This document from TI describes a method to allow use of bootstrapped N-Channel MOSFETS at high duty-cycles, by keeping the bootstrap capacitor "topped up" with an external charge pump, separate from the gate driver IC.
My question is: wouldn't this present a risk of exceeding the Vgs rating of the MOSFET? For example, say the maximum gate-source voltage on a FET is 16V, and it is being used as a high-side switch on a 12V load. Your average gate driver IC will (approximately) double the 12V to 24V, leading to a Vgs of 12, within the FET's rating. Say one now connects a constantly-running voltage doubler to the high side of the bootstrap capacitor. Now instead of the bootstrap mechanism doubling the 12V, it will double the 24V coming from the external doubler, leading to a Vgs of 36V.
Is this the case, or am I misunderstanding something? Is there an easy way to mitigate this? The only thing I can think of is disabling the charge pump whenever (probably slightly before) the high-side switch turns off, which sounds like a pain.
(Note that the gate driver I am working with has the bootstrap diode built-in, hence its absence form the schematic)
If it helps, here are the specific parts I'm hoping to use:
- Gate Driver: Micrel Inc. MIC4606-2YML
- Charge Pump: Texas Instruments LM2767M5
- FET: Texas Instruments CSD87588N
(Yes, this is for an H-bridge, not just a high-side switch. I am just confused about the bootstrapping part of it.)