Leaky thyristors

29.07.2010 19:52

As you might remember, my lab power supply in-the-making utilizes a thyristor pre-regulator stage. When I soldered the circuit together on the final version of the printed circuit board it had an interesting glitch. Occasionally it would get into a mode where the regulation circuitry completely failed to work, pegging the output voltage to the maximum with ill effects to the longevity of the linear regulator down-stream.

The solution, as you will see, was simple, but the exact cause still puzzles me.

This is the critical part of the circuit:

Simplified thyristor regulator diagram

Here you see the transformer with a 30 V secondary and two full rectifier bridges with a common return path. The lower bridge simply produces a pulsating voltage (SYNC) serving as a synchronization signal for the control circuit. The upper bridge creates the regulated voltage over the tank capacitor. It has thyristors instead of diodes on the positive side which are triggered in the right moment with current provided by the control circuit.

Now here's what it looks like on the scope:

Thyristor regulator oscillogram

The regulator is working correctly on the left half of the picture. Set point is 8 V (as shown by the yellow trace) and almost no load. The control fires thyristors when the AC waveform on anode falls a little bit above 8 V (just before the blue or gray trace crosses the yellow trace) to cover for the negligible loss of charge on the tank capacitor.

But what happens at the third cycle? The gray thyristor fires at the correct moment, however the blue thyristor fires as well as soon its anode voltage gets high enough, charging the output capacitor way above the set voltage.

The immediate reason for this fault is because the combined blue and gray lines never fell to 0 V at the same time. Here's a close-up of the oscillogram: you can clearly see that the blue and gray waveforms intersect at 8 V, not 0 V. Since the control circuit depends on the SYNC signal (which basically provides max(Ublue, Ugray) function) falling near zero it never switches off the trigger current and mistakenly triggers the blue thyristor too soon.

Thyristor regulator oscillogram, detail

So the blue thyristor firing is a secondary effect. The problem already shows itself well before that, when the gray voltage gets pegged at 8 V. So what's happening here?

If it weren't for the SYNC bridge the blue and gray voltages would be weakly defined when the voltage over the transformer secondary is less than 8 V because all diodes in the bridge would be closed. However, the SYNC bridge diodes are conducting down to 0.7 V above ground, so the voltages should not be floating at 8 V as the 10 kΩ resistor makes a good connection to the ground.

Take a look at the instant marked with the red line. Here are the voltages at that instant on all the active components:

Thyristor regulator diagram, annotated

Note that the voltage on the SYNC bridge's 10 kΩ load is 7.4 V, meaning 0.7 mA is flowing through it.

Kirchhoff's current law says this current must come from somewhere. Where? At that moment voltages on all elements point away from the bridge, so it can't be a large leakage current because it would point into the wrong direction.

The only exception is the blue thyristor, however it would have to leak pretty badly to account for almost a milliamp of current. These thyristors are rated at 1 mA reverse current at 800 V. And furthermore it appears as if the current starts the moment trigger current starts flowing into the gate.

As I said, the practical discussion ends with the finding that some stray source of current is affecting the circuit. The obvious solution is to decrease the load resistance of the SYNC bridge so far that it is able to sink the additional leakage current. And it worked - after decreasing the resistance to 5 kΩ the problem went away.

However, the underlying cause of this anomaly remains a mystery to me.

Posted by Tomaž | Categories: Analog

Comments

Beware of transitions when tyristors fires into large capacitor taking into account xfmr internal resistance. Good luck with analog electronics. I just glanced Python after Java tutorials. MIT Fourrier before that. Convolution is easy now. Even at 65 :-)

Posted by MMM

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