative actionwithin the triac. The first line of defence is to minimise theoccurrence of the noise in the first place. This is bestachieved by keeping the gate connections as short aspossible and ensuring that the common return from thegate drive circuit connects directly to the MT1 pin (orcathode in the case of a thyristor). In situations where thegate connections are hard wired, twisted pair wires oreven shielded cable might be necessary to minimisepickup.Additional noise immunity can be provided by adding aresistor of 1kΩ or less between the gate and MT1 toreduce the gate sensitivity. If a high frequency bypasscapacitor is also used, it is advisable to include a seriesresistor between it and the gate to minimise peakcapacitor currents through the gate and minimise thepossibility of overcurrent damage to the triac’s gate area.Alternatively, use a series H triac (e.g. BT139-600H).These are insensitive types with 10mA min I GT specswhich are specifically designed to provide a highdegree of noise immunity.

Rule 4. To minimise noise pickup, keep gateconnection length to a minimum. Take thereturn directly to MT1(or cathode). If hardwired,use twisted pair or shielded cable. Fit a resistorof 1kΩ or less between gate and MT1. Fit abypass capacitor in conjunction with a seriesresistor to the gate.Alternatively, use an insensitive series H triac.

(b) Exceeding the max rate of change ofcommutating voltage dV COM/dtThis is most likely to occur when driving a highly reactiveload where there is substantial phase shift between theload voltage and current waveforms. When the triaccommutates as the load current passes through zero, thevoltage will not be zero because of the phase shift (seeFig. 6). The triac is then suddenly required to block thisvoltage. The resulting rate of change of commutatingvoltage can force the triac back into conduction if itexceeds the permitted dVCOM/dt. This is because themobile charge carriers have not been given time to clearthe junction.The dVCOM/dt capability is affected by two conditions:-1. The rate of fall of load current at commutation, dICOM/dt.Higher dICOM/dt lowers the dVCOM/dt capability.2. The junction temperature Tj. Higher Tj lowers thedVCOM/dt capability.If the triac’s dVCOM/dt is likely to be exceeded, falsetriggering can be avoided by use of an RC snubber acrossMT1-MT2 to limit the rate of change of voltage. Commonvalues are 100Ω carbon composition resistor, chosen forits surge current handling, and 100nF. Alternatively, usea Hi-Com triac.

Note that the resistor should never be omitted from thesnubber because there would then be nothing to preventthe capacitor from dumping its charge into the triac andcreating damaging dIT/dt during unfavourable turn-onconditions.

Fig. 6. Triac dICOM/dt and dVCOM/dt.

(c) Exceeding the max rate of change ofcommutating current dI COM/dtHigher dICOM/dt is caused by higher load current, highermains frequency (assuming sinewave current) or nonsinewave load current. A well known cause of nonsinewave load current and high dICOM/dt is rectifier-fedinductive loads. These can often result in commutationfailure in standard triacs as the supply voltage falls belowthe back EMF of the load and the triac current collapsessuddenly to zero. The effect of this is illustrated in Fig. 7.

Fig. 7. Effects of rectifier-fed inductive load on phasecontrol circuit.

During this condition of zero triac current, the load currentwill be "freewheeling" around the bridge rectifier circuit.Loads of this nature can generate such high dICOM/dt thatthe triac cannot support even the gentle reapplied dV/dtof a 50Hz waveform rising from zero volts. There will thenbeno benefit in adding a snubber across the triac because

TimeOn-statevoltage

I V

Reverserecoverycurrent

Re-applied

Conductingcurrent

voltage

dIcom/dt

dVcom/dt

I T

I G

High dIcom/dt

Commutation failure causes

conduction at beginning of

half cycle

October 1997



dVCOM/dt is not the problem. The dICOM/dt will have to belimited by adding an inductor of a few mH in series withthe load. Alternatively, use a Hi-Com triac.

(d) Exceeding the max rate of change of off-statevoltage dV D/dtIf a very high rate of change of voltage is applied acrossa non-conducting triac (or sensitive gate thyristor inparticular) without exceeding its VDRM (see Fig. 8), internalcapacitive current can generate enough gate current totrigger the device into conduction. Susceptibility isincreased at high temperature.Where this is a problem, the dVD/dt must be limited by anRC snubber across MT1 and MT2 (or anode andcathode). In the case of triacs, using Hi-Com types canyield benefits.

Fig. 8. Triac turn-on by exceeding dVD/dt.

Rule 5. Where high dVD/dt or dVCOM/dt are likely tocause a problem, fit an RC snubber across MT1and MT2.Where high dICOM/dt is likely to cause a problem,fit an inductor of a few mH in series with theload.Alternatively, use a Hi-Com triac.

