VISHAY SILICONIX Power MOSFETs Application Note AN-957 Measuring Power MOSFET Characteristics TABLE OF CONTENTS Page 1. General Information .................................................................................................................................................................. 2 2. BVDSS ....................................................................................................................................................................................... 3 3. IDSS ........................................................................................................................................................................................... 3 4. VGS(th) ........................................................................................................................................................................................ 3 5. IGSS ............................................................................................................................................................................................ 4 6. gfs .............................................................................................................................................................................................. 4 7. RDS(on) ....................................................................................................................................................................................... 5 8. VSD............................................................................................................................................................................................. 5 9. Composite Characteristics ....................................................................................................................................................... 6 10. Transfer Characteristics ......................................................................................................................................................... 6 11. Measurement of Power MOSFET Characteristics without a Curve Tracer ............................................................................ 6 12. A Fixture to Speed-up Testing Time ...................................................................................................................................... 9 Document Number: 90715 Revision: 18-Nov-10 www.vishay.com 1 This document is subject to change without notice. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 APPLICATION NOTE This application note describes methods for measuring Power MOSFET characteristics, both with a curve tracer and with special-purpose test circuits. Application Note AN-957 Vishay Siliconix Measuring Power MOSFET Characteristics TOPICS COVERED: • Gate leakage The curve tracer used as an example in this application note is a Tektronix 576, since this instrument is in widespread use. However, the principles involved apply equally well to other makes and models. Figure 1 shows the layout of the controls of the Tektronix 576 curve tracer, with major controls identified by the names used in this application note. Throughout this applicationnote, when controls are referred to, the name of the control is printed in capitals. For all tests, when the power is on, the initialstate of the curve tracer is assumed to be as follows: • Transconductance • Left/right switch in “off” position • On-resistance • Variable collector supply at zero • Diode drop • Display not inverted • Characteristics in synchronous rectification • Display offset set at zero • Transfer characteristics • Step/offset polarity button out (not inverted) • Measurements without a curve tracer • Vert./horiz. display magnifier set at norm (off) • Device capacitances • The rep button of the step family selector should be in • Switching times • The aid button of the offset selector should be in • Gate charge • The norm button of the rate selector should be in • Converting the nomenclature from bipolars to MOSFETs • P-channel Power MOSFETs • Initial settings • Breakdown • Drain leakage • Gate threshold • Reverse recovery • A fixture to speed-up testing time • Related topics APPLICATION NOTE 1. GENERAL Curve tracers have generally been designed for making measurements on bipolar transistors. While Power MOSFETs can betested satisfactorily on most curve tracers, the controls of these instruments are generally labeled with reference to bipolar transistors, and the procedure to follow in the case of MOSFETs is not immediately obvious. This application note describes methods for measuring Power MOSFET characteristics, both with a curve tracer and with special-purpose test circuits. Testing Power MOSFETs on a curve tracer is a simple matter, provided the broad correspondence between bipolar transistor and Power MOSFET features are borne in mind. Table 1 matches some features of Power MOSFETs with their bipolar counterparts. The Power MOSFET used in all the examples is the IRF630. The controlsettings given in the examples are those suitable for the IRF630. The