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current into voltage doubler

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Finn Hammer

Sun Feb 08 2009, 08:56PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

This cost me a new variac:
It is the current into the voltage doubler of the v-twin.

As you will see, the peak current is a whopping 60amps, and this caused the wiper of the variac to smoke, then give up life.

I have tried to find a solution using microsim, but with no success.
Is there a standard method to deal with this kind of peaky current into a voltage doubler.

Cheers, Finn Hammer

Proud Mary

Sun Feb 08 2009, 09:33PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Oh dear! What a pity! I shouldn't like to have to buy another one

HV Enthusiast

Sun Feb 08 2009, 10:01PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

Its called an active PFC front end circuit.

GeordieBoy

Sun Feb 08 2009, 10:08PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Hi Finn,

To be honest that current waveform isn't too bad for a voltage doubler! I have seen a lot worse!

Any large capacitors charged directly from an AC line through a bridge rectifier will draw a current consisting of brief but very intense pulses.

The capacitors can only charge when the line voltage is higher than the capacitor voltage and the rectifier diodes become forward biased. If the capacitors are large their voltage does not fall much in between mains cycles, and therefore they only charge for a brief period near the peaks of the mains voltage waveform. Unfortunately all of the energy leaving the capacitors must be replenished during this short time so the brief current pulses are of very high magnitude as you found out.

The problem with this ugly current waveform is that it increases the RMS current to something that is more than is required to transfer the power you are using off the DC bus. This is known in engineering terms as "bad power factor" or more correctly "line current distortion." It causes a lot more heating in the supply wiring and variac, than an equivalently rated well-behaved resistive load.

There are two standard ways that industry tackles this problem. The first one is with something called a passive PFC choke. The second one is with an active PFC pre-converter.

In the first instance a fairly substantial inductor consisting of copper windings on a gapped stack of steel laminations is connected in series with the AC supply before the bridge rectifier. This inductor acts to delay the rate of rise of the current pulses. It essentially smoothes out the sharp current surges, by making the diodes conduct less current but for a longer time. Since you are running the system from a variac at the moment, the leakage inductance of the variac will have been doing this already to some extent, and that is likely why the current pulses are not that intense. Often supplies will draw much higher current pulses when they are run directly off the mains line without any intervening variac or isolation transformer!

Advantages of the passive PFC choke are that it is a relatively simple fix and easy to simulate.
Disadvantages of the passive PFC choke are that is is a large heavy iron component, and that it is not as effective as an active PFC design. (Most modern PC power supplies with the CE mark incorporate a passive iron PFC choke, so you can get an idea how big the component is for a 350W supply.)

The second option is really a dedicated switch-mode power supply. It is designed in such a way to charge the output capacitors to the desired voltage, but to draw a nice sinusoidal current waveform from the mains supply. It does this using a boost converter and a rather elaborate control scheme known as average current-mode control. The design of such an active PFC pre-converter is quite a complex undertaking even for seasoned SMPSU designers. There are many control loops that have complex interactions.

Advantages of active PFC are that it is very effective when done properly, and makes the ugly SSTC load look like a pure resistive load to the mains line. It can also be more compact and less heavy than an iron choke.
Disadvantages are the complexity of the power electronics, control loop and magnetics designs.

Hopefully I have given you enough information to make you aware of the two common solutions to this problem. You can do further reasearch into both options. Another option possibly worth considering is using 3-phase power? 3-phase rectifiers generally exhibit better power factor than single phase rectifiers, and you might find that this performs satisfactorily.

I hope this info helps,

-Richie,

Steve Conner

Sun Feb 08 2009, 10:33PM

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

This is a similar waveform to what I got with the doubler on my OLTC2.

My solution was to use an iron cored ballast choke of a few mH to provide some "passive PFC". This increased the PF from 0.6 to 0.8: the attached scope trace is with the choke. The price is more sag under load. The choke was the size of a (large) fist and weighed about 2kg.

The leakage inductance of a variac varies depending on the setting. When the output voltage is set the same as the input, the wiper is sitting right on the input tap, and the leakage inductance is practically zero. (was this how your variac was set when it blew up?)

Smaller capacitors also improve the PF, at the risk of possibly pushing them beyond their ripple current rating, since this is smaller for smaller caps. I used 1200uF, and while they were OK for short runs at up to 30A RMS line current, I wouldn't have cared to run it continuously.

I use "PF" casually to mean the ordinary "displacement power factor" denoted by cos phi, and also the thing that Richie calls "line current distortion", which I know as "harmonic power factor".

Harmonics influence power factor because they increase the RMS current but (by some Fourier type theorem or other) don't contribute real power to the load unless they have matching harmonic voltages. If you program your active PFC to draw 100A RMS at 150Hz from a line that delivers a pure sine wave at 50Hz, you'll get no real power, and have a harmonic PF of 0.0. If the line voltage were a 50Hz square wave, though, you'd get real power.

If you want to be really l33t, 3-phase power or active PFC is where it's at (apparently, I've never tried it myself)

Finn Hammer

Mon Feb 09 2009, 11:36AM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

Thank you for some very comprehensive answers.

3phase, yes that is my own favourite and in this case, it would have solved many problems in one (but the end user specifically called out for 1phase supply.)
A 240V rectifies to 338V which doubles to 676V. doubled again by charging choke to 1352: standoff of IGBT exeeded.
3phase (415V) rectifies to 587V and doubles in charging choke to 1175: IGBT still within standoff rating. So I could have done without variac alltogether.
But with single phase, I had to stick a Variac into the line. Which toasted.
Here is a parametric simulation showing the current waveform by adding increments of 8mH

Green is without choke, cyan is with 40mH: current down to 20A peak or circa 15A rms, which is what I designed for.
Thanks guys! Now I just have to wind a nice little choke, I`ve seen them at the scrapyart lots of times, might as well go there and pick one up.

Cheers, Finn Hammer

GeordieBoy

Mon Feb 09 2009, 02:08PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Looks like you have the problem solved already. Remember to look at the DC bus voltage after adding the passive PFC choke. As Steve C pointed out, it has the downside of wrecking the steady-state regulation of the DC bus voltage. The DC output will now sag when under load, or may go over-voltage if the load is suddenly removed. Half an hour with the simulator should show if the behaviour with a given PFC choke would be acceptable or not. (There doesn't seem to be much headroom between the intended operating voltage and the IGBT rating, so any DC bus voltage overshoot might be a problem.)

If you are looking to scrounge a suitable PFC choke from an industrial piece of equipment, they are often labelled as "line reactors" in things like 3-phase drives. It will look like a fairly hefty choke made with thick wire wrapped round a bobbin on a gapped core of iron laminations. Something resembling a small ballast inductor.

If you are going to wind your own, the simulations can tell you the RMS current through the choke's winding and the volt-second product across the winding. These are all you need to know to get a suitable passive PFC choke made.

Finally, you might consider splitting the passive PFC choke into two equal windings with half the number of turns. Each winding being connected in series with one AC input to the rectifier. Such a "balanced" choke is effective at stopping noise generated by your equipment getting back into the AC line.

-Richie,

PS. How many watts of real power does the system actually use?

Tom540

Mon Feb 09 2009, 06:34PM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
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Posts: 617

Couldn't you just turn up the variac slowly? Or does it peak this much current during use? What value are the caps?

-Tom

Finn Hammer

Mon Feb 09 2009, 07:34PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

GeordieBoy wrote ...

PS. How many watts of real power does the system actually use?

Calculated from BPS, Vin and Ctank, 2.2kW

Cheers, Finn Hammer

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