Figure 9-2 Negative Bias with Zener Diode on Iso-Bias Power Supply Output

Figure 9-3 shows another example which uses two supplies (or single-input-double-output power supply). Power supply V_A+ determines the positive drive output voltage and V_A– determines the negative turn-off voltage. The configuration for channel B is the same as channel A. This solution requires more power supplies than the first example, however, it provides more flexibility when setting the positive and negative rail voltages.

Figure 9-3 Negative Bias with Two Iso-Bias Power Supplies

The last example, shown in Figure 9-4, is a single power supply configuration and generates negative bias through a Zener diode in the gate drive loop. The benefit of this solution is that it only uses one power supply and the bootstrap power supply can be used for the high side drive. This design requires the least cost and design effort among the three solutions. However, this solution has limitations:

1. The negative gate drive bias is not only determined by the Zener diode, but also by the duty cycle, which means the negative bias voltage will change when the duty cycle changes. Therefore, converters with a fixed duty cycle (~50%) such as variable frequency resonant convertors or phase shift convertors which favor this solution.
2. The high side VDDA-VSSA must maintain enough voltage to stay in the recommended power supply range, which means the low side switch must turn-on or have free-wheeling current on the body (or anti-parallel) diode for a certain period during each switching cycle to refresh the bootstrap capacitor. Therefore, a 100% duty cycle for the high side is not possible unless there is a dedicated power supply for the high side, like in the other two example circuits.

Figure 9-4 Negative Bias with Single Power Supply and Zener Diode in Gate Drive Path