Magnavox DRV power supply SMPS

[johnfin]

Re: Magnavox DRV power supply SMPS

I meant resistor 2106 on the way up to the clamping section

[momaka]

Re: Magnavox DRV power supply SMPS

Are you talking about diode "Dclamp"? If yes, its anode is connected to pin 5 of the transformer and its cathode (stripe) is connected to parallel resistor-capacitor combo, "Clamp" and "Rclamp".

At least I don't see which other diode you're talking about. But please specify if it's something else. It's very important that you get the clamping network wired correctly in order to prevent blowing your ncp1200 chip.

Also, try opening the picture with Windows Picture Viewer and zoom in. The junctions should be a lot more clear that way. Anywhere where you see three or more wires coming together with a DOT over them, that's a junction. If wires cross each other with *no* dot, then there's no junction.

[johnfin]

Re: Magnavox DRV power supply SMPS

Momaka, just need to verify if the diode that goes between the ncp1200 and clamping module also attaches to the #3 on the tranny. Looks like a junction, cant tell.

[johnfin]

Re: Magnavox DRV power supply SMPS

yes, making mods. hope this frankenboard works. taking some time cutting the traces. pics soon

[momaka]

Re: Magnavox DRV power supply SMPS

If you do buy the MOSFET, might as well.

So looks like you're doing this after all.
Definitely post some pictures when you're done. Would be good to have for anyone following the thread or that happens to stumble on it.

[johnfin]

Re: Magnavox DRV power supply SMPS

do i need to replace q2101 to start or later?

[momaka]

Re: Magnavox DRV power supply SMPS

They're not needed. The NCP1200 has an internal power supply generated from pin 8 (HV), so the primary-side auxiliary winding on your transformer (pins 7 and 8) becomes unused.

[johnfin]

Re: Magnavox DRV power supply SMPS

how come 7 & 8 on the transformer input are not connected to anything?

[momaka]

Re: Magnavox DRV power supply SMPS

Good. Looks like there are better transistors out there now anyways. Here are two that have somewhat close current characteristics to what's in your PSU now while at the same time also much lower gate charge.

IPA70R600P7SXKSA1
https://www.mouser.com/ProductDetail...ER0AiaEg%3d%3d

IPA70R750P7SXKSA1
https://www.mouser.com/ProductDetail...pwGbRQPg%3d%3d

I say go with the first link above. If not, then the second.

[johnfin]

Re: Magnavox DRV power supply SMPS

Cant find MTD1n60e on mouser. Have another selection?

[johnfin]

Re: Magnavox DRV power supply SMPS

Thanks, now to get to work on the mod.

[momaka]

Re: Magnavox DRV power supply SMPS

And here it is:
https://www.badcaps.net/forum/attach...1&d=1528675214

Honestly, I thought this would take longer when I started this two days ago, but the NCP1200 chip simplifies things so much more - just look at how much cleaner that primary side is!

All of the changes for this mod are on the primary side - that is, mostly removing or isolating components that don't pertain to the NCP1200 chip. Just compare the old and modded schematic, and any component that is on the old one that is not on the modded schematic, you can remove. Then get an NCP1200 chip and make a small PCB board with it. Probably best to go with a DIP-8 packaged chip, as that will easily allow you to solder it on a perf. board. There are really only two other components that this whole NCP1200 chip mod needs: a 22 kOhm 1/2 Watt resistor (or two 10 kOhm, 1/4 Watt resistors in series) and a 10 uF capacitor. Actually, regarding those, we may need to change/tweak their values a bit, as this mod is still at an experimental stage for this power supply. You may also need to add a resistor from pin 1 (ADJ) to ground... but we will see.

Another important thing to mention regarding the modded schematic above: I added a clamp network (in blue), as suggested by the NCP1200 schematic (and as is also a standard practice for pretty much all PSUs I've worked on - so this could well be the reason why this power supply blew so many MOSFETs).

For this clamp network, you will need a diode, a resistor, and a 1-2 kV -rated ceramic capacitor.
For the clamp diode (D_clamp), something like BA159, FR157, PR1007, HER208, FR107, and PG18R should do the trick (i.e. fast recovery diode rated for 1 Amps or more and 800V DC blocking voltage or more).
For the clamp resistor and capacitor (R_clamp and C_clamp), I am just going to blatantly copy the Bestec ATX-250-12E circuit - i.e. you can try 100 kOhm, 2W resistor and 2.2 nF, 1 or 2 kV ceramic cap. There's actually a proper way to calculate those values, but it requires knowing the transformer inductance, among a few other circuit parameters, so I will stick with circuit values from other similar power supplies. Other possibilities for R_clamp and C_clamp include 47 kOhms and 10 nF... or 22 kOhms and 100 nF (though that last combo may be a bit harsh on the diode). In any case, any of these values should provide more damping/clamping for the transformer's primary than the original circuit (which has absolutely nothing! Seriously, what the heck )

Last but not least, notice the changed connections of the optocoupler on the primary side: that is, one side connects to primary-side ground (big cap negative lead) and the other to the NCP1200 pin 2 (FB / Feedback). Pay very close attention that you connect this correctly. Otherwise, the PSU may work in open-loop self-limit mode or not at all.

