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A Simple Inrush Current Limit Circuit With Minimal Power Dissipation

IP.com Disclosure Number: IPCOM000014101D
Original Publication Date: 2000-Dec-01
Included in the Prior Art Database: 2003-Jun-19

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Most of AC-DC switching regulators use NTC devices to limit inrush current at turn on of the switcher. The NTC resistor value is approximately 10 ohms when it is cold and drops below 1 ohm during normal operation of the switcher. This NTC device R1 is connected in series with the line as shown in figure 1. In figure1, V2 is an AC source, D2 is a full wave rectifier bridge, C2 is a high frequency decoupling capacitor, L1 is the Boost inductor, D3 is the bypass diode, Q1 is the switching transistor, D1 is the Boost diode, C1 is the Boost filter capacitor, U1 is the error amplifier. The pulse width modulator regulates the output voltage (VBOOST) by varying the duty ratio as a function of the input voltage and the output load current. At turn on time, the input current path will be through D2, R1, D3 and C1. The current is limited by the cold resistance of R1. The power loss in R1 during normal operation is given by; Ploss (Pout/n)/Vin}**2 x R1(hot). 1 D3 1