SMPS high side supply questions

[Danko]

Hello!


I was thinking of the floating supply for high-side MOSFET in a mains powered SMPS... For example a full bridge or half-bridge.

Can it happen, that the bootstrap-capacitor cannot charge up fast enough, and the high-side MOSFET can't turn ON?
If it can happen, how can I avoid this?


Thanks in advance!

[DaBit]

With the half bridge the problem probably does not exist. When the supply starts, the lower MOSFET is turned on first, charging the bootstrap capacitor. Or the upper MOSFET is turned on, but there is no gate drive voltage available, causing the MOSFET to stay off.

With the full bridge, there is always one MOSFET which won't turn on the first cycle since Vg can't raise above Vs. This is no problem since no power is dissipated. In the next cycle, the bootstrap capacitor is charged and the problem is over.

Thus, no problems there I think

[Eva]

The duty cycle has to be very small in order to prevent the bootstrap capacitor from charging properly after one or two cycles (assuming a gate drive circuit that does not draw current when it's off). But then again, at very low duty cycles transformer saturation due to asymetric drive is not likely to happen at all, so it's safe.

[Danko]

Thank! So, I have /almost:-)/ nothing to worry about.

Thanks Eva, DaBit!