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Keysight Technologies
Internal Gate Resistance Measurement Using the B1505A




                                Application Note
Introduction

     Power MOSFET and IGBT internal gate resistance is an important device paramteter, since it can limit the
     maximum switching frequency or determine the driving loss in switching converters and inverters. For the
     lower totem pole transistor of the converter output, it is also important to minimize the internal gate resitance
     to prevent device self turn-on. Device self turn-on is caused by transient currents injected into the gate
     terminal of the lower totem pole transistor through capacitive coupling when the upper totem pole transistor
     turns on (refer to igure 1).

     Since this phenomena can cause device destruction, controlling internal gate resistance variations
     is as important as minimizing the absolute gate resistance value. Unfortunately, most data sheets
     only show typical values for the internal gate resitance. Since the internal gate resitance has a strong
     temperature dependence, measuring it for many devices under actual use conditions is necessary to
     understand its true value and temperature behavior.

     Internal gate resistance is typically measured at a speciic frequency using an LCR meter. The Keysight
     Technologies, Inc. B1505A power device analyzer/curve tracer has an LCR meter module option
     for power device capacitance measurement. This module therefore gives the B1505A the ability to
     also measure internal gate resistance. This application note explains how to measure internal gate
     resistance using the B1505A and also shows an actual example measurement.


                                                     Driving Loss


                                                    D
                                       G                High Side
                                                        MOS FET

                                                    S
      Vin        Controller IC
                                                    D
                                       G                Low Side
                                                        MOS FET
                                                                                 Vout
                                                    S
                      Self -Turn -On



     FIGURE 1. Driving loss and self-turn-on induced by internal gate resistance in a DC-DC converter
03 | Keysight | Internal Gate Resistance Measurement Using the B1505A - Application Note



Measurement Fundamentals
Figure 2 shows the simpliied cross section of a trench gate type power MOSFET. Cgd is the
gate to drain capacitance, Cgs is the gate to source capacitance, and Cds is the drain to source
capacitance. In addition to these capacitances, the gate to channel capacitance (Cgc) is also
shown. The internal gate resistance, Rg(int), is in series with Cgd, Cgs and Cgc.

      S                             G

                                         Rg (int)


          n+                Cgs                Cgs              n+
                                     C
     p+
          Cds         Cgc                           Cgc   Cds
                                         Cgd
     n-


     n+




                                         D
FIGURE 2. Terminal capacitances and internal gate resistance of trench MOS-FET


An LCR meter extracts device capacitance and resistance using an equivalent circuit
model such as Cp-Rp, Cp-G or Cs-Rs (refer to igure 3).

To measure the internal gate resistance the Cs-Rs mode is used. However, to accurately
measure the internal gate resistance, it is important to use the appropriate connections,
bias conditions and measurement frequency.




    Cs
                                  Cp                             Rp
    Rs


            Cs-Rs                               Cp-Rp
                                                Cp-G
FIGURE 3. Equivalent LCR meter circuit models to extract capacitance and resistance
04 | Keysight | Internal Gate Resistance Measurement Using the B1505A - Application Note



Measurement Examples
Connections
Figure 4 shows a connection diagram to measure internal gate resistance using the B1505A's
Multi-Frequency Capacitance Measurement Unit (MFCMU) and N1259A test ixture. The
N1259A has two AUX inputs, which have BNC connectors. The outputs of the MFCMU are
connected to the N1259A's AUX inputs via BNC-T connectors.


                             1250 -2405
                                                         N1259A Test Fixture
     B1505A                  BNC 

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