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'[EE]: switching power supply and current'
2001\10\04@064654 by ckchan

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Greetings,

Dear all, recently i'm doing a switching power for SLIC (subscriber line
interface circuit) application. The problem now is that the MOSFET get
too hot, too rapidly (up to 100 degree celcius !). The circuitry is from
app. note of maxim. i'm using max774. Everthing follows the app. note
EXCEPT the grounding. i notice that the evaluation board grounding is
very large. How important grounding copper affect the operation ?? a
friend of mine said that the prototype board i built didn't have enought
ground plane. will ground plane affect the sourcing and sinking of
mosfet's current ?? is that why the mosfet got too hot ?? thanks.


regards,
ckchan

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2001\10\04@071812 by Alan Gorham

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Hello

I suspect that the ground plane on your evaluation board is being used as a
heatsink.
You should try to have the same area of copper on your own board ground
plane and this should
help!

Alan

{Original Message removed}

2001\10\04@072309 by Vasile Surducan

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Take a scope and see how looks the comutation: Vgs or Vload ( where load
is the transformer's primary etc.) , if Mosfet become hot probably is
working in linear region ( or comutation with extremely slow swings )
What is your frequency ? Connection carefully the ground plane could be
extremely important to avoid parasitical oscillations at large
frequencies.

Vasile

On Thu, 4 Oct 2001, ckchan wrote:

{Quote hidden}

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2001\10\04@105050 by Douglas Butler

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I assume you know about supernodes and single point grounding.  Just
copying the schematic may leave you open to VHF oscillations and other
naughties that may put your switching transistor in a linear region.
That would cause a lot of heat.

Sherpa Doug

> {Original Message removed}

2001\10\04@110333 by Roman Black

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ckchan wrote:
>
> Greetings,
>
> Dear all, recently i'm doing a switching power for SLIC (subscriber line
> interface circuit) application. The problem now is that the MOSFET get
> too hot, too rapidly (up to 100 degree celcius !). The circuitry is from
> app. note of maxim. i'm using max774. Everthing follows the app. note
> EXCEPT the grounding. i notice that the evaluation board grounding is
> very large. How important grounding copper affect the operation ?? a
> friend of mine said that the prototype board i built didn't have enought
> ground plane. will ground plane affect the sourcing and sinking of
> mosfet's current ?? is that why the mosfet got too hot ?? thanks.


I'd like to offer suggestions, and probably
many other people here would too. Can you post a
circuit, details of the mosfet (package, heatsink
type), in/out voltages and currents, and if you
have any cro data? I feel you have either under
heatsunk the fet or it's not switching quick or
hard enough. :o)
-Roman

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2001\10\04@112252 by Douglas Butler

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If you can run the switcher for a few seconds maybe you can measure its
efficientcy.  If the efficientcy is good then you are switching fine but
need more heat sinking.  If the efficientcy  before it overheats is poor
then you are probably not switching cleanly and generating too much
heat.  It would be worth building a fixture so you can make the
measurements fast.

Sherpa Doug

> {Original Message removed}

2001\10\04@120912 by ckchan

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Greetings,


> I'd like to offer suggestions, and probably
> many other people here would too. Can you post a
> circuit, details of the mosfet (package, heatsink
> type), in/out voltages and currents, and if you
> have any cro data? I feel you have either under
> heatsunk the fet or it's not switching quick or
> hard enough. :o)
> -Roman

i would suspect the pcb layout more thanthe components choice. we actually
built it according to the circuit and BOM list from Maxim's app. note. but it
maybe backdate though, the app. note is from 1997 :)

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