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High voltage SMPS with ultra-low ripple

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Enceladus:
Hello everyone! I would like to design a <100W switch mode power supply with ultra-low voltage ripple (2-5mV peak to peak). Usually it's not a big deal, there are plenty of circuits around, but this power supply also has to be a high voltage one (up to 400V). The input voltage is 25-28V. The output voltage will be fixed, the output currents are not high (<160mA) and the isolation of the output is not necessary (the ground will be the same). Ideally I would like to make it as efficient as possible so I can use passive cooling (heatsink).

So I wonder, what would be the best topology in this case? Flyback + LC filter? Boost + linear regulator? What PWM controller would you recommend?
Thank you!

T3sl4co1l:
Flyback is the way to go, yes.

Start with a 240:24V flyback transformer, either pulled from equipment or purchased new.  Laptop power bricks are probably a good reference.  Just run it in reverse, driving the secondary with a higher current transistor and current shunt, and the same controller if it's not directly offline but can be aux-powered (e.g., UC3842).  Probably use a regulator to supply the controller, since 20V+ is kind of high otherwise.  A dumb zener + transistor regulator is fine, or a 78xx or whatever.  The aux winding on the transformer (if applicable) can be left open.

Keeping common ground will help greatly with keeping noise low.  Having the ground return path right there makes filtering much easier.  Don't neglect filtering the input or the output.

Tim

hli:
4mV at 400V output voltage is 0.001%. It will be a stretch just to find a voltage reference accurate (and stable) enough for that. What are you building this for?

exe:

--- Quote from: hli on January 07, 2019, 02:45:21 pm ---4mV at 400V output voltage is 0.001%. It will be a stretch just to find a voltage reference accurate (and stable) enough for that. What are you building this for?

--- End quote ---

He was refering to ripple, not to DC accuracy.

T3sl4co1l:
What is DC accuracy but lower frequency ripple?

Presumably, the intent is to keep high frequency noise within those limits (which will be beyond average, but not unreasonable to solve), and hopefully mains frequency ripple (assuming the 28V source has any, which it might not -- that makes the error amp less challenging to design).

But without a spec, we cannot make a meaningful judgement, no.

Tim

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