Thyristor/SCR H-Bridge driver circuit Diagram

[mcmsat13]

The circuits below are simulation circuits for my SCR H-bridge driver with passives and discrete components.
I have have been looking for a way to drive thyristors/SCRs in DC applications but gating the devices, especially turning them off has been a poser to me. I have Googled for a standard driver circuit regarding this constraint and only saw little or no circuit relating to my application.

I know there are people here who are experts in electronic designs.

The circuit below shows when the switch is in mode A; the SCR D1 and D3 are switched on while D2 and D4 are turned off



The switch at mode B; the SCR D2 and D4 are switched on while D1 and D3 turned off.



Now, the circuit with a motor, switch in mode A If I turn the circuit on, it worked in any of the mode; A or B but if I switch from one mode to another then something bad happen!



With Motor, switch in mode B:



The circuit below shows when I switch to another mode.



Please I want your help here. this circuit I have been looking for but I could not see any. I design this circuit through thyristor theories base on Forced Capacitor forced commutation, C1 to C4.

I just design this but i know that there is something I am missing or some values that are underrated or overrated. Please I want some corrections in this circuit.

 

[BradtheRad]

From what I understand, you stop an SCR conducting with a brief voltage spike at one end.

To do this, the spike must be the same voltage and polarity which is applied at the other end.

Have you succeeded at simulating this with an SCR?

As you know, you cannot turn off an SCR by turning off the gate. You must stop current from going through the body of the device.

 

[betwixt]

I think in the schematic the method of operation relies on the saturation voltage of the commutation transistors being lower than the 'on' voltage across the SCR. Pulsing the transistors at the right time causes C1 - C4 to draw charge from the SCR voltage and drop it suficiently to stop it conducting. The resistors across C1 - C4 discharge the capacitors in readiness for the next cycle. I'm not sure what speed the circuit runs at but the CR time constant would be fairly critical, too fast may not draw enough current to cancel the SCR voltage and too slow may not allow the capacitors to discharge sufficiently.

Brian.

 

[mcmsat13]

Yes at all, This thread indicated my intention to switch 4 SCRs in H bridge topology. I posted this circuit to show my intention.

What I want here now is to know if someone in this famous EDA board can give me a working circuit that can reliably switch these SCRs at the right time in H-bridge. See, you can entirely ignore my own circuit. The circuit I expected may not necessarily look the same with my own. I say again I posted this my own for reference just for the engineers to know my intention.

Thanks in advance.

 

[rhnrgn]

which program is this you are using for simulation?

 

[BradtheRad]

Here's a demo of a simple way to turn off an scr with a pulse of negative 170V.

This is just to illustrate. I'm not saying it has to be the optimum method.

 

[schmitt trigger]

To turn off an SCR, the A to K current has to drop to zero for a certain amount of microseconds.
This condition is defined as naturally commutated (like phase control) or force commutated.

Since your bridge is force-commutated, the first thing is to employ an inverter grade thyristor. Don't use 50/60 Hz rated thyristors. Their turnoff time is NOT controlled.

Other than that...your circuit looks ok.
Be aware however, that the capacitor/transistor combination must carry the full load current for the amount of time the SCR turns off. In other words, the capacitor's value, and thus its stored energy, is dependent on the load current.

GE's SCR manual, which unfortunately is out of print since the early 80s, has a full chapter devoted to SCR commutation methods. You nay want to search the web to see if someone has scanned it.

 

[mcmsat13]

which program is this you are using for simulation?

I use circuit wizard to simulate it.

 

[Orson Cart]

In the 70's and 80's there was a lot of work done on how to turn off inverter grade SCR's with forced commutation by employing large-ish charged capacitors that are connected across the SCR's to force a reverse current slightly greater than the load current to give a small net reverse current in the SCR to ensure forced turn off, the problem then is to recharge these caps in time for the next turn off event - often a series L was usd to give a resonant current half cycle with a well defined peak current Vc / (root(L/C)), no doubt the GE scr manual referred to above has a lot of good circuits. When you see how involved these circuits are you will understand why almost everyone has shifted to IGBT's....

 

[D.A.(Tony)Stewart]

Here is an old presentation on an SCR upgrade to IGBT's FWIW


https://www.wmea.net/Technical Papers/AC IGBT Drives And Their Applications; DCS - Nov 08.pdf

 

[mcmsat13]

I started this thread. I thank everyone who contributed to this thread. I think this thread will be closed here. Though this has led me to many knowledge about SCRs/Thyristors. now, I can be able to tell more about theses rugged devices and the more I could tell about then is that SCRs/Thyristors are never for hobby thing when it comes to inverter application stuffs!

Thanks.