Using Ultrafast Rectifier in place of 60Hz Bridge

[Mr RB]

Using only two diodes in the bridge package should be fine.

I would just put a large power resistor (10W? 20W?) in series with the output. That should save the diodes from turnon inrush current and reduce peak currents. Running at 8A average on a good sized heatsink, a 50A diode will not be killed by the heat. It has to be peak currents.

 

[Western]

You can't hurt anything to do what KISS or Mr RB are talking about. Or a softstart as dr pepper recommends. The simplest softstart one can make is just a high wattage, low resistance NTC thermistor. These have a high resistance at first, so limit inrush current. But as the current starts to flow, they heat up. Having negative temperature coefficient (NTC) means they lower their resistance when they heat up. The subsequent drop in resistance allows for full system current with minimal circuit impedance. It's a single cheep part, I see them all the time in CRT displays. The only thing is they can get quite hot. Sometimes they would even get hot enough to unsolder, and thus need to be welded/socketed to be reliable.

*HERE* is what I'm talking about. If you have a business, you can even request free samples from them. But you can also order them from DigiKey, Newark, Farnell, etc. with the links on the left hand side. They make units that you can use to limit current for either the primary or the secondary. Either circuit location should work to limit inrush current through the diodes.

I'm really liking this suggestion.

I have seen these devices used occasionally, but have never learned enough about them to know when and where to implement them myself.

Thank you for the links, I'll spend some time getting acquainted with them ... I'm sure they will be worthwhile fitting in lots of situations.

Thanks everyone again. Wes.

 

[Western]

Running at 8A average on a good sized heatsink, a 50A diode will not be killed by the heat. It has to be peak currents.

Exactly my thoughts all along.

Using only two diodes in the bridge package should be fine.

I would just put a large power resistor (10W? 20W?) in series with the output. That should save the diodes from turnon inrush current and reduce peak currents.

Yes, I can see that working if they all had the same load, but my problem is they are for pulsing solenoids, and it depends on the facility as to how many solenoids they use.

Some places run right at maximum load and others may only run at 25%.

I am still going to study more on NTC's, though differing loads may also be an issue as well.

 

[chemelec]

I am still going to study more on NTC's, though differing loads may also be an issue as well.

NTC Thermistors Go Lower in resistance as they heat up with higher current.
I Doubt that will solve your problem.

I would put 10 or 20 watt, 1 Ohm Fixed Resistors between the Transformer each of the Diodes, to limit peak currents.
Possibly also a Movistor across the input to the two diodes.

I believe you said you used 600 Volt rectifiers?
But I Can't believe you have voltage Spikes that can burn out 600 Volt Diodes on such a Low Voltage line.

 

[Western]

I believe you said you used 600 Volt rectifiers?
But I Can't believe you have voltage Spikes that can burn out 600 Volt Diodes on such a Low Voltage line.

Exactly !! ... that's been my problem all along too ... why are most of the originals soldiering on 25 years later ... with 6A 400V (MR754) diodes ... while some I fitted bridges to, were coming back after 12 months ... and they have a 50A 600V bridge in them.

Admittedly, I don't have this problem any more, now that I fit two single 20A 800V D8020L T0220 diodes ... but I sure would like to know what has caused all the grief all this time.

Possibly also a Movistor across the input to the two diodes.

I'm just editing here ... but I do exactly that ... I implemented that at the same time I started with the TO220 diodes. It may have been that, that solved my problem.

I have 5 more on the bench to do today (never been looked at, and 25 yrs old) ... and while it's great work, it would be nice to know what causes it.

I read what you said about NTC's, so will need to study them properly.

Thanks.

 

[KeepItSimpleStupid]

Exactly !! ... that's been my problem all along too ... why are most of the originals soldiering on 25 years later ... with 6A 400V (MR754) diodes ... while some I fitted bridges to, were coming back after 12 months ... and they have a 50A 600V bridge in them.

Admittedly, I don't have this problem any more, now that I fit two single 20A 800V D8020L T0220 diodes ... but I sure would like to know what has caused all the grief all this time.

I can think I can off some light, hopefully.

If you take a negative spike > than the PRV the approprite diode will "temporarily regulate at the PRV". If you take a diode has a rating higher than the spike, it actually gets rectified and be turned around.

When breakdown is exceeded the diode acts as a Zener diode at the PRV, so a larger PRV may actually allow a spike to pass and get rectified rather than clamped.

Exceeding the PRV temporarily is not destructive.

Now, it's entirely possible that the entire system could be redesigned to use solenoid drivers. The benefit could be reduced energy consumption and longer lifetimes. A "real" solenoid driver would allow full power for a a short time and then throttle the current back to a "holding current". I once had the opposite problem. I wanted the solenoid to activate slowly. The cycle time was about on two minutes, off for 30 sec for an entire workday, manually operated. The solenoid was very close to a <1mm thick $800 USD piece of glass.

I ended up using the "soft start" feature of a PWM driver IC.

i also had the conventional problem too, but no energy saving heroics.

 

[chemelec]

I have a lot of POWER TYPE NTC Thermisters and have used them in many Ocassion.
They will give a Good Soft Start, But that is about it.
Be Aware, they can get PRETTY HOT.

 

[Timescope]

You can also add capacitors in parallel with the diodes to prevent damage from spikes as discussed here many years ago https://www.electro-tech-online.com/threads/capacitors-connected-to-rectifiers.10728/.

Timescope

 

[()blivion]

Yeah, caps across the diodes is a good move. I have seen that before, also helps with reverse recovery time.

Don't use half of a full bridge, it will (Edit: "Could") heat irregularly and warp/fail internally.
Use power NTC thermistor for soft start. (Or a fixed resistor if you don't care about having full power.)
Do use low value caps on the diodes.
Do put ZNRs on the filter caps.
Do fuse the output/input.

And every thing should be more than good.

 

[Mr RB]

...Don't use half of a full bridge, it will heat irregularly and warp/fail internally.
...

Sorry ()blivion I will argue that one.

I've seen this done in many heavy duty PSUs, and have done it myself. High power bridges are a cast metal unit that fully surrounds all 4 diodes, and if only two diodes are producing heat it won't matter at all as the temperature difference anywhere within that cast metal epoxy-filled area will be the same, within a tiny fraction of a degree.

The main cause of failure of bridge rect diodes is peak currents, usually because someone put megaFarad sized filter caps straight after the bridge rectifier without adding a sensible amount of resistance between the diodes and filter caps. If there's enough resistance there the bridge rect will last 30 years no problem.

 

[()blivion]

Edited.

I have some cast metal units also. And I really couldn't see this happen with them either. It's the plastic case ones I would be worried about. Though admittedly, they don't usually make them plastic when they are expected to get hot.