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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Aw, don't get up on this far.. *hugs and solaces Uzzors*
You should really get some kind of simple overcurrent protection going. If your diodes failed they are likely to draw mosfets with them.
You should be able to sample some of such diodes (even Schottky) from Vishay. If I needed really lot of power at lower voltages I would even consider a synchronous or at least semi-synchronous rectifier. Semi-synchronous rectifier is quite easy and robust:
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Hehe, thanks.
I have disconnected the secondary rectifiers and ran the smps open loop just now with new IRFP450s and nothing failed. So the rectifiers died and pulled the half-bridge down with them originally. I have the diodes required for center-tap rectification, so I'll rebuild it sometime. A synchronous rectifier is also a possible solution, but I'll save that until I'm out of proper rectifiers.
The obvious question now is how can the overcurrent protection be improved? I don't think a primary CT is necessary because that just covers up the symptoms. I want a stable system that will never kill the secondary side again. Did I set the RC delay too long? Tests with direct shorts earlier didn't cause a problem. Which values should I use when the rectifiers can take 250A for 8.3 ms? At the time of the failure I had a 1µF cap and 2.2k resistor.
Btw, I cut out the current limiting function entirely.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
CT is one of best ways for overcurrent shutdown, it guards both the mosfet's and rectifiers. I thought you weren't using it?
Shunt is problematic and for higher currents is usually replaced with hall probe, and it doesn't help you much unless you have some kind of switch after the filter cap to prevent it form dumping into load.
Just use a comparator and the triac (you can also make a latching switch out of a remaining comparator if you want.)
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
I haven't put the CT in just yet but it might be a good idea. I have two extra op-amps left, so why not? I pulled apart the rectifier and it turns out that just one diode died in the rectifier-bridge. This gets stranger yet as that diode was parallel with another and part of a dual diode package. I now think the overcurrent limit is set just too high, so I'll trim it down a bit.
I'll also add a CT. How is the primary/secondary current relationship? I don't mean turns ratio, but does the primary current spike or behave strangely due to primary inductance? Or is it a simple matter of 1:2,4 ratio, so 14/2,4 = 5,83 primary amps max?
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi Uzzors
I'll also add a CT. How is the primary/secondary current relationship? I don't mean turns ratio, but does the primary current spike or behave strangely due to primary inductance? Or is it a simple matter of 1:2,4 ratio, so 14/2,4 = 5,83 primary amps max?
CT's are very precise. By far biggest error will be due to burden tolerance after all. Fullwave rectifier is proffered for better resolution.
It may be better to use the rectifier before the burden, to eliminate temperature drift of diodes.
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Switch-mode bench supply problems
