Inrush current control method for switching power supplies

Abstract: This paper introduces the inrush current limiting method of switching power supply and provides a design example. Keywords: Inrush current, Inrush current control 1. Introduction The input of a switching power supply generally has a filter to reduce the ripple fed back to the input. The input filter is generally a Π-shaped filter composed of capacitors and inductors. Figures 1 and 2 show typical AC/DC power supply input circuits and DC/DC power supply input circuits, respectively. Since the capacitor can be considered as a short circuit in transient state, a very large inrush current is generated when the switching power supply is powered on. The amplitude of the inrush current is much larger than the steady-state operating current. If the inrush current is not limited, it will not only burn out fuses and connectors, but also interfere with nearby electrical equipment due to the common input impedance. Figure 3. Maximum inrush current limit of communication system (AC/DC power supply) Figure 4. Inrush current limit of communication system at nominal input voltage and maximum output load (DC/DC power supply) The European Telecommunications Standards Institute (ETSI) has specified the magnitude of inrush current for switching power supplies used in communication systems. Figure 3 shows the maximum inrush current limit of communication system when powered by AC/DC power supply [4], and Figure 4 shows the maximum inrush current limit of communication system when powered by DC/DC power supply at nominal input voltage and maximum output load [5]. In the figure, It is the transient value of the inrush current, and Im is the steady-state operating current. The magnitude of the inrush current is determined by many factors, such as the magnitude of the input voltage, the impedance of the input wire, the input inductance and equivalent impedance inside the power supply, and the equivalent series impedance of the input capacitor. These parameters vary depending on the power supply system and layout, and are difficult to estimate. The most accurate method is to measure the magnitude of the inrush current in actual application. When measuring the inrush current, the magnitude of the inrush current should not be changed by introducing a sensor. The recommended sensor is a Hall sensor. 2. Inrush Current Limiting Methods for AC/DC Switching Power Supplies 2.1 Series Resistor Method For low-power switching power supplies, the series resistor method, as shown in Figure 5, can be used. If the resistor is large, the inrush current is small, but the power consumption across the resistor is large. Therefore, a compromise resistor value must be chosen to keep both the inrush current and the power consumption across the resistor within the allowable range. Figure 5. Inrush Current Control Circuit Using Series Resistor Method (Applicable to bridge rectifiers and voltage multiplier circuits, with the same inrush current) The resistor connected in series in the circuit must be able to withstand the high voltage and large current during startup. Resistors with large rated current are more suitable for this application. Wire-wound resistors are commonly used, but they should not be used in high-humidity environments. This is because in high-humidity environments, transient thermal stress and wire expansion will reduce the effectiveness of the protective layer, and moisture intrusion can cause resistor damage. Figure 5 shows the typical locations of inrush current limiting resistors. For a 110V/220V dual-voltage input circuit, two resistors should be placed at positions R1 and R2. This ensures that the inrush current is the same when the 110V input connection is connected and when the 220V input connection is disconnected. For a single-input voltage circuit, a resistor should be placed at position R3. 2.2 Thermistor Method In low-power switching power supplies, negative temperature coefficient thermistors (NTCs) are commonly used at positions R1, R2, and R3 in Figure 5. During the initial startup of the switching power supply, the NTC's resistance is very high, limiting the inrush current. As the NTC heats up, its resistance decreases, reducing power consumption during operation. However, the thermistor method also has drawbacks. After the initial startup, the thermistor takes some time to reach its operating resistance value. If the input voltage is near the minimum operating value of the power supply at this time, the voltage drop is large due to the relatively high resistance of the thermistor during startup, potentially causing the power supply to operate in a malfunctioning state. Additionally, after the switching power supply is turned off, the thermistor needs a cooling time to raise its resistance to room temperature in preparation for the next startup. The cooling time varies depending on the device, installation method, and ambient temperature, but is generally one minute. If the switching power supply is turned on immediately after being turned off, the thermistor has not yet cooled down, and it loses its limiting effect on the inrush current. This is why power supplies using this method to control inrush current are not allowed to be turned on immediately after being turned off. 2.3 Active Inrush Current Limiting Method For high-power switching power supplies, the inrush current limiting device should be short-circuited during normal operation to reduce its power consumption. Figure 6. Active Inrush Current Limiting Circuit (Large Inrush Current with Bridge Rectifier) In Figure 6, R1 is selected as the starting resistor. After startup, R1 is bypassed using a thyristor. Because the resistor R1 in this inrush current limiting circuit can be very large, it is usually not necessary to change the resistance value for 110V input voltage multiplier and 220V input. The thyristor shown in Figure 6 is a bidirectional thyristor, but it can also be replaced by a thyristor or relay. The circuit shown in Figure 6 initially limits the inrush current to resistor R1. Once the input capacitor is fully charged, the active bypass circuit starts working, bypassing resistor R1, thus minimizing losses during steady-state operation. In this thyristor startup circuit, the thyristor can be easily powered by a coil on the main transformer of the switching power supply. The slow start of the switching power supply provides a delayed start for the thyristor, allowing the input capacitor to be fully charged through resistor R1 before the power supply starts. 