Re: power supply discharge

Chris Jones <lugnut808@spam.yahoo.com> wrote:

On 30/09/2024 1:23 am, john larkin wrote:

On Sat, 28 Sep 2024 22:28:07 -0700, Joerg <news@analogconsultants.com>
wrote:
 

On 9/27/24 8:07 AM, john larkin wrote:

>
Given a benchtop power supply, you can turn the voltage up and then
down, and it goes down. Most have a substantial amount of output
capacitance, and can be driving an external cap too. So something
pulls the output down.
>

>
Often the only internal load is the resistive divider for the regulator
loop feedback.
>
>

I guess that there are no standards for this, but I've never seen a
supply that just hangs high when it's cranked down.
>

>
I have some. They drop very slowly when there isn't much load on the
output.

 
Customers might whine if they ask for 10 volts and see 30. Amd that
may be mostly held up by their capacitive load.
 

>
>

I'm designing some programmable multi-channel power suplies and that
is one of many tangled issues in the project.
>

>
A synchronous buck architecture should work quite well if you need to
slew fast. I've used that on a driver that had to modulate a hard
capacitive load at several kHz and above 100V.

 
I'm doing some multichannel non-isolated supplies that will be sync
buck, using multiple TI DRV8962 chips.
 
One problem is that a sync buck can become a boost in the wrong
direction, and start charging my +48 supply. If it hits, say, 55
volts, I'll disable the switcher chips, and the outputs can hang. I
need to discharge the outputs. I'm thinking about 20 mA of depletion
fet per channel.
 

 
There is no need to concede defeat and disable your switcher chips, just
turn on a big load instead.
 
In variable speed drives for induction motors, the voltage of the DC
rail and bulk capacitance can also rise when the motor is slowing down
with a lot of inertia attached to the shaft. They have a switch built
in, which you are supposed to attach a big load resistor to. When the DC
rail rises above some threshold, it turns on your external load
resistor. It cycles on and off to keep the bulk capacitor voltage in an
acceptable range.

I once had to repair a large CNC milling machine where the load dump
resistor was on permanently because the controlling transistor had
failed short-circuit.  The resistor was mounted in a cage on the top of
the control cabinet and the machine eventually switched itself off when
the radiant heat from the red-hot resistor raised the internal
temperature of the cabinet to an unsafe value.

--
~ Liz Tuddenham ~
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