Triggering Snubberless Triac in QII and QIII with positive power supply

[Nazmul Hasan]

Posted on October 28, 2017 at 07:17

Hi

Can I connect the positive output voltage terminal of my off-line power supply using Viper16L to AC-main's (N) terminal (doesn't make sense though) to trigger a snubberless triac (T435-800T) as I drawn below. Is there any way to avoid isolated power supplies in such cases? Please let me know your opinion.

Thanking you.

#triac #snubberless

[Benoit RENARD]

Posted on October 30, 2017 at 09:26

Hi Hasan,

Isolated power supply is not mandatory.

I don't know if your schematic is correct as you placed the label A1 of the Triac on the A2 terminal... If you want to trigg the Triac in QII and QIII, please connect the +12 V to A1, also referenced to Neutral. And connect the load between Line and A2.

All you need to know is in this ST application Note : AN3168 regarding 'Non-insulated SCR / Triac control circuits'. Especially section 2.4 for Triac triggering with negative gate current and section 3.4 for buck design.

An another usefull AN on this subject : AN4564 regarding 'Is a positive power supply mandatory for my application, or could a negative output work also?'.

Let us know if you have additionnal question.

Benoit

[Nazmul Hasan]

Posted on October 31, 2017 at 05:16

Hello Benoit,

I regret for my bad, label A1 of the Triac. To be honest, I didn't even know earlier that gate terminal should be drawn from A1 terminal. Thank you so much for pointing me out.

Yes, I found it helpful now. I don't need an isolated power supply but in order to trigger Triac in QII and QIII, I can do so with a non-isolated power supply and  it should be negative one i.e. Buck-Boost converter for ease of implementation. Please correct me if I misunderstood something.

Nazmul

[Benoit RENARD]

Posted on November 02, 2017 at 12:23

Hello Nazmul,

You understand well. 

The figure 4 of the AN4564 is a easiest way for a non-insulated SMPS (buck converter), with the +5V connected to A1 for a QII/QIII driving.

Did your power supply only supplying the Triac gate ? If yes, you can have a look on resistive or capacitive power supply. 

Benoit

[Nazmul Hasan]

174845CGIL4

[Benoit RENARD · Accepted solution]

Posted on November 13, 2017 at 09:08

Hi Nazmul,

Happy to see that your design is working.

Only a small remark about your circuit : If you use the T435-800T Triac, you need to sunk at least the IGT = 35 mA from the gate.

I advise you to reduce the R10 resistor from 180 Ohm to 68 Ohm (IG ~ 5 V / R10), to apply twice the T435-800G IGT for proper operation.

Let me know if any other question.

Benoit

[Nazmul Hasan]

Posted on November 16, 2017 at 07:57

Hello Benoit,

Please find my calculation regarding current limiting resistor, R10.

           Worst-case Vdd (10% regulation of 12V) - VGT - VBE (Q1)      11 - 1.3 - 0.7

R10 = -------------------------------------------------------------------------------- = ------------------- = 180 Ohm

                  IGT (50 mA for safe operation instead of 35 mA)                         50

Yes, it's better to count twice of the rated value for safe operation. Thanks for remembering me of that.

Nazmul

[Benoit RENARD]

Posted on November 17, 2017 at 07:48

Perfect !! 

Yes twice the IGT, because there is also a derating of IGT versus junction temperature (IGT higher at cold temperature).