Inrush Current Limiting Circuit Design

When a power supply is connected to the mains, the bulk capacitor is empty and it looks like a short circuit for the first few milliseconds. The resulting inrush current is limited only by the wiring, and it can be tens of times the operating current of the product.

Why the Current Is So Large

The capacitor voltage cannot change instantly, so at the moment of connection the full mains voltage appears across the small resistance of the wiring and the rectifier. The peak current follows from that resistance, and it can reach hundreds of amps in a large supply.

The event is short, and it is repeated every time the product is switched on. The stress falls on the fuse, the connector, the rectifier and the printed tracks, and each has a different tolerance for it.

Repeated switching, such as a thermostat cycling a load or a product that is plugged in frequently, turns a tolerable event into a wear mechanism. The limiting circuit is designed for the number of events the product will see.

Passive Limiting with a Resistor

The simplest limiter is a resistor in series with the input, bridged by a relay or a thyristor contact that closes after the capacitors have charged. The resistor takes the initial current and is then bypassed so that it does not dissipate power in normal operation.

The resistor has to absorb the energy stored in the bulk capacitor, which for a large supply is several joules in a few milliseconds. A wire wound or a ceramic composition type with a high pulse rating is used, and a film resistor is not suitable.

The bypass contact has to close at the right time. Too early and the capacitor is not charged, so the current is only partly limited; too late and the resistor overheats. A timer or a voltage comparison across the capacitor sets the moment.

NTC thermistor Limiting

An NTC thermistor has a high resistance when cold and a low one when hot, which is exactly the characteristic needed. At switch on it limits the current, and the current through it then heats it and reduces its resistance.

The steady state resistance is still a few ohms, which dissipates power continuously and reduces the efficiency slightly. It also means that a supply that is switched off and on quickly has no protection, because the thermistor is still hot.

Placing the thermistor in the position where it sees the largest current gives the best limiting. Its location matters more than its exact value, and the same part in a different circuit position behaves quite differently.

Active Limiting

An active limiter uses a transistor or a thyristor in a controlled ramp, so that the capacitor charges over a defined period and the current never exceeds a set value. The circuit is more complex and it removes the steady state loss of a passive element.

A bypass thyristor triggered by a resistor and capacitor network is a common and robust implementation for a mains input. The thyristor carries the operating current after the ramp, and its forward drop is much smaller than a resistor.

The active circuit also handles the repeated switching case without the recovery time of a thermistor. Where the product may be switched on and off rapidly, that property matters more than the cost.

Sizing the Components

The energy that has to be absorbed is half the capacitance times the square of the final voltage. A four hundred and seventy microfarad capacitor charged to three hundred and twenty five volts stores about twenty five joules, which is a substantial pulse for a resistor.

The peak current without limiting is the mains peak divided by the total series resistance, which includes the wiring, the rectifier and the equivalent series resistance of the capacitor. Estimating that resistance is the first step in deciding how much limiting is needed.

The fuse rating has to survive the limited inrush without opening, and the I squared t of the pulse has to be below the melting value of the fuse. A slow blow fuse is used where the limited inrush is still significant.

soft start and the Bulk Capacitor

A soft start ramp on the switching converter does not limit the inrush into the bulk capacitor, because the capacitor charges through the rectifier before the converter starts. The two functions are separate and both may be needed.

Some designs place the bulk capacitor after an active switch and charge it slowly through a bypass path. That gives control of the inrush and of the converter start, at the cost of an extra switch in the main current path.

Where the input is a direct current supply rather than the mains, the same reasoning applies to the connector and the wiring, and the limiting is often a simple series resistor with a bypass contactor.

Rectifier and Connector Stress

The rectifier sees the inrush as a single high pulse, and its rating is quoted for a surge of a defined waveform. A bridge that is comfortable in normal operation can be damaged by a pulse that exceeds its surge rating even once.

The connector and the printed tracks have their own limits. A track that carries a few amps continuously can be destroyed by a hundred amps lasting a millisecond, because the energy is deposited in a small volume and there is no time for it to spread.

The fuse is often the component that fails first, not because it is weak but because it is designed to. A slow blow type with a suitable melting value survives the limited inrush and still protects the circuit against a genuine fault, which is the balance the design has to strike.

Measurement and Verification

Verify the design by measuring the input current with a current probe and an oscilloscope during switch on. The peak and the duration of the pulse are the two numbers to record, and both have to be compared with the ratings of the fuse, the rectifier and the connector.

Repeat the measurement at the worst case, which is the highest mains voltage with the product cold and the capacitors fully discharged. A test carried out after the product has been running does not show the worst case.

Repeat the switch on many times to check the component temperatures. A limiting resistor that is bypassed correctly stays cool, while one that is not dissipates power continuously and eventually fails. The release checks that keep such a design consistent are collected in our PCB design release checklist, the assembly points in judging PCB quality, and the thermal measures in our guide to PCB thermal management design.

FAQ

Can I use an NTC thermistor for a product that is switched on and off often? Only with care. A hot thermistor has no limiting effect, so repeated switching removes the protection.

How much energy does the limiter absorb? Half the capacitance times the square of the voltage. For a mains supply that is often ten to thirty joules in a few milliseconds.

Does soft start in the converter limit inrush? No. The bulk capacitor charges through the rectifier before the converter runs, so a separate limiter is needed.