*Protect circuits from high current until a capacitor charges*

A high-power offline supply is nothing more than a half- or full-bridge dc/dc converter. Rectifying the ac line yields a dc voltage that feeds the converter. At power-supply turn-on, the bulk capacitor of the uncontrolled rectifier is completely discharged. It results in a huge charging current for a high instantaneous line voltage because the discharged bulk capacitor temporarily short-circuits the diodes of the rectifier stage. 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. The high inrush current can trigger a mains circuit breaker, burn a fuse, or even destroy a power supply’s rectifier diodes unless you take precautions. The circuit in Figure 1 limits the inrush current.

Figure 1. |
A thyristor and a MOSFET control current to bulk capacitor C_{O}. This circuit limits the inrush current. |

At turn-on, if the instantaneous rectified ac-line voltage, V_{ACR}, is greater than approximately 10 V, Point A in Figure 2, MOSFET Q_{2} turns on, forcing thyristor Q_{1} off. In this situation, a little current flows through R_{1} and Q_{2}, injecting a small charge into bulk capacitor C_{O}, Path A to B in Figure 2.

Figure 2. |
If V_{ACR} is greater than approximately 10 V, MOSFET Q_{2} turns on; current flows through R_{1}and Q _{2}, injecting a small charge into bulk capacitor C_{O}. |

When V_{ACR} – V_{O} ≤ 8 V or so, where V_{O} is the output voltage, Q_{2} is off, letting Q_{1} conduct. In this situation, the bulk capacitor receives the necessary charge through Q_{1}, Path B to C in Figure 2, to match V_{O} to V_{ACR}. After this point, V_{ACR} falls below V_{O}, and the bulk capacitor alone must support any power the dc/dc converter demands until V_{ACR} – V_{O} ≥ 5 V or so, Path C to D in Figure 2. At Point D, V_{ACR} – VO ≈ 5 V and thyristor Q_{1} triggers, which conducts the capacitor’s charge current and the current the dc/dc converter demands until V_{ACR} matches the sinusoidal peak at Point E.

When V_{ACR} falls, thyristor Q_{1} cuts off, and the bulk capacitor alone feeds the dc/dc converter. The thyristor conducts again when V_{ACR} matches V_{O} to the sinusoidal peak. This process then repeats. Use a nonsensitive gate thyristor with a breakdown voltage of at least 400 V for an ac voltage of 220 V rms (root mean square) and with twice the rms-current rating of the rectifier diodes.

This circuit uses a TYN610 thyristor. You can calculate the value of R_{1} using

where V_{GT} is the minimum gate-cathode voltage necessary to produce the gate-trigger current for Q_{1} and I_{GT–20°} is the minimum gate current to trigger Q_{1} down to –20 °C. The NTD4815NHG MOSFET is suitable for this circuit. A MOSFET with a different threshold voltage may require different values for R_{2} and R_{3}.