In addition to that, you may need to change Q2101 (2SK3561) with a MTD1N60E or other MOSFET with similar Gate Charge parameters.... but again, we will see if that is needed or not. Same goes for the MOSFET sense resistor, R2108.

All in all, don't be discouraged if you make the mod, plug it into your PSU board, and it doesn't work - it is normal for mods to not always work as intended the first time. That said, the NCP1200 should at least shut down gracefully and not blow any parts up if something is not quite right. Of course, if you do have major power components wired incorrectly, then you can still blow something up. So, it should go without saying that it's best you double or even triple-check your work. It probably won't hurt to buy a few extra fuses for fuse F2001.... or a backup/second or even third spare for all of the parts required for this mod.
The use of a current-limiting device like an incandescent bulb also might be a good idea... though this time I am going to recommend something with a heating element, like a simple 300-800 Watt toaster oven or hair dryer... or even an incandescent/halogen work lamp (rated for 300 Watts or more). That should still limit the current while allowing plenty for the PSU to work, even with a small load.

Anyways, that is all for now. When you build the mod board, I may revise the schematic again, if we find something needs to be changed. I also attached the original schematic below (in JPG format) for easy reference. Enjoy!

[johnfin]

Re: Magnavox DRV power supply SMPS

Sounds great, cant wait to see your schematic.

[momaka]

Re: Magnavox DRV power supply SMPS

Meanwhile, can you tell us what the frankenstein board will accomplish.

The NCP1200 PWM chip basically drives the MOSFET on the primary side. Unlike the 2-transistor self-oscillator design that you have now, the NCP1200 has pulse-by-pulse control, which allows the PSU to shut-down safely in case of a fault rather than blowing up a ton of parts. It will also likely raise the efficiency of the PSU at no/low load. And best of all, it will simplify the primary side of your PSU drastically (i.e. there will be much less components). This is good, because if you can't figure what those diodes and Zener diodes were after removing them... well, you now won't have to worry at all, because they won't be needed.

[johnfin]

Re: Magnavox DRV power supply SMPS

Thanks, i am sure everyone with these ps will appreciate it. Meanwhile, can you tell us what the frankenstein board will accomplish.

[momaka]

Re: Magnavox DRV power supply SMPS

I looked at pdf schematic, you make it look simple and while the schematic is simple, integrating that circuit into what i have is another story. If i could get that to work, a bunch of people would be thankful because the original design is junk. Can you explain how that circuit splices into what i have.

I'll try to draw a diagram on the PSU's schematic to hopefully be able to show how to connect it. It may take me a few days to do that, though. Most likely, I will cross out which components should be removed and which moved to a different location... and new ones added.

[johnfin]

Re: Magnavox DRV power supply SMPS

Cant test them because i cant find them. Lol. See pics. D2108 is missing, D2109 on bottom. 2197 is typo, sb 2107, see pics. I looked at pdf schematic, you make it look simple and while the schematic is simple, integrating that circuit into what i have is another story. If i could get that to work, a bunch of people would be thankful because the original design is junk. Can you explain how that circuit splices into what i have.

[momaka]

Re: Magnavox DRV power supply SMPS

Zener diodes D2108 D2111 D2117 D2197 are missing from my board. They are printed on the circuit board, just not there.

That doesn't quite make sense, because in post #64 you mentioned this:

I checked those components, caps and diodes are large in size and easy to check. What I have been trouble checking is the small diodes on the bottom left of the schematic. D2108, D2109, D211, D2113

So D2108 and D2109 should be there, according to that earlier quote. Also, I don't see diodes D2117 and D2197 on the schematic. What would these correspond to from your power supply?

As far as the new circuit, I read the article but as far as integrating that into my circuit is cryptic. Circuit design is beyond my scope.

Well, the original PSU seems to have so many faults, that fixing it may also require circuit designing than actually doing the PWM chip mod. For the PWM chip mod, it's literally 13 components and the NCP1200 PWM chip. Some of these are already on your board and can be reused. Just copy the circuit from the PDF file in post #2 (only the part inside the dashed lines, that is) and do any modifications that the thread mentions later one. That's it.

Either that, or just send it to someone to fix.

[johnfin]

Re: Magnavox DRV power supply SMPS

Zener diodes D2108 D2111 D2117 D2197 are missing from my board. They are printed on the circuit board, just not there.

[johnfin]

Re: Magnavox DRV power supply SMPS

I will do those test on the diodes.
As far as the new circuit, I read the article but as far as integrating that into my circuit is cryptic. Circuit design is beyond my scope.