Author Topic: ATX Flyback Transformer  (Read 10092 times)

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Offline Gyro

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Re: ATX Flyback Transformer
« Reply #25 on: June 16, 2020, 10:05:44 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.

Sorry, I provide help to lots of people, your responses are too much of a turn-off to be worth the effort though.

... 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.
« Last Edit: June 16, 2020, 10:09:36 pm by Gyro »
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Offline engrguy42Topic starter

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Re: ATX Flyback Transformer
« Reply #26 on: June 20, 2020, 12:10:01 pm »
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     
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Offline Buriedcode

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Re: ATX Flyback Transformer
« Reply #27 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.
 

Offline engrguy42Topic starter

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Re: ATX Flyback Transformer
« Reply #28 on: June 20, 2020, 06:42:08 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.

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?

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- I'm always amazed at how many people "already knew that" after you explain it to them in detail...
 

Offline T3sl4co1l

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Re: ATX Flyback Transformer
« Reply #29 on: June 20, 2020, 07:06:33 pm »
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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Offline engrguy42Topic starter

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Re: ATX Flyback Transformer
« Reply #30 on: June 20, 2020, 07:44:27 pm »
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

Like I said, the input is a reverse engineer of the actual ATX supply. That's why D802 is there. Because, well, it's there. And like I said, there's more circuit to the story that I haven't gotten to yet. I just added some core stuff and tweaked it 'til it worked.

Regarding L1, you ask why it's so huge, then say it's about the right value. I just threw a number in there. And the "K4 L7 L8 1" statement is there, you just can't see it. And I tried reversing the switching transformer previously with a bunch of different ratios and it didn't make much difference in terms of improving response. And Z1 is there because, well, it's there. This is part a documenting of what's there and part a simulation to see how it all works.

Now that I've answered your questions, can you answer mine?

"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?" 
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Offline T3sl4co1l

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Re: ATX Flyback Transformer
« Reply #31 on: June 20, 2020, 07:55:55 pm »
Regarding L1, you ask why it's so huge, then say it's about the right value.

Yes.  In context, as language is wont to do. :) It's much too large for flyback (ideal value 0, typical value maybe 1-5uH as transformer leakage), and still probably too large for forward but in the right ballpark at least.

And again, a flyback this size (100s watts) will have Lp ~ 100s uH, and Ls that much smaller still, so L1 is obviously off magnitude in that case.  With values as shown, it's not a big deal, but it's also only good for a few watts.


Quote
I just threw a number in there. And the "K4 L7 L8 1" statement is there, you just can't see it.

Oh, great.  I suppose there are parasitics on components too?  Which LTspice isn't kind enough to display by default?  :rant: :rant: :rant:

Quote
Now that I've answered your questions, can you answer mine?

"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?"

I would guess the controller is doing a hiccup mode because it thinks it's overloaded.  Depends on shunt resistor (not so much on transistor size, and what you've got there is way more than enough for the currents shown) and how the control works.

Damping depends on compensation; or maybe it's switching ripple, depends on what components and time constants you're looking at.

Play with this a bit,
http://schmidt-walter-schaltnetzteile.de/smps_e/smps_e.html
and by all means, draw the waveforms by hand, they are quite simple to plot -- you should be able to see why, for example, less inductance means more power transfer in a flyback converter, or why duty cycle doesn't mean nearly as much in CCM as it does in DCM.

Tim
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Offline engrguy42Topic starter

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Re: ATX Flyback Transformer
« Reply #32 on: June 20, 2020, 08:11:17 pm »
Thanks. So riddle me this...

The actual supply provides about 170VDC to the switching section (120VAC, then the bridge). In my case I dropped it down to around 40VDC and it seems to work.

My confusion is regarding how to change this circuit to allow for 170VDC to presumably appear across the switching MOSFET and the switching transformer. And add to that the inductive switching overvoltages...

