"Goosing" with a higher voltage

E

eromlignod

Guest
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?

Don
 
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?
Click to expand...
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.

--
Regards,

John Popelish
 
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.
Click to expand...
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*.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
 
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*.
Click to expand...
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
 
Sounds like I can use a Schottky diode. I found one with a voltage
drop of about a half a volt. My chips downstream appear to be able to
handle that drop. I suppose I'll have some adjustment of the 5V at
the power supply too. If I can crank it as high as 5.5V I might be
able to compensate.

Thanks for the help guys.

Don
 
On Apr 28, 9:54 am, John Popelish <jpopel...@rica.net> wrote:
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- Hide quoted text -

- Show quoted text -
Click to expand...
I'd say Don should use OR-control, that way nothing can go wrong.
He already has 24 volts to drive a high-side mosfet.


24v
|
+-----+----,
| | |
| | |
|\ | 6k2 |
,------|+\| | |
| | \ | |
| LM393| >----+ |
| | / | o|
| ,--|-/| | |--power
| | |/ | | o| switch
10k 10k gnd | |
| | | |
| '-----------|----+
| | |
| ,------+---+ |
| | | | |
| 18v | |_ |
| zener 10k __ |
| | | || |
| 5v-+------+---''---+--load
| | s d
| | n-channel mosfet
| \ SPP80N03
| /
'--->\1k
/
\
|
|
50k
|
|
gnd

I didn't draw a .1 uF bypass cap for the comparator, but it should
probably have one.
The mosfet needs oriented source to the 5 volt supply and the drain to
the load.
The 1k pot nulls out any negative offset in the comparator, to make
sure it trips when the source voltage exceeds the drain voltage,
preventing the mosfet from ever conducting back into the 5 volt
supply. If the comparator has positive offset, it will oscillate at
light loads under a few tenths of an amp or so, which probably won't
hurt anything. But if one prefers, using an op amp instead of a
comparator would get around that. I like the comparator 'cause it's
fast, cheap and robust.
 
On Apr 28, 12:23 pm, gearhead <nos...@billburg.com> wrote:
On Apr 28, 9:54 am, John Popelish <jpopel...@rica.net> wrote:





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- Hide quoted text -

- Show quoted text -

I'd say Don should use OR-control, that way nothing can go wrong.
He already has 24 volts to drive a high-side mosfet.

              24v
               |
               +-----+----,
               |     |    |
               |     |    |
            |\ |    6k2   |
     ,------|+\|     |    |
     |      |  \     |    |
     | LM393|   >----+    |
     |      |  /     |    o|
     |   ,--|-/|     |     |--power
     |   |  |/ |     |    o|  switch
    10k 10k   gnd    |    |
     |   |           |    |
     |   '-----------|----+
     |               |    |
     |    ,------+---+    |
     |    |      |   |    |
     |   18v     |   |_   |
     |  zener   10k  __   |
     |    |      |   ||   |
     | 5v-+------+---''---+--load
     |    |         s  d
     |    |   n-channel mosfet
     |    \       SPP80N03
     |    /
     '--->\1k
          /
          \
          |
          |
         50k
          |
          |
         gnd

I didn't draw a .1 uF bypass cap for the comparator, but it should
probably have one.
The mosfet needs oriented source to the 5 volt supply and the drain to
the load.
The 1k pot nulls out any negative offset in the comparator, to make
sure it trips when the source voltage exceeds the drain voltage,
preventing the mosfet from ever conducting back into the 5 volt
supply.   If the comparator has positive offset, it will oscillate at
light loads under a few tenths of an amp or so, which probably won't
hurt anything.  But if one prefers, using an op amp instead of a
comparator would get around that.  I like the comparator 'cause it's
fast, cheap and robust.- Hide quoted text -

- Show quoted text -
Click to expand...

D'oh, can't have that gate-source zener and resistor that way. Do
away with the zener. Run the 10k resistor from gate to ground instead
of to the 5 volt supply, and change its value to 15k.
Hope that does it.
 
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?

OR control, latest version:
Click to expand...

              24v
               |
               +-----+----,
               |     |    |
               |     |    |
            |\ |    6k2   |
     ,------|+\|     |    |
     |      |  \     |    |
     | LM393|   >----+    |
     |      |  /     |    o|
     |   ,--|-/|     |     |--power
     |   |  |/ |     |    o|  switch
    10k 10k   gnd    |    |
     |   |           |    |
     |   '-----------|----+
     |               |    |
     |     gnd--15k--+    |
     |             |    |
     |   5v     |_   |
     |   |    __   |
     |    |         ||   |
     | +----------''---+--load
     |    |         s  d
     |    |   n-channel mosfet
     |    \       SPP80N03
     |    /
     '--->\1k
          /
          \
          |
          |
         500k
          |
          |
         gnd
Click to expand...
 
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