user must modify these values appropriately when testing adifferent device. The IRF630 was selected since it is a typical mid-range device with a voltage rating of 200 V and acontinuous current rating of 9 A (with TC = 25 °C). For measurements with currents above 20 A, or for pulsed tests notcontrolled by the gate, the Tektronix 176 Pulsed High Current Fixture must be used instead of the standard test fixture. The IRF630 is an n-channel device. For a p-channel device, all the test procedures are the same except that the position of the Polarity Selector Switch must be reversed - that is, for p-channel devices, it must be in the PNP position. www.vishay.com 2 Fig. 1 - Location of controls in a 576 curve tracer The accuracy of all tests is predicated on the correct use of the Kelvin connections, as indicated in the instructions for the curve tracer. This is particularly important for power semiconductors, as inductive and resistive drops across sockets and wiring are significant. Some tests require the use of high voltages. After the device is mounted in the test fixture as described for each test, the test fixture safety cover should be closed and the curve Document Number: 90715 Revision: 18-Nov-10 This document is subject to change without notice. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 Application Note AN-957 Vishay Siliconix Measuring Power MOSFET Characteristics tracer manufacturer’s safety warnings heeded. The exposed metal parts of many Power MOSFETs (for example, the tab of TO-220 devices) are connected to the drain and are therefore at the potential of the collector supply. As with any semiconductor device, some of the characteristics of Power MOSFETs are temperature dependent. For tests in which there is significant heating of the Power MOSFET, a low repetition rate should be used. For tests involving a slow transition through the linear region, a damping resistor of at least 10 should be connected in series with the gate, close to the gate lead to prevent oscillations. If frequent testing of MOS-gated devices is expected, the use of a test fixture that plugs directly into the curve tracer would save a significant amount time. Such a fixture is descibed in Section 12. MOS-gated transistors are static sensitive. Wrist straps, grounding mats and other ESD precautions must be followed, as indicated in INT-955. 2. BVDSS This is the drain-source breakdown voltage (with VGS = 0). BVDSS should be greater than or equal to the rated voltage of the device, at the specified leakage current. 1. Connect the device as follows: drain to “C”, gate to “B”, source to “E”. 2. Set the max. peak V to 350 V. PER V E R T DIV 50 µA PER H O R Z DIV 50 V Fig. 2 - Drain-source breakdown voltage 3. IDSS This is the drain current for a drain-source voltage of 100 % of rated voltage, with VGS = 0. This measurement is made in the same manner as BVDSS, except that: 1. The mode switch is set to “leakage”. 2. Connect the device using the left/right switch and adjust the collector supply voltage to the rated voltage of the Power MOSFET (200 V for the IRF630). Read the value of IDSS from the display (see figure 3). The vertical sensitivity may need altering to obtain an appropriately sized display. Often IDSS will be in the nA range and the current observed will be capacitor currents due to minute variations in collector supply voltage. 3. Set the series resistor to limit the avalanche current to a safe value (i.e., tens of mA). A suitable value in this case would be 14 k. PER V E R T DIV 1 ηA PER H O R Z DIV 50 V 4. Set the polarity switch to NPN. 5. The mode control should be set to norm. 6. Horizontal V/div. should be set at 50 V/div. on the “collector” range. 7. Vertical current/div. should be set at 50 µA/div. 8. On the plug-in fixture, the connection selector should be set to “short” in the “emitter grounded” sector. This action grounds the gate and disables the step generator. Fig. 3 - Drain-source breakdown voltage 4. VGS(th) This is the gate-source voltage which produces 250 μA of drain current (VDS = VGS). At this gate-source voltage the device enters the active region. In circuits where devices are connected in parallel, switching losses can be minimized by using device swith closely matched threshold voltages. This test requires the gate to be connected to the drain and conducted as follows: 1. Connect the device as follows: source to “E”, gate to “B”, drain to “C”. This connection arrangement may require the construction of a special test fixture. Bending of the device leads can cause mechanical stress which results in the failure of the device. Document Number: 90715 Revision: 18-Nov-10 www.vishay.com 3 This document is subject to change without notice. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 APPLICATION NOTE 9. Connect the device using the left/right switch. Increase the collector supply voltage using the variable collector supply control until the current (as indicated by the trace on the screen) reaches 250 µA. (see figure 2.) Read BVDSS from the screen. Application Note AN-957 Vishay Siliconix Measuring Power MOSFET Characteristics 2. Set the max. peak V to 15 V. 3. Set the series resistor to 0.3 . 