EMF from turning off mains mid cycle - Page 1
Forum section: electronics / beginners

author: TimB100
author_rank: Contributor
timestamp: February 01, 2022, 08:40:39 am
content: Hello good peopleFirst off I really appreciate this forum that allows me to periodically ask stupid questions. Thanks for readingA standard Triac mains speed/power control system works by turning on mid way through the sine wave and it turning itself off at next Zero cross. From experience the high turn on voltages create electrical noise.In a new circuit I want to rectify the AC to DC and using a mosfet turn on at Zero cross and off somewhere during the current half wave. My question is is that going to generate a lot of electrical noise and is it an inherently bad idea.BTW it is a resistive load and No I do not want to use the whole half wave pulse density control. It's too crude for my needs.Thanks Tim

author: Siwastaja
author_rank: Super Contributor
timestamp: February 01, 2022, 08:50:44 am
content: Stray inductance (inductance of wire and the resistive heater) tries to keep current flowing, increasing voltage as a result.But seeing you have a rectified DC link, the solution is trivial: instead of a single MOSFET, use a half bridge, i.e., add a freewheeling diode. This way, there always is a route for current: after turn-off, the diode conducts however long is needed to discharge the energy stored in inductance, utilizing it for the intended purpose (mostly heating the element).Circuit-wise, the diode goes in parallel with the load, but physically as close to the MOSFET as possible, with minimized length through the loop defined by: closest DC link capacitor positive - track - diode C - diode A - track - MOSFET D - MOSFET S - track - DC link capacitor negative.Optimally, the diode should be of schottky type to allow quick turn-on and turn-off, with no reverse recovery loss. For rectified mains voltage, SiC that is. Although it might be an overkill for such application, but they are not that expensive anymore.Correction: with just little stray inductance, obviously the circuit always operates in DCM (discontinuous conduction mode, current through diode falls to zero every cycle). Reverse recovery and turn-off speed are thus irrelevant: just use a diode with fast turn-on.This reduces the switched inductance to only that of the minimized switching loop.So the idea is to use the same strategy which is absolutely fundamental with a truly inductive load - a motor or the inductor of a buck converter, for example. In your case you have just much less inductance and "might" be able to do without freewheeling. It's still the simplest thing to do.Finally, limit the switching speed. This is trivial because you switch rarely (once every 20ms?) so even a long switching time (a few µs) isn't going to add significant switching loss penalty. Just be aware of the MOSFET's SOA curve in case you go to excessively long switching times (say, over 10µs). Gate series resistor is used to control switching speed.

author: TimB100
author_rank: Contributor
timestamp: February 01, 2022, 10:30:42 am
content: Many thanks for the answer. I'm really very grateful.I'm not sure the switch on will be an issue as the voltage will be near zero as it will be started at zero crossing. But definitely going to add the diode as it's always a sensible addition.I could also look at slow turn off as a method as long as, as you say I do not make it take to long.CheersTim

author: radiolistener
author_rank: Super Contributor
timestamp: February 01, 2022, 10:35:49 am
content: I suspect the current and duration of a short pulses near crossing zero will not be enough to feed the load.And yes, it will produce noise. The same as any PWM. Higher current of pulse leads to higher noise emission.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 01, 2022, 10:40:02 am
content: Oh, so you are not smoothing the rectified link with a capacitor, hence turn-on at zero voltage.I haven't completely thought this out but just a hunch, why not reverse the idea, and turnoffat zero current (with mostly resistive load, this is also at zero voltage), but turn on at random angle?This should work because zero voltage turn-on reduces inrush current (and high dI/dt) into capacitive load, but the heater has negligible stray capacitance. Same can't be said about stray inductance, which is significant.You can see this in practice: plug in a laptop power supply. Sparks ensue. Unplug it: no sparks. Then plug in a kettle. No sparks. But try to plug out the kettle while on!

author: David Hess
author_rank: Super Contributor
timestamp: February 01, 2022, 02:55:34 pm
content: Assuming similar dV/dT, I do not think the generated noise will be any different.The usual solution in thyristor designs is LC filtering to limit the dI/dT.

author: jonpaul
author_rank: Super Contributor
timestamp: February 01, 2022, 09:39:42 pm
content: The energy stored in an inductive circuit causes the fast spikes in V as current is interrupted.Common practice is an RC snubber across the switch eg ~ 100 Ohms 5 W flameproof in ser with cap 1000pf...10 nF Y rated for 250 VACJon