(e) Exceeding the repetitive peak off-state voltageVDRM

If the MT2 voltage exceeds VDRM such as might occurduring severe and abnormal mains transient conditions,MT2-MT1 leakage will reach a point where the triac willspontaneously break over into conduction (see Fig. 9).If the load permits high inrush currents to flow, extremelyhigh localised current density can occur in the small areaof silicon that is conducting. This can lead to burnout anddestruction of the die. Incandescent lamps, capacitiveloads and crowbar protection circuits are likely causes ofhigh inrush currents.Turn-on by exceeding the triac’s VDRM or dVD/dt is notnecessarily the main threat to its survival. It’s the dIT/dtthat follows which is most likely to cause the damage. Dueto the time required for conduction to spread out over thewhole junction, the permitted dIT/dt is lower than if the triacis correctly turned on by a gate signal. If the dIT/dt can belimited during these conditions to this lower value, whichis given in data, the triac is more likely to survive. Thiscould be achieved by fitting a non saturable (air cored)inductor of a few µH in series with the load.

If the above solution is unacceptable or impractical, analternative solution would be to provide additional filteringandclamping to prevent the spikes reaching the triac. Thiswould probably involve the use of a Metal Oxide Varistoras a "soft" voltage clamp across the supply, with seriesinductance followed by parallel capacitance upstream ofthe MOV.

Fig. 9. Triac turn-on by exceeding VDRM.

Doubts have been expressed by some manufacturersover the reliability of circuits which use MOVs across themains, since they have been known to go into thermalrunaway in high ambient temperatures and failcatastrophically. This is due to the fact that their operatingvoltage possesses a marked negative temperaturecoefficient. However, if the recommended voltage gradeof 275V RMS is used for 230V mains, the risk of MOVfailure should be negligible. Such failures are more likelyif 250V RMS MOVs are used, which are underspecifiedfor 230V RMS use at high ambient temperatures.

Rule 6. If the triac’s VDRM is likely to be exceeded duringsevere mains transients, employ one of thefollowing measures:Limit high dIT/dt with a non saturable inductorof a few µH in series with the load;Use a MOV across the mains in combinationwith filtering on the supply side.

Turn-on dI T/dtWhen a triac or thyristor is triggered into conduction bythe correct method via its gate, conduction begins in thedie area immediately adjacent to the gate, then quicklyspreads to cover the whole active area. This time delayimposes a limit on the permissible rate of rise of loadcurrent. A dIT/dt which is too high can cause localisedburnout. An MT1-MT2 short will be the result.If triggering in the 3+ quadrant, an additional mechanismfurther reduces the permitted dIT/dt. It is possible tomomentarily take the gate into reverse avalanchebreakdown during the initial rapid current rise. This mightnot lead to immediate failure. Instead, there would beprogressive burnout of the gate-MT1 shorting resistanceafter repeated exposure. This would show itself by aprogressive increase in IGT until the triac will no longertrigger. Sensitive triacs are likely to be the mostsusceptible. Hi-Com triacs are not affected as they do notoperate in the 3+ quadrant.

VD

200

400

600

800

1000

-200

-400

0

(V)

VD

October 1997



The dIT/dt capability is affected by how fast the gatecurrent rises (dIG/dt) and the peak value of IG. Highervalues of dIG/dt and peak IG (without exceeding the gatepower ratings) give a higher dIT/dt capability.

Rule 7. Healthy gate drive and avoiding 3+ operationmaximises the triac’s dIT/dt capability.

Asmentioned previously, a common load with a high initialsurge current is the incandescent lamp which has a lowcold resistance. For resistive loads such as this, the dIT/dtwould be at its highest if conduction commenced at a peakof the mains voltage. If this is likely to exceed the triac’sdIT/dt rating, it should be limited by the inclusion of aninductor of a few µH or even a Negative TemperatureCoefficient thermistor in series with the load. Again, theinductor must not saturate during the maximum currentpeak. If it does, its inductance would collapse and it wouldno longer limit the dIT/dt. An air cored inductor meets therequirement.A more elegant solution which could avoid therequirement for a series current-limiting device would beto use zero voltage turn-on. This would allow the currentto build up more gradually from the beginning of thesinewave.Note: It is important to rememember that zero voltageturn-on is only applicable to resistive loads. Using thesame method for reactive loads where there is phase shiftbetween voltage and current can cause "halfwaving" orunipolar conduction, leading to possible saturation ofinductive loads, damagingly high peak currents andoverheating. More advanced control employing zerocurrent switching and / or variable trigger angle is requiredin this case.