4. Set polarity to PNP. This causes the drain (collector) terminal to be negative with respect to the source (emitter) terminal. 5. Set the mode control to norm. 6. Set the vertical current/div. to 50 µA/div. 7. Set the horizontal V/div. to 500 mV/div. 8. Set the connection selector to “short” in the “emitter grounded” sector. 9. Display should be inverted. 10. Connect the device using the left/right switch. Increase the variable collector voltage until the drain current reaches 250 µA as indicated by the trace on the screen. Read the voltage on the horizontal center line (since this line corresponds to ID = 250 µA) (see figure 4). PER V E R T DIV 50 µA PER H O R Z DIV 500 mV supply control, but do not exceed 20 V, the maximum allowable gate voltage. It may be necessary to adjust the vertical sensitivity. Read the leakage current from the display (see figure 5). In many cases, the leakage current will be inthe nA range, in which case the trace will be dominated by currents which flow through the device capacitance as aresult of minute fluctuations in the collector supply voltage. 10. The above procedure is for determining gate leakage current with a positive gate voltage. To make the same measurement using a negative voltage, reduce the variable collector supply voltage to zero, change the polarity switch to the PNP position, and reapply the voltage (see figure 6). The trace will take time to settle because of the gate-source capacitance. PER V E R T DIV 1 ηA PER H O R Z DIV 5V Fig. 5 - Gate-source threshold voltage 6. gfs Fig. 4 - Gate-source threshold voltage 5. IGSS This is the gate-source leakage current with the drain connected to the source. An excessive amount of gate leakage current indicates gate oxide damage. 1. The device is connected as follows: gate to “C”, drain to “B”, source to “E”. This is not the usual connection sequence, and a special test fixture will be required if bending of the leads is to be avoided. APPLICATION NOTE 2. Set the max. peak V to 75 V. 3. Set the series resistor to a low value (for example., 6.5 ). 4. Set polarity to NPN. This is the forward transconductance of the device at a specified value of ID. gfs represents the signal gain (drain current divided by gate voltage) in the linear region. This parameter should be measured with a small AC superimposed on a gate bias and the curve tracer is not the appropriate tool for this measurement. Even with specific test equipment, as indicated in section 11, the DC bias tends to overheat the MOSFET very rapidly and care should be exercised to insure that the pulse is suitably short. PER V E R T DIV 1 ηA PER H O R Z DIV 5V 5. Set the mode switch to leakage. 6. Set the connection selector to the “short” position in the “emitter grounded” sector. 7. Horizontal V/div. should be set at 5 V/div. 8. Vertical current/div. should be set to an appropriately low range. Fig. 6 - Gate-source leakage current at - 20 V 9. Connect the device using the left/right switch. Increase the collector supply voltage using the variable collector www.vishay.com 4 Document Number: 90715 Revision: 18-Nov-10 This document is subject to change without notice. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 Application Note AN-957 Vishay Siliconix Measuring Power MOSFET Characteristics 7. RDS(ON) PER V E R T DIV This is the drain-source resistance at 25 °C with VGS = 10 V. Since RDS(on) is temperature-dependent, it is important to minimize heating of the junction during the test. A pulse test is therefore used to measure this parameter. The test is set up in the following manner: PER H O R Z DIV 1A 500 mV 1. Connect the device as follows: gate to “B”, drain to “C”, source to “E”. 2. Set the max. peak V to 15 V. 3. Set the series resistor to 0.3 . 4. The polarity switch should be set to NPN. Fig. 8 - Drain-source resistance 5. The mode switch should be set to “norm”. 8. VSD 6. Set the step amplitude to 1 V. 7. Set number of steps to 10. 8. Set offset mult to 0. 9. The current limit should be set to 500 mA. 10. The step multiplier button should be out - that is, 0.1X not selected. 11. On the pulsed steps selector, the 80 µs button should be in (or the 300 µs, if the 80 is not available). 12. On the rate selector, the 0.5X button should be in. 13. Set vertical current/div. at 1 A/div. (IRF630). This scales hould be chosen according to the on-resistance of the device being tested. 14. Set the connection selector to the “step gen” position in the “emitter grounded” sector. 15. Connect the device using the left/right switch and raise the variable collector supply voltage until the desired value of drain current is obtained. RDS(on) is obtained from the trace by reading the peak values of current and voltage (see figures 7 and 8). RDS(on) = VDS/ID. Logic level devices would have different settings for 6, 7, and 8 so that the on-resistance is measured at the specified gate voltage. 1A PER H O R Z DIV 500 mV 1. Connect the device as follows: gate to “B”, drain to “C”, source to “E”. 