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Diodes in series and hybrids

I'm looking at a project that requires a B+ of at least 600v, but not much current. So I was casting around the net to get some ideas for the power supply. This one has a B+ of 920v and an interesting feature: http://tinpan.fortunecity.com/saints/668/primer/211-se.gif

It uses a full bridge with double ss diodes in series, followed by a 5AR4. I assume that pairing the ss diodes in series is done to double the allowed voltage rating of the bridge. I also assume that putting the 5AR4 after the bridge is done to provide a slow warm-up, and perhaps minimize noise from the ss diodes?

It makes sense that diodes of the same type in series would allow double the voltage, as each one only "sees" half the total. How would that calculation apply to diodes of different type, such as ss followed by vacuum? I could imagine that one or the other would be exposed to more than half, since the shut off behavior might have a different time curve. Or would this only be an issue at higher frequency AC?

The real question is; can you use a tube diode at higher than it's rated voltage, if it's after a ss bridge?

Sheldon

Sheldon
 
Hi Sheldon,
I haven't actually done what you are suggesting, but the usual method for using a series string of SS diodes is to put high value resistors across each of them to divide the reverse voltage. Just need enough conductivity to overcome the junction capacitances, so I would look at the capacitances of the SS diodes with say 100 V reverse on them and check the capacitance of the VT diode to see if they are in the same ballpark. Since the SS diodes have higher capacitance with lower voltage on them they will tend to lag the tube in charging up, so may need to take the higher capacitance at low voltage into account in setting up the resistive divider. Maybe divide out more voltage on the SS diodes in proportion to capacitance. Good luck.

Don
 

EC8010

Ex-Moderator
2003-01-18 7:57 am
Near London. UK
If you were to make half of the bridge with a pair of 6CL3, then your only PIV problem is with the silicon department. The "off" capacitance across the 6CL3 is almost certainly lower than across any silicon. By Kirchoff, both diodes see the same charge, and Q = CV, so most of the voltage transient should appear across the valve (which can handle it). I really hadn't thought about this before, but it rather looks as though you only need to worry about the static conditions as smoking-amp suggests.
 
Thanks both,

It looks like it's workable then. I was actually thinking 6X4, so I can compare the specs. and see. The 6X4 has a PIV of 1250v and a max. DC rating of 450. I'll have to look around for the capacitance of the tube, as those aren't on the ordinary data sheets.

EC8010, the schematic I cited uses the tube diode after a full bridge, not part of it. The tube diode sections are paralleled. In this case the diode is not needed for rectification, so I assume it was put there only for delay or filtering purposes. If I used a "hybrid" bridge I assume that both the tube diode, and the ss diodes (or a series pair of ss dioded) would have to be rated at the final B+.

I'm partly just curious too. The ss diodes have a sharp cutoff at about 0.6V. I haven't seen data on how the tube diodes compare. They obviously have a more gradual cutoff, but how does it look near 0v?. The 0.6V ss shut off would suggest that the ss diode would shut off before the tube saw significant inverse voltage. If, true then it shouldn't much matter what the tube is rated at here. If it's the other way round, the tube would see most of the drop. Any pointers to such kind of info?

Sheldon
 

EC8010

Ex-Moderator
2003-01-18 7:57 am
Near London. UK
I doubt if 6X4 capacitance is noted anywhere, but I'll happily measure cold capacitance for you (shouldn't be too dissimilar to hot).

I realized that your circuit included the valve rectifier in addition to the silicon bridge, but why not combine them? If you do, you save two bits of silicon and the switching is softer. Yes, the normal bridge voltage ratings would apply, so each diode would need to be able to withstand the peak output voltage in reverse bias. 6CL3 is good for 3kV (I think). 6X4 is somewhat weedier, but still pretty impressive by silicon standards (remember that they were designed for use with centre-tapped transformers, and needed twice the peak inverse voltage rating for a given output DC compared to a bridge rectifier).
 
EC8010 said:
I doubt if 6X4 capacitance is noted anywhere, but I'll happily measure cold capacitance for you (shouldn't be too dissimilar to hot).(remember that they were designed for use with centre-tapped transformers, and needed twice the peak inverse voltage rating for a given output DC compared to a bridge rectifier).


Thanks,

I don't know if I need the capacitance, but I'm curious as to the relative magnitude.

As for the PIV - yes, looking at schematics I think I see what you mean. With a CT, the diode plate sees B+ at the output side and B- at the input side, when the cycle at the transformer is at the minimum, whereas with a bridge, it sees only B+ and 0. So the even though the 6X4 has a max rating of 450 DC, when used in a full wave bridge, it 's really 900V. Have I got the logic straight?

Sheldon
 
Another advantage to having a vacuum diode in the rectifier string is the vacuum diode is incapable of reverse current. With SS diodes there is a brief moment of reverse conduction before the diode opens and stops conducting. It is the short burst of reverse current that suddenly stops that excites the resonances of the power transformer and generates switching hash.

In either of the variations discussed above the vacuum diode will prevent the reverse current from occuring. This should make the power supply quieter.

In the hybrid bridge circuit you have a SS diode in series with the vacuum diode. In the full SS bridge with the vacuum diode after the bridge you will have 2 SS diodes in series with the vacuum diode. Either way the reverse current is blocked.

Only requirement for this to work is the vacuum diode has to be before the first cap or choke. I mention this as there is some information of dubious nature stating that a diode after the first choke or capacitor can reduce noise. It just doesn't work that way.

Gary
 
Thanks Gary,

Good explanation. I'd seen some of the stuff on hybrid bridges but hadn't seen that full explanation. That is partly why I was interested in the combination. Against the greater heat of the tube diode, you get less noise and a slow warm-up. That's not a bad trade.

One question though. If the tube diode stops the reverse voltage, it would actually see the full drop for a brief period, so should be rated for the full B+ (taking into account EC8010's point about bridges vs. full wave)?


Sheldon
 

EC8010

Ex-Moderator
2003-01-18 7:57 am
Near London. UK
You had me worried for a moment about reverse voltage ratings, but then I drew a diagram and thought about it...

When the sine wave is at its peak voltage, each diode that is switched off sees the peak voltage across it. (Draw a bridge and annotate it with diode drops etc, and you will see what I mean). In other words, if you had 141V across the reservoir capacitor, each "off" diode would also see that voltage across it. So that's not a problem. At the point when the diodes change over, the voltage leaving the mains transformer must be near enough 0V, so the maximum voltage that could be across any "off" diode is the voltage across the reservoir capacitor plus a bit for the other diode's "on" voltage. Even if the vacuum diode needs 15V to switch on, that's insignificant in the context of a few hundred volts.