Not too often you see one of those

[Sch3mat1c]

That's right, I'm building a tube oscilloscope, with a switching power supply.

It doesn't *do* anything right now. The HV works (hence the green dot), and the heaters work, but no circuit is hooked up.

A lot of people might be interested in the power supply, by the way. Sans HV generator, it's perfectly suited for the average tube amp. The main outputs are +/-250V 200mA (which could be rearranged for +500/+250) and 6.3VDC 10A, both filtered and regulated. Residual ripple under light load is on the order of 0.1Vp-p (120Hz and 100kHz), the former of which is significantly lower than almost any conventional tube circuit, and the latter of which is very easy to filter out.

Tim

[Wavebourn]

What about additional noise through ground loop?

[wrenchone]

Iv'e been running switching power supplies in two of my tube amps ("Shrine" and "Mighty Mite") for some time now with good results (and small size), and am working on several others. Having said that, I spent a lot of time in the EMI room at my job optimizing those supplies for low emissions. It wasn't easy, as the HV output winding is just as bad a source of noise as the primary winding.

[Sch3mat1c]

The HV winding waveform is ugly as sin. Lots of ringing from the flat wound construction -- didn't have room for bank winding, which I did for the filter inductor (choke input filter, it's a forward converter). It doesn't seem to be going very far thanks to the aluminum panels, and there doesn't seem to be much HF trash beyond the ~20MHz ringing of said transformer, at least on the scope.

I've taken adequate precautions with filtering, common mode and otherwise, so ground loop should be no problem at all. What's more, the voltages all come out on terminal strips, so I can nip any HF noise in the bud without it going places.

Tim

[TubeManiac]

Cool !

I have a Scope with tubes too !
It was a Philips "kit" back in to 60's or 70's.

[wrenchone]

A flyback converter is far more appropriate for HV applications than a forward, especially a ZVS flyback. 1) Since the flyback has boost capability as part of its transfer function, the transformer ratio need not be so extreme. 2) You get rid of the output choke 3) with ZVS flyback you operate in borderline discontinuous mode which greatly reduces the stress on, and noise from, output diodes. With ZVS flyback you also get the option of shaping both the leading and trailing edge of the primary waveform, further reducing EMI.

[Sch3mat1c]

The high voltage generator has a lovely waveform, a piecewise cross between sine, tilted triangle and square waves, plus a small amount of ringing. It's a blocking oscillator in the flyback domain, but the HV winding has enough capacitance that the rising/falling edges are quite gradual, and it operates in continuous rather than single-cycle (or burst / squegging) mode. The HV output is a doubler, which flattens the top and bottom peaks.

Flyback supplies are attractive for lower power levels. Forward converters start looking good around 100W, which is the size of this power supply. I don't have any BCM flyback chips, so it would've been with UC3842, which can be far noisier, especially if it enters CCM. So I went with forward, which has lower ripple current, useful around the high ESR HV electrolytics and giving an easier time for the high amperage heater winding, at the expense of a somewhat more complicated HV filter choke.

I am considering PFC + flyback for a SS scope. That PSU might end up a fairly large board: it needs filter, PFC, flyback, more filters, HV generator (Royer oscillator, ran from LV supply), and still more filtering.

Tim

[kenpeter]

I'd be tempted to use a high side power factor correction chip
to get boost regulated +390. Then chop 50%/50% duty into a
push-pull isolation transformer. Doubt I'd bother with any loop
feedback, let back EMF into the primary PFC take care of it...

Oh, Royer... I just looked that up. We on same sheet of music.
http://wiki.4hv.org/index.php/Royer_oscillator
Thinking 6080 here... Too much plate voltage? Too much Rpc?
Miller? Need separate switcher for isolated/unisolated filaments...
Maybe sand is better.

[kenpeter]

High side is perhaps a misnomer here?

[wakibaki]

The widemouth frog was going along one day and he met a wabbit.

'Hello, furry creature with the long ears and little white stumpy tail, what kind of a creature are you?'

And the wabbit said 'Oh, I'm a wabbit, I eat mostly grass, although I'm coprophagous and crepuscular to boot'

'Oh, right' said the widemouth frog, marvelling at these long words and smiling broadly 'I'm a widemouth frog, I eat flies and other insects, nice to meetcha'.

A little further on the widemouth frog came across a fox.

'Oh, swift creature with the red fur and pointy ears and impressive bushy tail, what kind of a creature are you?' said the widemouth frog.

'Oh' said the fox, 'I'm a fox, I run around staying out of the way of hounds, and among other things I eat wabbits, when I can spot the crepuscular little sods in the twilight. What kind of a creature are you?'

'Oh' said the widemouth frog smiling broadly in relief that he wasn't a wabbit, since the fox was rather fierce, 'I'm a widemouth frog, I eat flies and other insects, nice to meetcha'.

A little further on the widemouth frog came across a green snake.

'Oh, long and slithery green creature' he said 'what kind of a creature are you?'

'Ahh... sss,' said the green snake, hissing after the fashion of snakes. 'I am a green ssssnake, and I eat widemouth frogssss.'

'Ew' said the widemouth frog, pursing his lips as narrow as they would go, 'You don't see many of those around here...'

w