series input parallel output SMPS

Den torsdag den 29. marts 2018 kl. 15.20.36 UTC+2 skrev bitrex:
> I have a bunch of small 12 volt input ATX SMPS power modules each rated 
> for 120 watts. about 14 bux each, surplus. Fanless 200 watt modules are 
> harder to come by and significantly more expensive.
> 
> I'd like to slave two together to get ~200 watts out, with the inputs in 
> series. 24 volt 8 amp power bricks are pretty common.
> 
> I found this whitepaper on voltage balancing SMPS inputs in series but 
> it seems more applicable to from-scratch designs.
> 
> <http://www.deltartp.com/dpel/dpelconferencepapers/apec11_yj.pdf>
> 
> Got any suggestions?

https://www.amazon.com/Nextrox-Converter-Regulator-Step-Down/dp/B00BWKXTUU

bitrex wrote:

> I have a bunch of small 12 volt input ATX SMPS power modules each rated
> for 120 watts. about 14 bux each, surplus. Fanless 200 watt modules are
> harder to come by and significantly more expensive.
> 
> I'd like to slave two together to get ~200 watts out, with the inputs in
> series. 24 volt 8 amp power bricks are pretty common.
> 
> I found this whitepaper on voltage balancing SMPS inputs in series but
> it seems more applicable to from-scratch designs.
> 
Nope, won't work.  Paralleled outputs will likely not current share, one 
will put out limit current, the other will pick up the remaining current 
demand.  So, with delivered power not being shared equally, the voltage drop 
across the supplies will be different, but in series the input current MUST 
be the same.  So, the one carrying less output current will have less 
voltage drop, and it will increase until the other unit is starved for 
voltage.  It will be totally unstable, and the supplies will be pulsing on 
and off.

Jon

On Thursday, March 29, 2018 at 3:04:18 PM UTC-4, Jon Elson wrote:
> bitrex wrote:
> 
> > I have a bunch of small 12 volt input ATX SMPS power modules each rated
> > for 120 watts. about 14 bux each, surplus. Fanless 200 watt modules are
> > harder to come by and significantly more expensive.
> > 
> > I'd like to slave two together to get ~200 watts out, with the inputs in
> > series. 24 volt 8 amp power bricks are pretty common.
> > 
> > I found this whitepaper on voltage balancing SMPS inputs in series but
> > it seems more applicable to from-scratch designs.
> > 
> Nope, won't work.  Paralleled outputs will likely not current share, one 
> will put out limit current, the other will pick up the remaining current 
> demand.  So, with delivered power not being shared equally, the voltage drop 
> across the supplies will be different, but in series the input current MUST 
> be the same.  So, the one carrying less output current will have less 
> voltage drop, and it will increase until the other unit is starved for 
> voltage.  It will be totally unstable, and the supplies will be pulsing on 
> and off.
> 
> Jon

I've done it with bench power supplies.  (individual adjustments 
for voltage and current.)  I never thought much about it but it's 
worked fine the few times I've needed more current.  I see bitrex
is asking about DC-DC converters, which may behave differently.  

George h. 

Den torsdag den 29. marts 2018 kl. 21.32.33 UTC+2 skrev George Herold:
> On Thursday, March 29, 2018 at 3:04:18 PM UTC-4, Jon Elson wrote:
> > bitrex wrote:
> > 
> > > I have a bunch of small 12 volt input ATX SMPS power modules each rated
> > > for 120 watts. about 14 bux each, surplus. Fanless 200 watt modules are
> > > harder to come by and significantly more expensive.
> > > 
> > > I'd like to slave two together to get ~200 watts out, with the inputs in
> > > series. 24 volt 8 amp power bricks are pretty common.
> > > 
> > > I found this whitepaper on voltage balancing SMPS inputs in series but
> > > it seems more applicable to from-scratch designs.
> > > 
> > Nope, won't work.  Paralleled outputs will likely not current share, one 
> > will put out limit current, the other will pick up the remaining current 
> > demand.  So, with delivered power not being shared equally, the voltage drop 
> > across the supplies will be different, but in series the input current MUST 
> > be the same.  So, the one carrying less output current will have less 
> > voltage drop, and it will increase until the other unit is starved for 
> > voltage.  It will be totally unstable, and the supplies will be pulsing on 
> > and off.
> > 
> > Jon
> 
> I've done it with bench power supplies.  (individual adjustments 
> for voltage and current.)  I never thought much about it but it's 
> worked fine the few times I've needed more current.  I see bitrex
> is asking about DC-DC converters, which may behave differently.  
> 

