8/9/2019 AV02-0421EN_AN-5336_GateDrive_2014-02-25,0

http://slidepdf.com/reader/full/av02-0421enan-5336gatedrive2014-02-250 1/4

Gate Drive Optocoupler Basic Design for IGBT / MOSFETApplicable to All Gate Drive Optocouplers

Application Note 5336

Introduction

This application note covers the topic of calculating gatedriver power and thermal dissipation of the gate driveoptocoupler IC. Gate drive optocouplers are used todrive, turning-on and off, power semiconductor switches,MOSFETs / IGBTs. The gate drive power calculation can bedivided into three parts; power consumed or lost in theinternal circuitry of the driver, power sent to the powersemiconductor switches (IGBT/MOSFET) and power lostat the external component between the driver IC and thepower semiconductor switch, e.g. across external gateresistor. In the following example, we will discuss an IGBTgate driver design using the Avago ACPL-332J (2.5nApeakintelligent gate driver). This design guide is applicable forMOSFET gate drivers also.

IGBT/MOSFET Gate Resistor

When choosing the value of R G, it is important to lookfrom the point from both gate driver IC and the powersemiconductor switches, MOSFET/IGBT. For the gatedriver IC, we choose an RG that is within the IC maximumallowable power disippation rating while sourcing/sinking the highest possible driver current. From the IGBTor MOSFET point of view, the gate resistor inuences thevoltage change dV

CE /dt and current change di

C /dt during

the turn on and turn off period.

So it is important when a designer chooses an IGBT orMOSFET, the appropriate gate driver optocoupler is alsochosen as the current and power rating of this driverdetermine how fast the IGBT or MOSFETcan turn on orturn off.

Figure 1. Block Diagram of ACPL-332J

SHIELD

SHIELD

DRIVER

VE

DESAT

VCC2

VOUT

VCLAMP

VEE

VCC1

VS

FAULT

ANODE

CATHODE

VCLAMP

VLED

6, 7

5, 8

2

3

1, 4

13

11

14

9, 12

10

16

15

DESAT

UVLO

LED1

LED2

2

3

8/9/2019 AV02-0421EN_AN-5336_GateDrive_2014-02-25,0

http://slidepdf.com/reader/full/av02-0421enan-5336gatedrive2014-02-250 2/4

2

Figure 2. VOL vs IOL

Step I: Calculate RG minimum from IOL peak specication: To nd the peak charging l OL assume that the gate isinitially charged to the steady-state value V CC. For theACPL-332J, the voltage drop is linearly approximated as6.3 V for 2.5 A output at 70° C (Fig 2: VOL vs IOL). Thereforeapply the following relationship:

0

1

2

3

4

5

6

7

8

0 0.5 1 1.5 2 2.5

IoL - OUTPUT LOW CURRENT - A

V O L - L

O W

O U T P U T V O L T A G E D R O P

- V

_ _ _ _ 100 o C ______ 25 o C--------- -40o C

Gate Drive Power Operation within IC Maximum AllowablePower Ratings

The power dissipation of the gate drive optocoupler is acombination of output-side power to the IGBT/MOSFET,red circle, and the input-side power due to input LEDpower dissipation, blue circle. The power dissipation forthe second LED used for fault feedback is neglected asthe current to drive the open-collector transistor is small.

The calculation steps are:

1. Calculate the minimum desired R G according to themaximum peak gate current

2. Calculate total power dissipation3. Compare the input and output power dissipation

calculated in step #2 to the maximum recommendeddissipation for the IC. (If the maximum recommendedlevel has been exceeded, it may be necessary to raisethe value of RG to lower the switching power andrepeat step #2.)

In this example, the total input and output power dissi-pation of the ACPL-332J is calculated given the followingconditions:

• IG = ION, MAX ~ 2.5 A• VCC2 = 18 V• VEE = -5 V, (Note: VEE = 0 V if negative voltage supply is

not required in application)• f SWITCH = 15 kHz• Ambient Temperature = 70 ° C

Rg = (VCC2 – VEE – VOL)/ IOL = (18 V - (-5 V) - 6.3)/ 2.5 A = 6.68 Ω (approximately 6.8 Ω )Note: The value of the gate resistance has a signicantimpact on the dynamic performance of IGBTs/MOSFETs.

A smaller gate resistor charges and discharges the powertransistor input capacitance faster, reducing switchingtimes and switching losses. The trade off is that this couldlead to higher voltage oscillations. In the MOSFET andIGBT datasheet, there is usually a recommended gateresistor which is used for the datasheet characterization.However, a designer should be cautious not to over-drivethe gate drive IC by using the recommended gate resis-tance from the IGBT or MOSFET datasheet.

