High-side driver feeds IGBTs and MOSFETs

Carlisle Dolland, Honeywell Engines and Systems, Torrance, CA -- EDN, 11/23/2000

The low- to moderate-power system in Figure 1 provides the interface between a pulse-width modulator and a high-side IGBT (insulated-gate-bipolar-transistor) or MOSFET switch. You can use it to interface TTL or CMOS circuitry to an H-bridge if you buffer it by a FET driver, such as an ICL7667 or an MIC4423. When OA is positive, D₁ conducts, charging the capacitance of the FET through R₁. The value of R₁ and the output impedance associated with the drive signal determine the turn-on time of the FET. After the capacitance is charged, the voltage across R₁ is essentially 0V, and Q₁ is off. During the PWM dead time, the gate capacitance discharges through Q₁ and R₂. R₂ and the h_fe of Q₁ determine the turn-off time of the FET. This circuit achieves turn-on and -off times of less than 150 nsec.

In systems that require higher power, you can use a dual-FET circuit (Figure 2). R₁ and R₂ and the output impedance of the PWM or FET driver determine the turn-on time. R₁ and R₂ and the h_fe values for Q₁ and Q₂ determine the turn-off time. The Schottky diodes, D₃ and D₄, prevent current flow through the collector-base junctions of Q₁ and Q₂ when the drive signal is negative. You can obtain switching speeds higher than 50 nsec at turn-on and 100 nsec at turn-off with this circuit, depending on the output impedance of the transformer drive signal.