Rectifying 115V AC @ 400Hz 2

[mriechert]

Hello all;

I have just had a discussion with a collegue about rectfiying 115V AC @ 400Hz. The question came up if we can use a power supply rated for 115V AC @60Hz to rectify 115V AC @ 400Hz.

At first I thought you could since after the bridge diode rectifer the voltage would be DC and so therefore any components after that wouldn't "feel" the effect of the Alternating current whether it was 60Hz or 400Hz. and the voltage would be the same since RMS of 115VAC @ 60HZ is the same as RMS 115VAC @ 400HZ... so this would seem frequency independant.

However from experiance the rectifier operates at a higher temperature when operated at 400Hz rather than its designed 60Hz. Could this be because the diodes are required to cycle from fwd bias to reverse bias and higher rate???

cheers
mriechert

 

[richard1st]

Hi,

Absolutely right. If you look at diode information in the manufacturer's data, it will normally give details such as 'recovery time'. This is the time taken for the diode to begin 'blocking' current when voltage across the diode reverses. All to do with mobility of carriers, and intrinsic capacitance, etc.

50/60Hz diodes are about the cheapest available - that is they have long recovery times. Suggest you look for some in the same package, with similar current ratings, but with shorter recovery time. (Sometimes the same type of diode in higher voltage rating will be OK.)

If you continue to run with original diodes they may fail, which would be bad news for everything hanging on the DC rail. Diodes are cheap so why run the risk? If you can find a part number on the diode and post it here we can have a quick look for an alternative.

RAC.

 

[edison123]

Good post richard. You get a star.
Do you think extra cooling with fan will prevent the diodes from overheating and from failure? or will the diodes will still fail due to paucity of recovery time at 400 Hz?

 

[richard1st]

Thanks!

I reckon a fan is probably OK as a short-term fix, but junction could develop a hotspot. A tiny part of the junction ceases to be effective as a diode, which puts more strain on the remainder; this leads to either failure of the junction (thermal runaway) or of the bonding wire. Depends whether this is a lab lash-up or you want to put it out on site and 'fit and forget'.

I am in UK, so use RS Components. Their website

http://rs

http://www.com

lists rectifiers as standard, fast, ultrafast, etc. and correponding values for trr (reverse recovery time), which is a starting point.

Let me know how you get on.

RAC

 

[edison123]

richard,

I use diodes in brushless exciters for generators where the frequency of ac input to diodes is between 100 to 250 HZ. I have always used off the shelf diodes without due consideration of frequency till you pointed it out. Ofcourse, I have not had diode failures till date which could be due to the fact that they are well cooled due to rotation (and hence by previous post). Also, I invariably choose diodes with a high PIV (about 1000 volts). Could it account for absence of diode failures ? Anyway, from now on, I will include the frequency also as a parameter for choosing diodes.

 

[alehman]

One thought - transient recover times typically are a few microseconds which is many orders of magnitude faster than a 400Hz waveform would seem to require.

 

[richard1st]

Alehman,

I absolutely accept your point.

However, the question is regarding excess heat when apparently the only thing which has changed is the frequency. I interpreted it as 'I have observed this effect, can anyone suggest a reason?'.

Reverse recovery could be significant because there are two effects: firstly increasing frequency by a factor of eight means the diodes reverse eight times more often, and losses from this source increase in proportion. Secondly, for the same rms voltage, the rate of rise of voltage is eight time greater, which means that the rate of rise of (reverse) current through the diode increases, and peaks at a value several times higher: current is limited only by source impedance (transformer, forward diode, etc).

The two effects multiply, and losses from this source will probably be 10-20 times greater for increase in frequency of eight times.

My recent experience has tended to be operating in 100s of kHz, and I may be conditioned by that; losses from this source can be at least as high as forward conduction. Happy to keep an open mind.

Richard.

 

[alehman]

You may be corret. I was just trying to point out the counter-argument. I have also seen this at much higher frequencies but have never seen it occur or heard of it as a problem at audio frequencies or below. That's my only real basis.

 

[mriechert]

Thankyou all for giving me your input. It has been very helpful.

cheers
mriechert