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I have a requirement to drive a triac from the MCU powered by a DC/DC converter. The AC supply is typ 24 VAC @ Rc terminal with respect to C terminal. The triac is driven as follows. The MCU GND is always regulated at Rc - 3V3. The MCU output which drives the gate preferably will be connected straight, without the BJT. The gate current is II/III quadrants is typ 5 mA.

enter image description here

How can I configure the DC/DC converter and its input?

The problem is a real world design issue I am trying to solve. I can use an opto coupled triac driver to solve this. But this is a cost sensitive application, so I am trying to see if I can find a feasible solution without any isolation.

The triac drives a 24V A/C motor. When I tried the triac drive as MT1 connected to GND and MT2 to the load, I loose a feedback signal generated from the motor. I have no control over this, as this is inherent to the motor controller. So I have to wire-up the triac as MT1 to Rc (Live) and MT2 to the load. So instead of switching the 'C' (or neutral), I have to switch the 'Rc' (or Live). This is the constraint.

To further to reduce BoM cost, I am trying not to use any BJTs or FETs to drive the triac gate. To do this, and also for better switching, I want to operate the triac in II and III quadrants, where gate current is negative. I plan to use Z0103 as the triac and in II/III quadrants, I could trigger it @ -4mA (considering temperature range). My MCU (STM32F103) can sink/source 8mA from GPIO so this should be viable. This is my idea for the moment, I could be wrong.

This is where my initial design came from. I think this way might also be a viable solution. enter image description here

We can configure a standard buck as a negative output regulator, so this way the MCU GND will follow the Rc below 3V3.

The DC/DC Buck could be anything which can accept up to 60VDC and source at least 1A.

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  • \$\begingroup\$ Is this homework? Is there a DC/DC converter you are required to use? Can you provide any manufacturer part numbers possible please? \$\endgroup\$
    – Electrocol
    Commented Aug 2, 2023 at 7:06
  • \$\begingroup\$ Sounds like an XY problem. Do you need to time something to the AC cycle in your MCU? \$\endgroup\$
    – winny
    Commented Aug 2, 2023 at 8:21
  • \$\begingroup\$ If your MCU needs less than 20 mA you can use a negative linear regulator like LT3015EMSE-3.3. A better solution is an isolated DC/DC converter where you can connect +VIN and +VOUT (RC in the circuit). \$\endgroup\$
    – Jens
    Commented Aug 2, 2023 at 21:46

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Yes, this is an acceptable method.

Note that:

  • You generally want to drive with as much current as possible (given device ratings, thermal limits, driver capacity, etc.), to ensure rapid and complete turn-on.
  • Gate is the base of some back-to-back transistors that make up the thyristor (effectively); as such, it has voltage drop (a VBE) while conducting, in the direction of load current. So, I would put a diode or two in there, to introduce a little voltage drop, ensuring the MCU isn't reverse-biased while the load is reversed.
  • Gate drive doesn't need to be maintained (continuous) while an on-state is desired, but it should be pulsed to ensure the TRIAC doesn't turn off due to unexpected load current phase (which, a motor will be notably inductive, with phase depending on torque). A modest duty cycle (say, 10-20µs on-time, 10% duty) might help save 3V3 current consumption, say.
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  • \$\begingroup\$ Thanks Tim, I have prototyped it and seem to work. I have not yet tried full temp range yet, but I think that should be ok. Sorry for my delay in replying. \$\endgroup\$
    – Kaush
    Commented Aug 27, 2023 at 2:55

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