Re: power supply discharge

On Mon, 30 Sep 2024 11:49:54 -0700, Joerg <news@analogconsultants.com>
wrote:

On 9/30/24 11:24 AM, john larkin wrote:

On Mon, 30 Sep 2024 08:39:27 -0400, legg <legg@nospam.magma.ca> wrote:
 

On Sun, 29 Sep 2024 08:23:01 -0700, john larkin <JL@gct.com> 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.

>
You might consider overvoltage protection or a (switched ?)
internal minimum load.There's usuaally some point in the
control loop that's a good indicator of a pull-down requirement.
A single ovp or autoload on the input looks likely to serve
all of your many sync-bucks.
>
RL

 
An MOV on the bulk supply could limit the reverse-pump excursion until
the software can notice and shut things down.
 
MOVs can gobble a lot of joules, but their clipping is very soggy.
 

>
MOVs are usually cumulative. They can take a certain amount of
dissipation over their lifetime and then *PHUT* ... POOOF. Like a bank
account that runs dry.

What kills MOVs? Integrated joules? Time-temperature?

I don't expect a lot of joules per event. Just enough energy to keep
my supply voltage down until a slowish ADC and the software can shut
the buck switchers down. 15 milliseconds max, maybe.