Power Electronics

Hybrid IC IGBT Gate Driver Integrates DC/DC Converter

A fully isolated gate driver for IGBTs consists of the drive amplifier and an isolated dc-dc converter in a SIP outline package.

The VLA502-01 is a hybrid integrated circuit (Figure 1) intended as a gate driver for high power IGBT modules. This circuit has been optimized for use with Powerex NFH-Series IGBT modules. However, the output characteristics are compatible with most MOS gated power devices. The VLA502-01 features a compact single-in-line package design. The upright mounting design minimizes printed circuit board space to allow efficient and flexible layout. The VLA502-01 converts logic-level control signals into fully isolated +15V/-8V gate drive with up to 12A of peak drive current. Isolated drive power is provided by a built in dc-dc converter and control signal isolation is provided by an integrated high speed optocoupler. Short circuit protection is provided by means of destauration detection.

Figure 2 is a block diagram of a typical desaturation detector. In this circuit, a high voltage fast recovery diode (D1) is connected to the IGBT's collector to monitor the collector to emitter voltage. When the IGBT is in the off-state, VCE is high and D1 is reverse biased. With D1 off, the (+) input of the comparator is pulled up to the positive gate drive power supply (V+) which is normally +15V. When the IGBT turns on, the comparators (+) input is pulled down by D1 to the IGBT's VCE(SAT). The (-) input of the comparator is supplied with a fixed voltage (VTRIP). During a normal on-state condition the comparator's (+) input will be less than VTRIP and its output will be low. During a normal off-state condition the comparator's (+) input will be larger than VTRIP and its output will be high. If the IGBT turns on into a short circuit, the high current will cause the IGBT's collector-emitter voltage to rise above VTRIP even though the gate of the IGBT is being driven on. This abnormal presence of high VCE when the IGBT is supposed to be on is called desaturation. Desaturation can be detected by a logical AND of the driver's input signal and the comparator output. When the output of the AND goes high, a short circuit is indicated.

The output of the AND can be used to command the IGBT to shut down in order to protect it from the short circuit. 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 IGBT's 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 allowable tTRIP delay is limited by the IGBTís short circuit withstanding capability. In typical applications using Powerex IGBT modules the recommended limit is 10µs.

The VLA502-01 incorporates short circuit protection using desaturation detection as described above. A flow chart for the logical operation of the short-circuit protection is shown in Figure 3. When a desaturation is detected the hybrid gate driver performs a soft shutdown of the IGBT and starts a timed (tTIMER) 1.5ms lock out. 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 lock-out the driver pulls Pin 28 low to indicate the fault status. Normal operation of the driver will resume after the lock-out time has expired and the control input signal returns to its off state.

ADJUSTMENT OF TRIP TIME

The VLA502-01 gate driver has a default short-circuit detection time delay (tTRIP) of approximately 3µs. This will prevent erroneous detection of short-circuit conditions as long as the series gate resistance (RG) is near the minimum recommended value for the module being used. The 3µs delay is appropriate for most applications so adjustment will not be necessary. However, in some low frequency applications it may be desirable to use a larger series gate resistor to slow the switching of the IGBT, reduce noise, and limit turn-off transient voltages. When RG is increased, the switching delay time of the IGBT will also increase. If the delay becomes long enough so that the voltage on the detect Pin 30 is greater than VSC at the end of the tTRIP delay the driver will erroneously indicate that a short circuit has occurred. To avoid this condition, the VLA502-01 has provisions for extending the tTRIP delay by connecting a capacitor (CTRIP) between Pin 29 and VEE (Pins 21 and 22). The waveform defining trip time (tTRIP) is shown in Figure 4. If tTRIP is extended, care must be exercised not to exceed the short-circuit withstanding capability of the IGBT module.

ADJUSTMENT OF SOFT SHUTDOWN SPEED

As noted above, the VLA502-01 provides a soft turn-off when a short circuit is detected in order to help limit the transient voltage surge that occurs when large short circuit currents are interrupted. The gate driver's default shutdown speed will work for most applications so adjustment is usually not necessary. In this case CS can be omitted. In some applications using large modules or parallel connected devices it may be helpful to make the shutdown even softer, to minimize the effects of possible transient voltage surges. This can be accomplished by connecting a capacitor (CS) at Pin 27.

The VLA502-01 IGBT gate driver is housed in a 30-pin single-in-line package (SIP), as shown in Figure 5. This package outline minimizes the space required in the mounting area, to fit space-constrained designs.

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