Rule 8. If the triac’s dIT/dt is likely to be exceeded, anair cored inductor of a few µH or an NTCthermistor should be fitted in series with theload.Alternatively, employ zero voltage turn-on forresistive loads.

Turn-offSince triacs are used in AC circuits, they naturallycommutate at the end of each half cycle of load currentunlessa gate signal is applied to maintainconduction fromthe beginning of the next half cycle. The rules for IH arethe same as for the thyristor. See Rule 2.

Hi-Com triacHi-Com triacs have a different internal construction toconventional triacs. One of the differences is that the two"thyristor halves" are better separated to reduce theinfluence that they have on each other. This has yieldedtwo benefits:1. Higher dVCOM/dt. This enables them to control reactiveloads without the need for a snubber in most cases whilestill avoiding commutation failure. This reduces thecomponent count, board size and cost, and eliminatessnubber power dissipation.

2. Higher dICOM/dt. This drastically improves the chancesof successfully commutating higher frequency or nonsinewave currents without the need for a dICOM/dt-limitinginductor in series with the load.3. Higher dVD/dt. Triacs become more sensitive at highoperating temperatures. The higher dVD/dt of Hi-Comtriacs reduces their tendency to spurious dV/dt turn-onwhen in the blocking state at high temperature. Thisenables them to be used in high temperature applicationscontrolling resistive loads, such as cooking or heatingapplications, where conventional triacs could not be used.The different internal construction also means that 3+

triggering is not possible. This should not be a problem inthe vast majority of cases because this is the leastdesirable and least used triggering quadrant, so directsubstitution of a Hi-Com for an equivalent conventionaltriac will almost always be possible.Hi-Com triacs are fully described in two PhilipsFactsheets:Factsheet 013 - Understanding Hi-Com Triacs, andFactsheet 014 - Using Hi-Com Triacs.

Triac mounting methodsFor small loads or very short duration load current (i.e.less than 1 second), it might be possible to operate thetriac in free air. In most cases, however, it would be fixedto a heatsink or heat dissipating bracket.The three mainmethods of clamping the triac to a heatsinkare clip mounting, screw mounting and riveting. Mountingkits are available from many sources for the first twomethods. Riveting is not a recommended method in mostcases.

Clip mountingThis is the preferred method for minimum thermalresistance. The clip exerts pressure on the plastic bodyof the device. It is equally suitable for the non-isolatedpackages (SOT82 andSOT78) and the isolatedpackages(SOT186 F-pack and the more recent SOT186A X-pack).Note: SOT78 is otherwise known as TO220AB.

Screw mounting1. An M3 screw mounting kit for the SOT78 packageincludes a rectangular washer which should be betweenthe screw head and the tab. It should not exert any forceon the plastic body of the device.2. During mounting, the screwdriver blade should neverexert force on the plastic body of the device.3. The heatsink surface in contact with the tab should bedeburred and flat to within 0.02mm in 10mm.4. The mounting torque (with washer) should be between0.55Nm and 0.8Nm.5. Where an alternative exists, the use of self-tappingscrews should be avoided due to the possible swelling ofthe heatsink material around the fixing hole. This couldbe detrimental to the thermal contact between device andheatsink. (See 3 above.) The uncontrollable mountingtorque is also a disadvantage with this fixing method.6. The device should be mechanically fixed before theleads are soldered. This minimises undue stress on theleads.

October 1997



RivetingPop riveting is not recommended unless great care istaken because the potentially severe forces resulting fromsuch an operation can deform the tab and crack the die,rendering the device useless. In order to minimise rejects,the following rules should be obeyed if pop riveting:1. The heatsink should present a flat, burr-free surface tothe device.2. The heatsink mounting hole diameter should be nogreater than the tab mounting hole diameter.3. The pop rivet should just be a clearance fit in the tabhole and heatsink mounting hole without free play.4. The pop rivet should be fitted with its head, not themandrel, on the tab side.5. The pop rivet should be fitted at 90 degrees to the tab.(The rivet head should be in contact with the tab aroundits complete circumference.)6. The head of the rivet should not be in contact with theplastic body of the device after riveting.7. Mechanical fixing of the device and heatsink assemblyto the PCB should be completed before the leads aresoldered to minimise stressing of the leads.

Rule 9. Avoid mechanical stress to the triac when fittingit to the heatsink. Fix, then solder. Never poprivet with the rivet mandrel on the tab side.

