Background: (Proper galvanic isolation, e.g. opto-isolation, isolation transformer, etc., is assumed for the question)

When working with 110/220AC, switching elements are typically relays (solid state and electro-mechanical) or triacs/thyristors. Doing AC power control requires phase angle detection and zero crossing switching. Less frequently I've seen rectification+MOSFET as a control strategy. A previous question here links an Instructables tutorial which implements a possible DC switching method.

Both share some disadvantages, like any time an AC supply is switched, power factor is affected. Advances in MOSFET technology mean that prices for high-voltage transistors have evened out.

Question: For loads that are compatible*, what are some reasons against "rectify AC and use a high voltage MOSFET" and favor "triac phase angle control with zero crossing detection"?

*Some AC loads also function on DC: resistive bulbs or heating elements, universal motors, etc. and therefore maintaining AC is not a requirement. Assume higher non-RMS voltage is not an issue.

Side note: I'm analyzing different strategies for controlling heating elements in a SMD reflow oven-style device.

  • \$\begingroup\$ like any time an AC supply is switched, power factor is affected A diode-bridge + smoothing cap. also affects the power factor (assuming no PF improvement circuit is present). The TRIAC based solution can be done without a large (electrolytic) smoothing cap. and Elcos are a well known point of failure. I think TRIACs are more robust than MOSFETs. I also think TRIACs are cheaper. Especially for heating applications, on/off only is good enough, doing that with TRIACs is a well known and reliable solution. \$\endgroup\$ – Bimpelrekkie Jun 27 '18 at 12:47

One of the issues is that Rds(on) increases rapidly with breakdown voltage rating for MOSFETs (for a given die size) and it does not for triacs, SCRs or IGBTs, so high voltage high current MOSFETs tend to be expensive and/or require expensive protection against transients.

When you rectify you're also adding two diode drops' worth of losses. The triac avoids that.

Nothing is really perfect, but in the 10+A/240VAC range triacs (especially in the form of SSRs) and mechanical relays or contactors stand out. In the first case you have heatsinks to provide (which may cost more than the SSRs) in the second case you have lifetime to worry about. Solid state switches tend to fail 'on' and can easily be destroyed by a heater failure, even with expensive fuses, unless grossly overrated (and thus more expensive). It's fairly easy to protect a 40A triac with a < 5A load, but not so easy at higher currents.

Mechanical contactors can withstand a bit of abuse (transients, surges) but have a defined life. And they click, which may or may not be an issue. It would be nice if we could buy a solid-state device which could withstand a 2000V surge, a 500A momentary short AND had almost no losses, but they don't exist, at least not at an affordable price because acres and acres of silicon would be needed.

We used to use mercury displacement contactors because of the very long life, quiet and cool operation and only occasional explosions, but they are verboten these days in many situations.


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