When parasitic inductances are introduced by nonideal PCB layout and long package leads (such as TO-220 and TO-247 type packages), ringing in the gate-source drive voltage of the power transistor could occur during high di/dt and dv/dt switching. If the ringing is over the threshold voltage, unintended turn-on and shoot-through could occur. Applying a negative bias on the gate drive is a popular way to keep such ringing below the threshold. A few examples of implementing negative gate-drive bias follow.

Figure 9-3 shows the first example with negative bias turn-off on the output using a Zener diode on the isolated power-supply output stage. The negative bias is set by the Zener diode voltage. If the isolated power supply is equal to 20 V, the turn-off voltage is –5.1 V and the turn-on voltage is 20 V – 5.1 V ≈ 15 V.

Figure 9-3 Negative Bias With Zener Diode on Iso-Bias Power-Supply Output

Figure 9-4 shows another example which uses two supplies (or single-input, double-output power supply). The power supply across V_CC2 and the emitter determines the positive drive output voltage and the power supply across V_EE2 and the emitter determines the negative turn-off voltage. This solution requires more power supplies than the first example, however, it provides more flexibility when setting the positive and negative rail voltages.

Figure 9-4 Negative Bias With Two Iso-Bias Power Supplies