Trigger happy

24.03.2011 20:40

Here's one final note in what turned out to be a series of posts on thyristors.

In the case where the cathode of the thyristor isn't at the ground potential you usually need some way of galvanically separating the control circuit from the thyristor's gate potential. The most common components that can do that are a pair of coupled coils (impulse transformer), a capacitor or an opto-isolator. And in fact you can see all these three used in practice.

Here are examples of all three principles I've seen in real-life circuits with some comments:

SCR trigger using a coil

This appears to be the most common way of doing it in older devices. For instance, the Hewlett Packard 6286 power supply uses an impulse transformer to control its thyristor preregulator.

The control circuit sends a current pulse through the primary winding and the matching impulse on the secondary side triggers the gate. The resistors are there to limit the current and assure proper distribution to both gates.

The coil makes this an expensive solution.

SCR trigger using an optoisolator

This approach is usually seen in modern circuits. You can also get thyristor with the opto-coupler already built-in, which removes the need for an external component.

In the case above the opto-coupler acts like a switch that short-circuits the gate to the thyristor's anode, providing triggering current. The trigger current is again limited by the resistor and depends on the anode-cathode voltage. The resistor should be carefully chosen to keep the current within safe limits for all anticipated switching voltages.

SCR trigger using a capacitor

This is arguably the cheapest and most troublesome solution. While in the previous two solutions the trigger current was independent of the cathode voltage, this isn't so in this case thanks to the I = C dU/dt characteristic of the capacitor.

The galvanic separation of the control circuit is achieved by the 1 nF capacitor. Thyristors are triggered by letting it charge through the 2.4 kΩ resistor and the thyristor's gate inputs. After they are triggered the control circuit grounds its output and the capacitor discharges through the 1N4148 diode. Note that V+ doesn't need to be higher than cathode voltage for this to work.

So far so good. However there are several potential sources of problems here:

  • To achieve a powerful enough impulse to adequately open the thyristors either the capacitor should be large or the supply voltage should be high. However the same amount of charge must also flow in the other direction when the capacitor discharges. A large capacitor hence means a longer discharge time and longer delay before thyristors can be triggered again.
  • When a thyristor fires there is a positive step change in the capacitor voltage, which directly translates to a voltage spike on the control side. Without proper protection this can reduce the life time of ICs in the control circuit.
  • When a thyristor is opened during the second quarter of the sine wave the gate voltage has a negative slope before the thyristor commutates. This negative dU/dt charges the capacitor in the opposite way (say -0.7 V if there are protection diodes on the other side). After the first thyristor closes, the current flowing through the capacitor while the negative charge dissipates can falsely trigger the second thyristor, causing misfires.

(Update: that is a 1 nF capacitor in the last schematic, as Žiga noted in the comments)

Posted by Tomaž | Categories: Analog

Comments

The 0.001 capacitor is 0.001uF (not 1uF), though.
I've simulated the (part of the) circuit in LTSpice using an ON's spice model of the MCR706A thyristor (which seems to be a replacement for the C106 mentioned in the AN-32 app. sheet).
The thyristor is a "sensitive gate"-one (IGT=25uA), so the capacitor doesn't need to be that large (1nF does well).
I'm not sure about the du/dt thing. Truly, C*du/dt is so small that it defines charging current of the capacitor, and it can also get charged reversely. But, both thyristors are triggered by a positive current into the gate, so negative capacitor voltage should even help them stay closed.

Posted by Žiga

Of course you are correct about the capacitor size on that schematic, that was my mistake.

Although even for sensitive gate thyristors you might want to have a larger capacitor. This way you can get a higher gate current, which causes a faster switch-on. That can make an important difference in power dissipation when you have large inrush currents.

I guess I should have explained the last point more thoroughly. I can tell you first hand that it does happen. Look at it this way: after the first thyristor commutates, the right-hand side of the capacitor ends up at a negative potential in respect to ground. The right-hand voltage then slowly increases back to the ground potential through the control circuit (which is shorting it to ground). While that happens, current flows from the capacitor into the gates and that might trigger the second thyristor.

Granted the details of this depend on how the control circuit is implemented and perhaps I shouldn't discuss it without going into a particular implementation. But that is enough material for not one but several posts.

Posted by Tomaž

You can find more elaborate impulse driving circuits in bipolar SMPS with few diodes assuring fast turn-off. Beware of false triggering due to frequent mains spikes. Enjoy building space project PSU :-) KISS de S56A.

Posted by MMM

thank you, very helpful.

Posted by H B

hi
here in argentina
thanks for your article
ive been using the pulse transformer approach (built by me)and sometimes the opto (pc817) way
now investigating with IL300 opto scr
with pulse i even fire the big big scr (ice puck) on biphase (380vac ) mains
the main concern is zero crossing, isolation and getting paid
saludos
use atmel avr and bascomavr a great combo
GERALD

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