The purpose of the AC voltage measurement is for control algorithm implementation.
It's intended to be used in a PFC circuit and a DC-AC circuit.

 

It‘s in EN60335 (Domestic appliances), EN60950 (IT equipment) and EN60065 (Audio equipment), all of which are based on international standards. EN60065=IEC65 and I think the others follow suit. Those are the one I am familiar with - I suspect it crops up elsewhere.
There’s various different voltages. 4kV for double insulated, 2.5kV or 1.5kV for Class I.
I’m not sure about anywhere else other then Britain, and perhaps Germany, where it is also a part of the annual Portable Appliance Test or Prüfung ortsveränderlicher Elektrischer Betriebsmittel

Think the requirement for class 1 is 0.75mA leakage @ 1.5kV and class 2 is 0.25mA @ 4kV which suggests impedances of >2MΩ for class 1 and >16MΩ for class 2.

 

Think the requirement for class 1 is 0.75mA leakage @ 1.5kV and class 2 is 0.25mA @ 4kV which suggests impedances of >2MΩ for class 1 and >16MΩ for class 2.

Indeed, but the 2M resistor has to be specified so that it will withstand the voltage.

 

The purpose of the AC voltage measurement is for control algorithm implementation.
It's intended to be used in a PFC circuit and a DC-AC circuit.

Not sure that's enough info. Control of what? PFC doesn't usually require AC volts measurement.
For a DC-AC inverter, if that's what you mean, the requirement to measure output volts could be justified, but its more normal to have an isolated output side voltage reference which feeds a on/off digital signal back to the inverter.

 

Indeed, but the 2M resistor has to be specified so that it will withstand the voltage.

Of course, and that requires, for class 1, at least 4 SMD1206 devices rated @ 400v DC each, though I use 5 x 500k 0.1% in each leg for my DIY differential probe with board routing under each as well.

 

Not sure that's enough info. Control of what? PFC doesn't usually require AC volts measurement.
For a DC-AC inverter, if that's what you mean, the requirement to measure output volts could be justified, but its more normal to have an isolated output side voltage reference which feeds a on/off digital signal back to the inverter.

Yes, I mean inverter.
The voltage measured at the ADC is fed into the microcontroller for processing so it can adequately vary the PWM that is sent to the MOSFET driver, hence controlling the operation of the inverter

 

Yes, I mean inverter.
The voltage measured at the ADC is fed into the microcontroller for processing so it can adequately vary the PWM that is sent to the MOSFET driver, hence controlling the operation of the inverter

Why do it that way? Massive overkill. If you look at the reference designs from eg ST or AD for DC-AC inverters from 250W to 10kW not one of then use an actual measurement of the AC. They tap off then rectify a small sample to give a DC voltage related to the AC voltage, then use a comparator, or most commonly, a TL431 precision reference to switch the LED in an opto-coupler on and off which essentially blocks the PWM signal.

The one example I found that does use this technique doesn't use a microcontroller but a much faster TI DSP to implement the control algorithm - the DSP is needed for its hardware multipliers as the algorithm involves some complex transforms.

 

If it is running a PFC, then the microcontroller will have its V- pin connected to rectified mains negative, so it won't need isolation from the mains. It will just need isolation from everything else!
Implementing a PFC feedback loop in software is no mean feat. (It also seems like an immense source of frustration and waste of time when you can get an 8-pin IC to do the whole thing for you in the analogue domain for <£1.)

 

If it is running a PFC, then the microcontroller will have its V- pin connected to rectified mains negative, so it won't need isolation from the mains. It will just need isolation from everything else!

To be fair to the TS, a DC-AC inverter - as long as it isn't grid connected - doesn't have quite the same isolation requirements as something monitoring grid voltage, as you have much more control over the environment. But then, if it isn't grid-connected, I'm not sure where the PFC requirement comes from...