AC PFC & MPPT - Expert EE Analysis Required

Good day,

After years of reading and learning I thought I was there with PFC and MPPT but I’ve an outstanding question and it’s time to try my best to get it answered. I’ve tried on other forums but without luck so far.

This question/topic is around a generator, not solar, which complicates matters. Being a generator it’s fixed current and inductive.

The generator’s properties; L, series R, parallel R (modelling Eddie loss), poles are known. Omitting parallel R the impedance equation is straight forward and an ideal load can be calculated. It’s possible with parallel R also but maths is much more complex. The outcome doesn’t change a whole deal anyhow. None of this is related so I’ll move on, but it’s a bit of background for later.

There are two types of PFC for an inductive AC supply; phase correction and distortion correction.

Phase correction involves sticking in capacitance to counteract the inductance. Ideally it will make the phase angle equal zero. The aforementioned impedance of LCR should give the perfect resistive load value to gain maximum output (MPPT).

Now consider the following system.

Inductive AC → Capacitance → Rectification → Buck → Super-Capacitor/Storage → Boost → Output

Assume the super-capacitors are huge and can sink any current provided to them and start from 0V and peak at 5V. Assume there is very little capacitance after rectification (enough to counteract buck ripple). Assume the inductive AC core is small, say 0.5A and so sags easily under-load.

The aim of the game is to present the equivalent of a resistive load by changing the buck’s duty cycle. There will be a little distortion loss as the super-capacitors charge but that can be ignored here.

When we calculate the impedance for MPPT earlier I believe this is using the equivalent DC using AC RMS. If the generator current is known along with impedance and ideal voltage can be calculated. A proposal is to track this voltage either by modifying the duty or by hysteretic control (comparator) which will also vary the frequency. The latter doesn’t require a high MHz MCU to manage high resolution PWM steps and so can keep the buck inductor small.

With this proposal as the AC cycle progresses and voltage rises as soon as it hits the target the buck starts increasing duty and the voltage at the input is now seen as a near straight line. The current into the super-capacitors will increase up to the point at which the AC would be at the peak of it’s wave.

This introduces the question. First, this is not a 1:1 representation of a resistive load and so I don’t know if it’s actually a valid approach for MPPT. An alternative method, assuming the buck operates in CCM, is to use the calculated voltage with the storage voltage and equation duty = output voltage/input voltage.

Your thoughts are much appreciated!

Regards, Andrew

Sounds like you’ve done your research, and probably have already studied all the papers out there on this stuff, but in case you haven’t seen it, this is a fairly good one:

https://www.ti.com/seclit/ml/slup264/slup264.pdf

It’s primarily about buck, but briefly mentions boost, and gives quite a lot of other references.

You can see some pics of how well the EV manufacturers have done in their on-board chargers in this post. Those guys need to generate ~400V DC (or even 800 in some models).

That one is excellent and I hadn’t seen it. Going through it it mentions fixed duty for CCM which I now believe is the correct approach.