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Step-up/step-down current source charges batteries

-June 06, 1996

For battery charging, the highly efficient step-down (buck) configuration is usually the topology of choice. However, a different approach is necessary if the following conditions prevail: The supply voltage is less than the battery voltage, or, even worse, the supply voltage ranges above and below the battery voltage. The charger may need to accommodate one of several voltage sources, according to which is active. The charger may need to charge batteries with different cell counts. The circuit in Figure 1, which can charge one to 15 cells from an input of 4 to 15V, meets all of these requirements.

The topology in Figure 1 is the single-ended, primary-inductance converter, which is notable for its step-up/step-down capability. The controller (IC1) usually regulates an output voltage. However, in this case, the resistive dividers at pin 3 keep the feedback unsatisfied, which causes the system to produce current pulses at a level determined by the IC's current-limit circuitry. To regulate charging current, op amp IC2 adjusts Q1's current limit. IC2 does this by comparing R2's voltage, which is proportional to charging current, with a voltage derived from the reference in IC1. S1 and S2 let you set the level of charging current.

The maximum Q1 current of 4A that R1 sets is within the capability of L1 but allows some saturation and heating. If this peak inductor current is insufficient, IOUT falls gracefully short of the desired maximum value of 1A. If VIN is high and VOUT is low, you can obtain more charging current by changing resistor values at the op amp's inverting input. Otherwise, higher current requires that you set a higher peak current by lowering R1. In that case, L1, L2, C1, and C2 must be larger to withstand the higher currents.

The resistor values connected at pin 3 of IC1 set a maximum output voltage of 28V across the battery to limit the voltage stresses on Q1, C1, C2 and D1. You can extend this voltage by adjusting the resistor values. But, note that Q1 and D1 must withstand slightly more voltage than VIN+VOUT, and the coupling capacitor C1 must withstand VIN. The full charging current flows through C1, so be sure that any substitutes can handle the required voltage and the ripple current. C1 and C2 are nonpolarized ceramic capacitors, but you should heed the polarities the figure shows if you substitute polarized capacitors.

The maximum VIN is about 15V. This value can be higher if you limit the supply voltage applied to IC1's pin 2. Either add a linear regulator for this purpose, or replace the MAX770 with a MAX773, which takes its power from a built-in shunt regulator. Note that any coupling between L1 and L2 assumes the polarities shown by the dots, but circuit operation does not depend on such coupling. (DI #1874)

 

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