Mains Triac drive by isolated SMPS?

[cupoftea]

Hi,
Regarding driving mains triacs to supply kilowatt loads,
Why don't they bother using an isolated flyback SMPS, with
its secondary ground to the Triac A2 terminal. Than a simple
transistor switch, operated via a digital isolator, could make triac gate
current flow. It would be exactly the right amount of gate current, instead
of these phase controlled opto-triac drives whereby the amount of triac
gate current depends on where you are in the mains cycle at the point
of triggering.
So why don't they do it like this?

 

[Easy peasy]

Have you heard about GD transfromers, 100kHz, picket fence - does any SCR or Triac triggering you want, been around since 1955

 

[cupoftea]

Thanks, though with a bipolar isolated SMPS, you can drive the triac in any quadrant you like. Quads 1 and 3 give far less noisy operation.

 

[Easy peasy]

not sure about less noisy - there are 4 quadrant Triacs ( i.e. designed to be driven any which way ) - or back to back SCR's for higher powers.

 

[Easy peasy]

https://nz.mouser.com/datasheet/2/848/bt139-600e-1520116.pdf

4-Q

Triac

 

[FvM]

4-Q triacs have lowest, 3-Q triacs medium and SCR highest dI/dt and dV/dt rating.

 

[D.A.(Tony)Stewart]

All Triacs have BJT current gain thus load dependent, thus phase voltage dependent. High sensitivity types have higher hFE. Some have built-in snubbers.
The Miller capacitance tends to increase with current rating in all BJT's and Triac's thus making it more prone to false triggers from dV/dt when off.
Thus why SCR's are used in high current switches.

My 1st light organ in the '60's used DIY pulse transformers triggered from CD4000 series logic using magnet wire pairs wrapped around iron wire.

 

[Easy peasy]

Also if you are operating below 10 deg C - more and more gate current is required as the temp drops ( and vice versa )