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

Bill Beaty (beaty@chem.washington.edu)
Thu, 06 May 2004 18:00:26 -0700 



Rather than paying the $1500, I've repaired a couple of these.  
Want some info?

On the first one I saw, the main bridge rectifier and the big MOSFET
for the 400VDC Power Factor regulation were fried.  These sit right
on the 220VAC line, so any major surge can kill them.  The bridge is 
a weird thin little thing under the main PCB, part number D20XB60,
availible from Mouser Electronics, (600V 35amp bridge.)  The MOSFET 
was APT5025 from Advanced Power Technology, http://www.advancedpower.com/
http://www.advancedpower.com//communities/apt/products/5025AN.PDF
I found an APT5025, but probably a similar transistor would work, 
if you can find a similar package for 500V 20A .25ohm 230watt,
N-channel, gate threshold max 4v.



The second dead unit had a bad NPN transistor from ZETEX which, if I
recall right, drove the gates of the main switching MOSFETS driving the 
400VDC to 30VDC stepdown.  The MOSFETS were dead too.  No doubt killed
by surges on the 220VAC line.



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.




Also...

I discovered that the schematic for the 220VAC input section of this
supply is almost identical to the schematic shown in this application 
note for the UC3854 chip used in the supply's power-factor correction 
section.  It uses a MOSFET and inductor to massage any input voltage 
between 50VAC to 270VAC and puts out 400 volts DC (385V.)   The 400Vdc 
is later switched as a 200KHz Khz square wave and applied to the 
stepdown transformer to make 30VDC.  Schematic:

  Advanced Power Factor Correction Control ICs
  http://focus.ti.com/lit/an/slua177/slua177.pdf

The power supply ***WILL RUN*** on 120vac, at least for testing.  I wouldn't
run the NMR amp on 120Vac though, since the supply is probably out of spec 
for power factor and for 600W capability.

Other schematics for your reference:

   THE UC3823A,B AND UC3825A,B 
   ENHANCED GENERATION OF PWM CONTROLLERS
  http://focus.ti.com/lit/an/slua125/slua125.pdf


Others:
http://focus.ti.com/lit/an/slua196a/slua196a.pdf
http://focus.ti.com/lit/an/slua144/slua144.pdf
http://focus.ti.com/lit/an/slua172/slua172.pdf



Also see messages:


  question about Bruker amps and their power supply modules
  http://chemnmr.colorado.edu/ammrl/archives/February-2004/13.html

  question about Bruker amps and their power supply modules--Summary
  http://chemnmr.colorado.edu/ammrl/archives/February-2004/17.html

  Bruker BLAX/H Power Supply Cooling Fan
  http://chemnmr.colorado.edu/ammrl/archives/June-2001/7.html



((((((((((((((((((((((( (  (    (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