Built a 250W SMPS, possible to modify?

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
I know this isn't quite the same thing but the horizontal circuit of a TV requires a very fast turn on but the turn off isn't as important. That why I'm a bit unsure of what is required for a fet.

I've been reading SWITCHMODE™ Power Supply Reference Manual (SMPSRM-D.pdf) by on-semi, hence why I have implemted that design into my schematic. I will put the circuit you posted earlier in my schematic so when build I don't have :bawling: from toasted fets.

Is it required to have a cap in series in the secondary of the drive transfomer?

Agin, I'm still learning here so all your paciance and advice is extremly appriciated.

P.S: Hey Eva :innocent: :innocent: could you post or re-serve those pictures?

Thanks

Dave.
 
Let's not go off-topic... :D

I put the current sense transformer in series with the center point of the capacitors and the main transformer. On overload it limits the current (but doesn't shutdown? I haven't really tried overloading it THAT much).

However, if I short the output and THEN power it on, it (correctly) does not start... what it doesn't survive is a short AFTER power on...
 
Here're some shots of the gate drive waveform... directly on the primary (HV) side of the driver transformer. Note! Don't do this unless your scope has isolated inputs!

The spike looks worrying... what could be causing it? The circuit seems to be limiting the maximum duty cycle (the voltage starts to drop on overload) but it doesn't shut off.

1) No load
 

Attachments

  • noload.jpg
    noload.jpg
    44.5 KB · Views: 560
3) Overload (6x 12V 50W halogen bulbs)

Of course I could just make sure I don't short circuit the output... :D
Also what could be causing the little "ripple" during the off time? It's not causing any stray turn-ons of the FETs though...
I might use a proper RM core for the next project...
 

Attachments

  • over.jpg
    over.jpg
    50.4 KB · Views: 422
With no load pic look great. But as for othe two :whazzat: .You will need some snubber for those big spikes and that 2v ripple to minimize it as much as possible.Did you put that gate circuit to use yet?

The ripple can come from badly winded trafo

If you have to have over current, but this won't work there are allway overrated fuses that you can use.
 
Gate circuit? Please elaborate... :)

I might incorporate the snubbers from here...

The 'scope (Tektronix THS720) is going back to the office tomorrow however... :( it's isolated inputs are making me want to buy one :D

(product plug) Everyone who is even remotely serious about doing such projects should have one of these... :D (end product plug)
 
Thanks I might try that.
From casual inspection I suppose the diodes can be any small signal type such as 1N4148 and the transistors any small PNP type like a BC557/8?

I'm trying very hard not to buy one of those scopes... buying a pre assembled power supply is cheaper :D
 
simingx said:
Right now it's about 50kHz, but I hope to move to at least 100kHz for future projects.. :)

Is 50kHz the oscillator or switching frequency? Also, I went back and re-read the earlier posts: Your FETs are IRF730s, correct? Their peak I(d) is only 5A with an Rds(on) of 1W. You might consider upgrading to '740's or even IRFP450s or '460s. Just a thought.
 
sinimgx,

I've been following this thread and have a few points to add:

1) There really are no generic snubbers- every application needs to be dealt with on an individual basis depending on the ringing frequency and what damping needs to be done. You generally cannot just use values from schematic unless the PCB and all the parts are the same. You should be able to find notes on calculating snubber components from Unitrode/TI app notes.

2) The location where you have your current sense/protection will not protect the supply from FET shoot-through. Another words, you could have cross condution between the 2 FET's in the bridge and the current sense xfmr would not see it. By the looks of your gate waveforms (the ringing) at overload, there could be a chance of both FET's turning on at the same time which would cause smoke very fast. I think you should take a really good look at your FET drive circuit and try to eliminate the ringing. It's hard to elaborate any more on this without seeing your actual supply.

Hope this helps.
 
Huh? :confused:

Here's my pc AT supply conversion project: I have an old 235W AT supply, pretty typical: +5 @ 23A, +12V @ 9A, -5V @ -0.5A, and 1-2V @ -0.5A, nothing ground-breaking. That is, until I decided to modify it to power an as yet unbuilt LM4765T chipamp amplifier.

Now, my intention is to modify the output section to get +/-15V as follows: make +5V the -15V line, make +12V line the +15V line, eliminate the -5V & -12V lines all together, and have C-L-C output filtering.

After inspecting the main transformer, I found a separate low-current winding for -12V, so this will remain disconnected. I plan to re-wind the big yellow/white toroid bifilar with the same number turns as the existing +12V line. The two existing common-cathode rectifiers will be replaced wit hMUR1620CT (common-cathode) and MUR1620CTR (common anode) for the +/- outputs.

Since the PWM (TL494) & driver transformer section are already optimized for each other, I will leave them alone, but plan on upgrading the MJE-13007s (400V, 8A) to MJE-13009s (400V, 12A).

I polan on taking pics as I go along, and will post them here.

Steve
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.