connectin 2 switching power supplies, possible ?

[max_imum2000]

hello
i have 3 switching power supplies on hand.

all have 110/220 VAC input and

24V/14.6A output
the other 2 are 48V/7.5A output

now i want to have about 90V output
can i just connect 2 48v/7.5A togther in parrallel or maybe serial. what will be the output amperage ?
also can i connect 1 48V/7.5A and 1 24V/14.6A togther to produce 72V output and what will be the output amperage?

how can i connect them togther safly ?

will be any drawbacks ?

thank you very much

 

[Russlk]

You can connect the outputs in series for more voltage. The current will be limited to the lowest rated supply. Depending on the overload characteristic of the supplies, connecting them in parallel may or may not work. The output voltages are not exactly equal, so one will supply all the current up the max rating. If the supply is designed to fold back or turn off when overloaded, the other supply will take up the load, but it will be overloaded also.

 

[Sceadwian]

At the very least you're going to want to put diodes on the + leads in order to keep current from flowing between the two power supplies.

 

[max_imum2000]

how about connecting a diode between the 2 leads of the output ?

 

[dknguyen]

Wouldn't the diodes produce a voltage drop across them defeating the purpose of the regulation? But without them, the switching from one output might make it into the other converter and cause funny things to happen (I think).

Do the datasheets say anything about paralleling converters? Some converters at least can be paralleled just fine without any synching.

 

[Cabwood]

Before connecting them in series, make sure that at least one of them has a floating ground. When connecting them in parallel make sure there's plenty of distance between you and the supplies when you switch them on.

 

[Russlk]

Texas Instruments makes an IC designed to facillitate current sharing between two or more paralled switching power supplies. I don't remember the number, you will have to look it up.

 

[max_imum2000]

thanks all
what do ou mean by floating ground ?

 

[Russlk]

He means not connected to neutral (earth ground), free to be connected to a higher potential. Most AC input supplies include a transformer to isolate the output from the input.

 

[dknguyen]

Floating ground (or isolated converter) means the ground input and ground output don't have to be the same (unless you connect them together of course). It means that one converter gets to pick ground and the other converter's ground will follow that, rather than both converters fighting each other to use their ground.

 

[max_imum2000]

i gut the text below from faq about switching power supply (PSP) website
but i dont understnad what do they mean by "current limiting protection"
is it a diode, if so how can i connect it ??

thanks

" If you connect the S.P.S. to motors, light bulbs, or high capacitive loads, you will have a high output surge current when you turn on the S.P.S. and this high surge current will cause failure of start up. We suggest using S.P.S. with constant current limiting protection to deal with these loads. "

 

[dknguyen]

You can kind of think a capacitor (or capacitve load) as being a small resistor for fast changing currents, and a large resistor for slow changing currents. This means that when a capacitive load (a giant capacitor) first starts up (the voltage goes from zero to something big very fast and as a result, the voltage is changing very fast), it makes a lot of current flow through the capacitor. If the SMPS cannot supply this surging start up current, it will fail.

A capacitor will allow the high frequencies of an electrical signal to pass through it, and will block lower frequency signals. Put in another way, a capacitor has low impedence for high frequencies, and has high impedence for low frequencies. The larger the capacitor, the lower overall impedence it will have across all frequencies. So a large capacitor will pull a very large surge current when the voltage changes across it's terminals.

The resistor prevents increases the overall impedence of the circuit and limits current flowing through at all times (including that initial surge current). As a result, it will go in series with the load. It will cause some losses during operation but that's better than not starting up at all or blowing up the power supply. It has to be as small as possible to reduce losses, but just large enough to prevent the SMPS from failing on startup.

For your interest, an inductor (or inductive load) is the complete opposite. A capacitor makes large current spikes for fast changing voltages when starting up. An inductor will make large voltage spikes for fast changing currents when powering down. Inductors have higher impedence for high frequencies, and lower impedence for high frequencies - opposite of a capacitor. Large inductors have lower overal impedence across frequencies. Also, for comparison, capacitors force voltage changes to be gradual while inductors force current changes to be gradual.