it is the input side that is the problem

On Thursday, March 29, 2018 at 3:40:44 PM UTC-4, Lasse Langwadt Christensen wrote:
> Den torsdag den 29. marts 2018 kl. 21.32.33 UTC+2 skrev George Herold:
> > On Thursday, March 29, 2018 at 3:04:18 PM UTC-4, Jon Elson wrote:
> > > bitrex wrote:
> > > 
> > > > I have a bunch of small 12 volt input ATX SMPS power modules each rated
> > > > for 120 watts. about 14 bux each, surplus. Fanless 200 watt modules are
> > > > harder to come by and significantly more expensive.
> > > > 
> > > > I'd like to slave two together to get ~200 watts out, with the inputs in
> > > > series. 24 volt 8 amp power bricks are pretty common.
> > > > 
> > > > I found this whitepaper on voltage balancing SMPS inputs in series but
> > > > it seems more applicable to from-scratch designs.
> > > > 
> > > Nope, won't work.  Paralleled outputs will likely not current share, one 
> > > will put out limit current, the other will pick up the remaining current 
> > > demand.  So, with delivered power not being shared equally, the voltage drop 
> > > across the supplies will be different, but in series the input current MUST 
> > > be the same.  So, the one carrying less output current will have less 
> > > voltage drop, and it will increase until the other unit is starved for 
> > > voltage.  It will be totally unstable, and the supplies will be pulsing on 
> > > and off.
> > > 
> > > Jon
> > 
> > I've done it with bench power supplies.  (individual adjustments 
> > for voltage and current.)  I never thought much about it but it's 
> > worked fine the few times I've needed more current.  I see bitrex
> > is asking about DC-DC converters, which may behave differently.  
> > 
> 
> it is the input side that is the problem

Ahh, sorry.  
You know it's good to have a village idiot, 
we help all the other village idiots out there.

So what about if he has a real stiff source. :^)
(apologies again, I got home early and had a beer.)

George H. 

On Thursday, March 29, 2018 at 9:35:26 AM UTC-5, bitrex wrote:

> Yeah they would need the input voltages actively balanced, but even so 
> stacking these in series and taking the output in parallel is gonna be a 
> problem due to the input and output sides sharing a ground.

I thought I was cheap. 

First of all what is wrong with the inputs being in parallel ? Is voltage too high, or the available current too low ? 

Just what voltage and current (compared to the ratings) do you want out of this thingamajig ? 

On 03/30/2018 02:11 AM, jurb6006@gmail.com wrote:
> On Thursday, March 29, 2018 at 9:35:26 AM UTC-5, bitrex wrote:
> 
>> Yeah they would need the input voltages actively balanced, but even so
>> stacking these in series and taking the output in parallel is gonna be a
>> problem due to the input and output sides sharing a ground.
> 
> I thought I was cheap.
> 
> First of all what is wrong with the inputs being in parallel ? Is voltage too high, or the available current too low ?
> 
> Just what voltage and current (compared to the ratings) do you want out of this thingamajig ?

Basically I need something like this:

<http://www.mini-box.com/PW-200M-DC-DC-power-supply>

Compact fanless 200 watt-level ATX power supplies are hard to come by 
domestically they're always either out of stock like above or stupid 
expensive.

I could probably parallel the 120 watt units but then I need a bigger 
external power brick, 24 volt 8 amp power bricks are cheap ($10-15) and 
commonly available, 12 volt 18-20 amp power bricks are like 70 bucks.

I'll probably have to end up ordering what I need from China but it 
takes a month, sucks.

<https://www.ebay.com/itm/Pico-Sale-New-Atx-Personal-Supply-Itx-Pc-Psu-Mini-Car-12v-Supplies-Module-Power-/123005652480>

