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Limits of an unregulated supply

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Steve Conner

Fri Apr 02 2010, 09:27PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Radiotech: Those capacitors are getting a hammering, they're probably passing about 10 times their rated ripple current. So is the rectifier for the reasons Sulaiman explained.

The PFC choke needs to be a couple of millihenries, not microhenries.

I'm not sure I believe that rectifier is 80 amp. I've never seen anything over 25A in that package style. This link suggests that it's really 8 amp:

Those equations for Xc, Xl, and so on don't apply in this situation. The current in a rectifier like this is not a continuous sine wave. It flows in sharp pulses that (by Fourier analysis) consist of a whole spectrum of frequencies much higher than the line frequency.

Other possibility: Maybe you have inadequate RF bypassing, and high RF voltage got into the rectifier and blew it.

IamSmooth

Sat Apr 03 2010, 12:14AM

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

Boy...more stuff to consider.

Where is the best place for the choke? Is it from the rectifier to the smoothing cap? Is it from the smoothing cap to the inverter? Should I place it before the rectifier?

I have to remeasure my choke, but is is about 0.1 to 0.2 mH, wound with #10 wire.

What should I do for RF at these power levels?

radiotech

Sat Apr 03 2010, 07:17AM

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

The ballpark ripple voltage in fullwave filters is Il/2fc. What is your ripple voltage and what is the DC voltage at 23 amps?

If the ripple (p-p) on your scope is close to the dc voltage,
the capacitors do nothing except heat up.

Without filtering the DC voltage from the bridge is about 0.9
the RMS AC. The meter on DC reads average.

Russell Haley

Mon Apr 05 2010, 05:06AM

Registered Member #2478 Joined: Mon Nov 23 2009, 03:24AM
Location: Texas A&M University
Posts: 47

The less ripple you have on your DC supply, the smaller the fraction of the cycle over which the mains voltage exceeds the capacitor voltage. Larger filter capacitance results in higher peak current and more ugly harmonics on the mains.

With an induction heater, the thermal time constant for the load and power of the heater is almost certainly much much longer than the ripple period. As such, ripple doesn't matter. You still need a filter capacitor, but only to support the high frequency load current. Choose a cap to have low impedance at the switching frequency.

I'm not certain, but if you're using a half-bridge, the caps in the other leg might be entirely sufficient.

kell

Mon Apr 12 2010, 03:17AM

Registered Member #142 Joined: Sat Feb 11 2006, 01:19PM
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Posts: 102

Put the choke between the rectifier and the filter cap.
Use a choke that won't saturate.

radiotech

Mon Apr 12 2010, 03:42AM

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

Or you could use a choke that does saturate in which case the filtering will be less at high loads and better at low loads. The bottom line with such a 'swinging choke' is better regulation.

deja vu tells me that same issue was gone over a while ago.

IamSmooth

Mon Apr 12 2010, 05:26PM

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

I put a scope on the output with the choke (0.5mH) between the rectifier and the smoothing capacitor. Then, I placed the choke after the smoothing capacitor. I found the ripple was almost unaffected in the first case, and almost all gone when it was after the smoothing capacitor.

Any thoughts? It seems to make sense to me as the inductor is keeping the current drawn from the capacitors in a tighter range, as the inductor does not like changing currents.

Steve Conner

Tue Apr 13 2010, 09:04AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The choke is most effective in smoothing ripple when you put it after the smoothing capacitor. It forms a pi filter with your RF bypass caps.

However, you said that your goal was to improve power factor on the line, and reduce ripple current in the first smoothing cap, not reduce ripple voltage on the DC output. To achieve your goal you need to put the choke either before the rectifier, or between the rectifier and the first smoothing cap.

If the choke current is discontinuous, it doesn't matter which of these places you put it, but if it's continuous, the effect will be different. With only 0.5mH it'll be discontinuous for sure.

If you're using a doubler, there's no single place called "between the rectifier and the first smoothing cap", so your only two options are before the rectifier, or after the caps. Hence you have to put it before the rectifier, in the AC line.

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