Inrush Current Limiters In Switching Power

Supplies

The problem of current surges in switch-mode power supplies

is caused by the large filter capacitors used to smooth the

ripple in the rectified 60 Hz current prior to being chopped

at a high frequency. The diagram above illustrates a circuit

commonly used in switching power supplies.

In the circuit above the maximum current at turn-on is the

peak line voltage divided by the value of R; for 120 V, it is

be very large, and after the supply is operating, should be

reduced to zero. The NTC thermistor is ideally suited for this

application. It limits surge current by functioning as a power

resistor which drops from a high cold resistance to a low

hot resistance when heated by the current flowing through

it. Some of the factors to consider when designing NTC

thermistor as an inrush current limiter are:

• Maximum permissible surge current at turn-on

• Matching the thermistor to the size of the filter

capacitors

• Maximum value of steady state current

• Maximum ambient temperature

• Expected life of the power supply

Maximum Surge Current

The main purpose of limiting inrush current is to prevent

components in series with the input to the DC/DC convertor

from being damaged. Typically, inrush protection prevents

nuisance blowing of fuses or breakers as well as welding of

switch contacts. Since most thermistor materials are very

nearly ohmic at any given temperature, the minimum no-load

resistance of the thermistor is calculated by dividing the peak

input voltage by the maximum permissible surge current in

Energy Surge at Turn-On

At the moment the circuit is energized, the filter caps in a

switcher appear like a short circuit which, in a relatively

short period of time, will store an amount of energy equal

must flow through the thermistor. The net effect of this large

current surge is to increase the temperature of the thermistor

very rapidly during the period the capacitors are charging.

The amount of energy generated in the thermistor during

this capacitor-charging period is dependent on the voltage

waveform of the source charging the capacitors. However,

a good approximation for the energy generated by the

filter capacitor). The ability of the NTC thermistor to handle

this energy surge is largely a function of the mass of the

device. This logic can be seen in the energy balance equation

for a thermistor being self-heated:

~

Typical Power Supply Circuit

Input Energy = Energy Stored + Energy Dissipated

or in differential form:

where:

P = Power generated in the NTC

t = Time

H = Heat capacity of the thermistor

T = Temperature of the thermistor body

δ = Dissipation constant

During the short time that the capacitors are charging

(usually less than 0.1 second), very little energy is

dissipated. Most of the input energy is stored as heat in

the thermistor body. In the table of standard inrush

limiters there is listed a recommended value of maximum

capacitance at 120 V and 240 V. This rating is not

intended to define the absolute capabilities of the

thermistors; instead, it is an experimentally determined

value beyond which there may be some reduction in the

life of the inrush current limiter.

Maximum Steady-State Current

The maximum steady-state current rating of a thermistor

is mainly determined by the acceptable life of the final

products for which the thermistor becomes a

component. In the steady-state condition, the energy

balance in the differential equation already given reduces

to the following heat balance formula:

As more current flows through the device, its

steady-state operating temperature will increase and its

resistance will decrease. The maximum current rating

correlates to a maximum allowable temperature.

In the table of standard inrush current limiters is a list of

values for resistance under load for each unit, as well as

a recommended maximum steady-state current. These

ratings are based upon standard PC board heat sinking,

with no air flow, at an ambient temperature of 77° (25°C).

However, most power supplies have some air flow, which

further enhances the safety margin that is already built

into the maximum current rating. To derate the

maximum steady state current for operation at elevated

ambient temperatures, use the following equation: