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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I am building a flyback inverter for TV transformer. I want to have a potentiometer which sets maximum pulse width ("power control"), but also a feedback from an emitter current-sense resistor which will limit the pulse width when the peak current is too high (e.g. a short-circuit on the xfmr output).
Now I want to use a SG3525 ( ) SMPS controller but I have no idea how to set it up. It has an error amp, I know how would I set it just to limit the peak current, but I have no idea how to incorporate the power control... Maybe I should use another controller? Thanks for any help
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
You set your reference at the inverting input, as well as a gain setting R or RC network. Try simply shorting compensation to the inverting output to start with. Current feedback goes to the non-inverting input, and should equal the reference at the desired peak point. It might be the other way around with the SG3525, I've only used the TL494. Check this circuit for reference.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I'm not sure about one thing. They say that the Pin 9 has a pull-up current source, but according to the schematic there's a pull-down source. Is there an error in the datasheet?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Uzzors wrote ...
You set your reference at the inverting input, as well as a gain setting R or RC network. Try simply shorting compensation to the inverting output to start with. Current feedback goes to the non-inverting input, and should equal the reference at the desired peak point.
Yeah, but the problem is that the current will be limited to a different value dependent on the "power" setting. I want it to always limit to the same value, the power control sets only maximum allowed pulse width. IIRC the tl494 has two error amps, so it should be easier. I'll take a look at the datasheet (though I will need a gate driver as the tl494 outputs are not totem-pole).
EDIT: How to set the gain of tl494's op amps? They contain a diode on the outputs...
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Right, I forgot about that. You'll need a TL494 or some other chip with two comparators. I've never thought about the diodes, and they don't seem to have caused problems. Getting the right gain and loop compensation is so complex that it's best left for people who do it for a living. For your needs max gain would work alright since you're just setting the maximum duty cycle which will need to trip instantaneously. Compensation is only needed when stabilizing a control loop.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
UC3845 is the chip you want to be using for a flyback converter. It has the necessary internals to control the average power throughput, and also a fast over-current shutdown to set an absolute limit on peak current through the switch. This chip is found in most cheap offline switchmode power supplies below 70W where the flyback topology dominates.
It works just as you described. Current mode control effectively varies the duty cycle to control power throughput and keep the output voltage in regulation during normal operation. If the output is short-circuited, the current waveform becomes continuous, and the fast peak-current limiter kicks in to prevent the current integrating up to a destructive level. It is this action under s/circuit conditions that often leads to cycle-skipping and the familiar squeal from flyback converters.
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Peak current limiting in flyback inverter