Thermal resistanceThermal resistance Rth is the resistance to the flow of heataway from the junction. It is analogous to electricalresistance;i.e. just as electrical resistance R = V/I, thermal resistanceRth = T/P,where T is the temperature rise in Kelvin and P is thepower dissipation in Watts. Therefore Rth is expressed inK/W.For a device mounted vertically in free air, the thermalresistance is dictated by the junction-to-ambient thermalresistance Rth j-a. This is typically 100K/W for the SOT82package, 60K/W for the SOT78 package and 55K/W forthe isolated F-pack and X-pack.For a non isolated device mounted to a heatsink, thejunction-to-ambient thermal resistance is the sum of thejunction-to-mounting base, mounting base-to-heatsinkand heatsink-to-ambient thermal resistances.Rth j-a = Rth j-mb + Rth mb-h + Rth h-a (non isolated package).

The use of heat transfer compound or sheet between thedevice and heatsink is always recommended. In the caseof isolated packages, there is no reference made to"mounting base", since the Rth mb-h is assumed to beconstant and optimised with heat transfer compound.Therefore, the junction-to-ambient thermal resistance isthe sum of the junction-to-heatsink andheatsink-to-ambient thermal resistances.

Rth j-a = Rth j-h + Rth h-a (isolated package).

Rth j-mb or Rth j-h are fixed and can be found in data for eachdevice.Rth mb-h is alsogiven in the mounting instructions for severaloptions of insulated and non-insulated mounting, with orwithout heatsink compound.Rth h-a is governed by the heatsink size and the degree ofunrestricted air movement past it.

Calculation of heatsink sizeTo calculate the required heatsink thermal resistance fora given triac and load current, we must first calculate thepower dissipation in the triac using the following equation:

P = Vo x IT(AVE) + Rs x IT(RMS)2.

Knee voltage Vo and slope resistance Rs are obtainedfromthe relevant VT graph in data book SC03. If the valuesare not already provided, they can be obtained from thegraph by drawing a tangent to the max VT curve. The pointon the VT axis where the tangent crosses gives Vo, whilethe slope of the tangent (VT/IT) gives Rs.Using the thermal resistance equation given above:

Rth j-a = T/P.The max allowable junction temperature rise will be whenTj reaches Tj max in the highest ambient temperature.This gives us T.Rth j-a = Rth j-mb + Rth mb-h + Rth h-a. SC03 data gives us thevalues for Rth j-mb and Rth mb-h for our chosen mountingmethod, leaving Rth h-a as the only unknown.

Thermal impedanceThe above calculations for thermal resistance areapplicable to the steady state condition - that is for aduration greater than 1 second. This time is long enoughfor heat to flow from the junction to the heatsink. Forcurrent pulses or transients lasting for shorter than 1second, however, heatsinking has progressively lesseffect. The heat is simply dissipated in the bulk of thedevice with very little reaching the heatsink. For transientconditions such as these, the junction temperature rise isgoverned by the device’s junction-to-mounting basethermal impedance Zth j-mb.Zth j-mb decreases for decreasing current pulse durationdue to reduced chip heating. As the duration increasestowards 1 second, Zth j-mb increases to the steady stateRth j-mb value.The Zth j-mb curve for bidirectional and unidirectionalcurrent down to 10µs duration is shown for each devicein the SC03 data book.

Rule For longterm reliability, ensure that the Rth j-a is10. low enough to keep the junction temperature

within Tj max for the highest expected ambienttemperature.

Range and packagingPhilips thyristors range from 0.8A in SOT54 (TO92) to25A in SOT78 (TO220AB).

October 1997



Philips triacs range from 1A in SOT223 to 25A in SOT78.Conventional types (4-quadrant triggering) and Hi-Comtypes (3-quadrant triggering) are available. SOT54 typesare planned for 1996.The smallest package is the surface mount SOT223 forthe smaller thyristors and triacs (Fig. 10). The powerdissipation is governed by the degree of heatsinkingoffered by the PCB onto which it is soldered.The same respective chips are also available in SOT82which is a non isolated package (Fig. 12). The improvedheat removal of this package when heatsunk allowshigher current ratings and improved power dissipations.Figure 11 shows SOT54 in which the very smallestdevices are mounted. Smaller chips than thoseaccommodated by SOT223 go into this package, whichoffers the most compact non surface mount solution.

SOT78 is the most common non isolated package inwhich most of our devices are supplied (Fig. 13).Figure 14 shows SOT186 (F-pack). This has been thetraditional Philips isolated package. It offers an isolationvoltage of 1,500V peak between device and heatsinkunder clean conditions.The more recent SOT186A package (X-pack) shown inFig. 15 possesses several advantages over the oldertype.1. It has the same dimensions as the SOT78 package forpin spacing from the mounting surface, so it can directlyreplace SOT78 devices to provide isolation without theneed for modification of the mounting arrangement.2. It has no exposed metal at the top of the tab, andcreepage distances from pins to heatsink are greater, soit can offer an improved true isolation of 2,500V RMS.3. It is a fully encapsulated SOT78 replacement.

