Need to regulate Current of Power supply 0 to 30A

[shahbaz.ele]

dear all
I have two 48Volts switching mode power supply modules. They outs fixed 48VDC and 15A current.
I want to combine them to make 30A supply.
The problem I am facing is that I want to regulate the current output according to my desire.
can anyone suggest me how to control the output current of the power supply modules.
thanks in advance.

 

[Tunelabguy]

To control the current you would need to control either the voltage or the load resistance. From what you have said, the voltage is fixed and cannot be varied to force a certain amount of current. That leaves only the load resistance. Can you control that?

I'm guessing that you don't really want to regulate the total current. You just want to ensure that the two power supplies contribute current in a way that shares the current equally. Is that so? Because the first way of interpreting your request does not make any sense.

 

[crutschow]

Do you actually want to regulate the current or do you mean regulate the voltage? Those are regulated in different ways.

 

[shell.albert]

i am agree with #2.
combine the two power module,your purpose is want to supply more current.
and now the condition is fixed voltage,changed load resistance,to regulate current,the only way is change the load resistance dymaticly.

 

[shahbaz.ele]

Thanks Tunelabguy
If I say I can change or regulate voltage not the load resistance, then what will be the method to regulate voltage and current.
My problem is that I need 50VDC, 30A max current to supply an amplifier.
I want to save amplifier, so I thought to run it on lower voltage and current first, then increase both slowly and reach its maximum limit.
I have two modules of switching power supplies having 48VDC, 15A.

 

[Tunelabguy]

You cannot control both voltage and current to your amplifier independently. If you can control only the voltage, that should be good enough. The amplifier will probably draw less current when less voltage is used. So if you slowly increase the voltage, the current will take care of itself.

Most power supplies have current limiting. This is built-in to the power supply. This will measure the current and if the current goes above the limit for that power supply, the power supply will automatically reduce the voltage, perhaps to zero, to protect itself or the equipment that it is powering.

That still leaves open the question of controlling the voltage. Some power supplies have adjustable voltage output and some do not. If your power supply does not have adjustable voltage, then you should probably look for a different power supply.

The other problem is combining two power supplies. If the output voltage of one power supply is slightly higher than the other one (which is always going to be true) that power supply will supply most of the current and the other power supply will supply nothing. This is especially a problem if the supplies have very good output voltage regulation. If they have poor output voltage regulation that could actually benefit you, because if you connect the two supplies in parallel, and one supply starts supplying more current than the other, its voltage will drop so that the other supply will start supplying more current. If you can tolerate poor load regulation, that might be the way to combine them. But then you still have the problem of controlling the voltage in both of them the same.

Another way to gradually increase the current and voltage is to combine the power supplies together (hoping that their poor load regulation will force them to share current equally) and then connect a large adjustable rheostat in series with the power connection to the amplifier. The rheostat would have to be rated for about 800 watts and have a resistance range of about 2 Ohms. This will probably be more expensive than your power supplies.

 

[crutschow]

To simulate poor regulation you can add a small resistor in series with each output before you connect them together. For a 30A supply a 3mΩ resistor (about 3" of 16AWG copper wire) or so will help with the sharing of the load between the two supplies. The positive temperature coefficient of the copper wire improves the sharing since, as the current increases, its temperature and thus its resistance, goes up, causing the other supply to share more of the load..

 

[Tunelabguy]

To simulate poor regulation you can add a small resistor in series with each output before you connect them together. For a 30A supply a 3mΩ resistor (about 3" of 16AWG copper wire) or so will help with the sharing of the load between the two supplies. The positive temperature coefficient of the copper wire improves the sharing since, as the current increases, its temperature and thus its resistance, goes up, causing the other supply to share more of the load..

3 mOhms seemed a little small, so I worked up an example. Suppose both power supplies have ideal voltage regulation. One puts out 49 volts and the other puts out 50 volts. Suppose the total current demand is 30 amps. Discounting copper heating for a moment, the 1-volt difference between the power supplies needs to be entirely taken up in the difference in voltage drops across the 3 mOhm resistors. That means the current difference would have to be 1/.003 = 333 amps, which is clearly impossible. So we will have to pick a much smaller voltage imbalance. Let's say one supply puts out 49.99 volts and the other puts out 50.00 volts. The 10 mVolt difference would result in a current-sharing difference of 3.3 amps, which is now doable. So instead of each power supply putting out 15 amps, we have one putting out 16.65 amps and the other putting out 13.35 amps. That is a lot more equitable, however look what it took to get there: an inherent voltage imbalance of 0.01 volts at 50 volts. That is next to impossible to guarantee with any reasonable power supply. So I would say that 3 mOhms is definitely too small to do any good for current sharing. At 15 amps, such a resistor only drops 0.045 volts. This is hardly what I would call a "degraded" load regulation. So let's use your idea of series resistors, but let's make them as big as we can without losing too much load regulation. If the OP can tolerate a 2-volt drop at full current load (15 amps each), than makes the resistor 0.133 Ohms. That's a little harder to do with #16 copper, but it might not be too bad if done with Nichrome wire. They do have to dissipate 30 watts, though. Using such series resistors with each power supply means they could be off by as much as 1/2 volt and that would generate only 3.7 amps of current difference between the power supplies. If you derate the specs to say the combined supply only delivers 26 amps instead of 30, you can still stay within the maximum current specs of each supply.

 

[shahbaz.ele]

can I regulate voltage output of a fixed supply ?
I have 48VDC supply how can I regulate its output voltage by external arrangement of some components.
so that I can increase voltage from say 0VDC to 48VDC or 5VDC to 48VDC.

 

[Tunelabguy]

can I regulate voltage output of a fixed supply ?
I have 48VDC supply how can I regulate its output voltage by external arrangement of some components.
so that I can increase voltage from say 0VDC to 48VDC or 5VDC to 48VDC.

You can do that, but the arrangement of external components would be every bit as complicated and expensive as the power supplies themselves. You would be better off discarding the supplies you have now and buying one adjustable output power supply of the required specs.

 

[dwcookiemonster]

can I regulate voltage output of a fixed supply ?
I have 48VDC supply how can I regulate its output voltage by external arrangement of some components.
so that I can increase voltage from say 0VDC to 48VDC or 5VDC to 48VDC.

Yes you can. you can use dc-to-dc stepdown monolithic power switching regulator in the market (ex. L497x series from ST Microelectronics, it have adjustable 5.1 to 40 VDC output voltage range). just require few external components.

 

[Tunelabguy]

Yes you can. you can use dc-to-dc stepdown monolithic power switching regulator in the market (ex. L497x series from ST Microelectronics, it have adjustable 5.1 to 40 VDC output voltage range). just require few external components.

Not so fast! The OP wants a total of 30 amps. Those chips are rated for 3.5 amps. It won't be so easy.

 

[dwcookiemonster]

Not so fast! The OP wants a total of 30 amps. Those chips are rated for 3.5 amps. It won't be so easy.

You're right, sorry for not seen targeted current. Even L4710 model just can deliver 10Amps peak.