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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 oineiciency in the power transistor. Because those losses are a unction oa 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 othis two-part article series discussed the contribution osaturation 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 othe article. A detailed description is given othe 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 leand 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 obattery lie through resistor R6, diodes D8 to D10 and transistor Q7. Power indicator D8 will go out ithe voltage oeither 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 ored 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 obattery 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 othe 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 oproduction and no longer readily available. For most applications, the 2N3904 and 2N3606 transistors work well and are inexpensive. Working rom the input othe 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 aturatiottr to aurthaturatiovoltagowithig traitorauratly ithrohigh withig voltagor 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

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  • 8/6/2019 test Saturation Voltage

    1/3Power Electronics Technology April 2008 www.powerelectronics.com2020

    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 transistors saturation voltage, its 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 highvoltages 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 bytwo 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 mayseem 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 andvoltage 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 workwell 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 than9 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 batterys lie.Switch S1 and diode D4 allow theoutput to be zeroed when setting upthe oscilloscopes baseline, which isvery 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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    2/3www.powerelectronics.com Power Electronics Technology April 200821

    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 probes 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 approximately1.65 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-s 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 k

    Q7D9

    5.1-V zener

    5.1-V

    zener

    +

    +

    +

    Input

    Zero

    (press to zero)

    1--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 k

    1 mA

    22 F16 V

    R180.6

    C4

    C3

    22 F16 V

    C5

    0.1 F

    R7499

    R9499 R8

    1 k

    R1020 k

    Zero

    Q52N3906

    Q62N3906

    39 pFC1

    2 mAR3

    499

    9 V

    R539

    C20.1 F

    10 mAR4

    Q4

    100

    2N3904

    R11

    39

    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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