Switched-mode conundrum

Thread Starter

Ian0

Joined Aug 7, 2020
1,113
I have 18V DC and I need 200V DC @ 50mA (which shares a common ground with the 18V).
My choices are:
1) Push pull
2) Flyback - single winding
3) Flyback - auto-wound
4) Flyback - isolated winding
5) LLC
6) inductor-fed push-pull
7) Cockcroft-Walton multiplier (which is a non-starter).

#1 would be my first choice, but the output inductor is 20mH, so lots of turns of very fine wire, which will be a pain to make, and will probably have a lot of self-capacitance.
Which one would you suggest? or any other ideas?

(18V comes from a 25W plug-top power supply, 600mA goes to the heaters of three 6Н1П valves in series, and the 200V is for the anode supply)
 

AnalogKid

Joined Aug 1, 2013
8,804
You said input and output share common ground, so whats with all this complexity, you only need a simple boost converter, one simple inductor!
Agree. At 10 W, it should work well. Input current would be 0.6 A plus losses, well within a jillion power MOSFETs. With an I/O ratio of over 11 it will not be super efficient, but I don't see that as an issue.

ak
 
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Thread Starter

Ian0

Joined Aug 7, 2020
1,113
I'd not tried a flyback with such a high step-up ratio, which is why I thought I'd canvass a few opinions.
I see two, possibly three, disadvantages.
1) it's beyond the maximum m/s ratio of quite a few control ICs, so limiting my choice (but not the good old 3842). I wonder if the slope compensation may be troublesome.
2) The output is a series of 1A pulses 1us long (if it's discontinuous mode, lower in continuous mode), which might need some extra filtering (when you consider that the circuit being supplied really doesn't have any PSRR to speak of).
3) The power transistor needs a Vds rating of 250V and the power diode will have reverse recovery losses if it's continuous mode.
As you all seem unconcerned, I'll give it a go.

It's a valve preamp - a guitar pedal in fact. I'm not sure I wanted mains inside the pedal, and an external supply would need a weird connector between the two boxes. A 18V 25W power supply is only £10 - far less than I would have to pay for a custom-made mains transformer - and it keeps the hum out of the preamp.
 

ronsimpson

Joined Oct 7, 2019
1,121
First try. To go from 12 to 200V is a long way. I tried a center tapped inductor to help.
Q1 will see 3A pk and about 100V with ringing in top.
D5 needs to be fast and will see over 200V + ring so 400V part.
1609108675676.png
Coilcraft has dual inductors that will make a center tapped inductors.
It needs slope compensation which I did not include.
 

Thread Starter

Ian0

Joined Aug 7, 2020
1,113
That was my thought #3, and I knew about the dual inductors having used them in a SEPIC, but it never occurred to me to use it as an autowound inductor. Thanks for that!
 
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