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Simple inrush-current limiter mitigates surges

19 May 2016  | Gregory Mirsky

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A 12V electromechanical relay, such as Omron's G2RL-1, provides low-resistance contacts to bypass R1 (Reference 1). As an alternative, an optically isolated solid-state relay, such as the Carlo Gavazzi RP1A48D5, with a MOSFET or an SCR (silicon-controlled-rectifier) output device can replace S1, provided that the voltage drop across the output device introduces no substantial power loss (Reference 2).

Figure 3: The upper trace shows the current as measured with a current probe. The lower trace depicts the voltage drop that the input current across resistor R1 produces.

Figure 2 depicts the charging process's waveform as the voltage drop across R1. The exponential envelope and its subcycles represent components of the inrush process; R3 and C2 filter out the subcycles and produce a decreasing exponential voltage waveform across R4, holding Q2 on for the duration of the inrush process. Q2 suppresses the auxiliary power supply's operation by pulling its disable input low. At a few volts across R1, Q1 stops generating constant current and shuts down Q2 to enable the auxiliary power supply. Thus, the entire power supply waits until the inrush current attains a safe value that R2 sets. The power supply starts immediately after relay S1 trips and shorts out inrush resistor R1. The remainder of figure 1 comprises a conventional PFC but may also represent a part of any other power-supply configuration.

Figure 4: The input-current waveform shows the effects of the inrush-current limiter with the power supply driving a 2400W load.

Trace 1 in figure 3 depicts the start-up of a 2.4-kW power supply with the inrush-current limiter and a slow-start circuit, which allows the separation of the inrush and the start-up processes. The inrush-current value is 5A, a relatively low value for a 2.4-kW power level. Trace 4 shows the input current measured with a current probe. Figure 4 depicts a 2400W power-supply start-up. Its inrush current is intentionally approximately 5A, which is far less than its operating current of approximately 14A.

1. "PCB Relay, G2RL," Omron Corp.

2. "Solid State Relays PCB, 1-Phase ZS/IO Types RP1A, RP1B," Carlo Gavazzi.

About the author
Gregory Mirsky is with Juno Lighting Group, ModuLight Division.

This article is a Design Idea selected for re-publication by the editors. It was first published on August 2, 2007 in

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