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Lowering SMPS dissipation
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dzseki:
Here is an excerpt of an SMPS circuit. The design comes from the early 90’s. Q20 being the switching element is mounted on a rather small heatsink because of size constraits (possible design flaw, possibly could have done better) and the thing is running hot obviously. Utilizing a modern MOSFET (with much smaller RDSon) would be a good way to reduce the dissipation there.
I am not the first one tempted by this idea in this particular circuit however, other tinkerers have changed the Q20 MOSFET to IRF740 but the reported behaviour was the FET went short rather soon.
I haven’t tried this for myself, and I find it somewhat surprising. Gate capacitance is important for sure but the MC34151 should have no problem with driving that, what other parameters should be observed then?

spec:
Hi dzseki,

Just thinking out loud really.

You mention that the heatsink is not really adequate so considering that area:

Are you certain that you can't beef up the heatsink one way or another?
Look for an NMOSFET with a lower thermal resistance junction to case, as you mention.
Use the best new thermal grease.
Investigate thermal insulating washers with a low thermal resistance- I think aluminum oxide is the lowest, but ceramic is also pretty good.
In the same vein, if the transistors are T0220, consider a case clamp to lower thermal resistance to the heatsink or change to TO3.
You could consider making the heatsink live and thus eliminating the insulating washer completely. This should reduce the thermal resistance to the heatsink a fair amount, but don't know how practical this would be. You would have to be cautious about the extra capacitance and inductance this would introduce to the drain circuit too.
Fit a fan- this is not as radical as it sounds- you can use the small CPU type fans which are compact, but effective.

The MC34151/33151 driver chip has a fairly high drive current capability at 1A, but that is not an awful amount by modern standards, where some drivers can sink and source 4A to 7A. Modern MOSFETs with lower RDss, tend to have higher parasitic capacitances in general.
There is a spare section of the driver, so maybe you could parallel them up to get more drive. It would be best to fit a better driver though if you were seeking more drive current. But, the thing is that driving MOSFET gates is a complex task with quite a few variables to consider.
Fit a fan- this is not as radical as it sounds- you can use the small CPU type fans which are small, quiet, and compact, but effective.

And a real crazy idea- fit liquid cooling as used on some high end PCs. ::) I have looked at this approach for a few critical applications and it is possible to design quite a simple, low cost system with a bit of ingenuity.  Last time I looked, off-the-shelf liquid cooling kits were costly.

You could consider an altogether more beefy NMOSFET- IXUS make some real beasts.

Check and possibly upgrade the critical capacitors may be worth considering too.

And that is about it. :)
 
T3sl4co1l:
Yeah well, IRF740 isn't much improved from what they've got there...

Actually it's pretty damn close anyway.  If the original design is that borderline, I guess that would do it.

Try something newer like an, uh, FDPF12N50T or IPAN50R500CEXKSA1 or STP12N50M2 or something.

Could try replacing D21, D22 with schottky e.g. C3D03060F or something.  It's probably not running in hard switching so this shouldn't be a big deal.

The snub circuit (D23, R94, C56, etc.) could be changed too, depending on what all is happening.  In particular, a dV/dt snubber (R94 --> 0.0, place a ~1k in parallel with D23, C56 --> 1nF C0G maybe) can prove helpful for saving switching losses.

Mind, all these changes are contingent on the EMC remaining acceptable, both in terms of internal signals (don't want lines/dots/crawl in the video?) as well as external (emissions).  The latter is a bit harder to evaluate, and I might just as well suggest finding a useful supply inside (like the 6V output there, or a 12 or 24V supply elsewhere, maybe?) and running a fan off that.  Can also try shopping around for better heatsinks in the same space / footprint, or adding on more material to improve what's there, utilizing whatever nearby free space you can find.

Tim
T3sl4co1l:

--- Quote from: spec on December 01, 2018, 03:36:54 pm ---Are you certain that you can't beef up the heatsink one way or another?
Look for an NMOSFET with a lower thermal resistance junction to case, as you mention.
Use the best new thermal grease.
Investigate thermal insulating washers with a low thermal resistance- I think aluminum oxide is the lowest, but ceramic is also pretty good.
--- End quote ---

It sounds like it's a small on-board heatsink, in which case, none of these will be much use unfortunately -- RthHA (heatsink-ambient) is dominant by a huge amount.  Some kind of grease is still needed (a dry joint can be RthCH ~ 10 C/W easily, but greased is typically ~0.3), but beyond that, you're just beating yourself up.


--- Quote ---In the same vein, if the transistors are T0220, consider a case clamp to lower thermal resistance to the heatsink or change to TO3.
--- End quote ---

I would recommend against moving the transistor somewhere.  Stray lead length will cause RF oscillations and slower switching.

Tacking stuff onto the heatsink to make it bigger, or connect it (thermally) to the enclosure, is a good idea though!



--- Quote ---You could consider making the heatsink live and thus eliminating the insulating washer completely.
--- End quote ---

...Speaking of -- it may already be, so watch out if you add anything to it, don't accidentally short things out!



--- Quote ---The MC34151/33151 driver chip has a fairly high drive current capability at 1A, but that is not an awful amount by modern standards, where some drivers can sink and source 4A to 7A. Modern MOSFETs with lower RDss, tend to have higher parasitic capacitances in general.

There is a spare section of the driver, so maybe you could parallel them up to get more drive.
--- End quote ---

Wouldn't worry about it.  Paralleling is not a bad thought.  Wouldn't bother with anything bigger.

My power supplies using UC3842 (which has a very similar gate driver section) are just fine with transistors about this size.  Difference of course being, I have enough heatsinking for the transistors to survive. :-\


--- Quote ---Fit a fan- this is not as radical as it sounds- you can use the small CPU type fans which are small and compact, but effective.
--- End quote ---

Yup, even if it's just improving airflow within an enclosure, it's still something.  If it draws in more airflow through vent holes and such, even better!

Downside: it needs cleaning every once in a while, as the forced air deposits dust everywhere.  (Surprisingly, natural convection can stay very clean -- I have a CRT monitor that's been used for years and years, and it's still very clean inside.  Anything with a fan?  Gunked up in a few years.)



--- Quote ---And a real crazy idea- fit liquid cooling as used on some high end PCs. ::) I have looked at this approach for a few critical applications and it is possible to design quite a simple, low cost system with a bit of ingenuity.  Last time I looked, off-the-shelf liquid cooling kits were costly.

--- End quote ---

Hah!  I wouldn't worry about it for anything less than a couple kilowatts -- but if you had it handy and want to put the time and materials into it... why the heck not, I guess? :P

Might even be better than that, if it's a projector as it says, it may already have water cooling to deal with the lamps / CRTs / optics.  Tap into a nearby hose and there you go -- just mind the liquid conductivity, don't want that nasty high voltage and noise getting into everything else.

Tim
xavier60:
Also there are some odd things about the schematic. The values of R90 and R94 look wrong. The position of the snubber components under the transformer don't make much sense. Maybe C54 goes to ground?
EDIT: R90 could not possibly really go to Drain!
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