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Two-Switch converter bootstrap

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kathmandu

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Hello,

I've just build a two-switch flyback converter (Vin=48VDC, Vout=12VDC, Pout=50W) but the boostrap circuit for the high-side driver doesn't work quite well. That's it, during the Mosfet switches OFF period, the low-side freewheeling diode is only forward biased for a (too) small period of time (I suppose) hence the bootstrap capacitor doesn't get enough charge.

I've seen a workaround for this issue, by adding an extra switch across the low-side freewheeling diode and make it close (ON state) during main switches OFF periods. Anyway, I want to avoid this kind of circuit thus I was thinking of a way to make use of the freewheeling current.

This is the purposed circuit:

two-switch-bootstrap.png

As you can see, I moved the low-side freewheeling diode as to include the bootstrap capacitor on the freewheeling path. The downside of this circuit could be the increased current through the zener diode (but I could use an overrated one) and the increased voltage across the high-side Mosfet in OFF state (48V+12V instead of 48V) but, than again, the Mosfet I am using is already overrated (Vds=150V).

Is there any (other) problem with the circuit above? Btw, giving the rated output (50W), what could be the amplitude (max/average values) of the freewheling curent?

Thank you very much for your time.
 

Btw, I think I should use a TVS instead of that Z12 zener diode (due to the faster response). Is there a problem if the TVS is operating on a cycle-by-cycle limiting basis?

I thought of another variant, too:

two-switch-bootstrap1.png

This time, I kept the original circuit topology but I have added a circuit (double R+D) to have the bootstrap capacitor charged during Mosfet switches OFF period. Anyway, the RD circuit is disipative (moreover, it drains current all the time) hence it would affect the overall circuit efficiency.

Any other suggestions?
 

Hello,

I've just build a two-switch flyback converter (Vin=48VDC, Vout=12VDC, Pout=50W) but the boostrap circuit for the high-side driver doesn't work quite well. That's it, during the Mosfet switches OFF period, the low-side freewheeling diode is only forward biased for a (too) small period of time (I suppose) hence the bootstrap capacitor doesn't get enough charge.

I've seen a workaround for this issue, by adding an extra switch across the low-side freewheeling diode and make it close (ON state) during main switches OFF periods.
Looking at a few app notes, this seems to be the standard solution (aside from using a gate drive transformer instead of a bootstrapping driver).

This is the purposed circuit:

View attachment 143057
This doesn't seem to solve the problem though. Both circuits still rely on the freewheeling diodes to conduct for long enough for the bootstrap cap to be charged. If the energy stored in the leakage inductance is too small, then neither will work.
 
Both circuits still rely on the freewheeling diodes to conduct for long enough for the bootstrap cap to be charged. If the energy stored in the leakage inductance is too small, then neither will work.

Well.. at least, the whole leakage current pulse is going through the bootstrap capacitor.

A gate drive transformer could be a feasible solution too (although I have no experience with). Another working solution is to use a dedicated (secondary) winding to feed the high-side driver.

Anyway, I don't have enough time for experimenting right now hence I've decided to change the topology (two-switch forward instead of two-switch flyback). Seems like the forward "reset" time will be longer and I could go on with the current bootstrap technique.
 

Well.. at least, the whole leakage current pulse is going through the bootstrap capacitor.
Yes, eliminating the bottom catch diode will probably allow more current to be steered towards the bootstrap. But then you would have to worry about damage to the gate drive due to the bootstrap ground going too far below control ground. Something has to clamp that....

Anyway, I don't have enough time for experimenting right now hence I've decided to change the topology (two-switch forward instead of two-switch flyback). Seems like the forward "reset" time will be longer and I could go on with the current bootstrap technique.
Yes, that's certainly one way around the issue.
 

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