2. Set the max. peak V to 15 V. 3. Set the series resistor at 1.4 or a value sufficiently low that rated current can be obtained. 4. Set polarity to PNP. 5. Set the mode switch to “norm”. 6. The 80 µs button of the pulsed steps selector should be in (or the 300 µs, if the 80 is not available). 7. The connection selector should be set to the “short” position in the “emitter grounded” sector. 8. Horizontal V/div. should be on 200 mV/div. 9. Vertical current div. should be on 1 A/div. 10. The display button should be set to invert. 11. The device is connected using the left/right switch. Increase the variable collector supply voltage until rated current is reached (9 A for the IRF630). Read VSD from the trace (see figure 9). PER V E R T DIV 1A PER H O R Z DIV 200 mV Fig. 9 - Source-drain voltage (diode) Fig. 7 - Drain-source resistance, pulsed mode Document Number: 90715 Revision: 18-Nov-10 www.vishay.com 5 This document is subject to change without notice. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 APPLICATION NOTE PER V E R T DIV This is the source-drain voltage at rated current with VGS = 0. It is the forward voltage drop of the body-drain diode when carrying rated current. If pulsed mode testing is required, use high current test fixture. Application Note AN-957 Vishay Siliconix Measuring Power MOSFET Characteristics 9. COMPOSITE CHARACTERISTICS 2. Set horizontal V/div. on “step gen”. The forward and reverse characteristics of the Power MOSFET may be viewed at the same time. This display can beused to obtain an appreciation of the Power MOSFET’s behavior in applications in which current flows in the channelin either direction. such as synchronous rectifiers and analog waveform switching. The procedure is the same as for on-resistance except that: 3. The 300 µs button of the pulsed step selector should be in. 4. Increase the variable collector supply voltage to obtain the trace shown in figure 12. The transfer characteristic is outlined by the displayed points. PER V E R T DIV 1. Offset is set to zero. 2. The polarity control is set at “AC”. 3. The device is connected using the left/right switch. The variable collector supply voltage is increased to obtain the required peak value of ID. Beware of device heating. Figure 10 shows the trace obtained with the IRF630. To obtain the reverse characteristics of the diode alone, invert the step polarity. The FET is inoperative, and the display will resemble that shown in figure 11. The step polarity should also be inverted to obtain the composite characteristics of p-channel devices. PER V E R T DIV PER H O R Z DIV 2A 200 mV 1A PER H O R Z DIV Fig. 12 - Transfer characteristic (ID vs. VGS) 11. MEASUREMENT OF POWER MOSFET CHARACTERISTICS WITHOUT A CURVE TRACER Power MOSFET parameters can be measured usingstandard laboratory equipment. Test circuits and procedures fordoing this are described in the following sections, with the IRF630 used as an example. The test arrangement should be variedappropriately for other devices. 250 µA Fig. 10 - Operation in first and third quadrant (synchronous rectification) PER V E R T DIV PER H O R Z DIV 2A D BVDSS DVM G S 200 mV APPLICATION NOTE Fig. 13 - Test circuit for BVDSS BVDSS, Drain-Source Breakdown Voltage Fig. 11 - Operation in first and third quadrant without gate drive 10. TRANSFER CHARACTERISTICS The transfer characteristic curve of ID versus VGS may be displayed usingthe pulse mode. The test is set up in the same manner as the on-resistance test, except for the following: 1. Offset multiply should be set at zero. www.vishay.com 6 The test circuit is shown in figure 13. The current source will typically consist of a power supply with an output voltage capability of about 3 time BVDSS in series with a current defining resistor of the appropriate value. When testing high voltage Power MOSFETs it may not be practical or safe to use a supply of 3 times BVDSS. In such cases, another type of constant current source may be used. VGS(th), Threshold Voltage The test circuit is shown in figure 14. The 1 k gate resistor is required to suppress potentially destructive oscillations at the gate. The current source may be derived from a voltage Document Number: 90715 Revision: 18-Nov-10 This document is subject to change without notice. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 Application Note AN-957 Vishay Siliconix Measuring Power MOSFET Characteristics source equal to the gate voltage rating of the Power MOSFET and a series resistor. CP VDS(on), On-Resistance The test circuit is shown in figure 15. The pulse width should be 300 μs at a duty cycle of less than 2 %.The value quoted is at a junction temperature of 25 ºC. RDS(on) is calculated by dividing VDS(on) by ID. Connect the ground of the gate supply as close to the source lead as possible. 