test Saturation Voltage
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Power Electronic s Technology April 2008 www .powerelectroni cs.com 20 20 Test Saturation Voltage to Achieve High Efciency By Richard Dunipace, Principal Technical Marketer, Standard Products Group, Fairchild Semiconductor, Irving, Texas I n switch-mode power supplies, saturation losses represent the main source o ineiciency in the power transistor. Because those losses are a unction o a transistor’s saturation voltage, it’s important that power-supply designers be able to accurately me asure saturation voltage when evaluating particular devices as power switches or their designs. In the March issue, part one o this two-part article series discussed the contribution o saturation losses to power-supply ineciency, the relationship between satura- tion voltage and saturation losses, and a novel approach to accurately measuring saturation voltage even when high voltages or noise are present. Tat measurement technique can be applied by build- ing the low-cost tester described here in part two o the article. A detailed description is given o the circuitry and components required to construct the saturation voltage tester or probe. In addition, a procedure or calibrating the probe is given along with some tips on how to use the probe eectively. Building a Saturation Tester Fig. 1 shows the circuit or a saturation-voltage probe. In looking at the gure, the input rom the switching tran- sistor is on the le and the output to the oscilloscope, or dierential probe, is on the right. Te circuit, powered by two 9-V alkaline batteries, consumes approximately 14.7 mA and 12.4 mA or the 9-V and –9-V supplies, respective ly. Both batteries are moni- tored or end o battery lie through resistor R6, diodes D8 to D10 and transistor Q7. Power indicator D8 will go out i the voltage o either battery drops below 6.2 V. Power indicator diodes D8 and D6 are used to start the voltage reerence. Te voltage reerence sel-biases and will not start on its own. Te voltage reerence consists o red LED D7 plus the current source R9 and transistor Q6 (2 mA), the current mirror transistor Q3, resistor R3, and the current source transistor Q2 and resistor R2 (1 mA). While this may seem odd in that the voltage reerence is used to produce a precision current that is then used to bias itsel, overall it produces a highly stable supply that is largely independent o battery voltage and airly stable with temperature, while being low in cost and not using any special devices. Te current source plus current mirror is also used to bias the current source transistor Q4 and resistor R4 (10 mA), which in turn is use d to bias the output transi stor Q1. Te temperature stability o the current sources and voltage reerence can be improv ed by replacing transistors Q2, Q3, Q4, Q5 and Q6 with npn transistor arra y CA3096. However, this is a more expensive solution, and the CA3096 is out o production and no longer readily available. For most applications, the 2N3904 and 2N3606 transistors work well and are inexpensive. Working rom the input o the saturation probe and moving right, the signal rst reaches a 0.5-A ast use. Te use protects against excess reverse voltage (more than –9 V). From the use, we contact diode D3 (reverse pro- tection) and diode D4. D4 is used with zener-diode D5 to limit the maximum positive input swing. Tis limits the maximum output voltage and produces a consistent positive output swing throughout the battery’s lie. Switch S1 and diode D4 allow the output to be zeroed when setting up the oscilloscope’s baseline, which is very handy . Continuing to move to the right, resistor R1 is used to provide an ad- ditional voltage drop to balance the voltage dropped by diode D1 with Build a low-ot aturatio ttr to aur th aturatio voltag o withig traitor auratly i th r o high withig voltag or oi. Parameter Value Positive power- supply voltage 9 V at 14.7 mA Negative power- supply voltage – 9 V at 12.4 mA Rise time 12 ns Fall time 30 ns In put-volt age range – 9 V t o 1 kV Table 1. Specifcations or a saturation-voltage test probe to measure SMPS losses. Part Two
Transcript of test Saturation Voltage
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Test Saturation Voltage
to Achieve High EfciencyBy Richard Dunipace, Principal Technical Marketer,Standard Products Group, Fairchild Semiconductor,Irving, Texas
In switch-mode power supplies, saturation lossesrepresent the main source o ineiciency in thepower transistor. Because those losses are a unctiono a transistor’s saturation voltage, it’s important thatpower-supply designers be able to accurately measure
saturation voltage when evaluating particular devices aspower switches or their designs.
In the March issue, part one o this two-part articleseries discussed the contribution o saturation losses topower-supply ineciency, the relationship between satura-
tion voltage and saturation losses, and a novel approach toaccurately measuring saturation voltage even when high voltages or noise are present.
Tat measurement technique can be applied by build-ing the low-cost tester described here in part two o thearticle. A detailed description is given o the circuitry andcomponents required to construct the saturation voltagetester or probe. In addition, a procedure or calibratingthe probe is given along with some tips on how to use theprobe eectively.
Building a Saturation Tester
Fig. 1 shows the circuit or a saturation-voltage probe.In looking at the gure, the input rom the switching tran-sistor is on the le and the output to the oscilloscope, ordierential probe, is on the right. Te circuit, powered by two 9-V alkaline batteries, consumesapproximately 14.7 mA and 12.4mA or the 9-V and –9-V supplies,respectively. Both batteries are moni-tored or end o battery lie throughresistor R6, diodes D8 to D10 andtransistor Q7. Power indicator D8will go out i the voltage o eitherbattery drops below 6.2 V. Powerindicator diodes D8 and D6 are usedto start the voltage reerence. Te
voltage reerence sel-biases and will not start on its own.Te voltage reerence consists o red LED D7 plus the
current source R9 and transistor Q6 (2 mA), the currentmirror transistor Q3, resistor R3, and the current sourcetransistor Q2 and resistor R2 (1 mA). While this may seem odd in that the voltage reerence is used to producea precision current that is then used to bias itsel, overall itproduces a highly stable supply that is largely independento battery voltage and airly stable with temperature, whilebeing low in cost and not using any special devices. Te
current source plus current mirror is also used to bias thecurrent source transistor Q4 and resistor R4 (10 mA), whichin turn is used to bias the output transistor Q1.
