Paralleling DC-DC Converter Outputs

DC-DC converters provide efficient low cost power conversion with good input to output isolation, enabling designers to produce modular systems with power distributed where needed. Reliability 15 Watt or 25 Watt converters can be connected in parallel to increase output power. Different interconnections exist, each having advantages and disadvantages.

All of the 25 Watt converters can be adjusted by about 0.5V around their nominal output. When overloaded, current limiting begins about 50% past the rated current.

The 5 volt output 15 Watt converter can be adjusted from about +5.3V to less than +4.5V. When overloaded, its output voltage begins decreasing at about 4.5A. After 6.5A, the short circuit protection folds back the output to 0.2V and 1.4A.

Direct Paralleling

Up to three units of the same type can be connected directly in parallel as in Figure 1, but their output voltages must be trimmed to be equal. This method retains converter efficiency, input to output isolation and can have <20% unbalance in current sharing. Long-term current balance between units is not guaranteed.

Differences in current between units can cause unequal heating and output voltage changes. The highest output voltage will supply more of the current to the common load and this unit can become current limited while another converter is lightly loaded.

Equalizing Currents

The very best way to produce equal loading in paralleled power converters is to compare the output currents with an operational amplifier which adjusts the converter voltages to be equal.

This can be done in two ways: current sampling resistors can be put in the converter's positive output lead or in the negative output lead.

If the current sampling resistors are in the positive output lead of the converter, both op amp inputs will be at +5V with a 5V converter. The op amp cannot be powered from the converter output because both TLC271 inputs can only be at +3.5V or lower when power is +5V. The op amp power must be at least +1.5V above the converter output, or > +6.5V.

Two ways of supplying this higher voltage are: use a small converter of some type which adds cost and complexity, or power the op amp from the converter input voltage if it is high enough.

Input to output isolation is sacrificed if the converter input voltage is used, because the input and output grounds must be connected. If the converter input voltage is greater than the op amp rating you must limit op amp voltage with a resistor and zener diode.

Input to output isolation can be maintained while using only the converter output voltage for op amp power if the current sampling resistors are in the ground return of each converter. The small valued current sampling resistors can be made from runs on a printed circuit board. See references 1 and 2. An op amp like TLC271 is preferred in this connection because:

1. It operates from 3V to 16V supply voltage and can run on the converter output,

2. Output voltage can equal the negative supply voltage,

3. Input common mode voltage includes the negative supply voltage, and

4. It uses low operating current.

Figure 2 shows two 15 Watt converters paralleled this way for 30 watts, or 6 amps at 5V.

TLC271 op amp inputs can operate to .3V below the negative supply. In Figure 2, the .01W resistors bias the op amp inputs slightly above the op amp negative supply voltage to reduce any voltage swing below the negative supply. The TLC271 operates with high open loop gain and adjusts the 15 Watt trim to force points A and B to be equal in voltage: Va = Vb and Vab = Va - Vb = 0. The converter currents flowing through the .03W resistors produce the voltages at A and B (Vcd and Vfe): Vab = Vcd + Vfe = 0, so Vcd = -Vfe. The resistors should be matched for optimum current sharing.

Figure 3 shows two 25 Watt converters paralleled for 50 Watts, or 10 Amps at 5V. The 25 Watt unit has sense lines to monitor output voltage at the load.

Paralleling Non-Adjustable Converters

Non-adjustable converters can be paralleled with limited success by placing a small resistor between each converter output and the common load. This has lower output voltage, less efficiency and unequal current sharing, but retains input to output isolation. Output voltage and current sharing will vary with load changes.

If this method must be used, it is best to use converters with matched output voltages. Close matching gives better current sharing and allows smaller resistors with less loss.

For example, two 81% efficient 3 Amp converters having 5.1V and 4.9V outputs can be paralleled with 0.1W resistors between them and a 1ê load. The 5.10V unit will supply 3.38A and the 4.9V unit will supply 1.38A to the 1W load at 4.76 output voltage. The converters produce 24 Watts output, but only 22.7 Watts is delivered to the load with a 5.6% loss in efficiency and a 75.4% overall efficiency. Current is shared, but not well, and load voltage is low.

If the voltages were 5.05V and 4.95V, with 0.1W resistors the currents would be 2.88A and 1.88A with 4.76V out and 5.6% loss. For .05W resistors, the currents would be 3.44A and 1.44A with 4.88V at the load and 2.8% loss for 78.2% overall efficiency.

If power requirements and system constraints make it necessary to parallel converters, it is best to use adjustable units and use the op amp circuit to equalize current between units.

1. International Telephone and Telegraph Corp., "Reference Data for Radio Engineers," p. 5-35, Howard W. Sams & Co., Inc., Indianapolis, Indiana: 1981.

2. F. Sears and M. Zemansky, "College Physics", pp. 541-548, Addison-Wessley, Reading, Ma.; 1960.

APP Paralleling Converters.doc August 16, 2001