Effects of overtaxing IXYS diode bridge?

[reiver]

I've been experimenting with building my own bridge rectifiers from four IXYS 6A 600V diodes, to go into an audio amplifier.

My amp came with a 25A standard bridge rectifier, 'someone' told me that summing four 6A diodes would be fine when building a bridge rectifier to replace it.

Exactly what are the effects at high volumes if the 6A IXYS diodes can't handle the current? Do they overheat, or would they tend to 'clip' on heavy bass notes?

[kubeek]

I don´t really think that paralelling diodes in PSU rectifier is a good practice.
The diodes will probably different forward voltage drops, so one of them will carry the highest current, while the others a lot less.
Something will probably give up.

[infinia]

Originally posted by reiver

My amp came with a 25A standard bridge rectifier, 'someone' told me that summing four 6A diodes would be fine when building a bridge rectifier to replace it.

No... not a good idea at all! Use a standard recovery diode bridge similar to stock.

[wg_ski]

Using four 6A rectifiers of any variety to make a bridge results in a 12A capability, not 24A. They don't all conduct at the same time.

The use of high speed diodes in the rectifier bridge of standard 60 Hz power suppiles is part of the pseudo-science surrounding the latest generations of chip amps. They all seem to do it. The high charging currents of overkill electrolytics are murder on high speed rectifiers and tends to reduce reliability. If a rectifier bridge is generating too much RF for your liking, the proper soulition is a SNUBBER, not faster diodes.

[Eva]

This is all pseudo-sciencie.

Some of the diodes will just become shorted if they are stressed enough past their ratings. There aren't any other effects.

[Dr.EM]

Originally posted by wg_ski
Using four 6A rectifiers of any variety to make a bridge results in a 12A capability, not 24A.

Would have thought it would be 6A in fact? Each diode can have no more than 6A current through it and having 2 in series as they effectively are in a bridge won't change this. In parallel it should go up to 12A but parallel diodes is bad practice

[reiver]

That's kinda what I wanted to hear. I'm running 40,000mf of caps in the amps power supply, and four 6A diodes running as a bridge rectifier scares me a bit.

Vishay makes a quality "stock" 25A bridge that I've used before, I think I'm going to forgo the Schottky hype and switch back.

60hz isn't exactly the high speed switching that it's intended to help anyway...

-Bryan

[wg_ski]

Originally posted by Dr.EM


Would have thought it would be 6A in fact? Each diode can have no more than 6A current through it and having 2 in series as they effectively are in a bridge won't change this. In parallel it should go up to 12A but parallel diodes is bad practice

It's not parallel because the diodes don't conduct at the same time. When you buy a 25A bridge, the individual diodes are "12.5A Average forward rectified current. It is assumed that the bridge will be driven with AC, with one pair conducting on one half cycle and the other pair conducting on the other half cycle. That puts half the average load current in each leg. If you want 25A diodes, you gotta buy the 50A bridge.

[jan.didden]

Your average currenty is probably just 1 or 2 amps. Running a 100W amp with +/-45V supplies will, if you run it full power on a testbench, require something like 2-3 A depending on the amp. But that's under test conditions, under music conditions surely less than an amp. Do the math.

The reason why they use those 25A bridges is because the peak currents to charge the supply caps. 40.000 uF will probably lead to 40-50A peak charge current, BUT only for 5 or 10% of the time, and then only every other cycle. What you need to look at is the peak current of those IXYS's, not the 6A. That's overkill anyway.

Note that very large supply caps 1) stress your bridge unduly, b) leads to very high and sharp current pulses that generate lots of harmonics to the line freq that sprays all over your amp and 3) doesn't lower the hum in your amp anyway.

Have fun,

Jan Didden

[Dr.EM]

Originally posted by wg_ski
the individual diodes are "12.5A Average forward rectified current. It is assumed that the bridge will be driven with AC, with one pair conducting on one half cycle and the other pair conducting on the other half cycle. That puts half the average load current in each leg.

Ah, yes indeed, forgot that