Re: "Goosing" with a higher voltage

Joerg wrote:
John Popelish wrote:
eromlignod wrote:
Hi guys:

I have an application where I'm driving a device with 5 volts DC.
From time to time it is necessary to give it a short pulse of 24 Vdc
then revert back to 5V.

I have a power supply that has both 5V and 24V outputs. Can I just
pulse the 24V right onto the 5V line (with a power transistor or
relay) as long as I include a diode on the 5V output to avoid back
current? Or is this foolish?

It sounds like it could work. I would use a Schottky diode to minimize the voltage loss when the 5 volt supply was providing the load with current. I even that loss is too much, you might use a MOSFET as a synchronous rectifier (with the channel conducting in parallel with the body diode when the 5 volt supply was driving). But then, you have to produce a gate drive signal that is synchronized (non overlapping) with the disabling of the 24 volt drive control.


Be careful. If there are any bypass caps downstream the voltage on them might still be higher than 5V when the 5V sync-rectifier comes back on -> phssst ... *POOF*.

Yep, that was one of the reasons for the non overlapping warning. I would probably turn the MOSFET on only after the body diode was carrying the load during the 24 volt switch off, and turn the MOSFET off before turning the 24 volt switch back on. That assumes the system could handle those brief sags below 5 volts.

--
Regards,

John Popelish
.

Relevant Pages

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  • Re: "Goosing" with a higher voltage
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  • Re: Drivers with more source current than LM5112?
    ... current delivered or drawn from a 5 volt source, ... sink, source currents. ... and P-channel single-chip soic-8 MOSFET ICs are plentiful. ...
    (sci.electronics.design)