Weird HP 6012A Power Supply Fault

[Chris Quayle]

Hi,

The above model psu has a weird fault - bought s/hand with blown output
Mosfets, which were replaced, along with associated driver components.
However, unit won't deliver significant power without tripping, limit ~
10 volts at 2 amps. Neither the down programmer or the rest of the
protection circuitry is tripping, so the fault must be somewhere in the
feed back control or pwm logic.

The fault seems to lie in the logic board, but the circuit doesn't seem
to make sense in that the maximum fet on pulse width cannot exceed the
fet on monostable pulse width of 1.7 us, even when the pwm comparator
pulse width becomes wider. The pre mono logic pulse width increases with
load up to a point, but the on pulse mono appears to be clamping the
overall width on the driver gate.

Started with a scope, but now have a logic analyser hooked up to it,
which confirms the above analysis. Am I missing something here ?. Bit of
a long shot, but suggestions welcome from anyone familiar with this
unit...

Regards,

Chris

 

[Roy L. Fuchs]

Hi,

The above model psu has a weird fault - bought s/hand with blown output
Mosfets, which were replaced, along with associated driver components.
However, unit won't deliver significant power without tripping, limit ~
10 volts at 2 amps. Neither the down programmer or the rest of the
protection circuitry is tripping, so the fault must be somewhere in the
feed back control or pwm logic.

Well, if you have it set to "sense" on the back (if it has such an
animal), the voltage sense wires must be in place and be used. If the
switch is set to internal, it regulates at the output taps. The sense
is for more precise voltage control AT the device under test.

Anyway... That *could* be one cause for a trip. Though it seems
unlikely in this case.

The way to set current limit is with the output leads shorted at a
low voltage set point. The limit value would show up on the meter.

This supply is overkill for practically any home hobby job. Mice to
have, but you should have a smaller PS (perhaps even more accurate)
for most bench work.

Ours in the lab was only 30V at about 3 Amps max, but it was fully
programmable so I could set high line, low line, and nominal input
voltages (inputs to my device), and test a product for function at
those points,.

The fault seems to lie in the logic board, but the circuit doesn't seem
to make sense in that the maximum fet on pulse width cannot exceed the
fet on monostable pulse width of 1.7 us, even when the pwm comparator
pulse width becomes wider. The pre mono logic pulse width increases with
load up to a point, but the on pulse mono appears to be clamping the
overall width on the driver gate.

Started with a scope, but now have a logic analyser hooked up to it,
which confirms the above analysis. Am I missing something here ?. Bit of
a long shot, but suggestions welcome from anyone familiar with this
unit...

Sounds like the internal watchdog circuitry might be a bit hosed.
Normally it senses voltage on a shunt in the output line to monitor
current.

Ebay has manuals if yours didn't come with one.

 

[Chris Quayle]

Well, if you have it set to "sense" on the back (if it has such an
animal), the voltage sense wires must be in place and be used. If the
switch is set to internal, it regulates at the output taps. The sense
is for more precise voltage control AT the device under test.

Anyway... That *could* be one cause for a trip. Though it seems
unlikely in this case.

The way to set current limit is with the output leads shorted at a
low voltage set point. The limit value would show up on the meter.

This supply is overkill for practically any home hobby job. Mice to
have, but you should have a smaller PS (perhaps even more accurate)
for most bench work.

Ours in the lab was only 30V at about 3 Amps max, but it was fully
programmable so I could set high line, low line, and nominal input
voltages (inputs to my device), and test a product for function at
those points,.


Sounds like the internal watchdog circuitry might be a bit hosed.
Normally it senses voltage on a shunt in the output line to monitor
current.

Ebay has manuals if yours didn't come with one.

Thanks - The weirdness may be intentional. After looking at it some
more, the logic board seems to be ok, with a 1.7uS mosfet on pulse at
the start of the pwm period and a 1uS off pulse at the end. The pulses
are transformer coupled to the mosfet gates. The transformer has a
centre tapped secondary, one side used via a diode and other parts to
drive the gate on, with the other side used to drive a transistor to
short the gate capacitance, thereby ensuring fast turn off. AFAICS, the
only way that the thing could ever work with the above is if it's using
the mosfet gate capacitance to store a charge to hold the fet on during
the pwm on period, which has a max of 25 uS. I guess it's feasable,
especially as the resistance in the gate circuit is much higher than
expected at 270K. No clue about this in the manual at all.

Problem is that the original mosfets don't have a commercial equivalent,
just the 4+4 hp part number. Have tried 4 x MRFP450, which should be
enough to do the job power wise, but they don't work. It's too usefull a
psu to just scrap, so will probably redesign the driver etc to get it
going, even if some functionality is lost. Your right about overkill
though - the whole design is pretty laboured and the 6012B (Have manuals
for both) even more so. It's 1984 vintage, so we give them the benefit
of the doubt ...

Regards,

Chris