Right, I overlooked that you referred to capacitor current sensing in your previous post. Although feedforward is the ultimate solution, I'm not sure if it's really needed for the present application. I have it in designs with PWM frequency below 10 kHzmeasuring the capacitor current of the output capacitor parallel to the load is a feedforward scheme
Isn't measuring output capacitor current equivalent to measuring the derivative of output voltage, though? Looking at it that way, it still seems like feedback, not feedforward. Measuring load current, on the other hand, sounds like it could work.FVM: measuring the capacitor current of the output capacitor parallel to the load is a feedforward scheme, so I fully agree with you. Just a voltage control loop gives bad response (from analog experience with hysteresis controllers).
Unfortunately I don't have something at hand that will fit your needs.
Isn't measuring output capacitor current equivalent to measuring the derivative of output voltage, though? Looking at it that way, it still seems like feedback, not feedforward. Measuring load current, on the other hand, sounds like it could work.
Our output inductor is 47u and its peak current is 6.7A.
Yeah I'm with you so far.The main (fast) loop uses the inductor current. This is a first order behavior so can be easily handled by a hysteresis control scheme.
The voltage loop is the slow one and that controls the set point of the inductor current loop.
When the load current increases, this causes a voltage drop across the capacitor and this will be detected by the voltage feedback. Gradually the set point of the current loop will be increased to get a new equilibrium ("inductor current" = "output current").
If you know that the output current increases with 1 A, you may directly add 1A to the set point of the inductor current control loop. This bypasses the voltage control loop. In my opinion this is a feed forward scheme.
I think I get what you're saying... so the goal is to null capacitor current to zero, which should help keep output voltage constant. Sounds good in theory, but the current measurement would have to be filtered carefully in order to get rid of ripple and be usable. And that imparts phase shift on the feedforward, which could hurt stability. I'll have to try and simulate it sometime.Instead of measuring inductor current and output current separately (and sum/subtract them in the end), you can measure the capacitor current directly as capacitor current is "inductor current" – "output current".
I kind of feel the same. A hysteretic controller may give the fastest rise time on transients, but will suffer far more ripple and overshoot, and thus a much longer settling time than a well compensated linear feedback scheme. Personally I would try to simulate all options to get a rough idea of which is better for the application.Saying a hysteretic controller can work for the application, implies that a regular linear control loop can do to (and most likely better).
Yes, I was referring to filtering ripple caused by the switching of the inverter, not the on/off switching of the entire converter.Hello,
You shouldn't filter the measured current to get rid of the ripple due to the switching of the hysteresis block, you will lose the first order system behavior (for the current feedback loop).
You should only filter components due to higher order behavior (think of parasitic capacitance of the inductor and ringing effects). Of course grizedale has to remove the ripple due to switching of the half bridge inverter itself.
By the way Orson, where do you actually measure current in a half bridge ACMC scheme? Do you measure primary current and rectify it before taking the averaging, or measure the secondary choke current, or measure the switch currents? I can't find an answer in my reference books.The fastest loop response for a half bridge is most easily obtained with average current mode control inside a voltage loop.
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