Hybrid Circuits Optimize Gate Drive and Protect High Power IGBT Modules

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

High power IGBT module applications usually require a completely isolated gate drive, as shown In Figure 1. This circuit isolates the logic level control from fault feedback signals using optocouplers and separate isolated power supplies for each gate driver. Its advantages include:

• Stable on and off drive driving voltages that are independent of the power device switching duty cycle

• Ability to allow very high output currents for large IGBT modules

• Isolation of power circuit switching noise and high voltages from control circuits

• Local power that is available for protection circuits, such as desaturation detection.

Among the disadvantages of this type of driver are the cost, complexity and board space required for all the isolated power supplies. In addition, these circuits can be difficult to develop due to the severe requirements for noise immunity and high isolation voltage. To simplify the design of isolated gate drive several new hybrid ICs have been developed to supply gate drive, short circuit protection and isolated gate drive power.

To understand the new driver circuit details, first look at the typical gate drive circuit illustrated in the simplified circuit schematic of Figure 2. The primary function of the gate drive circuit is to convert logic level control signals into the appropriate voltage and current for efficient, reliable, switching of the IGBT module. An output driver stage consisting of small power MOSFETs or bipolar transistors convert the logic levels by alternately connecting the IGBT's gate to the appropriate on and off voltages (V_ON and V_OFF, respectively). The driver stage devices and series gate resistance, RG, must be selected to provide the appropriate peak current for charging and discharging the IGBT's gate. Most gate drive circuits also isolate the low level logic signals from the dangerous high voltage present in the power circuit. The driver must also be immune to the severe EMI produced by the fast switching, high voltage, high current IGBT power circuit. Careful layout and component selection is critical to avoid problems with coupled noise.

TURN-ON VOLTAGE (V_ON)

To ensure collector-to-emitter conduction in an IGBT module, a positive voltage must be applied to the gate. The absolute maximum voltage that can safely be applied tothe IGBT's gate is usually specified on the device data sheet. For H- and F-Series IGBT modules it is 20V. Applying more than 20V may break down the IGBT's gate oxide, causing permanent damage. The 20V upper limit must be restricted even further if short circuit survival is required. The short circuit withstand time (tw) of a given device is inversely proportional to the product of applied voltage and short circuit current. The short circuit current increases with increasing gate voltage, thus degrading the withstand time. The H- and F-Series modules are guaranteed to survive a low impedance short circuit for 10µsec with an applied gate voltage of 15V± 10%.

The usable lower limit for the on-state gate voltage is decided by the IGBT's transconductance or gain, and acceptable switching losses. Figure 3 shows a typical output characteristics for a 1200V, 100A F-series IGBT module; about 10V on its gate is enough to support its peak current rating (ICM=2×IRATED).

A 10V gate drive would be sufficient to turn the device on fully, but it may not be sufficient to obtain efficient switching. If 10V were used for VON, it would cause a long dynamic saturation (slow turn-on) because the gate voltage takes a long time to reach 10V as it exponentially charges through the series gate resistance. For optimum performance, a turn-on gate drive should be 15V±10%. Using a voltage in this range will ensure that the device stays fully saturated and switches on efficiently while maintaining good short circuit durability.

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