And I measured the inductance of the actual main transformer (which I posted previously...10-20 mH). So I'm trying to understand what the actual circuit will look like in order to work for this condition, and also allow the rated 10-20 amps to flow on the 12V output rail.

From what little I know of forward vs flyback there doesn't seem to be a big difference in components or configuration, though that comes from somone who is basically clueless.

I'm just having a hard time imagining being able to use anything similar to the circuit I modelled to work with 170VDC input and 12VDC and 20 amps output, using those MOSFETS and the little 3 winding transformer I showed in my first post. I'm guessing it's a 2-switch forward converter like in the schematic I found, but that doesn't answer it for me.

I've got a lot to learn.   
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Offline T3sl4co1l

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Re: ATX Flyback Transformer
« Reply #33 on: June 21, 2020, 06:02:51 am »
Only thing you're missing is the phasing of the transformer, and another diode from GND to L1.  This catch diode keeps L1's current flowing, from GND rather than from the secondary, while the transformer resets its flux.

The transformer also needs something to clamp said reset flux, usually an RCD clamp snubber.  Add a diode from the primary to an R||C to +V.  The R and C should be relatively large (ballpark 10k, 33nF).  Alternately, a diode into a zener (say UF4007 + P6KE200) which will clamp at a stable voltage rather than a voltage varying with PWM.

In short, the primary looks like a boost converter, and you're just burning its output in a load resistor or zener.

Tim
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Offline engrguy42Topic starter

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Re: ATX Flyback Transformer
« Reply #34 on: June 21, 2020, 11:46:55 am »
Thanks. This thing is driving me up a freakin' wall.  |O

Last night I modified it per the attached schematic, though without the R/C/Diode transformer reset circuit on the primary. And today I added that and still I'm getting what I've been getting all along. If you look at the attached, it's fine upon startup and hits 12VDC and stays flat, but when I apply a 2.4A, 30watt load the output drops to about 5VDC and stays there.

I've varied the values until I turn purple and I can't understand what would be affecting its ability to recover from a load switching on. Unless there's something about the controller I'm missing (which is clearly the case).

With minor load additions it recovers and damps fine, but with this it just loses it completely and gives up. Maybe it runs out of duty cycle?

Oh, and the output inductor value and N1/N2 values came from that calculator website you recommended. Though I've also varied those about 150 times with little change....
« Last Edit: June 21, 2020, 12:04:13 pm by engrguy42 »
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Offline engrguy42Topic starter

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Re: ATX Flyback Transformer
« Reply #35 on: June 21, 2020, 11:53:05 am »
And here's the traces for the gate voltage and the voltage on the diode side of the output inductor during the heavy overload. This controller supplies a fixed 200kHz, and the gate voltage is showing that 200kHz at a 30% duty cycle. And I recall for this config you need a duty cycle below 50%, so it looks reasonable. And it also looks like it's not running out of duty cycle.

Oh, and you can also see the Vout trace at 5V, when it's supposed to be at 12V...
« Last Edit: June 21, 2020, 11:55:25 am by engrguy42 »
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Offline engrguy42Topic starter

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Re: ATX Flyback Transformer
« Reply #36 on: June 21, 2020, 02:07:49 pm »
First of all, LT data sheets SUCK!!!  :--

They tell you how to size the voltage divider resistors to give 1.2v into the FB pin at the desired output volts, but otherwise they tell you freakin' NOTHING!! Nothing on how to size the C for threshold (Ith) pin, or the R for the Vcc pin, or the size of the sense resistor, or what to do with the Run/SS pin.  |O

Anyway, I plotted the feedback voltage as I start the power supply, then apply the big load at around 20mSec, then remove it at around 40mSec and everything returns to normal.   

Attached is the FB voltage (this controller wants 1.2V here at the desired output) and the Ith threshold pin voltage. When output volts is good the Ith volts go to 0.9, and when the output is heavily loaded it slams to the ceiling of 2.0V, while the FB voltage drops down to around 0.7 volts (which means 7V output), then then decays down to around 0.3v.

