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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Electra wrote ...
Don't know what you call bogus signals, If you mean want to run it open loop there's a test circuit for that in the data sheet, you have to set up the op-amp part, simple dc feedback just treat it as any ordinary op-amp ( pins 1,2,3). Turn a pot and see the pwm waveform change.
PWM works by comparing sawtooth wave (that's your ramp input pin) and a control voltage (the output from the op-anp part) using a comparator. At the start of each cycle a latch is set turning the output high, soon as the ramp exceeds the dc level of the control voltage the comparator changes state re-setting the latch, the outputs go low. So increasing the control voltage increases the duty cycle.
Ok let me build the open loop circuit.
This is a totally new incomplete schematic. I think that red group of components (red in the example circuit, not this one) is for the peak current mode control. so I modding it for pulse by pulse.
Do you guys think I'm putting the CT result into the right IC pins ?
Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
Well it's not the wrong pin, but remember these things are designed to work with an inductor after the power transformer or as part of the load, so it going to work best with a wave that has a sloping top, if you hit it with a rectangular wave with a resistive load the comparator is either going to be 'on' early on in the cycle or it isn't. If it starts randomly skipping half cycles could be a bit unpredictable. There's only one way to find out..
Don't forget to connect you ramp pin to ct 6+7 it hadn't been drawn but you said it was incomplete.
edit, the page timed out or it got lost, crappy internet connection sorry if it's a double post
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
website on switching transistors. mental note: comeback to this later.
these two examples show a vertical rise, a ramp upward, then a vertical fall. So the ramp pin and comparator figure this out I think. but when you mention this inductor... is it a power inductor or a small signal inductor ?
Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
Yes I was referring to the power inductor at the output from the transformer, the current rises steadily as it stores the energy during the 'on' period, if its already has continuous current flowing in it you get the instant vertical rise to that point. The off period of course just circulates the current through the secondary diodes to the output, so it doesn't reflect anything back to the primary.
It's hard to speculate more at this stage, you're better off building an experimental version to get feel of how it all works.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
WHOA i NEVER would have figured that out ! the OUTPUT inductor ! Not in a million years. you just SAVED me bro. but now does that mean isolation is more difficult ?
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Its looking like electrical isolation may not be possible in this design.
the only exception I can find is if its possible to use the power inductor, as a sensing transformer, with a single loop feeding back to the pin1/inv op-amp section.
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
No, there doesn't need to be an electrical connection to the secondary side, Electra was just describing how the current fluctuations at the output inductor "ripple back" through the circuit and cause the current fluctuations at the input that the controller is expecting but, on a test rig, might not get.
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Need help with current limiting.
