That’s correct. Which reminds me that there’s another kind of flicker you need to pay attention to, which is causing voltage dips, or “flicker” on the line when you switch a heavy load. This was the very first type of EMC regulation ever legislated, back in ancient days.
You’re right; they fooled me by being drawn in different orientations on the page. They are not both ON at the same time, and the circuit is very similar to circuits that use back-to-back SCRs on alternate half-cycles to act much like a DIY TRIAC.
Any capacitance added to gates will slow the devices down a bit, but too much delay will increase EMI by causing turn-off to occur past the zero-crossing.
While the discussion so far has been concerned with MOSFETs vs TRIACS etc. I would propose a different approach entirely. Instead of chopping the mains voltage I would simply rectify the mains voltage and then use PWM to chop the DC at a much higher and more easy filtered frequency. This approach, being only slightly more complicated, would give you all the resolution you wanted. You could even use a synchronized rectifier to reduce any harmonic distortions on the mains supply. For the output stage you could use MOSFETs or even IGBTs.
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I had thought about suggesting rectification but wouldn’t it be as noisy on the PWM as sharply switched AC? At least there’s frequency control over your rectified PWM, I suppose. Any other pros / cons to switching rectified mains in EMI terms?
Switching the AC mains is done at the line frequency, especially when using TRIACs. Using PWM to switch DC can be done at 10-20KHz easily, much easier to filter. Depending on the current, the PWM is unlikely to get back on the mains with a sufficiently sized capacitor on the DC bus. There will be some harmonic distortion on the mains when using just a plain diode bridge rectifier, again depending on the current. This is how most servo drives and inverters work, although this application should be much simpler as it is driving a resistive load with no back EMF to deal with. There would certainly be no “flicker” and the system could be designed with as much resolution as required.
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Using PWM to switch DC can indeed be a good solution if, by changing the switching frequency, the electrical noise it generates is easier to squelch than the phase-modulated AC switching with a TRIAC. That said, selecting capacitors, inductors, and other components to reduce EMI (both radiated and conducted) is very often quite difficult. In short, even seemingly simple designs can be rife with complications for the designer.
But PWMing rectified DC doesn’t necessarily mean blunter edges than PWMing AC? The benefits are basically the relative isolation of the noise from the AC side in the rectified approach?
This benefit shouldn’t be downplayed, as conducted emissions and other line-side contaminations can be a serious and difficult to mitigate problem.
I endorse @1.21Gigawatts’s suggestion, but as the OP asked for AC control solutions I didn’t go into DC.
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No, no, I agree they are worthwhile. Just wanted to make sure I wasn’t missing an unspoken benefit in addition to this.
Yes, the topic and the initial question are focused on phase angle control of AC and a resistive load, specifically with TRIACs. However, after the op’s second post this seemed more like a description of the load and his current approach and less of a constraint. So I chose to go “outside the box” and address the main concerns he raised in that second post.
Namely:
No, it does not, in fact it will generate EMI. But that EMI is going to be much easier to mitigate than polluting the AC mains. As mentioned, depending on the power requirements, the rectifier will introduce harmonic distortion on the AC mains which is significant problem to be reckoned with but still manageable.
Obviously switching a TRIAC on mid-cycle is not a good approach from an EMI perspective. A “cleaner” approach using TRIACs would be to switch at zero-crossings and control the number of half cycles during which the load is on during the control window. For example, using a window of 1 second would give 100/120 half cycles giving a resolution of approx. 1%. However, the load would be switched on/off at a rate of 1 Hz and, depending on the power required, could result in significant flickering of other lighting etc. Reducing the window size to 10/12 half cycles reduces the control resolution and increases the rate of flicker to 10 Hz.
The proposed solution eliminates all of these issues and results in much more manageable EMI. The EMI is constrained to the local system, lines to the load could be kept short and shielded if necessary. The switching circuitry could be contained within a faraday cage. The higher frequency of the PWM is more easily filtered.
I am not sure that is correct
For Triac you are switching at 100Hz, 1500 times below the conducted emission start at 150kHz,so the harmonics are way down, easy to filter (no filter)
For switching at a DC supply, you have the ESR of the electrolytic and the remaining noise needs a filter. Not a simple filter if the load is high. (probably over 500W)
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Also,now you are above 75W, so then you need to do PFC.
The switching edges of the two solutions are not the same either. For 100Hz a slow edge is not a problem since switching losses relate to 100Hz. For a HF DC switch, you need fast edges or efficiency will be bad
You don’t want to smooth the rectified DC, just PWM the rectified AC.
Soft switch (> 1uS rise and fall times) the power devices and use a fairly low switching frequency to keep the switching losses under control.
The suggestion listed using DC cap
If not, ii is even worse. Conducted emissions at 150kHz would be very high?
Heaters have a high inertia so the speed of switching is less relevant. I do not know the wattage of these but I assume we are talking kilowatts. At such wattage, dissipation in your semiconductor elements can become an issue as well as EMI problems.
An approach i did not see come along is switching with a normal relay with a long PWM period to control the power to the heater.
In case you are concerned about EMI or lifespan of such a relay I got the following suggestion:
- Place a triac in parallel with the relay contacts.
- With a micro controller you fire these a couple of cycles before you switch ON and during you switch OFF the relay.
This way you achieve the following advantages: - The ON time of the triac is very small so no heatsink required.
- You switch during the zero crossing so no EMI.
- Re relay contacts take over which results in close to zero losses in the switching element (relay contacts).
- The relay contact never see the stress of contacts closing and opening under load which means they achieve a long life span.
Such solutions are used in controllers for injection molding machines which cycle day and night heavy resistive loads.
Both FCC and CISPR 22 compliance testing are only concerned with radio interference so it makes sense that their testing methods start at 150KHz. For the purposes of power quality in an AC system we’re far more concerned with THD at frequencies much closer to the line frequency.
While the op hasn’t posted the specs of his heater elements I would not be surprised if they are a couple orders of magnitude greater that 75W. And I did mention that the use of a simple bridge rectifier might result in THD that might need to be managed, i.e… PFC.
Slow edges might be good for EMI but bad for power dissipation when switching several kW. 10KHz is not exactly HF.
The proposed solution, while not necessarily perfect, is common in the power electronics industry.
You might want to revisit this statement after considering when a TRIAC turns off.
Assuming it would work, you would consider adding this complexity to avoid a heatsink?
Thank you for all of the suggestions! I hadn’t considered rectifying the AC. I’ll have to do some more research into efficiency and power factor correction with that approach.
Corrected: you switch the triac during the switching off of the relay.
It is not only the heatsink cost savings. You avoid EMI suppression components which can become quiet bulky and costly at high power levels. So in terms of component count/ product size and heat generation a relay combination with a relative light triac might be a good solution.
This very low frequency type of PWM would be a very good solution if the heater had a large thermal mass, but the heater is just a halogen lamp with very low mass.