I dunno, something is limiting the ability of the controller to boost the very low voltage.  :-//
« Last Edit: June 21, 2020, 02:44:02 pm by engrguy42 »
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- I'm always amazed at how many people "already knew that" after you explain it to them in detail...
 

Offline T3sl4co1l

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Re: ATX Flyback Transformer
« Reply #37 on: June 21, 2020, 02:13:26 pm »
Can't see the schematic; did you edit the attachment by any chance?  I see the thumbnail but not the full size, something got out of sync.

Tim
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Offline engrguy42Topic starter

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Re: ATX Flyback Transformer
« Reply #38 on: June 21, 2020, 02:15:42 pm »
Can't see the schematic; did you edit the attachment by any chance?  I see the thumbnail but not the full size, something got out of sync.

Tim

Yeah, for some reason it's not uploading to the server. I got a message it's loading too slow or the file's too big. Which it isn't.

EDIT: Okay, there it is...some sort of server hiccup I suppose
« Last Edit: June 21, 2020, 02:44:43 pm by engrguy42 »
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Offline engrguy42Topic starter

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Re: ATX Flyback Transformer
« Reply #39 on: June 21, 2020, 08:32:05 pm »
BAM !!!! I think I got it...

On a hunch, I assumed that with a higher turns ratio (N1:N2) I'd get more secondary amps and therefore more stored energy to feed the load. So I cranked the turns ratio up to about 4.5:1 (ie, a L1:L2 ratio of 20:1). And the result (it's still plotting) is just a tiny blip in output voltage as I slam a 2.4 amp, 30 watt load on the 12 output. And it instantly recovers.

BTW, I also added a reset winding with diode to discharge the transformer rather than the RC/diode circuit on the existing primary, and it seems to work great.

I'll post an image of the circuit and traces when it finishes. Sometime next month probably. One thing I hate about LTSpice is it's so freakin' slow on complex circuits.
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- I'm always amazed at how many people "already knew that" after you explain it to them in detail...
 

Offline engrguy42Topic starter

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Re: ATX Flyback Transformer
« Reply #40 on: June 21, 2020, 09:35:55 pm »
LTSpice sure doesn't like nano-henries does it? Freakin' slow as molasses on a cold day in January at the South Pole...

Anyway, here's what I have so far. I used mH for the transformer inductances to speed things up. At 100mSec I slam on the 2.4 amp load to the 12V output, and it's just a tiny blip. I'm gonna let it run for a while (like 0.5 seconds) to see if it holds or it drops off..
« Last Edit: June 21, 2020, 11:20:23 pm by engrguy42 »
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- I'm always amazed at how many people "already knew that" after you explain it to them in detail...
 

Offline HackedFridgeMagnet

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Re: ATX Flyback Transformer
« Reply #41 on: June 21, 2020, 11:14:24 pm »
LTSpice sure doesn't like nano-henries does it? Freakin' slow as molasses on a cold day in January at the South Pole...

u got your polarity wrong.
 

Offline engrguy42Topic starter

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Re: ATX Flyback Transformer
« Reply #42 on: June 21, 2020, 11:18:13 pm »
WTF??? This isn't fair.

So I fixed the ability to handle switching on load and maintain output voltage, but now if I run the sim longer 8mSec after it picks up load the freakin' controller shuts down. WTF?? I never told it to shut down !!! Apparently when the "Ith" (threshold) pin voltage gets up to 2 v it shuts down. And it stays off for like 50mSec, then turns on again. And keeps repeating the cycle (BTW, load is still applied the entire time).

WTF?? Now it's just trying to piss me off.

So now I have a freakin' science project to determine what happens with the "Run/SS" and "Ith" pins and what that crap all means. I have no clue how it works. And of course the datasheet for the LTC3873 doesn't tell you directly. I suppose you have to search for clues and figure it all out.

