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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Heres the basic plan for my attempt at PFM. I cant find the IC which was meant specifically for this purpose i once found it on Mouser, but i dont know its part number. Im sure they still sell it too. Ive gone through every search parameter possibe.
In any case, id like to try this for supression near other sensitive circuits. Airbourne for example.
BAck to the IC's, the first IC which i believe i mentioned here on the forum a year or two ago, and the recent one i found a week ago, seem to disapear from my memory. I should have written them down. But one supported two outputs the other only one. for the IC which has one output, i chose to put a JK flip flop in which two divides by 2 and alternate the two half bridge or push-pull transistors.
Im real tired, ive been working long hours and im sick, so im pretty mentally blurred at the moment. But i plan to pursue this over the next 3 weeks or so.
Registered Member #2529
Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600
If you do this, board layout is critical; the MOSFETs have to switch very fast, and the stray inductances of the tracks and the capacitances of the MOSFETs routinely cause large transients than can blow stuff up.
Registered Member #816
Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156
Out of curiosity which kind of way are you applying the frequency modulation? There is the method where either the on or off period is fixed in length , whereby as you increase frequency you also increase dutycycle so to speak.
Or there's the resonant kind often using a series L/C where your shifting the frequency up and down one of the sides of the resonant curve, something like slope conversion, the frequency translates into amplitude variation. The duty cycle is just left at 50% (square wave). At least I think that's how it goes.
Maybe there's other ways I've not heard of too. I don't know what the chips are either.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
I assume the PFM here is the type when a pulse, or series of pulses is sent each time the feedback loop reqisters undervoltage on the secondary side.
I have also seen the App Notes for these chips, but thought they were only used for flyback topology. Maybe you could adapt a flyback driver with a flip flop. Maybe that's the idea here.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
i dont see why i cant use a push-pull or half bridge. right? it seems to be just a lack of the ICs meant for two outputs. im only concerned with the possibility of shoot through, due to a lack of dead time.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Patrick wrote ...
i dont see why i cant use a push-pull or half bridge. right? it seems to be just a lack of the ICs meant for two outputs. im only concerned with the possibility of shoot through, due to a lack of dead time.
Wouldn't it be simpler to just use flyback topology?....How many watts do you need?....I think flyback has a smaller parts count if it's going to be airborne.
Will you get 'flux walking', or whatever it's called if you use push-pull?.....I seem to remember reading something along these lines, but I've mostly been researching flyback stuff recently.
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Attempt at a pulse frequency modulated SMPS.