Fig. 10. SOT223.

handbook, full pagewidth

6.76.3

0.950.85

2.30.800.60

4.6

3.12.9

3.73.3

7.36.7

A

B

0.2 A

1.80max

1616o

max

10o

max

0.100.01

0.320.24

4

1 2 3

MSA035 - 1(4x)

0.1 BM

M

S seating plane 0.1 S

o

October 1997



Fig. 11. SOT54 (TO92).

MBC015 - 1

2.544.8max

4.2 max

1.6

0.660.56

1

2

3

5.2 max 12.7 min

2.5 max(1)

0.480.40

0.40min

Fig. 12. SOT82.

1/1 page = 296 mm (Datasheet)27 mm

0.88max

2.29

1 2 3

3.75

11.1max

15.3min

3.12.5

7.8 max2.82.3

2.54max

(1)

1.2

4.580.5

MSA286

October 1997



Fig. 13. SOT78 (TO220AB).

5.9min

1.3

4.5max

15.8max

10.3max

3.7

2.8

3.0

13.5min

3.0 maxnot tinned

1.3max(2x)

0.6

2.42.54 2.54

0.9max(3x)

MSA060 - 1

1 2 3

Fig. 14. SOT186 F-pack.

MBC667

4.4max

2.9max

7.97.5

17max

13.5min

0.55 max

1.3

3.23.0

5.7max

10.2 max

4.44.0

0.90.5

3.5 maxnot tinned

dimensions withinthis zone areuncontrolled

4.4

seatingplane1.5

max

1 2 3

0.90.7 M0.4

2.54

5.08

October 1997



Fig. 15. SOT186A X-pack.

recessesO 2.5 x 0.8 max

depth (2x)

10.3 max

3.23.0

4.6max

2.9 max

6.4

15.8max19

max

3 maxuntinned

dimensions withinthis zone areuncontrolled

2.5

3

13.5min

5.08

2.54

2.8

0.6

0.5

2.5

seating plane

0.4 M

1 2 3

1.0(2x)

1.3

0.90.7

MSA178

October 1997



The Ten Golden Rules Summarised

Rule 1. To turn a thyristor (or triac) ON, a gate current ≥ IGT must be applied until the loadcurrent is ≥ IL. This condition must be met at the lowest expected operatingtemperature.

Rule 2. To turn off (commutate) a thyristor (or triac), the load current must be < IH for sufficienttime to allow a return to the blocking state. This condition must be met at the highestexpected operating temperature.

Rule 3. When designing a triac triggering circuit, avoid triggering in the 3+ quadrant (MT2-,G+) where possible.

Rule 4. To minimise noise pickup, keep gate connection length to a minimum. Take thereturn directly to MT1 (or cathode). If hard wired, use twisted pair or shielded cable.Fit a resistor of 1kΩ or less between gate and MT1. Fit a bypass capacitor inconjunction with a series resistor to the gate.Alternatively, use an insensitive series H triac.

Rule 5. Where high dVD/dt or dVCOM/dt are likely to cause a problem, fit an RC snubber acrossMT1 and MT2.Where high dICOM/dt is likely to cause a problem, fit an inductor of a few mH in serieswith the load.Alternatively, use a Hi-Com triac.

Rule 6. If the triac’s VDRM is likely to be exceeded during severe mains transients, employone of the following measures:Limit high dIT/dt with a non saturable inductor of a few µH in series with the load;Use a MOV across the mains in combination with filtering on the supply side.

Rule 7. Healthy gate drive and avoiding 3+ operation maximises the triac’s dIT/dt capability.

Rule 8. If the triac’s dIT/dt is likely to be exceeded, an air cored inductor of a few µH or anNTC thermistor should be fitted in series with the load.Alternatively, employ zero voltage turn-on for resistive loads.

Rule 9. Avoid mechanical stress to the triac when fitting it to the heatsink. Fix, then solder.Never pop rivet with the rivet mandrel on the tab side.

Rule 10. For longterm reliability, ensure that the Rth j-a is low enough to keep the junctiontemperature within Tj max for the highest expected ambient temperature.

October 1997



SURFACE MOUNTED TRIACS AND THYRISTORSIssue date September 1997 Document number 9397 750 02622

October 1997

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