2 Design and Potential Risk in Certain Application Scenario

In common half-bridge gate driver, bootstrap circuit is frequently used to drive two FETs with one power supply. But half-bridge gate driver with bootstrap circuit configuration is unable to drive high side FET with 100% duty cycle. Because the low side FET must be on for a certain time to let power supply charges the bootstrap capacitor up through bootstrap diode. If not, bootstrap capacitor can be unable to provide power for driving high side FET.

For one design to achieve 100% duty-cycle using half-bridge gate driver is to add extra power supply between HB and HS pin of gate driver. Figure 2-1 shows specific schematic. Then the energy to drive high side FET can be provided by this extra power supply instead of bootstrap capacitor. Then the high side FET is able to operate with 100% duty cycle.

The potential risk mainly appears from two reasons:

1. High duty cycle causes high current stress in bootstrap diode.
   For example, in the pass-through mode of optimizer, duty cycle of high side FET can gradually increase up to 100%. So, by end of transition process, a high duty cycle condition that persists for a period of time can let bootstrap diode withstand high current stress.
2. Influence by the extra power supply which is connected in HB, HS pin.
   Indeed, adding one extra power supply can let gate driver drive high side FET with 100% duty cycle. But the process still has some design consideration to make sure gate driver works properly. Improper design can increase bootstrap diode current stress during the transition process which is mentioned in reason 1.

These two reasons can be analyzed in the following details.