3. Inrush Current Limiting Methods for DC/DC Switching Power Supplies 3.1 Long-Short Pin Method The circuit shown in Figure 7 is an inrush current limiting circuit using the long-short pin method. When the DC/DC power supply board is inserted, the long pin contacts, and the input capacitor C1 is charged through resistor R1. When the power supply board is fully inserted, resistor R1 is short-circuited by the broken pin. C1 represents the total capacitance of the DC/DC power supply. Figure 7. Inrush Current Limiting Circuit Using the Long-Short Pin Method The drawback of this method is that the insertion speed cannot be controlled. If the insertion speed is too fast, the short pin will contact before the capacitor C1 is fully charged, resulting in poor inrush current limiting. Thermistor method can also be used to limit inrush current, but because the input voltage of DC/DC power supply is low and the input current is large, the power consumption on the thermistor is also large, so this method is generally not used. 3.2 Active Inrush Current Limiting Method 3.2.1 Limiting Inrush Current Using MOSFETs Using MOSFETs to control inrush current can overcome the shortcomings of passive limiting methods. MOSFETs have the characteristics of low on-resistance Rds_on and simple driving. With the addition of a few components, an inrush current limiting circuit can be made. MOSFETs are voltage-controlled devices, and their inter-electrode capacitance equivalent circuit is shown in Figure 8. Figure 8. Inter-electrode capacitance equivalent circuit of N-type MOSFET with external capacitor C2 The inter-electrode capacitance, gate-drain capacitance Cgd, gate-source capacitance Cgs, and drain-source capacitance Cds of MOSFET can be determined by the following formula: The values of feedback capacitance Crss, input capacitance Ciss, and output capacitance Coss of MOSFET in the formula can be found in the MOSFET datasheet. The charging and discharging speed of the capacitor determines the turn-on and turn-off speed of the MOSFET. To ensure that the state transitions of the MOSFET are linear and predictable, an external capacitor C2 is connected in parallel with Cgd. If the external capacitor C2 is much larger than the internal gate-drain capacitance Cgd of the MOSFET, it will reduce the effect of the internal nonlinear gate-drain capacitance Cgd during state transitions. The external capacitor C2 is used as an integrator to precisely control the switching characteristics of the MOSFET. Controlling the linearity of the drain voltage allows for precise control of the inrush current. Circuit description: Figure 9 shows a self-starting active inrush current limiting circuit based on a MOSFET. MOSFET Q1 is placed at the negative voltage input terminal of the DC/DC power supply module. At the moment of power-on, the voltage level of pin 1 of the DC/DC power supply module is the same as that of pin 4. Then, the control circuit reduces it to a negative voltage at a certain rate. The rate of voltage drop is determined by the time constant C2 * R2, and this slope determines the maximum inrush current. C2 can be selected using the following formula: R2 is determined by the allowable inrush current: where Vmax is the maximum input voltage, Cload is the sum of C3 and the internal capacitance of the DC/DC power module, and Iinrush is the magnitude of the allowable inrush current. Figure 9. Active Inrush Current Limiting Circuit. D1 is used to limit the gate-source voltage of MOSFET Q1. Components R1, C1, and D2 are used to ensure that MOSFET Q1 remains off when it is first powered on. After power-on, the gate voltage of the MOSFET rises slowly. When the gate-source voltage Vgs reaches a certain level, diode D2 conducts, so all the charge charges capacitor C1 with a time constant R1×C1. The gate-source voltage Vgs rises at the same rate until MOSFET Q1 conducts and generates an inrush current. The following are the formulas for calculating C1 and R1: where Vth is the minimum threshold voltage of MOSFET Q1, VD2 is the forward voltage drop of diode D2, and Vplt is the gate-source voltage when the Iinrush inrush current is generated. Vplt can be found in the product information provided by the MOSFET supplier. The following parameters are crucial for selecting MOSFETs in active inrush current limiting circuits: * **Drain Breakdown Voltage Vds:** The MOSFET must have a Vds higher than both the maximum input voltage Vmax and the maximum input transient voltage. For MOSFETs used in communication systems, a Vds ≥ 100V is generally chosen. * **Gate-Source Voltage Vgs:** Zener diode D1 protects the gate of MOSFET Q1 from overvoltage breakdown. Therefore, the gate-source voltage Vgs of MOSFET Q1 must be higher than the maximum reverse breakdown voltage of Zener diode D1. A typical MOSFET gate-source voltage Vgs is 20V; a 12V Zener diode is recommended. * **On-Resistance Rds_on:** The MOSFET must be able to dissipate the heat generated by the on-resistance Rds_on. The heat dissipation calculation formula is: where Idc is the maximum input current of the DC/DC power supply, determined by the following formula: where Pout is the maximum output power of the DC/DC power supply, Vmin is the minimum input voltage, and η is the efficiency of the DC/DC power supply when the input voltage is Vmin and the output power is Pout. η can be found in the datasheet provided by the DC/DC power supply supplier. The Rds_on of the MOSFET must be very small so that the voltage drop it causes is negligible compared to the input voltage. Figure 10. Example of waveform design for active inrush current limiting circuit with 75V input and no load DC/DC output. Given: Vmax=72V Iinrush=3A Select MOSFET Q1 as IRF540S Select diode D2 as BAS21 Calculate according to formula (4): C2>>1700pF. Select C2=0.01μF; Calculate according to formula (5): R2=252.5kW. Select R2=240kW, select R3=270W

# Switching power supply | Inrush current

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Inrush current control method for switching power supplies

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