Inrush current, the surge of current when a switchmode power supply (SMPS) is powered on, can cause significant stress on components and interfere with other equipment sharing the same power supply line. This is especially problematic in direct-off-line SMPS configurations, where the line input is switched directly to the rectifier circuit. In such designs, the power supply lines, switches, rectifiers, and capacitors experience high inrush currents upon initial power-up. These surges can damage components, reduce system reliability, and shorten component lifespans. To avoid such issues, various methods of inrush current control are employed. Among the most common are series resistors, NTC thermistors, and active limiting circuits.
1. Series Resistors for Inrush Current Limiting
Overview:
In low-power SMPS applications, series resistors are one of the simplest and most cost-effective methods for inrush current limiting. These resistors(R1,R2,R3) are positioned in the power supply lines between the input point and the reservoir capacitors to limit the initial surge of current when the system is powered on.
Design Considerations:
In low-power designs, selecting resistors with an appropriate balance between resistance value (to limit inrush current) and power dissipation during normal operation is critical. Surge-rated resistors, particularly wire-wound types, are commonly used for this purpose.
Resistive Inrush Current Control Circuit Diagram:
For single input voltage operation the resistor R1 is used as shown below.
If dual input voltages are required, two resistors Ru and Rl can be used for parallel operation at low-voltage inputs and series operation at high-voltage inputs. Such resistive inrush current control circuit diagram is shown below.
Advantages:
- Simple and inexpensive solution for low-power applications.
- Effective for controlling inrush current during initial power-up.
Challenges:
- Heat Dissipation: A higher resistance value reduces inrush current but increases power dissipation as heat during normal operation. Finding a balance between acceptable inrush current and operational losses is essential.
- Component Stress: The series resistors must withstand the initial high voltage and current stresses when the supply is first switched on.
- Moisture Sensitivity: In humid environments, wirewound resistors can degrade over time due to moisture ingress, leading to premature failure.
For further details on designing high-output shunt voltage regulators, which can be used in tandem with these techniques, check out this guide: How to Design a High Output Shunt Voltage Regulator.
2. Thermistor Inrush Limiting
Overview:
NTC thermistors (Negative Temperature Coefficient thermistors) are widely used for inrush current limiting in low-power SMPS systems. These thermistors have a high resistance when cold, which helps limit the inrush current upon power-up. As the thermistor heats up, its resistance decreases, allowing the system to operate normally without significant power loss.
NTC Therminstors Inrush Current Control Circuit Diagram:
Like in the resistive method of inrush current circuit diagram, the NTC thermistors are often used in positions R1, R2, or R3 in the power input stage of SMPS designs, where they limit the inrush current at power-on and reduce dissipative losses once the system reaches normal operating conditions.
For single input voltage operation and dual input voltage operation the circuit diagram of controlling the inrush current using NTC Therminstors are shown below.
Advantages:
- Self-Regulating: As the thermistor heats up, its resistance decreases, reducing dissipation losses once the system stabilizes.
- Cost-Effective: NTC thermistors are inexpensive and commonly used in a wide variety of SMPS designs.
Challenges:
- Warm-Up Delay: The thermistor's resistance decreases gradually as it heats up. If the supply voltage is near the minimum threshold, the system may not reach full regulation until the thermistor is fully warmed.
- Rapid Cycling: If the system is switched off and on rapidly, the thermistor may not have time to cool down, compromising its ability to limit inrush current effectively.
Despite these challenges, NTC thermistors are widely used in small units where rapid on-off cycling is less of an issue, making them ideal for compact SMPS designs.
For more details on designing basic shunt regulators and their role in power supply design, visit: How to Design Basic Shunt Regulator and Zener Circuit.
3. Active Limiting Circuits (Triac Start Circuits)
Overview:
For higher-power SMPS systems, active limiting circuits offer a more efficient solution to inrush current. In these circuits, a series resistor initially limits the inrush current when the system is powered on. Once the input capacitors are fully charged, an active switching device—such as a triac, thyristor, or relay—shorts the resistor, reducing the power dissipation during normal operation.
Design Considerations:
For high-power or low-voltage DC-to-DC converter applications, the power loss in the triac may be unacceptable. In such cases, relays can be used instead of triacs, though this introduces additional design complexities due to the required timing circuits. The system must ensure that the input capacitors are fully charged before the relay is activated.
Circuit Diagram:
Insert resistive inrush-limiting circuit with triac bypass for improved efficiency diagram here.
Advantages:
- Reduced Losses: Once the input capacitors are fully charged, the series resistor is bypassed, significantly reducing power losses during normal operation.
- Suitable for High-Power Applications: This method is particularly effective for high-power converters, where traditional resistive methods would lead to excessive losses.
Challenges:
- Timing Issues: Proper timing is critical in active limiting circuits. The input capacitors must be fully charged before the switching device (e.g., triac) bypasses the series resistor. If the system starts converting before the capacitors are fully charged, additional inrush currents can occur.
- Circuit Complexity: Active limiting circuits are more complex to design, as they require precise control and timing of the switching devices to prevent further inrush current once the capacitors are charged.
For more advanced designs that integrate shunt voltage regulation in power supply systems, see: Custom Power Supply for Arduino with Shunt Regulator.
Conclusion
Controlling inrush current is crucial in switchmode power supplies (SMPS) to prevent component stress, reduce interference, and improve overall system lifespan. The three main techniques—series resistors, NTC thermistors, and active limiting circuits—offer varying levels of effectiveness depending on the application.
- For low-power designs, series resistors or thermistors are sufficient for inrush current limiting.
- For high-power or complex systems, active limiting circuits like triac or relay bypass offer more effective control with reduced losses during normal operation.
By carefully selecting the appropriate inrush current control method based on the power requirements and operational conditions, engineers can design more reliable and efficient power supplies that ensure longevity and performance.
Related Resources:
- Learn how to design high-output shunt voltage regulators for effective power management in SMPS systems: How to Design a High Output Shunt Voltage Regulator
- Explore the basics of shunt regulators and how they can improve the performance of your power supply designs: How to Design Basic Shunt Regulator and Zener Circuit
- Build a custom power supply for Arduino using a shunt regulator for stable and efficient power delivery: Custom Power Supply for Arduino with Shunt Regulator