I. Introduction
A large capacitor exists at the input of a switching power supply. Due to the characteristic that the voltage across a capacitor cannot change abruptly, the capacitor is equivalent to a short circuit when the device is switched on. This results in a large inrush current in the switching power supply input circuit at the moment of power-on. When the input inrush current is too large, it may trigger the overcurrent protection of the upstream power supply equipment or the tripping protection of upstream air switches, circuit breakers, etc. Therefore, designing a suitable input inrush current suppression circuit can effectively prevent the upstream equipment from triggering protection and stopping operation at the moment of power-on, thereby improving the reliability of the system.
Keywords: input inrush current, power supply, reliability, circuit comparison
II. Common Methods
Introduction to common input inrush current suppression principles:
Generally, when the equipment is turned on, a certain impedance R1 is connected in the input circuit to suppress the current-limiting input inrush current. The maximum suppressed current value is Is≈Vin/R1 (Vin is the voltage at the input terminal), which makes it lower than the overcurrent protection value of the front-end power supply equipment or the operation protection value of the front-end air switch, circuit breaker, etc., to avoid triggering the protection and affecting the reliability of the system. After the equipment is turned on, the input capacitor voltage of the switching power supply reaches Vin. At this time, the impedance R1 in the input circuit is removed or reduced to improve the efficiency of the system.
The following introduces several common input inrush current suppression circuit schemes and analyzes their advantages and disadvantages:
(1) Option 1: Insert a thermistor in series
This solution involves inserting a negative temperature coefficient thermistor in series in the input circuit. This method requires fewer components, is simple in principle, and is low in cost.
Working principle analysis:
Before the equipment is turned on, the thermistor has no current flowing through it, its temperature is low, and its resistance is high. At the moment the equipment is turned on, it is equivalent to a large impedance being introduced into the input circuit, which can effectively suppress the peak value of the input inrush current. After the equipment is turned on, the thermistor generates heat as current flows through it, and its resistance decreases, reducing losses during normal operation.
Advantages and disadvantages analysis:
(2) Option 2: Connect cement resistors in series and then in parallel
This solution is a modification of the circuit in Solution 1, which involves connecting a cement resistor in parallel with the negative temperature coefficient thermistor to improve the problem of poor low-temperature start-up.
Working principle analysis:
At low temperatures, the thermistor NTC is connected in parallel with the cement resistor R1 to prevent the thermistor NTC from having an excessively high resistance at low temperatures, which would cause power supply current limiting and poor startup. At high temperatures, the resistance of the thermistor decreases, reducing losses during normal operation.
Advantages and disadvantages analysis:
(3) Option 3: Insert MOSFET
This solution involves inserting a MOSFET in series in the input circuit (requiring an external control circuit) and using the variable resistance region of the MOSFET to suppress current, making it suitable for suppressing low-power input inrush current.
Working principle analysis:
When the equipment is turned on, the MOSFET in the input circuit is controlled by the control circuit to enter the variable resistance region, and the variable resistance effectively suppresses the peak value of the input inrush current; after the equipment is turned on, the MOSFET conducts normally to reduce losses.
Advantages and disadvantages analysis:
Of the three solutions above: Scheme 1 and Scheme 2 both use thermistors to suppress input inrush current. Under high current, thermistors will suffer certain losses, fail at high temperatures, and require cooling before they can restore their input inrush current suppression function during a second startup; while Scheme 3 uses the variable resistance region of a MOSFET to suppress input inrush current, but its applicable range for suppressing input inrush current is narrow.
The following are feasible solutions to address the issues of temperature failure and narrow range for suppressing input inrush current:
(4) Option 4: Inserting an inrush current suppression circuit
This solution involves inserting an inrush current suppression circuit into the input circuit. The circuit consists of a MOSFET, a cement resistor, and a control circuit, which can effectively solve the problems of temperature failure and narrow range of input inrush current suppression.
Working principle analysis:
When the equipment is turned on, the MOSFET TR1 in the input circuit is in the off state, and the input inrush current is effectively suppressed by the cement resistor R1; after the equipment is turned on and the back-end switching power supply is working normally, the MOSFET is turned on, and the cement resistor is cut out of the main power circuit to reduce losses.
Advantages and disadvantages analysis:
III. Summary
Summary of input inrush current suppression circuit solutions:
Based on the analysis of the principles, advantages and disadvantages of the four commonly used circuit schemes above, Scheme 1 and Scheme 2 have fewer components, simpler principles, and lower costs, and can be used in operating conditions where temperature and loss requirements are not high; Scheme 3 has a narrower range of applications and can be used in low-power products with high temperature requirements; Scheme 4 has higher costs, more complex control, and requires a certain amount of board space, but its advantages are also obvious, such as low loss, no temperature failure, short secondary start-up interval, and wide adjustable range for suppressing input inrush current.
Mornsun has integrated the control section of Solution 4 into a single module for customer convenience, such as the FS-A(C)xxP(-N) series current suppression module suitable for railway power supplies.
Mornsun continues to respond to market trends, remains steadfast in its commitment to power supply technology innovation, and works diligently to promote the national industrial brand, providing customers with higher-quality products and contributing to the rise of domestic power supplies.
