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.
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)
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.
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...?