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Mains switching research break out

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wizard69:

--- Quote from: oschonrock on August 03, 2020, 12:19:32 am ---
--- Quote from: OM222O on August 02, 2020, 10:56:48 pm ---what on earth are you trying to achieve  :palm:

Don't use fets and simple opto coupler. You should look into using opto triac and triacs for the actual switch. This class AB amplifier you made has some significant drawbacks and most likely overheat the fets (not to mention zero crossing distortion etc) you also forgot protection features like movs, X and Y class rated caps depending on phases / netrual, common mode choke / ferrites for noise.


--- End quote ---

Perhaps I should have been more explicit. The object is go beyond triacs, which don't work with many modern lamps types to start with, and create evil harmonics (leading edge on, zero cross off).

--- End quote ---
I know it is old fashion but why not use a relay?   Use an opto isolator to turn in on that can drive the coil directly.   

--- Quote ---I was clearly concerned about the commutation (class AB as you called it), hence the question on that. The idea for that architecture is not from me ;-)

But if it's seriously flawed please be specific. What's the alternative? Separate and retrigger?

--- End quote ---
I'm not here to call it flawed but do question if it make sense in this use case.   If I understand your goals, the idea is to sell this to people that might have any sort of use you can imagine.   So why not make it as simple and rugged as is possible?   Frankly an ideal device would take a 5 VDC signal at what ever current the opto requires and that is it.

--- Quote ---The other most common/sane architecture (other than full blown industrial AC drive style DC Bus and re-chop) is MOSFET wrapped in a in a bridge rectifier, which has the same challenges ( I believe) with lots more parts that don't add anything..

Thanks for your thoughts and the ideas on protection protection features particularly. I had not yet added any, as trying to validate architecture first ("early sketch..."), but those are all certainly valid suggestions.

--- End quote ---

I look at something like this and frankly don't see a Sane reason to use any sort of solid state switch to drive the output.   This mainly because you have no idea what the load will be.

BrianHG:
https://www.digikey.com/product-detail/en/recom-power/RAC03-15SK/945-3443-ND/10131802
Still not as cheap as the dc-dc converter.

oschonrock:

--- Quote from: BrianHG on August 03, 2020, 03:03:28 am ---According to your circuit, your making an AC PWM on/off switch, not a variable output DC PWM supply.

A variable DC PWM output supply has a topology where when you turn on the low side and high side mosfet simultaneously, you short the V+ rail to your GND.  You are not making this, so in the TI datasheet, the example circuit on page 1 with the added diodes and resistors to tame the turn on and turn off speed do not apply to your design as both mosfets are either on, or off at the same time.  All you need is a series resistor from each output to each gate.


--- End quote ---

Yes, that's the intention. ie, I am not making an AC motor drive. They have a full rectification stage first then a DC bus with huge caps and then an H-Bridge type switch topology. The advantage of that is that they can make AC of any frequency, hence "variable speed drive". My circuit can't do that. All it can do is operate in three  modes (I know you have understood this, but I am catching up on what was clearly an inadequate circuit description in original post which caused confusion):

1. ON / OFF multi cycle period (say 1-2 seconds), the uP can (optionally) do the careful timing to ensure it switches the MOSFETs off during zero cross and hence causes less harmonics ... doing the job of an SSR - if this is all I wanted I should use an SSR - this is the relatively trivial part

2. What is often referred to as chopping / dimming / phase angle switching etc. Means turning the devices on / off once per 50/60Hz half-cycle. This can be done on the leading edge, ie wait after zero cross before switching on, or on the trailing edge, ie turn the devices off early before the half cycle completes. The former (ie leading edge) is what triac based dimmers do, but they can't do trailing edge which has advantages in many circumstances.

3. PWM of the sine wave at frequencies of multiple kilohertz. This is different to the AC variable speed motor drive because it is not switching DC to make AC of any frequency, it is switching the AC sine wave. That means we can "vary the effective amplitude" of the sine wave, but we cannot change it's frequency.

So proper induction motor control is beyond the scope of this "research break out board". It's purpose is to introduce hobbyists / students to the various kinds of AC switching you can do. It should be able to demonstrate the above 3 modes. The purpose is to educate and provide information on strengths weaknesses, from a "ready made box" which can do all 3 modes and be safe. Someone above mentioned current/voltage monitoring and that would be a great addition. Non-trivial due to the isolation requirements, but I might need to go there.

(I might edit original post to put this description there)

So yes, the 2 MOSFETs will always "switch on at the same time". Actually that is not quite correct. In the "upper half cycle"  (when Vline > Vneutral the lower MOSFET on my schematic (Q2) will be in "body diode bypass", ie whatever you send to its gate is irrelevant.  In that upper half cycle the "triggering which is important is on Q1. In the lower half cycle, the roles are reversed.

