Design of clamping protection circuit for switching power supply
Function of clamping protection circuit
The clamping protection circuit in a switching power supply primarily functions to clamp and absorb voltage spikes generated by the leakage inductance of the high-frequency transformer when the power switching transistor (such as a MOSFET) is turned off, preventing damage to the MOSFET due to overvoltage. This protection mechanism is crucial for improving the reliability and stability of the switching power supply.
Design Principles
The clamping protection circuit is designed based on the voltage clamping principle. That is, at the instant the MOSFET is turned off, a clamping diode (such as a transient voltage suppressor (TVS)) clamps the drain voltage within a safe range, thereby preventing damage to the MOSFET from voltage spikes. Simultaneously, the clamping circuit also needs to consider its coordination with components such as blocking diodes, clamping capacitors, and clamping resistors to achieve the best clamping effect.
Implementation method
A typical clamping protection circuit consists of a transient voltage suppressor (TVS), a clamping capacitor (C), a clamping resistor (R1), a damping resistor (R2), and a blocking diode (VD). The following is a specific design example:
Select components:
TVS: Select a transient voltage suppressor with appropriate clamping voltage and power, such as P6KE200.
Clamping capacitor C: Select an appropriate capacitance value based on the switching frequency and clamping voltage requirements, generally between a few nanofarads and tens of nanofarads.
Clamping resistor R1: The resistance value is determined by calculation to match the time constant of the clamping circuit with the switching cycle.
Damping resistor R2: Used in conjunction with clamping capacitor to form an RC attenuation circuit to absorb some of the leakage inductance energy.
Blocking diode VD: Select a fast recovery or ultra-fast recovery diode to improve reverse recovery speed.
Circuit design:
A TVS is connected in parallel between the drain and source of the MOSFET to clamp the drain voltage.
The clamping capacitor C and the clamping resistor R1 are connected in series and then in parallel across the TVS to form an RC clamping circuit.
A damping resistor R2 is connected in series before the clamping circuit to improve the clamping effect.
The blocking diode VD is connected in series before the clamping circuit to prevent reverse current from affecting the circuit.
Parameter calculation:
Based on the switching frequency f and the clamping voltage UB, calculate the values of the clamping resistor R1 and the clamping capacitor C, so that the time constant τ is equal to 9.47 times the switching period.
Considering that the clamping protection circuit only operates when the MOSFET is turned off, calculate the actual power consumption of R1 and select an appropriate rated power.
To verify whether the withstand voltage of the clamping capacitor meets the requirements, it is generally recommended to use a value at least 1.5 times the withstand voltage of the capacitor.
Precautions
When designing clamping protection circuits, the drain-source breakdown voltage U(BR)DS and drain voltage UDmax of the MOSFET should be fully considered to ensure that the clamping voltage is lower than U(BR)DS.
The value of the damping resistor R2 should be selected appropriately to balance the clamping effect and power efficiency.
The selection of the blocking diode VD should take into account its reverse recovery time and forward peak current capability.
Heatsink design
The function of radiator
The role of a heatsink in a switching power supply is to dissipate internal heat promptly, preventing components from being damaged by overheating. Good heat dissipation design is crucial for improving the reliability and extending the lifespan of the switching power supply.
Design Highlights
Heat flux density of heat source: The heat flux density is calculated based on the power consumption and heat-generating area of the heat-generating components to determine the heat dissipation capacity of the heat sink.
Temperature requirements for heat-generating components: The heat dissipation effect of the heat sink is determined based on the highest operating temperature and allowable temperature rise of the components.
Product internal space dimensions: The shape and size of the heat sink are designed according to the overall size and layout of the product.
Heat sink installation tightness: Ensure tight contact between the heat sink and heat-generating components to improve heat conduction efficiency.
Industrial design requirements: Consider factors such as the appearance, materials, and cost of the radiator to meet the overall design requirements of the product.
Design Methodology
Determine heat dissipation requirements: Calculate the required heat dissipation area based on the power consumption and allowable temperature rise of the heat-generating components.
Choosing radiator materials: Commonly used radiator materials include aluminum and copper. Choose the appropriate material based on heat dissipation requirements and cost.
Designing the heat sink shape: The shape and size of the heat sink are designed based on the internal space dimensions of the product and the heat dissipation requirements. Generally, the base plate area of the heat sink should be larger than the heat-generating area of the heat-generating components to increase the heat dissipation area.
Optimize heat dissipation structure: Increase the heat dissipation area by adding heat dissipation components such as heat sinks and fans, and by using structures such as fins and ribs, thereby optimizing the heat conduction path and improving heat dissipation efficiency.
The clamping protection circuit and heat sink design of a switching power supply are crucial for ensuring its stable and reliable operation. A well-designed clamping protection circuit can effectively prevent damage to critical components such as MOSFETs due to overvoltage; while a carefully designed heat sink can dissipate heat generated inside the switching power supply in a timely manner, preventing component failure due to overheating. Regular maintenance and upkeep are also necessary measures to ensure continued effective heat dissipation. With continuous technological advancements and evolving application requirements, the clamping protection circuit and heat sink design of future switching power supplies will continue to innovate and improve, providing more efficient and reliable power solutions for electronic devices.
