TRIAC Switching Trailing Edge

[nis2311]

Hello Team,
I want to drive the traic like trailing edge mode. In this mode how to turn off the TRIAC before the zero cross. Need to turn ON the TRIAC at starting and need to stop at 50% of duty cycle of the sine wave. But TRIAC turns off naturally at zero cross. I need to turn off the TRIAC before zero cross.




Regards,
Nishanth M

 

[FvM]

It's not feasible with TRIAC or SCR, they can only turn off at current zero crossing.

 

[KlausST]

Hi,

But TRIAC turns off naturally at zero cross.

more exactly CURRENT zero cross.

In this mode how to turn off the TRIAC before the zero cross.

I simply won´t do it. Because it´s against the typical triac behaviour. The triac itself can´t be turned OFF, one needs external cirtuitry to reduce current to close to zero for the triac to turn off.
It surely can be done ... but the effort is rather high.

***

Need to turn ON the TRIAC at starting

At starting of what?
Starting of voltage? Then the gate pulse need to be long enough to ensure the current did rise to the holding threshold.
And in case you have not purely resistive load ... it becomes even more difficult.

and need to stop at 50% of duty cycle of the sine wave

Sine wave has no duty cycle ... we talk about phase angle of voltage (or current) instead.
So did you mean @ 90° of voltage?

****
All in all we could help you a lot more if we knew true application requirements and specifications.
Without use case and numbers ... all we can do is guess.

Klaus

 

[FvM]

Trailing edge dimmers are available on the market, usually implemented with anti-serial connected MOSFET.

 

[dick_freebird]

Only specially designed "GTO" SCRs can be forced off while conducting. This needs a real strong gate shunt, active.

Butterfly that (in concept - structural reality differs) and you need two flying shunt switches that are OK with high reverse voltage, where's the supply / drive for that, yadda yadda.

 

[danadakk]

Saw this on a forum :

you can turn off the TRIAC by short circuit TRIAC main terminals, connect a BJT in parallel to the TRIAC MT1 and MT2, just pulse the base of the BJT for a very short time to short the TRIAC then the TRIAC is off, remember if the current is very high use a power BJT.

But does dV/dT negate this ?

SCR (commutation) :

SCR Turn OFF Methods: Natural, Forced Commutation

Different SCR Turn OFF Methods like Natural, Forced Commutation are explained. In Forced, Class A, B, C, D, E. Dynamic turn OFF Characteristics.

www.electronicshub.org

Regards, Dana.

 

[BradtheRad]

The link about SCR commutation has a method (among several) employing a capacitor in series with the load. The capacitor charges partway through each half-sine, halting conduction. Perhaps a similar method can be used with a triac. It requires careful selection of cap value and neighboring components.