Circlotron
Newbie level 4
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 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.