hybrid rectifiers and PIV rating

[Sheldon]

I have a CT transformer and I'd like to make a sort of hybrid full wave rectifier with a ss diode and tube (indirectly heated) section in series for each leg. The main reason to do this is for the slow turn on, as I'd like that for the DHT output tubes. My PSUD sims show that I am right at the limit for PIV for the tube rectifier. If I were using only SS diodes, I could simply put them in series to half the PIV for each diode. But what happens with a tube diode and SS diode in series? Can I assume the the voltage drop will be significantly less for the tube than without the SS diode in series? Any thoughts here?

I could just make a full wave SS rectifier and then put the tube in series after rectification, but that would lose the possible benefit of soft shut off. Or I could take apart the transformer and rewire for a hybrid bridge (assuming the two ends of the center tap are within reach), in which case the PIV would be half of that for the full wave. But I'd rather avoid that if I can.

Sheldon

[Eli Duttman]

Sheldon,

MAYBE SS diodes in series with the vacuum rectifier would work. What's the end to end voltage of the rectifier winding and which tube are you using? Changing the tube rectifier type seems a lot less problematic to me than series SS diodes or trafo modification.

[Sheldon]

Originally posted by Eli Duttman
Sheldon,

MAYBE SS diodes in series with the vacuum rectifier would work. What's the end to end voltage of the rectifier winding and which tube are you using? Changing the tube rectifier type seems a lot less problematic to me than series SS diodes or trafo modification.

Thanks,

Transformer is 480-0-480. Should give me 600VDC. I'd have to fish out the leads and parallel the secondary sections to go with a bridge. I'm trying to use a 6X4 (yeah, I know it's marginal). I don't see many alternatives in a small tube (7 or 9pin) with two sections that would have ratings much higher for PIV. I could use a damper diode after rectification (6CK3, etc.), just for the warm-up. I don't want to go larger if I can help it, cause I don't have much room. Heck, I may try it and see what happens. If the 6X4 lives, it lives. If not, on to plan B.

Sheldon

[Eli Duttman]

480-0-480 is WAY over spec for a 6X4.
6X4 Data Sheet


"Monkeying" with the trafo and a hybrid bridge seems best to me.

[Sheldon]

Oops, my unedited reply was way off. I misread the data sheet. I don't see where things would be way off here. I would surely be on the limit for PIV, but with slow warm up, maybe not. One of the examples shows a 900V plate to plate supply. The only thing I can see is the plate to cathode rating of 450V. But couldn't that be dealt with by letting the heater winding float?

Sheldon


second edit: sorry for not thinking THEN writing.
third: oops, meant heater winding not cathode

[Eli Duttman]

Sheldon,

Your trafo puts approx. 1358 V. on the 6X4's anode. The data sheet says 1250 V. max. That's a design center number; so, a little more with a choke I/P filter rates to be fine. However, your PSU is cap. I/P. I'm WORRIED about arc over.

[Sheldon]

Yes, definetly on the edge. I'll see what the transformer holds.

Happy Holidays all,

Sheldon

[Sheldon]

Actually, thinking about this a little more (danger, danger). It seems that with identical devices, the potential should be more or less halved if they are in series. In this case, with a SS diode and a tube diode, it would seem that the tube would see most of the inverse voltage drop. The tube is a slow cut off device and cannot conduct in the reverse direction. As I understand it, the SS diode has the property of a small initial reverse current, which is what can lead to the transient spikes that people are concerned with in these application. Therefore, it seems the the tube will control the cut off (hence the desireability of using it in a hybrid), and therefore see more of the reverse voltage potential across the two devices in series. Does that make sense? More importantly, it correct?

Sheldon

[Eli Duttman]

Sheldon,

I've got an idea. It just may solve the problem. Put a UF4007 in series with each of the 6X4's anodes. Wire resistive voltage dividers across the combinations to the cathode. Instead of 50/50 equalizing, as is "commonly" seen, use 40/60. Let the 1000 PIV SS parts handle 40% of the voltage and the tube the remaining 60%.

[Sheldon]

Thanks Eli,

That seems like it should get around the PIV limitations. But would the SS diodes now shut off normally, with the attendent reverse recovery spike? BTW, I'm a complete agnostic on the issue of whether or not that matters. I don't have the experience to have an informed opinion one way or the other. It is a practical way to get slow turn on with just one small tube, which is my main objective here. I'll still check the transformer to see if it would be easy to parallel the secondaries and go with a bridge, but if not, I'll give this a shot.

Sheldon