Te temperature stability o the current sources and voltage reerence can be improved by replacing transistorsQ2, Q3, Q4, Q5 and Q6 with npn transistor array CA3096.However, this is a more expensive solution, and the CA3096is out o production and no longer readily available. Formost applications, the 2N3904 and 2N3606 transistors work well and are inexpensive.
Working rom the input o the saturation probe andmoving right, the signal rst reaches a 0.5-A ast use. Te
use protects against excess reverse voltage (more than–9 V). From the use, we contact diode D3 (reverse pro-tection) and diode D4. D4 is used with zener-diode D5to limit the maximum positive input swing. Tis limits
the maximum output voltage andproduces a consistent positive outputswing throughout the battery’s lie.Switch S1 and diode D4 allow theoutput to be zeroed when setting upthe oscilloscope’s baseline, which is very handy.
Continuing to move to the right,resistor R1 is used to provide an ad-ditional voltage drop to balance the voltage dropped by diode D1 with
Build a low-ot aturatio ttr to aurth aturatio voltag o withig traitorauratly i th r o high withigvoltag or oi.
Parameter Value
Positive power-supply voltage
9 V at 14.7 mA
Negative power-supply voltage
– 9 V at 12.4 mA
Rise time 12 ns
Fall time 30 ns
Input-voltage range – 9 V to 1 kV
Table 1. Specifcations or a saturation-voltage
test probe to measure SMPS losses.
Part Two
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the voltage drop o transis-tor Q1. It is also used withcapacitor C1 to adjust thetransient response time.Te voltage drop o diodeD1 is only about 0.55 V
due to the small 2-mA biascurrent, while the voltagedrop on output transistorQ1 is about 0.7 V at 10 mA.Finally, moving to the rightthrough resistors R1 andR5, the signal reaches thebase o the output transis-tor Q1. Tis output goesthrough resistor R11 to theoutput connector.
Figs. 2 and 3 show
the completed saturationprobe, which was builtin an o-the-shel plasticbox. Te probe’s specica-tions are shown in Table 1,while the parts are listed inTable 2.
CalibrationOnce the circuit is built,
it should operate readily.
Put the batteries in and turn it on. Te ront-panel LEDshould be lit; i not, check the wiring. Te voltage dropacross resistors R2, R3, R4, R7 and R9 should be around1 V. Te LED reerence voltage should be approximately 1.65 V. Remember, the reerence will not start i the powerlight is not lit. Short the input and measure the output volt-age. Adjust the “zero” trimmer to set the output voltage to0.000 V, or as close to that as possible. Connect the inputto a known positive voltage rom 0 V to 5 V. Te outputshould be within a ew tens o millivolts o this voltage.Reverse the input leads. Te voltage should be very closeto the same magnitude but simply
reversed in polarity.Next, connect a pulse genera-
tor to the input and set it to 2 V,a 2-μs pulse width and a re-quency o 100 kHz. Te outputshould reproduce the inputwithin the rise-time and all-time specications. I betterrise times and all times arerequired, simply turn up thecurrent in the circuit. Cau-tion: Te saturation probeis polarity sensitive, so besure to connect the probecorrectly to the circuit.
RecommendationsTe saturation probe provides a low-cost solution to theneed to measure saturation voltage plus other voltages thatare required to evaluate the design o a switching circuitin a high-eciency power supply. Without proper switchtransistor operation, the power supply could ail to achieveoptimum eciency and reliability.
Te present design o the saturation probe is simple andcost eective, can be easily built and oers good peror-mance, but it does require the use o a dierential probe
–9 V
–9 V
+9 V
+
RG2.7 kΩ
Q7D9
5.1-V zener
5.1-V
zener
+
+
+
Input
“Zero”
(press to zero)
1-½-A fuse
Fast
S1 1 W5.1-V zener
D5
D4
UF4007
UF4007D1 D3
1N4148
D6
1N4148
Red
LED
D8
Power
D7
Red LED
2N3904
2N3904Q2
2N3904Q3
R21 kΩ
1 mA
22 F16 V
R180.6 Ω
C4
C3
22 F16 V
C5
0.1 F
R7499 Ω
R9499 ΩR8
1 kΩ
R1020 kΩ
Zero
Q52N3906
Q62N3906
39 pFC1
2 mA R3
499 Ω
–9 V
R539 Ω
C20.1 µF
10 mA R4
Q4
100 Ω
2N3904
R11
39 Ω
Q1
2N3904
Output
All diodes and transistors:
Fairchild Semiconductor
9-V battery
9-V batteryS2
On/off
+9 V
–9 V
Fig. 1. Build this circuit o a saturation voltage probe or accurate measurements in switching power supplies.
Fig. 2. A small box houses the
fnished low-cost saturation
voltage tester.
Fig. 3. An inside look o the test box shown in Fig. 2 with the
cover removed.
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