NTC Inrush Current Limiter Thermometrics Thermistors Features UL Approval (UL 1434 File E82830) Small physical size offers design-in benefits over larger passive components Low cost, solid state device for inrush current suppression Best-in-class capacitance ratings Low steady resistance and accompanying Applications power loss Control of the inrush current in switching power Excellent mechanical strength supplies, fluorescent lamp, inverters, motors, etc. Wide operating temperature range: -58F to 347F (-50C to 175C) Suitable for PCB mounting Available with kinked or straight leads and tape and reel to EIS RS-468A for automatic insertion Amphenol Advanced SensorsDC/DC Converter 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. Typical Power Supply Circuit In the circuit above the maximum current at turn-on is the Input Energy = Energy Stored + Energy Dissipated peak line voltage divided by the value of R for 120 V, it is or in differential form: approximately 120 x 2/R . Ideally, during turn-on R should I I Pdt = HdT + (T T )dt be very large, and after the supply is operating, should be A where: reduced to zero. The NTC thermistor is ideally suited for this P = Power generated in the NTC application. It limits surge current by functioning as a power t = Time resistor which drops from a high cold resistance to a low H = Heat capacity of the thermistor hot resistance when heated by the current flowing through T = Temperature of the thermistor body it. Some of the factors to consider when designing NTC = Dissipation constant thermistor as an inrush current limiter are: T = Ambient temperature A Maximum permissible surge current at turn-on Matching the thermistor to the size of the filter During the short time that the capacitors are charging capacitors (usually less than 0.1 second), very little energy is Maximum value of steady state current dissipated. Most of the input energy is stored as heat in Maximum ambient temperature the thermistor body. In the table of standard inrush Expected life of the power supply 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 Maximum Surge Current thermistors instead, it is an experimentally determined The main purpose of limiting inrush current is to prevent value beyond which there may be some reduction in the components in series with the input to the DC/DC convertor life of the inrush current limiter. 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 Maximum Steady-State Current nearly ohmic at any given temperature, the minimum no-load The maximum steady-state current rating of a thermistor resistance of the thermistor is calculated by dividing the peak is mainly determined by the acceptable life of the final input voltage by the maximum permissible surge current in products for which the thermistor becomes a the power supply (V ). component. In the steady-state condition, the energy peak/Imax surge balance in the differential equation already given reduces to the following heat balance formula: Energy Surge at Turn-On At the moment the circuit is energized, the filter caps in a 2 Power = I R = (T T ) A switcher appear like a short circuit which, in a relatively short period of time, will store an amount of energy equal 2 As more current flows through the device, its to 1/2CV . All of the charge that the filter capacitors store steady-state operating temperature will increase and its must flow through the thermistor. The net effect of this large resistance will decrease. The maximum current rating current surge is to increase the temperature of the thermistor correlates to a maximum allowable temperature. very rapidly during the period the capacitors are charging. In the table of standard inrush current limiters is a list of The amount of energy generated in the thermistor during values for resistance under load for each unit, as well as this capacitor-charging period is dependent on the voltage a recommended maximum steady-state current. These waveform of the source charging the capacitors. However, ratings are based upon standard PC board heat sinking, a good approximation for the energy generated by the 2 with no air flow, at an ambient temperature of 77 (25C). thermistor during this period is 1/2CV (energy stored in the However, most power supplies have some air flow, which filter capacitor). The ability of the NTC thermistor to handle further enhances the safety margin that is already built this energy surge is largely a function of the mass of the into the maximum current rating. To derate the device. This logic can be seen in the energy balance equation maximum steady state current for operation at elevated for a thermistor being self-heated: ambient temperatures, use the following equation: I = (1.14250.0057 x T ) x I 77F (25C) derated A max