Pudu
Junior Member level 1
push pull converter
Hi all,
some time ago I bought a 12V to 220V true sine wave inverter, rated at 3500W. When it arrived, I hooked it up, did the basic checks, including load tests at up to 2000W. It created a lot of EMI, but otherwise all seemed fine, with an efficiency of 87% at 2000W, which is pretty good for an inverter with such a low input voltage. I left it running, at essentially no load (less than 10W), and before two hours were over, the unit self-destructed. Several blown electrolytic caps, all MOSFETs blown, along with their gate resistors, and a few other parts blown too.
Sending the unit back to the manufacturer, half around the world, wasn't very practical. Shipping both ways would have cost more than the value of the unit! Instead, the factory sent me the required spare parts. I installed them, did some checks, the unit worked again, but I discovered so many design flaws, that I embarked on a major process of improving this inverter, before feeling confident using it.
I have now ironed out most of the obvious flaws, including the one that caused the voltage run-away that blew the caps, but one big problem remains, and on this I would like advice from someone fluent in control loop principles. This inverter uses a 12V to 340V DC-DC converter, configured as a push-pull circuit, using six converter blocks with 4 MOSFETs and a transformer each, with the secondaries in series. There is no current sensing whatsoever in this converter! And the error amplifier is a plain simple integrator. It has a crossover frequency of about 1kHz, while the converter's LC filter resonates roughly at 300Hz, and the swicthing frequency is 40kHz clock, 20kHz on the transformers.
As is to be expected, this circuit is totally unstable, due to the sudden 0 to 180 degree phase transition of the LC filter, along with the constant 90 degree phase shift of the integrator . While there is no load, it idles in hiccup mode, with bursts of pulses exceeding 200A of input current, followed by long times having no pulses at all. When loaded more heavily, the current bursts get into the kiloampere range, and the frequency at which the system self-oscillates goes up.
I cannot cure the instability by simply reducing the gain, because this application needs to have enough gain at 100Hz to follow the pulsed load presented by the sine wave chopper that follows the DC-DC converter.
I made a simulation of the whole loop, and came up with component values for a type 2 error amplifier (one pole and one zero), which according to the simulation should provide good phase and gain margins, and thus be stable, along with having enough response speed. The problem is that this error amplifier would have rather high gain at 20kHz. While the total loop gain at 20kHz would be way below unity, due to the LC filter, a high gain of the error amplifier at 20kHz means that any little pickup of the transformer signal would get to the PWM strongly amplified, making the drive pulses for one side of the transformers much longer than those for the other side, which would result in flux stepping and consequent saturation of the cores, blowing the whole thing up again!
Anything I try to do to reduce the error amplifier gain at 20kHz, results in phase shifts that would make the system unstable.
Can anybody suggest what to do, short of trashing the whole thing and building a new DC-DC converter from scratch, using current-mode control? Is it possible at all to achieve enough performance in a push-pull converter for an inverter, using voltage mode control? If yes, how is it done? I'm at the end of my wits with this!
Hi all,
some time ago I bought a 12V to 220V true sine wave inverter, rated at 3500W. When it arrived, I hooked it up, did the basic checks, including load tests at up to 2000W. It created a lot of EMI, but otherwise all seemed fine, with an efficiency of 87% at 2000W, which is pretty good for an inverter with such a low input voltage. I left it running, at essentially no load (less than 10W), and before two hours were over, the unit self-destructed. Several blown electrolytic caps, all MOSFETs blown, along with their gate resistors, and a few other parts blown too.
Sending the unit back to the manufacturer, half around the world, wasn't very practical. Shipping both ways would have cost more than the value of the unit! Instead, the factory sent me the required spare parts. I installed them, did some checks, the unit worked again, but I discovered so many design flaws, that I embarked on a major process of improving this inverter, before feeling confident using it.
I have now ironed out most of the obvious flaws, including the one that caused the voltage run-away that blew the caps, but one big problem remains, and on this I would like advice from someone fluent in control loop principles. This inverter uses a 12V to 340V DC-DC converter, configured as a push-pull circuit, using six converter blocks with 4 MOSFETs and a transformer each, with the secondaries in series. There is no current sensing whatsoever in this converter! And the error amplifier is a plain simple integrator. It has a crossover frequency of about 1kHz, while the converter's LC filter resonates roughly at 300Hz, and the swicthing frequency is 40kHz clock, 20kHz on the transformers.
As is to be expected, this circuit is totally unstable, due to the sudden 0 to 180 degree phase transition of the LC filter, along with the constant 90 degree phase shift of the integrator . While there is no load, it idles in hiccup mode, with bursts of pulses exceeding 200A of input current, followed by long times having no pulses at all. When loaded more heavily, the current bursts get into the kiloampere range, and the frequency at which the system self-oscillates goes up.
I cannot cure the instability by simply reducing the gain, because this application needs to have enough gain at 100Hz to follow the pulsed load presented by the sine wave chopper that follows the DC-DC converter.
I made a simulation of the whole loop, and came up with component values for a type 2 error amplifier (one pole and one zero), which according to the simulation should provide good phase and gain margins, and thus be stable, along with having enough response speed. The problem is that this error amplifier would have rather high gain at 20kHz. While the total loop gain at 20kHz would be way below unity, due to the LC filter, a high gain of the error amplifier at 20kHz means that any little pickup of the transformer signal would get to the PWM strongly amplified, making the drive pulses for one side of the transformers much longer than those for the other side, which would result in flux stepping and consequent saturation of the cores, blowing the whole thing up again!
Anything I try to do to reduce the error amplifier gain at 20kHz, results in phase shifts that would make the system unstable.
Can anybody suggest what to do, short of trashing the whole thing and building a new DC-DC converter from scratch, using current-mode control? Is it possible at all to achieve enough performance in a push-pull converter for an inverter, using voltage mode control? If yes, how is it done? I'm at the end of my wits with this!
