NTC Thermistor Inrush Limiter Design

An NTC thermistor used as an inrush limiter is a resistor whose value falls as it heats. It is one of the simplest ways to limit the current into a bulk capacitor, and it is also one of the most frequently misapplied components in a power supply.

The Ratio That Does the Work

The device is specified by two resistances: the value at twenty five degrees and the value in the hot state at a stated current. The ratio between them is the parameter that matters, and it is often ten or twenty to one.

At switch on the thermistor is cold and its resistance limits the current to a value the rectifier, the fuse and the connector can accept. The current then heats the element and its resistance falls to a small fraction of the cold value.

That self heating is the mechanism, and it is also the weakness. The limiting only works when the device is cold, and a supply that is switched off and straight back on has no protection at all.

Choosing the cold Resistance

The cold resistance sets the peak current. A supply with a four hundred volt peak and a thermistor of ten ohms limits the initial current to forty amps, which is still substantial and may be more than the rectifier can take.

A larger cold resistance limits the current further and dissipates more power in the steady state, which is the trade. The device is usually selected so that the limited inrush is within the surge rating of the rectifier and the melting value of the fuse.

The energy stored in the bulk capacitor is absorbed by the thermistor during the charge, and the device has to be rated for that energy as well as for the steady state current. The datasheet gives a maximum capacitance for a given device.

steady state current and Self Heating

In normal operation the current through the thermistor keeps it hot, and the resistance settles at a value that depends on that current. A device used at a current below its rating runs cooler and has a higher resistance.

steady state current below the rated value means a higher resistance in normal operation, which means a larger voltage drop and more power loss. Selecting the device from the rated current rather than from the actual one is the practical approach.

The power dissipated in the steady state is the square of the current times the hot resistance. At five amps through a thermistor with a hot resistance of a tenth of an ohm that is two and a half watts, which is a continuous loss in a device that is already hot.

recovery time and Repeated Switching

The device cools with a time constant of tens of seconds in still air. A supply that is switched off and on within that time is unprotected, and the second inrush is limited only by the residual resistance.

For a product that is cycled frequently, a bypass relay or a thyristor that shorts the thermistor after the capacitors have charged removes both the loss and the recovery problem. The thermistor then only sees the first charge.

The bypass contact has to close after the capacitor has charged and before the thermistor has cooled. Its control comes from a voltage comparison across the bulk capacitor or from a timer, and the timing is easy to get wrong by a factor of two.

Placement in the Circuit

The thermistor belongs in the position where it sees the full inrush current, which is usually in series with the mains input before the rectifier. Placed after the rectifier, it carries the same current and sees a direct current that heats it differently.

Placing it in the return conductor is electrically equivalent and mechanically convenient in some designs. What matters is that the current path is the one that charges the bulk capacitor.

The device gets hot in normal operation and its surface temperature can reach two hundred degrees. It must be mounted away from other components, away from wiring insulation and with clearance from anything that can be affected by heat.

Comparison with Other Methods

A fixed resistor with a bypass contact has no steady state loss and no recovery time, at the cost of the contact and its control. It is the better choice where the product is cycled often or the efficiency matters.

An active limiter with a transistor or thyristor gives a controlled ramp and no recovery time. It costs more components and it is used where the inrush has to be tightly controlled.

The thermistor is the cheapest and the simplest solution, and it is adequate for a product that is switched on occasionally and where a small continuous loss is acceptable. That describes a large part of the market.

Thermal and Safety Considerations

The thermistor is a hot component in the sense that it is designed to run at a high surface temperature. A product that has to meet a touch temperature requirement has to keep it away from accessible surfaces.

The device also fails, and its failure mode is usually an open circuit. A product whose supply depends on the thermistor simply stops working, which is safe but requires a service call rather than a fuse replacement.

A shorted thermistor removes the limiting and exposes the rectifier to the full inrush. The failure is less common and it is worth considering in a product where the inrush is close to the rating of the following components.

Energy Rating and Capacitance

The datasheet usually states the maximum capacitance the device can charge, together with the mains voltage that figure assumes. Exceeding it deposits more energy in the element during the charge than the element can absorb without damage.

That energy is half the capacitance times the square of the voltage, and comparing it with the pulse rating of the device is a two minute calculation that prevents most of the field failures seen with this component.

Verification and Faults

Verify the limiter by measuring the input current during switch on with the product cold, and again straight after a switch off. The second measurement is the one that shows the loss of protection.

Measure the case temperature of the thermistor after an hour of operation. A device running hotter than its specification is either oversized for the current or badly placed, and its resistance will be lower than expected.

A product that fails its inrush test in production after a repair usually has a thermistor that was still hot when the test was repeated. 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

Why does the protection fail when I switch off and on quickly? The thermistor is still hot. Its resistance is low and there is no limiting until it cools.

Can I bypass the thermistor? Yes, with a relay or a thyristor that closes after the bulk capacitor has charged. That removes both the loss and the recovery problem.

How hot does an NTC limiter run? Its surface can exceed two hundred degrees in normal operation, so it needs clearance from wiring and from anything heat sensitive.