Six ways to suppress the startup surge current of switching power supplies

Startup surge current of SMPS

The inrush current of switching current refers to the peak current flowing into the power supply device at the moment the power supply is turned on, as shown below. Due to the rapid charging of the input filter capacitor of the charger, the peak current is much greater than the steady-state input current.

The power supply should limit the surge level that the AC switch, rectifier bridge, fuse and EMI filter device can withstand. The AC input voltage should not damage the power supply or cause the fuse to blow due to repeated switching circuits.

In addition, surge current also refers to the non-repetitive maximum forward overload current that causes the junction temperature to exceed the rated junction temperature due to circuit abnormalities.

SMPS startup current with and without inrush current limiting

The following is the startup surge current in the switching power supply.

As shown in the figure below, the input voltage is first filtered out by interference, then converted to DC through bridge rectification, and then the large electrolytic capacitor smoothes the waveform before it is counted into the real DC-DC converter.

The input surge current is generated when the electrolytic capacitor is first charged. Its magnitude depends on the amplitude of the input voltage at the beginning of power-on and the total resistance of the loop formed by the bridge rectifier and the electrolytic capacitor. If it is started at the peak of the AC input voltage, a peak input surge current will appear.

Startup surge current in switching power supplies

How to limit surge current in switching power supplies

Series negative temperature coefficient thermistor (NTC)

Series NTC (negative temperature coefficient) thermistor current limiting resistor is the simplest way to suppress surge current.

Since the resistance of NTC thermistor decreases with the increase of temperature, when the switching power supply is started, the NTC thermistor is at room temperature and has a high resistance value, which can effectively limit the current. After the power supply is started, the NTC thermistor will quickly heat up to about 10°C due to its own heat dissipation, and the resistance value will be significantly reduced to about one-fifteenth of that at room temperature.

Selecting an NTC thermostat with appropriate resistance-temperature characteristics can greatly reduce the power loss of the switching power supply during normal operation.

Series Negative Temperature Coefficient Thermistor (NTC)

Advantages: Simple and practical, low cost.

Disadvantages: The current limiting effect of NTC thermistors is greatly affected by ambient temperature; when starting at low temperature, if the resistance is too large and the charging current is too small, the switching power supply may not start. If it starts at high temperature, the resistance of the thermistor is too small, and the effect of limiting the input surge current may not be achieved.

When the power is briefly interrupted, only part of the current limiting effect can be achieved. During this short terminal period, the electrolytic capacitor has been discharged, and the temperature of the NTC thermistor is still high and the resistance is small. When the power supply needs to be restarted immediately, the NTC thermistor cannot effectively limit the startup surge current.

The power loss of the NTC thermistor reduces the conversion efficiency of the switching power supply.

Use power resistors to limit surge current

When designing a low-power switching power supply, you can directly use power resistors to limit surge current.

However, this method is not energy-saving, so the resistance is almost constant both during startup and in the working stage, so constant power is applied during the entire operation of the power supply.

Use power resistors to limit inrush current

Advantages: simple circuit, low cost, and the inrush current limitation is almost unaffected by high and low temperatures.

Disadvantages: only applicable to low/micropower switching power supplies; has a great impact on conversion efficiency.

Use resistors at startup and remove them immediately after startup

Using resistors to limit the startup current in power supplies rated above a few watts is not very efficient. As long as the system is powered on, the resistors will consume power, that is, the resistors are only needed when the system is started. Therefore, the resistors must be removed immediately after the startup is completed.

There are many ways to achieve this function, such as connecting a relay, an NTC thermistor or a MOS tube in parallel with the power resistor, as shown below.

NTC thermistor bypass resistors limit startup inrush current in switch-mode power supplies

Relay bypass resistors limit startup inrush current in switch-mode power supplies

When starting at room temperature, the total resistance of the power resistor and thermistor in parallel is large enough to limit the inrush current.

When starting at low temperature, the resistance of the NTC thermistor is large enough, so the total resistance of the parallel resistor and the NTC thermistor is sufficient to effectively limit the inrush current.
As the NTC thermistor heats up rapidly, its resistance drops sharply, playing the role of the power resistor being shunted by the NTC thermistor.

Advantages: simple and practical, can be used at both room temperature and low temperature.
Disadvantages: greater impact on efficiency; high temperature surge current is large.

Use a series fixed resistor with a thyristor to limit the input surge current

When the power is turned on, Vs is cut off, and the current passes through R1, which plays a current limiting role. When certain conditions are met, VS is turned on and R1 is open. Efficiency loss is greatly reduced.

Thyristor bypass resistor limits the startup surge current in the switch mode power supply

Advantages: low power consumption; surge current limitation is almost unaffected by high and low temperatures.

Disadvantages: large size and high cost.

Using MOSFET switch tube and delay network circuit to suppress surge current

The basic principle of circuit operation is: Since the input end of the DC-DC switching power supply is connected to a capacitor filter circuit, when the power is turned on, the input capacitor needs to be charged instantly, so a large capacitor filter circuit is required. Instantaneous surge current occurs. The drain and source of the MOSFET (T) inserted into the bus input ground line is not conducting.

The gate of the MOSFET (T) is powered on through a delay circuit consisting of two resistors, a capacitor and a Zener diode.

The drain and source of the MOSFET (T) are gradually turned on, thereby effectively reducing the surge current value generated by the input capacitor filter circuit when the power is turned on.

When the circuit enters a stable working state, its drain and source are always turned on.

Since the actual switching power supply product design has different surge current suppression effects, different surge current suppression effects can be obtained by adjusting the specific parameters of the filter capacitor.

MOSFET Return Limits Startup Inrush Current in Switch Mode Power Supplies

PTC (Positive Temperature Coefficient) Thermistors

PTC (Positive Temperature Coefficient) Thermistors are sometimes the best solution for limiting inrush current in certain situations.

The ambient temperature is high, in which case the NTC thermistor has a lower resistance at system startup, greatly reducing the effectiveness of inrush current limiting. Conversely, the PTC thermistor has a larger resistance at higher temperatures, so using the PTC in this case has better results than using the NTC thermistor.

The ambient temperature is low, so the resistance of the NTC thermistor will be high, thus unfavorably limiting the power supply current to less than the minimum current required for startup. In this case, the PTC thermistor is preferred.

In the system, some devices must be turned on and off frequently. In this case, it generates multiple instances of inrush current peaks. The time between the two instances is very short, which poses a risk to the system if the NTC thermistor is used. The NTC thermistor needs time to cool down, and if the cooling is not sufficient, the resistance value will be low. When a restart is requested and the NTC is in a low resistance state, an excessive inrush current will be encountered.

When a short-circuit fault occurs, the system current increases dramatically and the NTC thermistor heats up quickly. When the resistance of the NTC thermistor is low, it allows more current, accelerating short-circuit damage.

In the above situation, it is better to use a PTC thermistor to limit the inrush current.

Relay bypasses PTC thermistor to limit startup inrush current in switch-mode power supplies

PTC thermistors cost more than NTC thermistors. In addition, PTC-based current limiting circuits require a bypass mechanism to remove the PTC from the power loop when it becomes hot.

As shown in the figure above, a relay is used to bypass the PTC thermistor when the power current is detected to be below the threshold.

Despite the disadvantage of high cost, PTC is still favored in many applications, such as DC motors and solenoids, because PTC thermistors have self-protection characteristics and their resistance increases when the current is too large.

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