push-pull converter
>how did u get 87% efficiency with KiloAmp pulses happening?
Probably only because these MOSFETs have such extremely low RdsON, and there are so many in parallel! Total RdsOn of the paralleled FETs is below 0.7 milliohms on each side of the push pull! And the connections are made with heavy copper bars. No problems there, at least.
I expect that the efficiency should end up slightly above 90%, once the circuit is stable. And I really hope so, because 13% loss, if it holds true at the 3500W level, means over 500W of dissipation in the inverter, and I can hardly believe this thing would survive that amount of heat for longer than a few minutes! It does have a large fan, but only very small holes on the other end for air to get in... And the heatsink fins are on the OUTSIDE, while the fan sucks air through the INSIDE! Very clever...
> i wouldn't have thougt voltage mode was worth thinking about...unless its got current limiting...
Same for me. But the engineer there in Taiwan thought it was fine, and here I'm stuck with this contraption!
> but u say no current sensing anywhere?
That's right, no current sensing anywhere in the DC-DC converter. There IS a current sensor at the 220V 50Hz output of the sine wave chopper section, which might be used to protect the inverter against shorts and overloads, but of course this is not usable in any way to avoid short-term overcurrent in the FETs, nor to aid in loop stability of the DC-DC converter, nor in avoiding flux stepping!
I added a basic transformer-type current sensor, in the secondaries of the DC-DC converter transformers. I wired this up to shut down the converter when the current exceeds a safe value. So far this has avoided burning out more FETs, but I need to cure the loop instability. As it is now, any significant load will make the loop unstable, making the DC-DC converter running in bursts, until these bursts reach the limit of my added current sensor, and shut down the beast. This is happening at about 200W load at this time! At that low load, the secondary current already bursts up to 15A! Which means that the input current at 12V is going up to over 600A for those short bursts.
> in some countries, no engineer help any other at all,
That's very unfortunate. But I have found that those who really know, usually also help! The ones who don't help are mostly those who know very little, and want to keep that to themselves, fearful of creating competitors for themselves!
> the poor guy who designed that probz now sacked and slogging away 16 hours a day to earn a few pennys to feed himself.
Who knows... maybe he is still designing poor equipment, and making a decent living on it! Or maybe he is learning to do better... I hope so.
> how did you simulate...using block diagrams or just by simulating the compenents themselves....if so. it would have taken ages to simulate
I simulated the real components for the LC filter, load, voltage divider, all of the error amplifier, but instead of simulating the PWM, drivers, FETs, transformers, and rectifiers, I used a simple gain block, representing the gain these circuits have when the filter inductor is operating in continuous mode, and the 12V input is at nominal level. I'm aware that this is imprecise when the 12V vary, and specially when the inductor gets into discontinuous mode at low load, and it's also neglecting the delay through this part of the circuit, but it's good enough for the moment, for frequencies up to 1kHz or so. Anyway the instability is happening only when the circuit operates in continuous mode, and the oscillation is at quite low frequencies. The proof that the simulation is good enough is that the oscillation in the real circuit happens pretty much at the frequency predicted by the simulation.
I included estimated values for ESR and inductor loss in the simulation.
The simulation software I use is 22-year-old Microcap 2! I also tried to simulate this in the evaluation version of Microcap 9, but wasn't able to get any meaningful result! I need to learn that software first. When I was desperate, I tried to simulate a simple battery, and the software told me zero volts, all the time! I'm obviously doing something wrong in version 9. But in the old version, I have ample experience, and know when to trust the results, and when not!
Do you think it makes sense to use my added current sensor (which is rather crude, and located on the secondary side of the transformers) to turn this circuit into a sort-of current mode one? I don't think I could do real pulse-by-pulse current limiting with it, because of delays, and certainly I can't detect magnetizing current with it, but maybe I could gain those additional 90 degrees of phase advance I need in the loop to obtain stability, without having to add high frequency gain to the error amplifier! What do you think?