How does the reverse recovery time of a diode affect energy efficiency?

一, The physical essence of reverse recovery time: the game between charge storage and release
During the switching process of a diode from forward conduction to reverse cutoff, the minority carriers stored in the PN junction (such as electrons in the P region and holes in the N region) cannot disappear instantly, but need to undergo a charge release process. This process can be divided into two stages:

Storage stage (ts): After the reverse voltage is applied, the carrier concentration gradient drives the charge to diffuse in the reverse direction, forming a peak reverse current (IRM).
Descent stage (tf): The charge is gradually recombined or extracted, and the reverse current decays exponentially to the leakage current level (Irr).
The duration of the entire process is the reverse recovery time (trr=ts+tf). Taking a typical fast recovery diode (FRD) as an example, its TRR is usually in the range of 50-500ns, while Schottky diode (SBD) can shorten the TRR to the nanosecond level or even close to zero due to the absence of minority carrier storage effect.

二, Loss mechanism: how reverse recovery devours energy efficiency
The reverse recovery process leads to energy loss through three pathways, directly affecting system efficiency:

1. Switching loss
In high-frequency switching applications, power devices such as diodes and MOSFETs conduct alternately. When the diode is not completely turned off, the MOSFET begins to conduct, forming a "cross conduction" phenomenon, resulting in instantaneous short-circuit current.

2. Conductivity Loss
During the reverse recovery process, the diode is subjected to reverse voltage while still experiencing conduction voltage drop

3. Electromagnetic interference (EMI) losses
The rapid change of reverse recovery current (high di/dt) will generate voltage spikes on the parasitic inductance of the circuit, forming conduction and radiation interference. For example, in PFC circuits, an excessively long TRR of the boost diode may result in a 30% increase in the volume of the EMI filter, further reducing the overall efficiency of the system.

三, Temperature dependence: efficiency collapse effect at high temperatures
The reverse recovery time has significant temperature sensitivity, and its variation pattern presents a "double-edged sword" effect:
Reverse recovery stage: High temperature will prolong the carrier lifetime and significantly increase TRR. For example, a 600V ultrafast recovery diode has a trr of 35ns at 25 ° C, but extends to 120ns at 125 ° C, resulting in a 240% increase in switching losses.
This non-linear characteristic is particularly dangerous in industrial power supplies. A customer reported that the efficiency of their 48V/50A server power supply decreased by 5% in high temperature environments. After investigation, it was found that the secondary rectifier diode experienced a significant increase in cross conduction losses due to TRR temperature rise. By replacing it with a silicon carbide Schottky diode (SiC SBD), not only is the trr stable within 15ns, but the junction temperature tolerance is also increased to 175 ° C, and the system efficiency is restored to over 94%.

四, Engineering Practice: Efficiency Optimization Strategies from Selection to Design
1. Device selection: a revolution in materials and structures
Silicon carbide (SiC) diode: With its wide bandgap characteristics, SiC diode achieves zero reverse recovery (trr ≈ 0ns), improving efficiency by 3-5% in high-frequency topologies such as PFC and LLC. A case study of a photovoltaic inverter shows that after adopting SiC diodes, the system efficiency increased from 97.2% to 98.1%, and the annual energy savings were equivalent to reducing CO ₂ emissions by 12 tons.
Soft recovery diode: By optimizing doping concentration and junction depth, the slope of reverse recovery current decrease (df/dt) is reduced by 50%, reducing voltage spikes. For example, when a motor driver adopts a soft recovery diode, the volume of the EMI filter is reduced by 40%, and the system efficiency is improved by 1.2%.
2. Circuit design: Collaborative optimization of topology and control
Synchronous rectification technology: Replace freewheeling diodes with MOSFETs to eliminate reverse recovery losses. After adopting synchronous rectification, the efficiency of a certain laptop adapter increased from 85% to 92%, and the temperature rise decreased by 25 ° C.
Dead time control: By precisely adjusting the dead time of the MOSFET drive signal, cross conduction is avoided. After adopting adaptive dead zone control, a certain industrial power supply reduced switch losses by 60% and increased efficiency to 95%.
3. Thermal management: from passive heat dissipation to active design
Packaging optimization: Using low thermal resistance packaging such as DFN and TO-247 to reduce the impact of junction temperature on TRR. A certain car charger uses DFN8 × 8 packaging to maintain stable TRR of SiC diodes at 150 ° C.
Heat dissipation path design: When multiple tubes are connected in parallel, a current sharing resistor or a thermal coupling structure is added to avoid local overheating. A certain communication power supply has optimized its heat dissipation design to control the temperature difference of parallel diodes within 5 ° C, resulting in a 20% increase in efficiency stability.