EDN Access--09.01.97 IC and capacitor improve isolated supply

September 1, 1997

IC and capacitor improve isolated supply

Kurk Mathews, Linear Technology Corp, Milpitas, CA

Many isolated power supplies typically use an optocoupler and a reference to provide feedback to a controller to maintain a constant output voltage. Alternatively, you can use an isolated flyback regulator, IC₁, to eliminate the optocoupler and provide a ±15V power supply with 1500 V ac of isolation (Figure 1). Line and load regulation is better than ±2%. IC₁ accomplishes this regulation by averaging the reflected output voltage on the transformer's primary during a portion of the converter's off-time. R₁ introduces a correction factor proportional to the average switch current to compensate for errors due to the transformer's parasitics, the output diode's voltage drop, and the output capacitor's ESR.

Cross-regulation issues arise when one output is lightly loaded and another is heavily loaded. The method of feedback (optocoupler or isolated flyback) has little effect on the main source of the problem, which is secondary leakage inductance. When the main switch turns off, current divides between the two outputs in proportion to the leakage inductance associated with each se condary. The two outputs track each other as long as the load currents follow this proportion. Unfortunately, when the load currents differ, the output voltages diverge to redistribute current to the appropriate output. For example, when C₁ is absent, the secondary currents with 15V/70-mA and 15V/0A loads are different (Figure 2, traces C and D).

However, under the same load conditions with C₁ included, the secondary currents are almost identical (Figure 2, traces A and B). Half of the difference in load currents (35 mA, including 3 kilo-ohm preloads) flows through C₁ and C₂ (the 15V output capacitor). The extra current in C₂ then circulates through both windings and C₁ during the main switch on-time. This current accounts for the dc offset in traces A and B. As a result, only a 230-mV difference between the outputs exists because of diode drops and secondary resistances. C₁ cancels the effects of leakage inductance on cross-regulation. (DI #2071)

Figure 1

IC₁'s isolated flyback regulator eliminates the need for an optocoupler and provides 1500V ac of isolation. Adding C₁ cancels leakage-inductance effects.

Figure 2

17D20712

Without C₁ in Figure 1, when the 15V output has a 70-mA load and the 15V output has no load (0A), the secondary load currents (traces C and D) are different. Under these same load conditions with C₁ in place, the load currents (traces A and B) are almost identical except for some dc offset.

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