Paralleling Switching Power Supplies? Can it be done.

[Brian M.]

I have two (identical) 24v 4.5A switching power supplies. Can they be
safely paralleled together to give me 24v at 9A.

Thanks
Brian

 

[Tim Wescott]

I have two (identical) 24v 4.5A switching power supplies. Can they be
safely paralleled together to give me 24v at 9A.

Thanks
Brian

They have to be made specifically for the job. Otherwise one will
inevitably have a slightly higher setpoint than the other and will do
all the work.

Supplies that are designed for paralleling have some sort of interface
to gang the two supplies together and to configure one as master. When
they're ganged the master regulates the voltage, the slave just delivers
as much current as the master asks for.

It's up to you to spelunk through your documentation to see what you've got.

 

[mook johnson]

If you diode couple them (not very efficient with 3 watts loss each), when
one supply hit its current limit and the voltage drops below the other, the
second one will then supply the additional current above the CL of the first
supply. you will lose about a volt in the process.

that is about all this can be expected from two separate power supplies.

 

[Paul Hovnanian P.E.]

They have to be made specifically for the job. Otherwise one will
inevitably have a slightly higher setpoint than the other and will do
all the work.

Supplies that are designed for paralleling have some sort of interface
to gang the two supplies together and to configure one as master. When
they're ganged the master regulates the voltage, the slave just delivers
as much current as the master asks for.

It's up to you to spelunk through your documentation to see what you've got.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Its possible to parallel two (or more) supplies that have remote sense
inputs. You'll have to cobble together a circuit that will bias the
sense inputs to achieve load sharing. Designing such a circuit to ensure
system stability isn't trivial, but it can be done.

 

[petrus bitbyter]

I have two (identical) 24v 4.5A switching power supplies. Can they be
safely paralleled together to give me 24v at 9A.

Thanks
Brian

Brian,

As almost always: It depends. Some power supplies have the provision to be
paralleled. Just read the manual. Some cannot be paralleled unless you want
to start the fireworks. Most of the times it can be done when the settings
are as equal as you can get them *and* the load is significant more then one
supply can handle.

petrus

 

[James Meyer]

They have to be made specifically for the job. Otherwise one will
inevitably have a slightly higher setpoint than the other and will do
all the work.

Malarkey (polite way of saying bullshit).

The one with the higher setpoint will do all the work, but only until
the load current causes its voltage to drop a little. The two supplies won't
divide the load current exactly in half at all load currents, but that is not
really necessary. There is no reason why two nearly identical switching
supplies can't be paralleled with impunity.

Jim

 

[James Meyer]

If you diode couple them (not very efficient with 3 watts loss each), when
one supply hit its current limit and the voltage drops below the other, the
second one will then supply the additional current above the CL of the first
supply. you will lose about a volt in the process.

that is about all this can be expected from two separate power supplies.

Diodes are unnecessary. You can expect the output impedance of each
supply, including the resistance of the wires connecting the supplies, to
perform the same action as diodes and without the voltage regulation and
efficiency losses.

Jim

 

[James Meyer]

Its possible to parallel two (or more) supplies that have remote sense
inputs. You'll have to cobble together a circuit that will bias the
sense inputs to achieve load sharing. Designing such a circuit to ensure
system stability isn't trivial, but it can be done.

Spoken like a true PE, Paul. Take a trivial "problem" that isn't really
a problem and make it so complicated that only a PE can come up with a solution.

Jim

 

[Allan Herriman]

Brian,

As almost always: It depends. Some power supplies have the provision to be
paralleled. Just read the manual. Some cannot be paralleled unless you want
to start the fireworks. Most of the times it can be done when the settings
are as equal as you can get them *and* the load is significant more then one
supply can handle.

In particular, ones with synchronous rectification shouldn't be
paralleled, since they can both sink and source current and will
"fight" unless a load sharing circuit such as the LCT4350 has been
used.

'4350 data sheet:
http://www.linear.com/prod/datasheet.html?datasheet=753

Regards,
Allan.

 

[Tam/WB2TT]

In particular, ones with synchronous rectification shouldn't be
paralleled, since they can both sink and source current and will
"fight" unless a load sharing circuit such as the LCT4350 has been
used.

What if he puts an inductor in series with each supply? Problems I see are :
If the two supplies are not adjusted close enough in voltage, the first one
might shut down before the second one supplies any current; if the load is
variable, as in an SSB transmitter, hard to tell how the dynamics work out.

Tam

 

[Brian M.]

Please correct me if I'm wrong but I'm guessing by using diodes you
mean something like this

Both supplies are 24v 4.5a

supply1-----diode----\
+------------ +24v @~9A (minus diode losses)
supply2-----diode----/

thanks
Brian