Diode reverse recovery is the brief interval in which a forward-conducting diode continues to carry reverse current after the circuit applies reverse voltage. In a PN diode, stored minority carriers must be removed or recombine; in a Schottky diode, recovery charge is associated with junction capacitance instead. The current pulse, its charge, and how abruptly it ends can affect a converter’s switching loss, voltage overshoot, ringing, and electromagnetic interference (EMI).
What happens during reverse recovery?
A diode does not always stop conducting the instant its voltage changes from forward bias to reverse bias. When a PN power diode has been carrying forward current, charge stored in the junction must be removed before the diode can block. During that transition, reverse current flows through the diode. Once the stored charge has been cleared, the current falls and the diode supports reverse voltage.
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The transition is transient, not steady-state reverse conduction. Its shape depends on the diode and on how it is being switched, including the forward current before commutation, the rate of current change, and junction temperature. Schottky diodes have a different recovery mechanism: their recovery charge is associated with junction capacitance rather than the same minority-carrier storage found in a PN diode.
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What do trr, Irrm, Qrr, Err, and softness mean?
| Term | Meaning | What to check |
|---|---|---|
| trr | Reverse-recovery time: a measured interval during which the diode transitions from forward conduction to blocking. | Read the datasheet’s start and end definitions. For example, ST defines the interval from the diode-current zero crossing until current returns within 25% of peak recovery current. |
| Irrm or IRR | Peak reverse-recovery current during the transient. | It varies with initial forward current, commutation slope, temperature, and device. |
| Qrr | Reverse-recovery charge: the area under the reverse-current waveform over a specified interval. | Check the integration interval and test conditions; values using different definitions are not directly comparable. |
| Err | Reverse-recovery energy, reported in some datasheets or measured during switching. | It relates to a particular voltage/current transition and its measurement conditions, rather than time alone. |
| Softness | A description or ratio of how the current falls after its peak relative to its rise or charge-removal interval. | Confirm the source’s definition. A soft waveform decays more gradually; a snappy waveform terminates abruptly. |
A triangular waveform approximation is Qrr ≈ ½ × Irrm × trr. It can help build intuition, but real waveforms are not necessarily triangular, and datasheets may define trr differently. Use the manufacturer’s stated test conditions and charge-integration definition for comparisons.
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Why recovery affects converter performance
When a diode is commutated off, its reverse-recovery current adds to the current handled by the switching device. That can increase switching energy loss. Qrr multiplied by switching frequency offers a first-order indication of why repeated recovery events matter, but it is not a complete loss calculation: circuit voltage, current and voltage waveforms, switching behavior, and parasitic elements also influence the result.
The way the recovery current ends matters as well. A rapid fall creates high di/dt. Parasitic inductance can turn that current change into voltage overshoot and ringing, which may contribute to EMI. A softer fall can reduce the abruptness of these transients, but softness can involve a loss trade-off. Selecting a diode solely because it has the lowest listed trr can therefore lead to a poor fit for the actual circuit.
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Why datasheet conditions matter
Recovery measurements depend strongly on the operating point. Renesas notes that the characteristics are highly dependent on forward bias current IF, diF/dt, and junction temperature. Reverse voltage, measurement endpoint, current path, and test setup also matter. A trr or Qrr value without its test conditions is not enough to predict behavior in a different converter.
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For scale, STMicroelectronics’ AN4544 gives example switching characteristics for a diode at IF = 40 A, VR = 400 V, VGE = 15 V, and di/dt = 1000 A/µs. At 25 °C, the reported values are trr = 41 ns, Qrr = 440 nC, Irrm = 21.6 A, and Err = 151 µJ. At 175 °C under the stated test conditions, they are trr = 109 ns, Qrr = 2400 nC, Irrm = 44.4 A, and Err = 718 µJ. These are example values for the tested device and conditions, not general specifications for power diodes.
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Other published figures are equally specific. Microchip’s “Brief Overview of Silicon Carbide” gives an illustrative silicon-diode example with about 2200 nC at 25 °C and approximately 14 W at 65 kHz; those figures describe the waveform and calculation in that article, not a general prediction for silicon diodes. The same overview reports about 20% higher Qrr at 175 °C than at 25 °C for its SiC example at very high di/dt. That observation should not be generalized to every SiC diode.
How double-pulse testing measures recovery
Double-pulse testing is a common way to observe switching behavior under controlled conditions. A switching device drives an inductive load, allowing the freewheeling diode to be commutated while current and voltage waveforms are captured. Measurements may include trr, Irrm, Qrr, Err, and di/dt. Tektronix describes double-pulse testing as the preferred method for measuring switching parameters of power devices under a controlled junction temperature.
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Results still depend on the test circuit and measurement method. Probe arrangement, parasitics, current path, operating conditions, and the measurement window all affect the captured waveform or calculated values. Compare test points and definitions like for like; values from manufacturer datasheets should not be treated as interchangeable when their conditions differ.
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- Start with the circuit requirements. Establish the diode’s required reverse-voltage and current ratings, thermal limits, forward drop, package constraints, and any application qualification requirements. The right ratings depend on the design, not on recovery speed alone.
- Compare recovery at the relevant operating point. Look at Qrr and Irrm alongside trr. Check forward current, reverse voltage, di/dt, junction temperature, and the manufacturer’s measurement endpoint and definitions.
- Consider the current waveform, not just a headline number. Review softness and likely overshoot, ringing, and EMI behavior in the intended switching circuit and layout. A lower charge or shorter time does not, by itself, establish the best system-level result.
- Match the technology to the application. PN fast- or soft-recovery rectifiers and Schottky options have different charge mechanisms and trade-offs. A technology label, including Schottky or SiC, does not replace checking the complete datasheet.
- Validate in the switching circuit. Where practical, assess recovery under representative conditions using a suitable measurement setup. For double-pulse measurements, instrument and probe suitability depends on the voltage, current, bandwidth, and probing requirements.
Without the circuit’s operating point and constraints, no exact part can be selected responsibly. The useful comparison is between complete datasheet definitions and waveforms under conditions close to the intended application.
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Sources
- Power Integrations, AN-301 (January 2011)
- Renesas Electronics, “Power Diodes: Descriptions of the Electrical Characteristics and Usage,” Rev. 1.00 (November 13, 2024)
- Tektronix, double-pulse testing application note
- STMicroelectronics, AN4544
- Microchip Technology, “Brief Overview of Silicon Carbide”
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