In the design and application of power electronics, inrush current, also known as startup current, is a critical phenomenon that cannot be overlooked. It refers to the peak current generated when a device is initially powered on, often exceeding the normal operating input current by a significant margin. Although inrush current only lasts for a very short duration, its impact can be substantial. For example, it can lead to premature aging of components and, in extreme cases, cause circuit failure, jeopardizing the overall safety of the system. Therefore, effectively suppressing inrush current is key to ensuring the stable operation of power electronic devices. Next, we will explore 6 common and effective methods to limit inrush current.
Method 1: Series NTC Thermistor
This is one of the simplest and most effective methods to limit inrush current. An NTC thermistor is a temperature-sensitive component that operates based on the negative temperature coefficient characteristic, meaning its resistance decreases as the temperature increases.
When the circuit is first powered on, the NTC thermistor is at a lower temperature, and its resistance is high. This high resistance effectively limits the current surge, protecting other components in the circuit. As the circuit continues to operate, the NTC thermistor heats up due to the current flow, causing its temperature to rise and its resistance to decrease, allowing current to pass through smoothly and reducing energy loss during normal operation.
Advantages:
- Simple and efficient.
- Low cost.
Disadvantages:
- Heavily influenced by environmental factors, making it less suitable for low and high-temperature environments.
- If the main power grid experiences a brief interruption, the NTC thermistor may not have fully cooled down, thus providing limited current-limiting effects.
- Reduces the overall conversion efficiency of the equipment to some extent.
Method 2: Series Fixed Power Resistor
As an alternative to NTC thermistors, another method is to use a fixed power resistor to limit inrush current. Since the resistance value of a fixed power resistor is constant, it does not have energy-saving characteristics throughout its use. Therefore, this method is typically applied in low-power devices with power ratings of only a few watts, where power loss does not significantly impact the device.
Advantages:
- Simple and efficient.
- Not sensitive to ambient temperature.
Disadvantages:
- Only suitable for low-power devices; power loss can be significant in high-power applications.
Method 3: Parallel Fixed Power Resistor and NTC Thermistor
When dealing with circuits with higher rated power, using only a fixed power resistor or NTC thermistor can lead to energy waste and reduced efficiency since the resistor will always be present in the circuit, continuously dissipating power. To optimize this, a parallel connection of a fixed power resistor and an NTC thermistor can achieve better results.
In the initial stage of powering the circuit, whether at normal or low temperatures, the combined resistance of the fixed power resistor and NTC thermistor in parallel is high enough to limit the inrush current effectively. As the circuit operates, the NTC thermistor heats up due to the current flow, causing its resistance to decrease. This change allows the NTC thermistor to shunt the fixed power resistor under high-temperature conditions, reducing the energy loss of the fixed power resistor.
Advantages:
- Simple and efficient.
- Can simultaneously meet the requirements of limiting inrush current under normal and low-temperature conditions while avoiding power loss during normal operation.
Disadvantages:
- May not be effective in limiting inrush current in high-temperature environments.
Method 4: Series Fixed Value Resistor and Thyristor
For low-power consumption projects, using a series connection of a fixed value resistor and a thyristor to limit inrush current is an effective solution. When the power is first turned on, the gate voltage of the thyristor is in a cut-off state, and the thyristor does not conduct, forcing the current to pass through the fixed value resistor, thereby limiting the current. As the current or other conditions reach a set threshold, the gate voltage is applied, turning on the thyristor. This process effectively bypasses the resistor, significantly reducing power loss.
Advantages:
- Not affected by ambient temperature.
- Completely avoids power loss in the resistor after the circuit is turned on.
Disadvantages:
- Requires more space for system design.
- Relatively higher overall cost.
Method 5: MOSFET and Delay Network Circuit
Due to the connection of a capacitor filtering circuit at the input of DC-DC switching power supplies, when the power is turned on, the capacitor needs to be charged, resulting in a significant inrush current. In this case, the initial state of the MOSFET (T) on the bus input ground is non-conductive.
The delay network circuit, composed of two resistors, a capacitor, and a zener diode, controls the gate voltage of the MOSFET (T). The capacitor gradually charges through the delay circuit, causing the gate voltage of the MOSFET (T) to gradually rise, thereby gradually turning on the drain-source junction of the MOSFET (T). This process effectively reduces the inrush current caused by the capacitor filtering circuit when the power is turned on.
Once the circuit reaches stable operation, the drain-source junction of the MOSFET (T) remains fully conductive. By adjusting the specific parameters of the filtering capacitor, different degrees of inrush current suppression can be achieved in actual switching power supply designs.
Advantages:
- Allows for customizable suppression of inrush current based on specific needs.
- Suitable for applications requiring fine control.
Disadvantages:
- The circuit design is more complex.
Method 6: Using PTC Thermistor
The PTC thermistor has characteristics opposite to the NTC thermistor. The PTC thermistor has a larger resistance at higher temperatures, making it a more prudent choice in environments with very low temperatures and applications requiring frequent switching. However, PTC thermistors are generally more expensive than NTC thermistors, and the design is more complex, possibly requiring additional bypass mechanisms to improve efficiency.
Advantages:
- Effective in low-temperature environments.
- Suitable for frequent on/off applications.
Disadvantages:
- Higher cost compared to NTC thermistors.
- More complex design requirements.
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