On 03/30/2018 09:11 AM, bitrex wrote:
> On 03/30/2018 02:11 AM, jurb6006@gmail.com wrote:
>> On Thursday, March 29, 2018 at 9:35:26 AM UTC-5, bitrex wrote:
>>
>>> Yeah they would need the input voltages actively balanced, but even so
>>> stacking these in series and taking the output in parallel is gonna be a
>>> problem due to the input and output sides sharing a ground.
>>
>> I thought I was cheap.
>>
>> First of all what is wrong with the inputs being in parallel ? Is 
>> voltage too high, or the available current too low ?
>>
>> Just what voltage and current (compared to the ratings) do you want 
>> out of this thingamajig ?
> 
> Basically I need something like this:
> 
> <http://www.mini-box.com/PW-200M-DC-DC-power-supply>
> 
> Compact fanless 200 watt-level ATX power supplies are hard to come by 
> domestically they're always either out of stock like above or stupid 
> expensive.
> 
> I could probably parallel the 120 watt units but then I need a bigger 
> external power brick, 24 volt 8 amp power bricks are cheap ($10-15) and 
> commonly available, 12 volt 18-20 amp power bricks are like 70 bucks.
> 
> I'll probably have to end up ordering what I need from China but it 
> takes a month, sucks.
> 
> <https://www.ebay.com/itm/Pico-Sale-New-Atx-Personal-Supply-Itx-Pc-Psu-Mini-Car-12v-Supplies-Module-Power-/123005652480> 
> 

I'm also skeptical as to whether these "250 watt" units from China can 
actually deliver the rated power.

George Herold wrote:

> On Thursday, March 29, 2018 at 3:04:18 PM UTC-4, Jon Elson wrote:
>> bitrex wrote:
>> 
>> > I have a bunch of small 12 volt input ATX SMPS power modules each rated
>> > for 120 watts. about 14 bux each, surplus. Fanless 200 watt modules are
>> > harder to come by and significantly more expensive.
>> > 
>> > I'd like to slave two together to get ~200 watts out, with the inputs
>> > in series. 24 volt 8 amp power bricks are pretty common.
>> > 
>> > I found this whitepaper on voltage balancing SMPS inputs in series but
>> > it seems more applicable to from-scratch designs.
>> > 
>> Nope, won't work.  Paralleled outputs will likely not current share, one
>> will put out limit current, the other will pick up the remaining current
>> demand.  So, with delivered power not being shared equally, the voltage
>> drop across the supplies will be different, but in series the input
>> current MUST
>> be the same.  So, the one carrying less output current will have less
>> voltage drop, and it will increase until the other unit is starved for
>> voltage.  It will be totally unstable, and the supplies will be pulsing
>> on and off.
>> 
>> Jon
> 
> I've done it with bench power supplies.  (individual adjustments
> for voltage and current.)  I never thought much about it but it's
> worked fine the few times I've needed more current.  I see bitrex
> is asking about DC-DC converters, which may behave differently.
> 

Sure, paralleling the OUTPUTS of well-behaved supplies should work fine.
Paralleling the inputs to DC-DC supplies should also work.  The OP wants to 
parallel the outputs and SERIES the inputs!  As far as I can see, that can 
NEVER work in the real world.

Jon

On 03/30/2018 04:13 PM, Jon Elson wrote:
> George Herold wrote:
> 
>> On Thursday, March 29, 2018 at 3:04:18 PM UTC-4, Jon Elson wrote:
>>> bitrex wrote:
>>>
>>>> I have a bunch of small 12 volt input ATX SMPS power modules each rated
>>>> for 120 watts. about 14 bux each, surplus. Fanless 200 watt modules are
>>>> harder to come by and significantly more expensive.
>>>>
>>>> I'd like to slave two together to get ~200 watts out, with the inputs
>>>> in series. 24 volt 8 amp power bricks are pretty common.
>>>>
>>>> I found this whitepaper on voltage balancing SMPS inputs in series but
>>>> it seems more applicable to from-scratch designs.
>>>>
>>> Nope, won't work.  Paralleled outputs will likely not current share, one
>>> will put out limit current, the other will pick up the remaining current
>>> demand.  So, with delivered power not being shared equally, the voltage
>>> drop across the supplies will be different, but in series the input
>>> current MUST
>>> be the same.  So, the one carrying less output current will have less
>>> voltage drop, and it will increase until the other unit is starved for
>>> voltage.  It will be totally unstable, and the supplies will be pulsing
>>> on and off.
>>>
>>> Jon
>>
>> I've done it with bench power supplies.  (individual adjustments
>> for voltage and current.)  I never thought much about it but it's
>> worked fine the few times I've needed more current.  I see bitrex
>> is asking about DC-DC converters, which may behave differently.
>>
> 
> Sure, paralleling the OUTPUTS of well-behaved supplies should work fine.
> Paralleling the inputs to DC-DC supplies should also work.  The OP wants to
> parallel the outputs and SERIES the inputs!  As far as I can see, that can
> NEVER work in the real world.
> 
> Jon
> 

As TW pointed out not if the switchers input and output share a ground. 
If they're an isolated topology don't see why you couldn't "in theory", 
but you'd need some kind of active feedback circuit to keep the input 
rail split evenly.