Hybrid Circuits Optimize Gate Drive and Protect High Power IGBT Modules

Jun 1, 2010 12:00 PM
ERIC R. MOTTO Powerex Inc., Youngwood, Pennsylvania

A delay (tTRIP) must be provided after the comparator output to allow for the normal turn on time of the IGBT. The tTRIP delay is set so that the IGBTs VCE has enough time to fall below VTRIP during normal turn on switching. If tTRIP is set too short, erroneous desaturation detection will occur. The maximum tTRIP delay is limited by the IGBT's short circuit withstanding capability. For H-Series and F-Series IGBT modules this limit is 10µsec.

Several hybrid IC gate drivers implementing desaturation detection are single-in-line packaged and have outlines similar to the M57962L shown in Figure 8. When desaturation is detected the hybrid gate driver performs a soft shut down of the IGBT and starts a timed (tRESET) 1.5msec lockout. The soft turn-off helps to limit the transient voltage that may be generated while interrupting the large short circuit current flowing in the IGBT. During the lockout a fault feedback signal is asserted and all input signals are ignored. Normal operation of the driver will resume after the lockout time has expired and the control input signal returns to its off state.

RTC DETECTION

F-Series trench gate IGBT modules have a built in RTC (Real Time Control) circuit that limits short circuit current and maintains 10µsec short circuit durability. When excessive current flows in the device, the RTC circuit activates and reduces the gate voltage to limit the short circuit current. The RTC circuit shown in Figure 9 is similar to the desaturation detector except that the comparators (-) input is connected to the gate of the IGBT module. The (+) input of the comparator is supplied with a fixed voltage (VTRIP) set at about 3V below the positive gate drive supply voltage. In the normal on state, the gate of the IGBT is at nearly the positive supply voltage, which exceeds VTRIP and makes the comparator output low. In the off state the gate voltage is at nearly the negative gate drive supply voltage, which is less than VTRIP, making the comparator output high. If a short circuit occurs, the RTC circuit inside the F-Series IGBT module activates and pulls the gate voltage down. If the gate voltage becomes less than VTRIP when the IGBT is being commanded on, the RTC activates. RTC operation is detected by a logical AND of the gate driver's input signal and the comparator's output. When the output of the AND gate goes high, a short circuit is indicated. The output of the AND gate is used to command the IGBT to shut down to protect it from the short circuit. A delay (tTRIP) must be provided after the comparators output to allow for the normal rise of gate voltage at turn on. The tTRIP delay is set so that the gate voltage has enough time to exceed VTRIP during normal turn on switching. If tTRIP is set too short erroneous short circuit detection will occur. The maximum tTRIP delay is limited by the IGBT's short circuit withstanding capability. For F-Series IGBT modules this limit is 10µsec.

The M57160L-01 hybrid gate driver circuit implements RTC This protection scheme is superior to conventional desaturation detection, because it avoids the need for high voltage components and reduces spacing requirements on the gate drive pc board.

Fully-isolated gate drive circuits with short circuit protection, like the one shown in Figure 1, can be realized by combining the hybrid gate drive circuits with the hybrid dc-dc converter described above. Figure 10 shows a complete gate driver constructed with only eight components using hybrid circuits.

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