need high resolution PWM

[tatus1969]

You can always consider a hybrid system where you have a charger & dc load for the low frequencies and a typical LCR meter circuit for the higher frequencies. This will greatly simplify your system.

You're assuming that it is a battery analyser. I'll be able to tell about what it is soon, but at the moment I afraid I can't. Your assumption seems to be leading to false conclusions here.

A ceramic or polyester film capacitor can handle quite a bit of current as long as the ESR isn't causing problems. In a typical half-bridge forward converter (like any PC power supply) the current through the bridge is fed through a capacitor to prevent DC current through the core. Depending on the impedance of the measurement system you use 10uF to 100uF should get you in the single digit Hertz range.  I recommend using a PET capacitor because these have an ultra low leakage current.

My concern of the capacitor wasn't so much ESR, but it's actual value. For it to have let's say 10V of voltage drop at 1mHz and 5A, the required capacitance would be ~ 80 Farads.

[nctnico]

Still if the load is a voltage source you'll need to be able to sink and source current from it. I don't see that happening with a switching mode amplifier with a big DC offset. You'll need to dissipate the power you pull from the load somewhere if you want to send a true AC current through the load while dealing with a large DC offset.

[tatus1969]

Still if the load is a voltage source you'll need to be able to sink and source current from it. I don't see that happening with a switching mode amplifier with a big DC offset. You'll need to dissipate the power you pull from the load somewhere if you want to send a true AC current through the load while dealing with a large DC offset.

That switching mode amplifier is basically a step-down DCDC converter with forced continuous conduction, these are inherently two-quadrant capable and feed back energy into their supply. The instrument then has a dissipative bus voltage limiter that starts PWMing a MOSFET, which in turn connects a power resistor across the main supply.

[PCB.Wiz]

So it is a battery analyser. That means the output swing (in voltage) is actually very low and so is the power. I think you are going at this the wrong way entirely. I have designed/made similar devices (some that needed to go up to 300VDC) and I always used a capacitor in series with the current source to get rid of the DC offset (with a fast settling circuit at startup). That way you can use a simple but precisely controlled linear current source.

I tend to agree, this is best designed as a current amplifier, not a voltage amplifier and could even be done as a 4 terminal system that uses a series resistor 'dummy load' approach to avoid the need to try to drive millivollts into milliohms just to try to get your Stiff-DC-Source, into the teensy transfer zone. ie Even if you can do that, the noise-floor will dominate the current result.,
A 4 terminal system means you need 2 good ADCs and a current meter, tho that could be a 3rd ADC across the known and stable series resistor.

A quick Digikey search finds
CSR2512C0R05F   Riedon   RES 0.05 OHM 1% 3W 2512   16,000 - Immediate   $0.18354/2,000 50 mOhms   ±1%   3W   ±50ppm/°C
5A into 50mOhms is 1.25 Watts and needs 250mV of swing (and that lands you in some nice current-ADC specs, high volume parts)

My concern of the capacitor wasn't so much ESR, but it's actual value. For it to have let's say 10V of voltage drop at 1mHz and 5A, the required capacitance would be ~ 80 Farads.

yes, OUTPUT end AC coupling gives big numbers, but it does not need to be applied at the output pin.
I'm not sure a cap is needed, as you can build a current amplifier and manage the voltage swings that way. The test software can emulate AC coupling as needed.
Having a finite, series R helps greatly in all of this.

[Yansi]

Still if the load is a voltage source you'll need to be able to sink and source current from it. I don't see that happening with a switching mode amplifier with a big DC offset. You'll need to dissipate the power you pull from the load somewhere if you want to send a true AC current through the load while dealing with a large DC offset.

Class-D amplifier is a pretty nice synchronous buck (or even boost, if the load is an energy source) and will pretty happily push energy back from load to the supply. (Ever heard about "bus pumping" with single ended stages?)

[tatus1969]

I tend to agree, this is best designed as a current amplifier, not a voltage amplifier

Thanks for pointing this out (+ntcnico), this could be an interesting alternative. The instrument has a sense resistor at its output (actually, 50 mOhms ), I could easily add an opamp there and use the amplified signal to control the PA. The DAC output would then become a current, and this would naturally solve my resolution problem as you guys point out. This would make voltage regulation for higher impedance loads more difficulty though. Maybe even a hybrid approach with selectable configuraton would make sense. (EDIT: and also likely hard to tame no-load output voltage that I have to maintain before the load is connected through a relais.) (EDIT2: thinkg further... this may not work as doing so would include the phase delay caused by the output filters in the feedback loop, which may lead to instability.)

and could even be done as a 4 terminal system that uses a series resistor 'dummy load' approach to avoid the need to try to drive millivollts into milliohms just to try to get your Stiff-DC-Source, into the teensy transfer zone. ie Even if you can do that, the noise-floor will dominate the current result.,
A 4 terminal system means you need 2 good ADCs and a current meter, tho that could be a 3rd ADC across the known and stable series resistor.

You're practically describing many aspects of how the instrument works - it uses 4 terminal voltage measurement (one isolated ADC) and shunt resistor based current measurement. At lower frequencies, I currently get > 15 ENOB for current and > 16 ENOB for voltage - and it is a total mess in terms of signal integrity.

EDIT: that also made me realize that the PA cannot see less than 50mOhms load, which is almost relaxing.

Ever heard about "bus pumping" with single ended stages?

I had this very topic in a recent project but the reason for that is actually different. It is the power conversion nature of a DCDC, being able to deliver more current at its output than it takes from its supply, in combination with a symmetrical supply having the load tied to midpoint. When the amplifier delivers e.g. positive current into the load, and output voltage is low, then the lower side supply capacitor sees the small negative supply current superimposed with the larger positive output current, and by is getting charged. The solution is to use two amplifiers in BTL.

[nctnico]

Either way some form of feedback will take care of reducing non-linearities.

[tatus1969]

The prototype PCB is ordered, and I'm very curious to see if my "class AD" power stage idea is any good. As you can see from the first picture, the H bridge driver is far more complex than usual. The other picture shows the PWM generator that I have implemented in the end. Sorry, no sigma delta, in the hope that I haven't made a bad decision