Power
Easy load sharing for two to four supplies
Using multiple small power supplies is often more economical and more reliable than using a single large power supply. For instance, separate batteries can be used for higher reliability. Design time is shortened by combining available small supplies rather than building a large one from scratch. Supply lifetime is extended by spreading the heat across multiple supplies as opposed to concentrating it in a single supply. In such a multi- supply system, it is important that the load is equally shared across supplies. In this article, Vladimir Ostrerov, applications engineer, and Pinkesh Sachdev, product marketing engineer, mixed signal products, Linear Technology Corp., shows how to easily load share up to four supplies by cascading LTC4370 controller circuits
T
he LTC4370 controller enables current sharing between two supplies with up to 0.6V difference between their output voltages. Unlike other sharing methods, the LTC4370 does not require extra connections for a share bus or supply access to trim its voltage. Figure 1 shows two nominally 12V supplies separated by 300mV during operation. To balance the current in both sides, the controller regulates the source-drain forward voltage of an N-channel MOSFET placed in series with each supply. The MOSFET acts as an adjustable ideal diode, blocking reverse current from back-feeding the supply, with the forward voltage drop adjustable from 25mV to 625mV. In the Figure 1 circuit, the MOSFET forward voltage in the lower 11.9V rail is regulated to 25mV, while that
in the higher 12.2V rail is regulated to the supply difference plus 25mV, i.e., 325mV. This equalises the OUT1 and OUT2 voltages to 11.875V. (Note that the output voltage is determined by the lower supply voltage.) Matched sense resistors from OUT1 and OUT2 to the common load force equal current flow in the two branches. For a defined ratio between rail currents, the sense resistors are sized in inverse proportion to the currents such that I1 • RS1 = I2 • RS2. With an unconnected RANGE pin, the
LTC4370 compensates for a voltage difference between two rails of up to 0.6V. If the expected voltage difference is less than 0.6V, the LTC4370 sharing range can be lowered by adding an appropriate resistor on the RANGE pin. When the supply difference exceeds the sharing range, the higher voltage supply's forward drop is clamped to the maximum value, and more of the load current is steered to the higher supply.
Load sharing three supplies Figure 2 shows a 3-input, 12V system delivering 10A. Notice that one LTC4370 (U1) performs equal current sharing between supplies V1 (12.4V) and V2 (12.0V) by employing equal sense
Figure 1. The LTC4370 current-balancing controller enables balanced load sharing between two supplies, even when their voltage outputs are different
24 June 2016 Components in Electronics
resistors (2m in both branches), while the second LTC4370 (U2)
implements a 2:1 relation between the output current of U1 and the current of a third supply, V3 (12.0V), by employing 1:2 ratioed sense resistors (2m and 4m). Thus, each supply contributes one-third of the total load current. The output voltage at the load is less than the minimum of the supply voltages V1, V2 and V3. Because there are two stages of cascading, it is possible to have more than a 1V difference between V3 and V1, or V3 and V2, if the difference between V1 and V2 is already at the 0.6V limit.
Load sharing four supplies Employing three LTC4370 controllers (Figure 3) in two stages allows four supplies to share the load. In the first stage, U1 and U2 force equal sharing between a pair of supplies, where the output current of U1 is I12 = I1 + I2, and the output current of U2 is I34 = I3 + I4. A third LTC4370 (U3), the second stage, keeps I12 = I34. Thus, each supply contributes one-fourth of the total load current. The two stages, as above, allow U1's supply inputs to differ by more than 1V from U2's inputs. However, the V1 and V2 difference must
Figure 2. Two LTC4370s can be cascaded to enable current sharing of three supplies
(maximum): determines OUT1 and OUT2 voltage matching and the sense resistor tolerance: worst-case for one per cent resistors is two per cent overall. Sharing error attributed to the error amplifier input offset decreases with increasing sense resistor voltage drop, but resistor power dissipation increases. For the circuit of Figure 2, where ideal load sharing means the load is distributed into 1⁄3ILOAD and 2⁄3ILOAD, it is easier to estimate the worst-case imbalance via an expression of the maximum and minimum current of each supply.
Figure 3. Four supplies can each support an equal share of a load by using three LTC4370s in a 2-stage cascade
be within the sharing range of U1, while the V3 and V4 difference must be within U2's sharing range.
Limitations The main error sources that affect perfect current sharing are the LTC4370 error amplifier input offset, ±2mV
Conclusion By not requiring a share bus or access to the supply, the LTC4370 provides a simple method to balance multiple supply currents. Cascading the shared output of one LTC4370 with another LTC4370 enables three or more supplies to
provide equal current to the load. While this principle can be extended to more than four supplies, each cascaded stage adds voltage drop of an N-channel MOSFET and sense resistor between the supply and the load.
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