MOSFETs have an inherent body diode between the drain and source, which becomes significant in applications where the MOSFET switches between on and off states rapidly. The reverse recovery time (trr) of this diode plays a crucial role in circuit performance, especially in high-frequency switching applications like motor drives, inverters, and power supplies.
What is Reverse Recovery Time (trr)?
When a diode is conducting in the forward direction and is then suddenly switched off (reverse biased), it does not stop conducting immediately. Instead, a reverse current flows for a short time due to the stored charge in the junction before it fully turns off.
This time interval is called the reverse recovery time (trr).
Why Reverse Recovery Time is Important in MOSFETs?
The body diode of a MOSFET plays a key role when the MOSFET is used in half-bridge, full-bridge, and synchronous rectification circuits, where current commutates through the diode during switching. The reverse recovery time affects performance in the following ways:
- Increases Switching Losses
- When the MOSFET turns on while the body diode is recovering, the stored charge in the diode causes a large reverse current, leading to higher power dissipation.
- This reduces efficiency in high-frequency applications.
- Causes Voltage Spikes and EMI Issues
- The sudden flow of reverse recovery current interacts with circuit inductances, leading to high-voltage spikes and ringing.
- These spikes can cause EMI (Electromagnetic Interference) and even damage components.
- Limits High-Frequency Operation
- In high-speed switching applications, a longer trr means the diode remains in conduction for longer, affecting efficiency and response time.
- Fast-recovery diodes or MOSFETs with low trr body diodes are preferred for such applications.
How Reverse Recovery Time of Diode Matters for an Application?
The impact of trr depends on the switching frequency and the type of application:
| Application | Effect of Reverse Recovery | Solution |
|---|---|---|
| DC-DC Converters (Buck, Boost, Flyback, Forward, etc.) | Increases switching losses, causes EMI and voltage spikes. | Use Schottky diodes or MOSFETs with low trrt_{rr}trr. |
| Motor Drives (Half-Bridge, Full-Bridge Inverters) | Causes heating and inefficiency in high-speed PWM operation. | Use SiC or ultra-fast recovery diodes. |
| Synchronous Rectification | Increases conduction losses and affects efficiency. | Use MOSFETs with fast body diodes or add external fast-recovery diodes. |
| RF and High-Frequency Switching Circuits | Reduces response time, affects signal integrity. | Use low-charge recovery diodes or fast MOSFETs. |
Example: MOSFET Selection for Low trr
For a high-frequency inverter or SMPS, selecting a MOSFET with a fast-recovery body diode can significantly improve efficiency.
Example:
- SiC MOSFET (e.g., C3M0065090J – 900V, 65mΩ)
- Has an extremely low reverse recovery charge (Qrr), reducing switching losses.
- Ideal for high-voltage, high-frequency applications.
Conclusion
- Reverse recovery time (trrt_{rr}trr) is critical in power electronics applications, especially at high switching frequencies.
- A long trr leads to higher losses, voltage spikes, and EMI problems.
- To improve performance:
- Use MOSFETs with fast body diodes.
- Use external Schottky or SiC diodes to handle recovery efficiently.
- Optimize gate drive and snubber circuits to minimize unwanted effects.