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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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