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Limit inrush current in low- to medium-power applications

A MOSFET and IGBT guide current to safe places

JB Castro-Miguens, Cesinel, Madrid

When switched-mode power supplies, including those for notebook computers, turn on, the bulk capacitor of the uncontrolled rectifier is completely discharged. This can result in a large charging current for a high instantaneous line voltage because the discharged capacitor temporarily short-circuits the power supply’s diode bridge.

With a large bulk capacitor, the current spike can trigger the mains breaker or even destroy rectifier diodes. Capacitor and line ESRs (equivalent series resistances) and inductances help to reduce the initial spike. Even so, current peak can reach tens of amperes. The rectifier-diode selection must account for this nonrepetitive spike. An initial spike also affects the lifetime of the bulk capacitor. The circuit in Figure 1 lets you avoid the large initial spike.

Figure 1. You can use this circuit to limit inrush current and clamp the output voltage to values lower than 380V.

At turn-on, if the instantaneous rectified ac-line voltage is greater than about 14V, MOSFET Q1 is on, ensuring that IGBT (insulated-gate bipolar transistor) Q2 is off. In this situation, no current flows through charging the bulk capacitor.

Whenever the rectified ac-line voltage is lower than the voltage across the bulk capacitor plus approximately 14V (V1 = VIN − VOUT ≤ 14V), Q1 is off, and Q2 turns on through R3, connecting the capacitor and RLOAD to the rectifier. Q2 remains on thereafter, making Q1 useless.

In the steady state, whenever the rectified input ac voltage matches the voltage across the bulk capacitor, Q1 is off and Q2 is on, making charging of the capacitor possible.

The current-limiting circuit lets you implement straightforward overvoltage protection. When the rectified output voltage is higher than 380V, the reference- to-anode voltage of IC1 is higher than its internal reference of 2.495V, making the anode-to-cathode voltage drop to approximately 2V. In this situation, the cathode sinks the current across R3, turning off Q2.

When the rectified line voltage is lower than 380V, the cathode current of the TL431 is approximately 0A. Thus, Q2 turns on through R3, connecting bulk capacitor C and RLOAD to the full-wave rectifier if V1 = VIN − VOUT ≤ 14V.

All the components have a small power dissipation. The GP10NC60KD transistor, with an input of 230V rms and a load as high as 500W, is suitable for Q2.

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