I hate electronics.   
« Last Edit: June 21, 2020, 11:23:57 pm by engrguy42 »
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Offline Buriedcode

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Re: ATX Flyback Transformer
« Reply #43 on: June 22, 2020, 12:04:15 am »
Look at your transformer.  This is a flyback converter, remember what I said about current not flowing in both the primary and secondary at the same time?

I realize you're getting quite into this, but others have suggested you start at the basics, but you don't seem to like links to articles.  Do you want someone to provide a full explanation?
 

Offline engrguy42Topic starter

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Re: ATX Flyback Transformer
« Reply #44 on: June 22, 2020, 12:22:53 am »
Look at your transformer.  This is a flyback converter, remember what I said about current not flowing in both the primary and secondary at the same time?

I realize you're getting quite into this, but others have suggested you start at the basics, but you don't seem to like links to articles.  Do you want someone to provide a full explanation?

A while back (my post from yesterday) I changed to a forward converter in order to get a higher output and showed the updated schmatic. I'm applying a 30 watt, 2.4 amp load and therefore need (I believe) a forward converter. And it seems to work okay other than my latest problem with shutting off.

And yes, I'm reading a lot of info on this, most recently the rather hilarious App Note 19 from Linear Technologies, "LT1070 Design Manual".

EDIT: BTW, my favorite troubleshooting hint from that app note:

12. Didn’t Read the Data Sheet
Then you shall have no pie.   

:-DD
« Last Edit: June 22, 2020, 12:28:40 am by engrguy42 »
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Offline T3sl4co1l

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Re: ATX Flyback Transformer
« Reply #45 on: June 22, 2020, 08:38:10 am »
Simulations can be misleading like that -- the data comes in thousands of times slower than real time, so it can seem to be working, but if you've not checked something (like SS, Ith or Vcc) in that time, it can keep on doing things that you maybe weren't expecting.

The power supply stopping and restarting isn't unintended behavior, much of the time; it's advertised as "hiccup" overload response.  What does seem to be unintended, is you've got nominal supply and load, and it should be running fine.  What's up with that?

Datasheet: https://www.analog.com/media/en/technical-documentation/data-sheets/3873fb.pdf

The controller isn't magic, it needs power to run.  It's got that 150k pullup from, whatever it is, 160V bus? (Can't seem to see the AC input section anymore, that attachment got mangled. :palm: )  That gives about 1mA.  Which will charge the 10uF bypass to startup voltage (p.3, UVLO rising: 8.4V) in about 84ms.

Assuming you've set it to zero; you may want to speed up simulations by setting initial conditions on the DC link and VCC caps, and remove AC ripple for now by using a DC source, not AC + rectifier.  Fewer things to mix up on the sim, eh?

And the controller needs 45-400uA to run, depending on condition; in and of itself.  That should be plenty, right, you've got ~1mA?

Well hold on there, that's with no load attached.  Useless right?  Well, it wouldn't be very useful if they characterized it with one particular load, so they don't do any.  Page 9 is your first hint on this I think, "since RSTART cannot supply enough current to operate the external MOSFET..."

How much current does it need?  Depends on the gate.  The STP18NM60N claims 35nC total gate charge, at 10V.  Supply max is 9.5V, I guess that's close enough.  (The min/max will differ quite a bit anyway, and it's rather irritating they don't give limits.  It's probably more like 50nC max.)  To fully charge and discharge a capacitance, to some charge Q, at frequency F (~200kHz), requires current I = Q F.  Or 10mA.

Which is 10 times the startup current, so we'd expect it starts up, runs for about 4ms, then stops, waits 40ms, and so on.  (Your simulation seems to be doing much better than this, which is interesting!)

Referring to page 1, they have a flyback configuration, with a diode D1 from output to VCC.  Since the output in that example is between 5-9V, they can use a single diode and that's that.

