Electronics > Projects, Designs, and Technical Stuff
ATX Flyback Transformer
Gyro:
--- Quote from: engrguy42 on June 16, 2020, 09:48:26 pm ---"Read a book" or "watch a video" is of zero help. That shit is beyond obvious. If you have nothing to add because you don't understand the subject then fine. stay out of the discussion.
Geez you guys give me a headache.
--- End quote ---
--- Quote from: Gyro on June 16, 2020, 07:25:28 pm ---Sorry, I provide help to lots of people, your responses are too much of a turn-off to be worth the effort though.
--- End quote ---
... and I'm sure a lot of other potentially helpful folks feel the same. I don't know why you bother with this forum, you seem to have so little regard for other members.
engrguy42:
Okay, well I'm in the beginning stages of making an LTSpice model of an ATX power supply, based on one I have lying around and I decided to reverse engineer. Though I suck at reverse engineering PCB's, especially ones as complicated as this.
The image below is where I am so far. The left side of the schematic is probably 80% accurate modelling of what's on the input side, and to the right of the 330uF cap it's just a kludge to get a basic 12VDC with a converter. And the controller was the first one I found in the LTSpice library (LTC3873), not the apparently unobtainable actual one (a Champion CM6800). Though I did find models of the actual STP18NM60N's, though I doubt they're not the best match for the LTC3873. Though it does seem to work, provide a flat-as-a-pancake 12VDC output with only about 25mA of load.
Since the actual circuit is working at around 170VDC out of the rectifier, and the LTC3873 specs talk about much less than that for Vin, I added an AC transformer on the front end to drop the DC down to around 40VDC. And I kludged a resistor value to feed the Vcc, though it seems to work. And the switching transformer has a different, much lower ratio than the actual (2 vs. 12.5).
I also added a switch to slam about 15watts of load to see how it responds, but at this point the voltage just wiggles +/- a couple volts with no damping. So clearly I have more work to do in figuring this out.
As you can see in the LTSpice traces, the MOSFET gate is a fixed 200kHz out of the 3873, and it's around 30% duty cycle at this point. I also plotted the transformer input and output currents, as the input current ramps up, then the output current discharges.
So at least I have a somewhat working 120VAC to 12VDC system. Now I need to do some more detective work to reverse engineer the switching part. Since there are 4 identical MOSFETS on the heat sink, I'm guessing it's a 2 switch converter of some sort. And then I suppose I'll have to locate an LT controller for a 2 switch converter (LT3705 maybe?).
This is gonna take a while.... :D
Buriedcode:
So you have modeled a flyback converter, which is I suppose a good stepping stone up to the ATX type converter - I believe ATX supplies use a smaller flyback converter for the standby supply.
Note that in a flyback converter the "transformer" is really a coupled inductor, that stores energy - current doesn't flow in the primary and secondary at the same time. This isn't the case with forward converters or half bridges, which is why the " transformer" is deigned differently.
engrguy42:
--- Quote from: Buriedcode on June 20, 2020, 04:25:23 pm ---So you have modeled a flyback converter, which is I suppose a good stepping stone up to the ATX type converter - I believe ATX supplies use a smaller flyback converter for the standby supply.
Note that in a flyback converter the "transformer" is really a coupled inductor, that stores energy - current doesn't flow in the primary and secondary at the same time. This isn't the case with forward converters or half bridges, which is why the " transformer" is deigned differently.
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Yeah, as I mentioned previously this supply is very similar to an ATX schematic I found, and it has a flyback transformer for the standby.
And yeah, if you look at the traces from my simulation I specifically plotted the transformer coil currents to show the current ramping up on the primary, then discharging from the secondary when the gate signal shuts the MOSFET off.
I guess what I'm grappling with now is the relationship between the turns ratio, the input Vdc, and the duty cycle of the gating PWM. And whatever else comes into play that, for larger loads, causes the output voltage to shut down (or drop to a very low value), while for lighter loads it just gets a small ripple that quickly damps and returns to the flat 12VDC. Or maybe the MOSFET has to be sized correctly to match the controller response?
T3sl4co1l:
The heck is up with that input? I don't see a Kx L7 L8 y statement. What's D802 for? Does C1 have a .IC set, is that what's powering this thing? Why is L1 so huge? Why include Z1, are you also testing transient immunity..?
Again as mentioned, it's a forward converter transformer; you won't get much power out of it this way, some watts I guess. Easy enough solution: turn around L10 (or set K5 = -1), double up D5 (use a diode from GND to L1), and use a, well, L1 is already there and roughly the right value so I guess that's alright once these changes are made.
Primary side doesn't need to change because the transistor sees the reflected current charging into L1 -- the current ramp, and I suppose LTC3873 has slope compensation too -- which is basically the same story there as for flyback. :-+
Tim
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