Now my original circuit with the optocoupler & +12V supply referenced to joined sources and that pull down resistor meant that the uP didn't have to care which half cycle it was in. It would just look at the zero cross info from U1 and decide when to trigger U2 - upper or lower half cycle, don't care.

Your suggestion for a proper MOSFET driver is very good, just what I was looking for, and will be needed to achieve Mode3 = PWM > 1kHz. However we are now talking about separate +/-15V gate triggers... (maybe not...see below)


--- Quote from: BrianHG on August 03, 2020, 03:03:28 am ---
I made a little mistake, you only need 1 cheaper single DC-DC 15v isolated converter and a single mosfet driver as your mosfet sources are connected together.  You only need to go from 0v to 15v from the 'source' on both.


--- End quote ---

Let me paraphrase for clarity. I initially understood your requirement for a +/-15V supply like this: Output stages of a driver like the UCC21220 are "functionally isolated from each other". ie they have VDDA/OUTA/VSSA and quite separately VDDB/OUTB/VSSB. in this case the sources are linked and therefore we would therefore join VSSB and VSSA together. I had originally thought: "Ah but we need to trigger one with +15V and one with -15V", but I am now realising (the same as you did?), that because they are both N-Channel devices and the gate we care about (ie the one which needs triggering, and not the one in body diode conduction) will always need a "positive trigger relative to the common sources".

Have I understood this correctly and am I correct in thinking that feeding what will be "an effectively -15V pulse" to the "non-active device in body diode conduction" will not bother that device at all...? It's "off", and turning it off some more, is still off. Do you have a part number for an equivalent single driver? Is it "normal" to use an isolated driver like this, on 2 MOSFETs, one of which has reverse Vds on it? I suspect it's not very common - except in high power trailing edge dimmers perhaps. Does it matter?

So then yes. single driver and single +15V auxiliary, isolated SMPS. Much easier - and actually kind of rather similar to what I had orginally, but with a proper driver which includes the job of the U2 optocoupler and drives the gates much more competently, allowing "HF" switching, and hence PWM. 

Thanks for the links to the SMPSs - easier now as you say. Some great options there.

Sorry for the long post. More words = more clarity...?

oschonrock:

--- Quote from: wizard69 on August 03, 2020, 03:43:58 am ---
I'm not here to call it flawed but do question if it make sense in this use case.   If I understand your goals, the idea is to sell this to people that might have any sort of use you can imagine.   So why not make it as simple and rugged as is possible?   Frankly an ideal device would take a 5 VDC signal at what ever current the opto requires and that is it.

--- End quote ---

I am sorry, I now realise that my original post had insufficient detail of what the purpose of this circuit was and how it is intended to operate. I have now put a fuller description of those things in the OP.

I am also not yet clear "whether I want to sell this". It is an area of research which interests me, and on which I see many videos/blogs which do many things wrong or are even outright dangerous. So if ultimately people find it interesting and want to "buy something", sure, maybe... but it's not yet a goal as such. In fact, as you correctly point out, I don't know what the load will be and the whole point is to demonstrate how different loads respond to the different modes of operation (see updated description in OP). So whatever the final design, it will not be "optimised for the load", so it will necessarily fall short of a custom design for a particular type of load in either cost or performance or both.

BrianHG:

--- Quote from: oschonrock on August 03, 2020, 09:44:52 am ---So yes, the 2 MOSFETs will always "switch on at the same time". Actually that is not quite correct. In the "upper half cycle"  (when Vline > Vneutral the lower MOSFET on my schematic (Q2) will be in "body diode bypass", ie whatever you send to its gate is irrelevant.  In that upper half cycle the "triggering which is important is on Q1. In the lower half cycle, the roles are reversed.

--- End quote ---

No, it is important that both mosfets are on.  Relying on a body diode means a 1v to 1.5v drop in high current applications.
Say 2.5kw switch.  1.5v drop x 10 amp means 15 watts of heat generated on an off mosfet while using it's body diode to conduct current.  Having a rugged mosfet turned on in this case means when using something like a 'STB40N60M2' at 88mOhm on means a 0.88v drop at 10 amps generating only 8 watts of heat.  Also, with high frequency load transients, that mosfet body diode may switch on and off adding a degree of signal noise while having the mosfet off relying on that internal diode, with the mosfet on, it acts like a fixed resistor conducting current swing transients in both directions, IE, no 1v crossover glitch.

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