If your output voltage is higher, you might consider using a voltage divider, sized to sink >>10mA, and giving a Thevenin voltage of say 8-9V, and then connect a diode from the divider to VCC.  (You can't use just a divider, because that would load the startup resistor and nothing happens.)  The voltage divider is poor efficiency, but expedient.  (Better efficiency can be obtained say with an LDO, one that has low shutdown current and tolerates backwards voltages.  Works the same way, sourcing current from the output to VCC once the output starts up, while not drawing the excess current a divider does.)

On page 9, they show an auxiliary winding providing VCC; this is out in the present case (your transformer doesn't have one), but quite useful when available.


As for Ith: that's the error or compensation node.  When output voltage (sensed by Vfb) differs from the setpoint, a current proportional to that error is driven in/out the pin.  This charges the capacitor, which integrates the current into a voltage; for small and brief disturbances, not much happens, but for large disturbances over long times, V(Ith) changes dramatically, and consequently the setpoint current (this is a peak current mode controller, so this is the peak switch current).  Thus responding to load.  Simple enough, right.  Often a resistor is placed in series (e.g. page 1) to afford better compensation.  In general, an R+C is much better, and you can simply adjust values up and down until you zero in on a good step response.


You may want a resistor in series with SW, to introduce slope compensation and filtering.  This is discussed on page 10.

Vfb divider is also shown on page 10.


The L2-D8 strategy is an equivalent winding-flux-reset network, obviously only an option if you have the extra winding of course.  Since L2 and L9 are 1:1, it behaves the same as a 2-switch converter does.  The RCD clamp shown earlier was drawn correctly, also.


Hehheheh... also... page 18, can you spot the error?  :-DD Ehhh...yikes!

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline engrguy42Topic starter

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Re: ATX Flyback Transformer
« Reply #46 on: June 22, 2020, 10:15:25 am »
Tim,
Thanks much. I really appreciate your getting into this.

In response to your wonderfully detailed response...HUH?  :D  Mostly over my head.

Though I did extract something that might be pertinent. I plotted the Vcc voltage across the 10u cap, and superimposed my Vout voltage on the load, and it seems that the shutdown is occurring when/because Vcc is dropping to 4V, and starting/re-starting occurs when it gets up near 9V.

So I guess I'll disconnect the AC/DC bridge and insert a DC source of 160V and see if that fixes it. Which begs the question:

Why so much wigglies in the DC supply?

- The best engineers know enough to realize they don't know nuthin'...
- Those who agree with you can do no wrong. Those who disagree can do no right.
- I'm always amazed at how many people "already knew that" after you explain it to them in detail...
 

Offline engrguy42Topic starter

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Re: ATX Flyback Transformer
« Reply #47 on: June 22, 2020, 10:19:44 am »
BTW, here's a zoom-in of the input circuit, including a trace of the Vin feeding the controller circuit.
- The best engineers know enough to realize they don't know nuthin'...
- Those who agree with you can do no wrong. Those who disagree can do no right.
- I'm always amazed at how many people "already knew that" after you explain it to them in detail...
 

Offline T3sl4co1l

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Re: ATX Flyback Transformer
« Reply #48 on: June 22, 2020, 10:50:49 am »
I have covered everything.  Take your time, read it by paragraph if you have to.  I kept the paragraphs short too, which should help.

Cheers!

Tim
Seven Transistor Labs, LLC
Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline engrguy42Topic starter

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Re: ATX Flyback Transformer
« Reply #49 on: June 22, 2020, 10:52:18 am »
Anyone else notice the image attachments are all screwed up? Sometimes it shows the correct thumbnail, but when you expand it's a different image.  :wtf:  I think the server was having issues yesterday with uploading too.

Anyway, I replaced the whole bridge input section with a rock solid 160V DC supply to power this, and the shutdown cycles are still there.

I don't get it.
- The best engineers know enough to realize they don't know nuthin'...
- Those who agree with you can do no wrong. Those who disagree can do no right.
- I'm always amazed at how many people "already knew that" after you explain it to them in detail...
 


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