[SOLVED] SG3525 oscillator asymmetry when error amp is maxed out.

Hi all.
I have an interleaved voltage-mode boost converter that uses both sides of a SG3525 to drive two MOSFETs in anti-phase for use in an automotive environment. Inductors are off the shelf 220uH powdered iron toroids. Input voltage is nominally 12V and output can vary between 15-25V. Output is 20 amps and switching frequency is 22KHz.

When there is a high step up ratio the pulse width goes out to maximum (~45%) and the error opamp swings fully high. It is then out of regulation of course, but in this application it is not a big deal.

The problem is, when the error amp goes fully high the sawtooth PWM clock is not perfectly spaced every cycle. Every second cycle is about 10% shorter and this causes a great deal of current imbalance between the the two inductors. One will run hot and the other almost cold.

Has anyone else seen this problem?
I know there are better controller chips available but I'd really like to stick with the SG3525.

- - - Updated - - -

Further to the above, I disconnected the inductors to make sure there is no switched current floating around the board to upset things. In any case the board has a ground plane on the top and bottom layers.

The oscillator sawtooth swings from 0.6V up to 3.4V.
When the error amp output goes higher than 3.2V the sawtooth gets jittery.

I have seen something like this in the past, and it was due to noise getting back into the ramp generator.
Its the same identical ramp that is supposed to drive both alternate cycles steered by a flip flop.

A quick and dirty fix is to reduce the resistance of the timing resistor, and increase the value of the timing capacitor by the same amount.
This lowers the impedance and makes the ramp oscillator less susceptible to noise.
One down side of doing this, is that it also increases dead time, and that may or may not be a big deal for you.

Anyhow, it is something you can try.

Hi Warpspeed, thanks for that.
I spent the whole afternoon fiddling with it and I had some success by changing the value of the timing cap discharge resistor. Was using zero ohms with 1nF which should have been alright according to the datasheet, but now I have 390 ohms and there is a tiny flicker as the opamp voltage goes through the critical range but once above this range it all comes good again. So, as far as I am concerned it is fixed.