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Designing a 48VDC to 30VDC 3A isolated power supply

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Pawelr98:
A push pull or half-bridge topology uses a transformer so the open loop voltage is not that much different from the voltage under load.
Winding resistance and diode drops are the main causes. If only rough voltage is needed then open loop is not that bad. Especially if considered how simple the whole thing becomes.

30V at 3A is 90W.
Generally speaking flyback is less desired at such power level and conditions.
Voltage kickback which has to be clamped means lower efficiency and other problems.

Classic 494 can be used to implement a stabilised push-pull.
But I guess there are more modern chips that can be used for this as well.

T3sl4co1l:
I'd be plenty happy with one- or two-switch forward or flyback.  Push-pull is undesirable due to the lower current draw and doubling of voltage.  I think transformers are also less available (unless that doesn't matter because it's a one-off, I don't know).  Half bridge would be okay as well, but kind goes too far the other way (dividing the voltage in half and doubling current).  And full bridge is just too much complexity for so little power output, I wouldn't bother.

So, what's been mentioned above, or an old fashioned UC3842, or a handful of other things that don't immediately come to mind that I'd have to shop for, would be likely suspects.  For the two-switch inverter, use a dual bootstrap gate driver.

Definitely a buck regulator isn't suitable here; the SW node is never rated much more than 2V below ground, and only for short transients at that.  Likewise a boost regulator won't do, but a flyback/SEPIC/Cuk capable type will.  External switch, boost or buck, non-synchronous types may also be suitable.

FWIW, active-clamp forward seems to be the most popular among fractional-brick DC-DC modules (which you might look into if you don't need to make this circuit for special purposes).  Though they also seem to have terrifyingly bad efficiency at light loads, at least the couple I've seen.  Noisy too -- that's another thing the two-switch configuration helps with, reduces EMI.

Tim

Decee1:

--- Quote from: T3sl4co1l on August 15, 2020, 07:39:59 pm ---I'd be plenty happy with one- or two-switch forward or flyback.  Push-pull is undesirable due to the lower current draw and doubling of voltage.  I think transformers are also less available (unless that doesn't matter because it's a one-off, I don't know).  Half bridge would be okay as well, but kind goes too far the other way (dividing the voltage in half and doubling current).  And full bridge is just too much complexity for so little power output, I wouldn't bother.

So, what's been mentioned above, or an old fashioned UC3842, or a handful of other things that don't immediately come to mind that I'd have to shop for, would be likely suspects.  For the two-switch inverter, use a dual bootstrap gate driver.

Definitely a buck regulator isn't suitable here; the SW node is never rated much more than 2V below ground, and only for short transients at that.  Likewise a boost regulator won't do, but a flyback/SEPIC/Cuk capable type will.  External switch, boost or buck, non-synchronous types may also be suitable.

FWIW, active-clamp forward seems to be the most popular among fractional-brick DC-DC modules (which you might look into if you don't need to make this circuit for special purposes).  Though they also seem to have terrifyingly bad efficiency at light loads, at least the couple I've seen.  Noisy too -- that's another thing the two-switch configuration helps with, reduces EMI.

Tim

--- End quote ---
I've never heard about two-switch forward. Old technology?
Seems like this concept I'm working on is getting more complicated than i'd imagine it would. Typical..  ;D

Yeah i did realize the buck converter solution doesn't work and thinking about it it also makes really good sense. If the PWM reaches 90% it would have a high voltage output but driving it as a flyback would just not energize the secondary side enough

So do i understand correctly that active-clamp forward is simply just a flyback with another FET that is activated in the switching FETs off period with a bypass capacitor to clamp the inductor leakage inductance?

T3sl4co1l:
Maybe.  But then, what's old is new?..

2-switch forward and flyback are popular in newer computer supplies, probably in part because the higher voltage set by the active PFC input stage would be annoying to do other ways, and because EMI is lower than 1-switch (both ends of the primary move at the same time, the common mode (average) is ~zero).

The unipolar drive means the magnetic core can't be driven over a wide operating range, but probably the switching frequency has risen enough that that isn't a meaningful restriction anymore.  (Old PSUs might've ran at 30-100kHz, half bridge forward; at such low frequencies, it's advantageous to drive the core with bipolar flux, up to doubling the available operating range before hitting saturation.  Up over 100kHz, to keep core losses down, the core flux density has to be low enough that you can't run anywhere near saturation anyway, and so you have no benefit from bipolar drive.)

Tim

NiHaoMike:

--- Quote from: T3sl4co1l on August 16, 2020, 01:44:01 pm ---2-switch forward and flyback are popular in newer computer supplies, probably in part because the higher voltage set by the active PFC input stage would be annoying to do other ways, and because EMI is lower than 1-switch (both ends of the primary move at the same time, the common mode (average) is ~zero).

The unipolar drive means the magnetic core can't be driven over a wide operating range, but probably the switching frequency has risen enough that that isn't a meaningful restriction anymore.  (Old PSUs might've ran at 30-100kHz, half bridge forward; at such low frequencies, it's advantageous to drive the core with bipolar flux, up to doubling the available operating range before hitting saturation.  Up over 100kHz, to keep core losses down, the core flux density has to be low enough that you can't run anywhere near saturation anyway, and so you have no benefit from bipolar drive.)

--- End quote ---
Nowadays, half bridge or full bridge is the norm for PC PSUs over 400W, with full bridge most common over 700W.

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