What are the practical considerations to be made if I have to connect two or more DC power sources?

Question

I need to connect two (or upto six at most) DC power sources (step down transformer+bridge rectifier) in parallel to increase the output current capacity to the load. The transformers will be connected to 230VAC line input, output being 15VAC @2Amp (each). I want each transformer's secondary to have it's own bridge, so that there is no parasitic load on one transformer's secondary winding due to slightly unequal impedances with the other transformers. Now, if I connect the output from each of the bridge rectifiers in parallel, I should theoritically be able to get about Nx2Amp in the output, N being the number of supplies I connect in parallel.
In case (and very likely in practice), there is this slight mismatch in the output rating of each transformer's secondary/bridge rectifiers, how is it going to affect the overall output voltage and current? What sort of voltage and current I can expect at the output of the supply? Will the bridges be at risk?

Answer 1

Small transformers generally have fairly poor load regulation - the output voltage drops significantly between no load and full load. Assuming your power supplies are identical, the poor load regulation is an advantage because if one transformer hogs the load, its voltage will drop to bring it more in line with the others. You can get an idea of how well this will work by measuring the voltage vs. current characteristic of each of your power supplies over the full current range and then modeling each power supply as a resistance in series with a fixed voltage source. From that, you can calculate the current drawn from each when they are all in parallel driving a single load resistor.



For example, suppose you have 3 power supplies with the following characteristics:

1) No load voltage = 16 V, Voltage driving a 7.5 Ω resistor = 14 V (that's ~ 30 Watts, so be careful!)

2) No load voltage = 17 V, Voltage driving a 7.5 Ω resistor = 14.5 V

3) No load voltage = 15 V, Voltage driving a 7.5 Ω resistor = 13 V



You would model each as:

1) 16 V source, Rseries = (Rload)(Vdrop) / (Vout) = (7.5)2/14 = 15/14 = 1.07 Ω

2) 17 V source, Rseries = (7.5)2.5/14.5 = 1.29 Ω

3) 15 V source, Rseries = (7.5)2/13 = 1.15 Ω



Wired in parallel and driving a 2.5 Ω load, for a total current Itotal = I1 + I2 + I3 we get three equations:

(1) 16 V = (1.07 Ω)I1 + (2.5 Ω)(I1 + I2 + I3)

(2) 17 V = (1.29 Ω)I2 + (2.5 Ω)(I1 + I2 + I3)

(3) 15 V = (1.15 Ω)I3 + (2.5 Ω)(I1 + I2 + I3)



Converting to matrix form:

--I1---I2---I3--

3.57 2.5 2.5 | 16

2.5 3.79 2.5 | 17

2.5 2.5 3.65 | 15



Using an online equation solver, such as:

http://www.bluebit.gr/matrix-calculator/linear_equations.aspx

we solve the 3 simultaneous equations (6 in your case), and get:

I1 = 2.037

I2 = 2.465

I3 = 1.026



In this example, power supply 3 is not doing its fair share, so you could balance things by adding equalizing resistors to the other two power supplies

Answer 2

If there is a mismatch, the current will not be evenly shared between the supplies, for example, if you have 3 connected and the load is 3 amps, the currents could be 1.2, 0.9, 0.9, or any other set of numbers.



You will get the voltage 15 volts, higher of course at lower currents, lower at higher currents. You don't mention any filters, so there will be a lot of ripple.



You should not have to worry till you get close to 2 amps out of each, then the imbalance may push one supply over 2 amps.



But supplies have a lot of leeway. The rectifiers are rated (presumably) for more than 1 amp each, as that would be too marginal. The transformers usually have lots of excess capacity, they just get hotter with small overloads, and the voltage goes down, which lessens the unbalance.



The fact that the voltage goes down on higher currents will tend to keep the currents balanced.



I'd be sure to use lots of fuses in each leg, both in the DC side (2.5 amp fuse) and the primary 0.5 amp.



Bottom line, if the rectifiers have some excess capacity, and the transformers are nominally identical, you will be fine.

Answer 3

yes if the voltages are not equal then there will be a circulating current between the transformers connected which means that your transformers and the bridge system will still be drawing current from the mains even you have not connected the load to the n parallel transformers system. and accidentally

if you have given the wrong transformers connections it may lead to direct short circuit and any faults in the any bridge can cause hell lot of problem .

i suggest you to get a higher rated transformers of 230V which in single can provide the rated current for you. so you can be more bothered in the main part of your project rather than transformers system and its connection