ISO5852S-Q1: Boot Strap Power Supply

Manoj,

Thanks for your question.

In fact, for an inverter design, you can even supply the lowside drivers from single isolated supply and use bootstrap on the highside.

The advantage of bootstrap supply is reduced cost and complexity compared with transformer solution.

The key disadvantage is that it cannot support keeping mosfets on for 100% of the time, as the bootstrap cap is replenished every off-cycle. If the off-cycle gets smaller and smaller, the bootstrap cap charge will not be able to be replenished fully. In this case, a seperate supply for high-side should be used.

If you wish to have negative bias on your high-side using zener, the topology is different than if you used a seperate supply for high side and has limitations.

Care must be used to properly select bootstrap capacitor, current limiting resistor, as well. The below report is an excellent reference on bootstrap circuit design.

• App report on bootstrap circuit components selection

Alternative isolated supplies for high-side include push-pull, isolated flyback, etc. The advantage of using magnetic solutions, apart from allowing high duty cycle of the gate driver control, is that multiple rails can be generated via transformer in order to generate the negative bias supplies directly.

We also have reference designs using isolated flyback as well as SN650x push-pull converter to generate isolated (highside) supplies. Compared with the aforementioned isolated flyback, this option has a smaller footprint.

• Reference design using ISO5852S w/ isolated flybuck
  • Isolated flybuck used in above design

• TI Training video w/ insights on negative-turn-off using bootstrap.

If this has answered your question, pleas let me know by clicking the green button. Otherwise, let me know if you have any additional questions.

Best

Dimitri