Issue with Flyback PWM signals

Thread Starter

fchastel

Joined Aug 25, 2011
4
Hi all,

I've designed a discontinuous current mode Flyback converter. I'm now testing it with fixed loads. At max. load, it works fine: the output voltages are at the expected value and the PWM controller output signal consists of pulses with a constant width. At nominal load, I still get the correct output voltages but the PWM output signal is not as I would expect: it consists of one wide pulse followed by several much narrower pulses. When I look closely at the regulated output voltage, I can see it going up when the wide pulse occurs and then down until the next wide pulse occurs. I've attached a sketch of the signals. I hope it helps.

Is it a known issue? Where is the issue coming from?

Thanks in advance for your help! I'm currently completly stuck with this issue.

Cheers
Fred
 

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pwdixon

Joined Oct 11, 2012
488
Does the PWM pulse have enough resolution to differentiate between the wide and narrow pulses? ie. is it possible to have an intermediate pulse width?
Perhaps the error detection window is restricted and it can only generate these two widths?
Can you post the schematic?
 

crutschow

Joined Mar 14, 2008
25,659
My crystal ball is a little cloudy but it sounds like you may have a feedback loop instability problem.
Can't help further without the schematic.
 

ronv

Joined Nov 12, 2008
3,770
Hi all,

I've designed a discontinuous current mode Flyback converter. I'm now testing it with fixed loads. At max. load, it works fine: the output voltages are at the expected value and the PWM controller output signal consists of pulses with a constant width. At nominal load, I still get the correct output voltages but the PWM output signal is not as I would expect: it consists of one wide pulse followed by several much narrower pulses. When I look closely at the regulated output voltage, I can see it going up when the wide pulse occurs and then down until the next wide pulse occurs. I've attached a sketch of the signals. I hope it helps.

Is it a known issue? Where is the issue coming from?

Thanks in advance for your help! I'm currently completly stuck with this issue.

Cheers
Fred
It's hard to tell without a schematic and some parts specs. My guess since it happens at higher current is that the inductor current rating is to low and it is starting to saturate. When that happens to increase in voltage can happen faster than the circuit response.
 

ronv

Joined Nov 12, 2008
3,770
Sorry, I had to quickly draw the schematic and missed the diode... it's also a 1N5822 but in the other direction.
This is only a "SWAG", but try adding a few nf across the 15k going to the op amp to give a little more high frequency gain.
 

umphrey

Joined Dec 1, 2012
39
Is it a resistive load? If you calculate what the duty cycle should be which does it match (the skinny pulses or the large pulse or somewhere in between)? Can you pin down a range of current or power where it is erratic and the range that it is not?

I don't know the answer, but DCM power supplies tend to have a lot of ring in the voltages and if the chip reads that it could cause this. I would try to pin down the states of everything when it happens, maybe when you hit 0 inductor current it rings the output voltage and the chip reads that.
 

Thread Starter

fchastel

Joined Aug 25, 2011
4
Is it a resistive load? If you calculate what the duty cycle should be which does it match (the skinny pulses or the large pulse or somewhere in between)? Can you pin down a range of current or power where it is erratic and the range that it is not?

I don't know the answer, but DCM power supplies tend to have a lot of ring in the voltages and if the chip reads that it could cause this. I would try to pin down the states of everything when it happens, maybe when you hit 0 inductor current it rings the output voltage and the chip reads that.
At max. load, the theoretical Ton is 2.63us (duty cycle = 26%) and the meas. Ton is 2.68us. It therefore matches quite well.
At nom. load, the theoretical Ton is 1.03us (duty cycle = 10%) and the meas. Ton is about 2.8us for the large pulse and 0.5us for the small pulses.

I've attached two snapshots of the drain-source voltage (VDS) of the switching MOSFET, showing the unstability.
 

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