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Forum: Analog Circuits Strange problem with a switching regulator


Author: Kike (Guest)
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Hello Folks

I am running out of ideas. I have designed a PCB with three Traco Power 
switching regulators: TSR1 2450. After an initial test with the three SW 
Regulators mounted, the PCB was broken because an overcurrent. Then I 
isolated one SW regulator in a Protoboard (see Pic1) and the SW 
regulator regulated the voltage rightly (Vin=10V, Vout=5V).

However when I mounted again the same regulator on the board (see Pic 
2), the SW regulator did not work properly and I measured the output 
voltage and it was the same voltage (aprox.) like the input voltage 
(Vin=10V, Vout=9.6V). It is like the regulator short circuits the input 
with the output. Anyone has any idea about what can it be happening?.

Thanks in advance.

Author: lasag (Guest)
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Kike wrote:
> voltage and it was the same voltage (aprox.) like the input voltage
> (Vin=10V, Vout=9.6V).

Please verify that you are using the proper pins.
I can't tell loking at pic1 - it would be easier if you could use 
different wire colors.

Author: Homo Habilis (Guest)
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Hey Kike,

at least there is one thing that you have to realize
(but maybe there are two of them ...):

Certainly there´s a "little bit" more resistance,
both at the "power"-lines, as at the eventual additional
measurement laces...

(...and - what surely could be the case - the two circuits
could simply be "a little" different... what could be
surprisingly hard to find out).

Please check that thoroughly.

Author: Homo Habilis (Guest)
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lasag wrote:
> Please verify that you are using the proper pins.

Dear lasag, did you mean a mix-up of two pins
only at the PCB (and right pinning at the BBoard)?

Author: lasag (Guest)
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Homo Habilis wrote:
> lasag wrote:
>> Please verify that you are using the proper pins.
>
> Dear lasag, did you mean a mix-up of two pins
> only at the PCB (and right pinning at the BBoard)?
Yep.  Also, I think there is a remark (24V) near the front of the board 
and another (5V) in the depth - which is contrary to OUT-GND-IN (front 
to back) pin order of the regulator if mounted as shown in pic2.

Author: Kike (Guest)
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I am unsing the proper pins verify a lot of times...

I figure out that there is a problem with the impedance of the power 
planes:

Pic1 with the yellow board: Z(24V-GND) = 100K. Notice that there is not 
short circuit but the Impedance is very low...
Pic2 without the yellow board and the SW regulator mounted directly on 
the PCB: Z(24V-GND) = 200Ohm.

I guess that there is design failure anyway,  but I would like to know 
if there is anyway to increase the plane impedance to solve this problem 
momentarily and go on with other tests...

thanks in advance.

Author: lasag (Guest)
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Kike wrote:
> I am unsing the proper pins verify a lot of times...

I have no other explanation for the reported symptoms(*). Limiting the 
current from the power supply, it should be no problem to mount the 
regulator rotated by 180° - just to be shure.

Without an actual schematic and PCB layout, it is impossible to give you 
any other hint.

(*) variable input resistance by mouning the correct pins either by wire 
or directly would indicate a failure whithin the socket/board soldering.

Author: Kike (Guest)
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The SW regulator was mounted in the wrong way on the PCB and there was a 
mess because that....Begginer failure :-( Now it works perfectly :-)

Greetings!!

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