Step II: Calculate total power dissipation in the gate driver: The total power dissipation (P T) is equal to the sum of theinput-side power (P I) and output-side power (P O) dissipa-tion:

P T = PI + PO

PI = IF(ON) ,max * VF,max

where,

IF(ON),max = 12 mAVF,max = 1.95 V

The IF(ON) can be found in the recommended opertingconditions and V F can be found in the ACPL-332J data-sheet, Table 5 of the electrical specications.Electrical Specication

Min Typ Max Units

Input Forward Voltage, VF 1.2 1.6 1.95 V

PO = PO(BIAS) + PO(SWTICH)

= ICC2.MAX * (VCC2–VEE ) + ∆VGE * QG * f SWITCH

where,

PO(BIAS) = Steady-state power in the driver due tobiasing the device.

PO(SWITCH) = Driver power for charging and discharging

of device gate capacitances.ICC2.MAX = Supply current to power internal circuity∆VGE = VCC2 + |VEE|QG = Total gate charge of the IGBT or MOSFET

as described in the manufacturer speci-cation (Illustrated in Figure 3) = 240 nC(approximation for a 100 A IGBT)

f SWITCH = Switching frequency of application

8/9/2019 AV02-0421EN_AN-5336_GateDrive_2014-02-25,0

http://slidepdf.com/reader/full/av02-0421enan-5336gatedrive2014-02-250 3/4

3

The output detector junction temperature is given by:

TJ = PD *(q J-P + q P-A) + TAUsing the q J-P = q 9-12 = 30° C/W (ACPL-332J Table 7,Package Characteristic) and a TA of 70° C and assumingthe thermal resistance from pin to ambient, q P-A is50° C/W:

TJ = 197.8*(30+50) + 70° C = 85.8° CIf the juntion temperature is higher than the maximum junction temperature rating (in this case 125 ° C), thedesired specication must be derated according.

Designers should note that the thermal resistancebetween pin to ambient is also the PCB heatsink thermalresistance. This thermal resistance is then dependent onthe area on the PCB and the free air-ow.

Further Topics:Higher Output Current Using an External Current Buffer: To increase the IGBT gate drive current, a non-invertingcurrent buffer (such as the NPN/PNP buffer shown inFigure 75 of HCPL-316J data sheet) may be used. Invertingtypes are not compatible with the desatura-tion faultprotection circuitry and should be avoided. To preservethe slow IGBT turn-off feature during a fault condition,a 10 nF capacitor should be connected from the bufferinput to VEE and a 10 Ω resistor inserted between theoutput and the common NPN/PNP base. The MJD44H11/MJD45H11 pair is appropriate for currents up to 8 Amaximum. The D44VH10/ D45VH10 pair is appropriatefor currents up to 15 A maximum.Thermal ModelMost of the steady state thermal models for gate driveoptocouplers can be found in AN1087. The thermal resis-tance values given in this model can be used to calculatethe temperatures at each node for a given operatingcondition.

Figure 3. Typical IGBT Gate Charge Curve

VGE(V)

QG(C)

+VGE (VCC2)

-VGE (VEE)

QG

Step III: Compare the calculated power dissipation with theabsolute maximum values in the IC:For the ACPL-332J, the maximum power dissipation canbe found in Table 3 of the ACPL-332J data sheet. Also, itrequires a derating of 10 mW/ ° C if the operating temper-ature is above 90 ° C (Note. 2 ACPL-332J datasheet).Absolute Maximum Rating

Min Max Units

Output IC Power Dissipataion, Po 600 mW

Input IC Power Dissipation, PI 150

PI = 23.4 mW < 150 mW (abs. max.) ) OK PO = 197.8 mW < 600 mW (abs. max.) ) OK

Therefore, the power dissipation absolute maximumrating has not been exceeded for the above example.Note: Heat dissipation for different packages require derating whenthe operating temperature exceed a certain level. For ACPL-332J,as operating temperature is below 90° C, derating is not necessary.Operating temperature is different for different products

Another method to check if the device is within themaximum limits is to calculate the junction temperatureof the device. We continue to use the example providedearlier.

Power dissipation, PD = 23.4 + 197.8 = 221.2 mW

Figure 4. Current Buffer for Increased Drive Current

16

15

14

13

12

11

10

9

VE

VLED2+

DESAT

VCC2

VC

VOUT

VEE

VEE

10 Ω

HCPL-316J

100 pF

10 nF

MJD44H11 or D44VH10

4.5 Ω

2.5 Ω

MJD45H11 or D45VH10

15 V -5 V

Using the above information, we calculate both P I and PO below:

PI = 12 mA * 1.95 V = 23.4 mWPO = PO(BIAS) + PO(SWITCH)

= 5.0 mA * (18 V – (–5 V)) + (18 V + 5 V) * 240nC * 15 kHz = 115 mW + 82.8 mW = 197.8 mW

8/9/2019 AV02-0421EN_AN-5336_GateDrive_2014-02-25,0

http://slidepdf.com/reader/full/av02-0421enan-5336gatedrive2014-02-250 4/4