RE: Bruker Elba supply on BLAX or BLARH amp, SMP/P600/BRV

Bill Beaty (beaty@chem.washington.edu)
Mon, 10 May 2004 16:07:47 -0700 


More news!

The design weakness in that power supply is NOT in the capacitor values 
as I thought.  But the problem is nearby.  It causes the capacitor values
to slowly decrease over time, so the supply fails early.  There's a 
10V Zener diode which probably causes trouble, and it might help things 
if we change it to a smaller value ( such as 4.3V 1N4731.)

The little voltage doubler on the big series inductor winding (the two 
diodes and three capacitors) is only supposed to supply its 20Vdc to the 
LM7812 for a couple of seconds; only until the main +30Vdc comes on line.  
The output from this voltage doubler is passed through a diode, as is the 
main 30Vdc output, both are applied to the LM7812 regulator input pin, 
and whichever is higher, that one powers the regulator.  This lets the 
main +30V take over from the volt-doubler when it later wakes up.

But unfortunately the manufacturer has put a 10Vdc Zener diode in series 
with the 30Vdc to drop it down to 20V (no doubt because the LM7812 without 
heat sink gets quite hot when given 30V input, and the series zener shares 
some of the thermal wattage.)

So the little volt-doubler and the main 30V are both set to 20V, and if you 
happen to be unlucky and have just the wrong circuit values, then BOTH are 
always powering the LM7812.  Or perhaps the voltage doubler "wins" and 
becomes the main supply for the UC3285 on that daughter board.   This is bad 
news for the capacitors in the voltage doubler, since they normally see 
two-ampere pulses at around 50KHz, and they will run hot.  Over the months 
they get baked out, their values decrease, their 20Vdc output voltage 
decreases, and finally it falls below the 9.0Vdc required by the UC3825 
main 30Vdc regulator chip.  It also doesn't help that the capacitors are 
right up against the very hot LM7812 regulator; and that might even be 
the real trouble here after all.  But regardless, the temporary bootstrap 
supply gets too low, and the system gets flakey during power-up and can't 
wake up anymore.  However, if it's ALREADY running, the bootstrap supply 
does nothing, and the system will run fine...  so just never turn it off!

:)

Or, if your BLAX or BLARH apparently dies right after a power-off, be aware 
that the bootstrap supply is still VERY close to the correct voltage.  
Perhaps try turning it off and back on a couple of times (with luck it may 
"catch" and start working.)

The cure we used (your milage may vary!):  replace the two 10uF and the 
one 100uF capacitors (they're all glued together, positioned next to that 
LM7812 voltage regulator approximately in the middle of the main board.)   
Replace them with low-ESR, 105 deg C types.   But that doesn't fix the real 
problem.  So also look for a chain of resistors right at the edge of the main 
board (labeled DZ1, DZ2, DZ3.)  One is a 10V zener diode, the others are zero-
ohm jumpers.  I replaced the 10V zener diode with a one-watt 4.3V zener.   
This lets the poor little voltage-doubler circuit turn off when it's not 
needed.   But it makes the LM7812 regulator run even hotter than before.  If 
I see another one of these dead supplies, I think I'll also be putting a 
couple of little bitty TO-220 heatsinks on the LM7812 regulator (HS214-ND 
from digikey.)




At 12:51 PM 5/10/2004 -0500, you wrote:
>Now our third dead unit was very interesting.
>
>There is a design weakness in the 30V section of the power supply.  When
>the supply first comes up, the 30VDC switching regulator chip UC3825
>needs at least 9V to wake up and start making DC.  In normal operation it
>creates it's own supply voltage from the 30VDC output.  But at startup
>it needs another supply.   It gets this from a little "bootstrap power
>supply" winding on that big iron tapewound inductor which is part of the
>power-factor switching circuit.  This winding is voltage-doubled with two
>diodes and three capacitors (partly hidden under a transformer,) makes
>20VDC, and it supplies a LM7812 regulator which supposedly puts out 12VDC
>for the UC3825 chip.  But the UC3825 draws 33mA, which drags the 12v
>supply voltage of the bootstrap/LM7812 down to 8.9V...   and sometimes
>the UC3825 goes into continuous repeating reset and never starts.  It's
>waiting for its bootstrap supply to rise up above 9.0V.  THis all depends
>on temperature and on many component values.   This is a bad design.  They
>should have given plenty of leeway (like designing it to actually put out
>12VDC, even when it draws 33mA as it does.)
>
>About the idea that heat can kill these supplies...  yep, if any of the
>three voltage-doubler electrolytics which supply the 12V (or 8.9V!) to the
>UC3825 chip should get baked out, so their capacitance value drops, or
>their internal leakage gets bad...   then this pushes things over the
>edge and the supply cannot wake itself up anymore.  Our dead supply had
>a 100uF 50V electrolytic capacitor which had changed itself to 20uF over
>the years.  When replaced, the supply worked fine again.
>

((((((((((((((((((((((( (  (    (o)    )  ) )))))))))))))))))))))))
William J. Beaty              http://staff.washington.edu/wbeaty/
Research Engineer             UW Chem Dept,  Bagley Hall RM74
beaty@chem.washington.edu     Box 351700, Seattle, WA 98195-1700
ph:206-543-6195 fax:206-685-8665