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Limits of an unregulated supply

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IamSmooth

Thu Apr 01 2010, 11:12PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Right now I have an 80A/1000v rectifier with two 450v/1000u capacitors set up as a voltage doubler.

After running about 40A through it the bridge rectifier blew. The capacitors were warm.

I don't want to have capacitors blowing, too. How large do they need to be to be safe? Is it ok if they get warm? Should I parallel another set to cut the current each one sees in half?

Luca

Fri Apr 02 2010, 07:41AM

Registered Member #2481 Joined: Mon Nov 23 2009, 03:07PM
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Posts: 134

Hi,
probably peak and rms current are too high. Peak current causes diodes blowing while rms current is responsible for capacitor heating.
Capacitor can get quite warm but not hot. Otherwise it means that you are exceeding rated rms current. Parallelling two caps will help.

Regards

Luca

Sulaiman

Fri Apr 02 2010, 10:36AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I think the main problem is that the current is 'flowing' in short pulses,
(like in a typical bridge rectifier plus capacitor ac/dc converter)
Try adding a few mH of inductance in series with the ac line.
A few mH for >40 A will be large.

Imagine your system has unity power factor (current proprtional to voltage at all times)
Heating will be I-squared x R (R can be cap esr, wire resistance etc.)
If current only flows for 1/10th of a cycle it needs to be 10xI,
heating will be 10x more, 10-squared multiplied by duty (1/10)

If the ripple on the dc output looks like a symetrical triangle then all is well, if it has a rising part much steeper than the falling part then add inductance to the ac line.
Try to get the rise time not less than 1/2 of the fall time.

IamSmooth

Fri Apr 02 2010, 11:23AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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I'll give it a try. I have a core wound with #10 wire just for this purpose.

By a few do you mean < 10uH?

I'm also going to increase the size of my rectifier caps to decrease the ripple. I took my test up to about 40A going through the rectifier and the caps were getting very warm. I also plan to split the current through 4 rectifiers.

Dr. Dark Current

Fri Apr 02 2010, 11:34AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Keep in mind that increasing the cap size increases the stress on the rectifier diodes (that is if you don't have any PFC in front of the rectifier)

IamSmooth

Fri Apr 02 2010, 02:07PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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How does increasing the cap size increase the stress?

Can I run an induction heater without any caps and just use the rectifier?

Dr. Dark Current

Fri Apr 02 2010, 04:04PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Increasing the capacitance decreases the "charge duty cycle" of the caps, and the diodes must switch higher peak currents.

Yes you can run an induction heater without filtering, but the power will be about half (with everything else the same), or if you tune it to the same power as with the caps, the system will see higher peak currents and the efficiency will probably be lower than with filtered supply.

IamSmooth

Fri Apr 02 2010, 05:20PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Dr. Kilovolt wrote ...

Increasing the capacitance decreases the "charge duty cycle" of the caps, and the diodes must switch higher peak currents.

Is this because Xc is lower for higher capacitance, so there is a greater load across the rectifier? Would I then calculate peak current as Vpeak/Xc for sizing my rectifier?

radiotech

Fri Apr 02 2010, 05:58PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

Iamsmooth wrote

Right now I have an 80A/1000v rectifier with two 450v/1000u capacitors set up as a voltage doubler.

After running about 40A through it the bridge rectifier blew. The capacitors were warm."
At 40 amps output, the capacitors would contribute nothing measurable if operating at power line frequency.Please show schematic with V, F, Rl and I.

IamSmooth

Fri Apr 02 2010, 06:31PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

The inverter is drawing power off of the rectifier/capacitors at 69khz. The autotransformer was providing about 23A at 100vac to my rectifier/capacitor.

I would not have thought the rectifier would fail. It was not even warm, but smoke blew out of the epoxy casing.

Here is a picture of the rectifier that I used:

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