Noise of a straining transformer

02.09.2010 17:38

My lab power supply is now almost complete. It passes most tests I throw at it with flying colors except for one. When the output is even moderately loaded the toroid mains transformer is giving off a loud 100 Hz hum.

This is usually a sign that the windings are badly overloaded. Another warning sign that pointed in that direction were the thyristors in the preregulation stage that were dissipating more heat than I anticipated. This called for further investigation.

This is how the voltage on transformer secondary (blue trace) and capacitor (yellow) look like when the power supply is in short-circuit with current limit set to maximum. The gray trace shows transformer voltage when the power supply is idle for comparison.

Transformer and capacitor voltage oscillogram

The preregulator is working correctly here - it fires the thyristors in the correct moment so that the sine wave charges the 4700 μF tank capacitor just enough to replace charge lost due to load current during the previous cycle.

But that voltage drop on the transformer looks worrisome. Adjusting my SPICE simulation so it matches these measurements it gives peak transformer current at 17 A and RMS of almost 7 A. This is quite a bit above the 4 A RMS specification of the transformer.

Although I don't trust these simulation results completely they do confirm that components are getting overloaded. This is much more current than I accounted for in the design - I was counting on the stray inductance of the transformer and capacitor ESR to dampen the response a bit. With such current spikes it's no surprise thyristors are heating up.

The solution I'm looking into right now is to put a couple of power inductors between thyristors and the tank capacitor. They should provide enough reactance to smooth out the current.

Unfortunately to lower the RMS current all the way down to 4 A I would need a pretty big inductor. Bigger in fact than I'm willing to invest into right now, considering these things cost and arm and a leg to ship to this end of the world. For a start I'm ordering a pair of cheap bobbin-types from eBay. Although they shouldn't be enough according to SPICE at least I will be able to more accurately estimate the correct value.

Here's the simulation with the 680 μH inductor in place:

Simulated transformer and capacitor voltages

Green - voltage on transformer secondary. Blue - voltage on the tank capacitor.

Simulated transformer current spike

Current through the transformer secondary.

So, a few lessons learned: with a thyristor regulator RMS current can be even 3.5 times higher than the rectified DC current (compare that to the 1.6 derating factor you usually see on transformer datasheets). To get that down to a reasonable level you need expensive inductors, which eliminates one of the advantages over switch-mode regulators.

Posted by Tomaž | Categories: Analog

Comments

You really need a pair of 2A bobbins for your project but inductance would be halved! Welcome to the world of power electronics and bad capacitor surges. Choke it :-)

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