250 µA VGS(th) + 50 V 100 Ω - OSCILLOSCOPE Fig. 16 - Test circuit for transconductance Figures 17, 18 and 19 show the circuit connections for the three capacitances that characterize Power MOSFETs. D DVM 1KΩ G + S 2000 pF - HIGH Fig. 14 - Test circuit for BVDSS CK gfs, Transconductance CB Connect a 50 V power supply between the device drain and source, as shown in figure 16. Use a current probe to measure ID. A signal generator operating at low duty cycle to prevent heating of the device, is used to obtain 80 μs pulses of the required voltage (VGS) to obtain the following currents: 0.015 x ID , 0.05 x ID , 0.15 x ID , 0.5 x ID , and 1.5 x ID where ID is the rated value at TC = 25 ºC. Plot a graph of VGS vs. ID.The transconductance is equal to the slope of. S Fig. 17 - Test circuit for input capacitance D + ID 1.0 µF G - HIGH D CK CB OSCILLOSCOPE or DVM 10 MΩ LOW 2000 pF BVDSS 1.0 µF G 10 MΩ S PULSE COMMAND G LOW + 10 V S Fig. 18 - Test circuit for output capacitance Fig. 15 - Test circuit for drain on-state voltage Turn-on Delay Time, Rise Time, Turn-Off Delay Time, Fall Time - A 1 MHz capacitance bridge is used for all these tests. The capacitance to be measured is connected in series with a capacitance of known value to provide dc isolation. If Cu is the unknown capacitance, Ck is the known capacitance, and Cm is the measured capacitance, then Cu can be calculated as follows: Ck x Cm C u = ---------------------Ck - Cm Document Number: 90715 Revision: 18-Nov-10 (1) Data sheet value are for a resistive load, as shown in figure 20, as well as individual data sheets. The gate pulses should be just long enough to achieve complete turn-on, with a duty cycle of the order of 0.1 %. The series resistor is as specifyed in the datasheet. The definitions of rise, fall and delay times are given in figure 21. Power MOSFETs can switch in ns. Unless the test circuit is laid-out with RF techniques, the measurements will be totally unreliable. Switching time measurements frequently amount to a characterization of the test circuit, rather than the device under test. Gate charge provides a better indication of the switching capability of Power MOSFETs. www.vishay.com 7 This document is subject to change without notice. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 APPLICATION NOTE Ciss, Coss, and Crss. Output, input and reverse transfer capacitances Application Note AN-957 Vishay Siliconix Measuring Power MOSFET Characteristics 2000 pF D 10:1 LOW CB or DVM ID G 25 V 1.5 mA VDS ≤ 0.8 x 200 V S HIGH 10M OSCILLOSCOPE D Fig. 22 trr, Qrr body-drain diode reverse recovery time and reverse recovery charge Fig. 19 - Test circuit for transfer capacitance RD Rg TO THE OSCILLOSCOPE G + 0.5 x RATED VDS VDS D Several test circuits are commonly used to characterize these parameters. Some have been qualified by JEDEC. The datasheet indicates the test method used for the specific device. VGS S 10 V VGS - 10 V V2 Fig. 20 - Test circuit for switching times Qg, Qgs, Qgd Total Gate charge, Gate-Source charge, Gate-Drain charge The total gate charge has two components: the gate-source charge and the gate-drain charge (often called the Miller charge). INT-944 gives more details on this test. Figures 22 and 23 show the testcircuit and waveforms. From the relationship Q = i, the following results are obtained: Qg = (t3 - t0) ig, Qgd = (t2 - t1) ig, Qgs = Qg - Qgd V1 t0 t1 t2 t3 t Fig. 23 - Gate charge waveforms S OSCILLOSCOPE or DVM G IS VDS 90 % APPLICATION NOTE D Fig. 24 - Test circuit MOSFET diode drop 10 % VGS td(on) tr ON DELAY TIME RISE TIME tdoff tf OFF DELAY TIME FALL TIME Fig. 21 - Switching time waveforms VSD Body-Drain Diode Voltage Drop The current source may consist or a voltagesource and a series resistor, as shown in figure 24. The voltage should be applied in short pulses (less than 300 μs) with a low duty cycle (less than 2 %). www.vishay.com 8 12. A FIXTURE TO SPEED-UP TESTING TIME The most commonly tested parameters in a MOS-gated transistor are gate-source leakage (IGSS), drain-source resistance (RDS(on)), breakdown voltage (BVDSS), drain current (IDSS), source-drain voltage (VSD), threshold (VGS(th)), and soon. These tests can begreatly simplifyed with the fixture shown in figure 25. Document Number: 90715 Revision: 18-Nov-10 This document is subject to change without notice. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 Application Note AN-957 Vishay Siliconix Measuring Power MOSFET Characteristics 1 TO CURVE TRACER 2 TO PVT 3 C 4 1 CSENSE 2 B 3 330 Ω DSENSE G 4 1 ESENSE E 30 V 2 3 30 V 4 SSENSE S Fig. 25 - Test fixture for Power MOSFET test Position Measurement Comment 1 IGSS 2 3 4 RDS(on) BVDSS/IDSS/VDS VGS(th) C sense disconnected, Drain Source S/C connected, Collector Voltage applied to gate via 330 resistor. Note: Gate protected by back to back 30 V zeners. Collector Voltage applied to Drain Based Voltage applied to Gate via 330 resistor. Collector Voltage applied to Drain, Gate Source S/C connected via 330 resistor. Collector Voltage applied to Drain, Gate Drain S/C connected via 330 resistor. Related topics: • Parameter definition in IGBTs • Gate Charge • Thermal characteristics • ESD sensitivity • ESD test methods www.vishay.com 9 This document is subject to change without notice. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 APPLICATION NOTE Document Number: 90715 Revision: 18-Nov-10