Diode reverse recovery is a brief but consequential commutation event in a Class D output bridge. A MOSFET body diode that conducted during dead time continues carrying reverse current when its complementary MOSFET turns on. That current adds to the new switch current, increasing turn-on loss, voltage overshoot, ringing, EMI and device stress. The best design uses the shortest dead time that still prevents cross-conduction under worst-case voltage, current, temperature, gate-drive and layout conditions.
What reverse recovery means
A forward-biased diode contains stored charge. When the applied voltage reverses, the diode does not block immediately: reverse current flows while that charge is removed, then the current falls to zero and the diode recovers its blocking ability. Toshiba defines the principal quantities under specified test conditions: reverse-recovery time (trr), peak reverse-recovery current (Irr) and reverse-recovery charge (Qrr) (Toshiba reference).
trr is a time interval, Irr is the peak reverse current, and Qrr is the area under the reverse-current waveform:
Qrr = ∫ irr(t) dt
Err is the actual energy dissipated during recovery. It cannot be inferred exactly from one headline data-sheet number because commutation voltage, current, di/dt, temperature, gate resistance, parasitic inductance and the opposing switch all change the waveform.
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Where recovery occurs in a Class D bridge
A synchronous half bridge has high- and low-side MOSFETs, a gate driver, a switching node, an output inductor or LC filter, a load and a DC bus. Complementary gate commands are separated by dead time so the two channels are not on simultaneously.
Because filter-inductor current cannot change instantaneously, the current needs a path while one MOSFET is off and the other has not yet turned on. Depending on current polarity, either body diode can conduct. A typical commutation is:
- The conducting MOSFET turns off.
- Dead time begins and inductor current moves the switch node toward the opposite rail.
- The opposite MOSFET’s body diode conducts temporarily.
- The opposite gate receives its turn-on command.
- Its channel takes over and drives the diode into reverse bias.
- Reverse-recovery current flows through the bridge leg until the diode blocks.
Reverse the load-current polarity and the complementary diode conducts instead; the mechanism is the same but the voltage and current directions change. In a full bridge, both legs can experience this event.
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Why the event creates extra loss and stress
During recovery, the newly turned-on MOSFET carries approximately Iswitch ≈ Iload + Irr for part of the transition. It therefore has substantial voltage and current at the same time:
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The current pulse can resemble a short bridge fault, producing supply-current spikes and stressing silicon, package leads, the driver and PCB copper. A first-order screening estimate is Prr ≈ VcommutationQrrfsw (TI describes this conventional charge-times-voltage method at TI). It is only an estimate: do not add it to a switching-energy specification that already includes recovery.
For example, with a hypothetical 48 V bus, 20 nC of recovery charge and 400 kHz switching, the estimate is 48 × 20 nC × 400 kHz ≈ 0.384 W. This excludes diode forward loss, channel switching, output-capacitance loss, gate-drive loss and layout-related ringing.
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Dead time is a compromise, not a safety margin to maximize
| Dead-time choice | Benefit | Cost or risk |
|---|---|---|
| Too short | Less body-diode conduction and usually less dead-time distortion | Residual turn-off current can overlap the other MOSFET, causing shoot-through |
| Optimized | Enough nonoverlap for worst-case timing while limiting diode conduction | Requires validation over voltage, current, temperature, tolerances and layout |
| Too long | More margin against cross-conduction | Longer diode conduction, more forward loss, more stored charge and potentially more recovery and zero-crossing distortion |
Infineon notes that reducing dead time can reduce body-diode current duration and recovery charge, but excessive reduction risks shoot-through (Infineon AN-1070). Analog Devices likewise recommends the shortest dead time that reliably avoids cross-conduction because it often minimizes distortion (Analog Devices).
Efficiency, ringing, EMI and reliability effects
- Efficiency: recovery adds turn-on energy, while dead time adds body-diode forward-conduction loss.
- Ringing: the steep recovery current excites package and PCB inductance. The resulting voltage is approximately VL = Lparasiticdi/dt, followed by parasitic LC ringing.
- EMI: high-frequency current and voltage transients increase conducted and radiated emissions.
- Reliability: overshoot can exceed MOSFET ratings, disturb gate voltage through Miller coupling and trigger false turn-on or protection circuits.
Infineon discusses recovery-related transients and layout-sensitive EMI in AN-1071. Do not assume every ringing waveform is recovery: common-source inductance, gate-loop inductance, Coss resonance, poor decoupling, excessive gate-drive strength and probe artifacts can dominate.
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What it does to audio distortion
Reverse recovery does not map to a fixed THD value. The result depends on current magnitude and polarity, modulation, feedback location and bandwidth, output-filter behavior, switching frequency, supply voltage, parasitics and dead-time compensation.
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During dead time, current flows through a diode or reverse-conduction path instead of an ideal switch. That voltage error is nonlinear around current zero. Recovery then adds a short high-frequency transient to the switching node, potentially modulating the filter, supply or feedback signal. TI presents its LMG5200 80 V integrated GaN half bridge and LMG1210 driver as examples aimed at reducing body-diode-related effects; TI specifies a 0–20 ns adjustable dead-time range for the LMG1210, which is not a universal specification for GaN drivers (TI article).
Choosing silicon, SiC, GaN or a Schottky diode
| Technology | Recovery behavior | Main trade-offs |
|---|---|---|
| Silicon MOSFET | Conventional pn body diode can have meaningful stored charge | Low cost and broad availability; compare Qrr, Irr, trr and forward voltage at real conditions |
| SiC MOSFET | Generally faster body-diode recovery than ordinary silicon | Body-diode forward voltage can be high; Microchip reports about 4 V for its cited family, not a universal SiC value (Microchip) |
| GaN FET | No conventional silicon body-diode minority-carrier recovery | Reverse-conduction voltage, output capacitance, fast dv/dt, dead time and gate-drive layout still matter (Analog Devices) |
| Parallel Schottky | Free of conventional minority-carrier recovery | Trades recovery loss for forward drop, capacitance, BOM cost, area and loop inductance |
Toshiba describes faster SiC recovery and devices that integrate a SiC Schottky barrier diode to reduce body-diode conduction (Toshiba SiC guidance). SiC is usually more compelling at high voltage or power than in ordinary low-voltage audio stages. A parallel Schottky is worthwhile only when its forward and capacitive losses are lower than the recovery problem it replaces, and it must be placed directly in the commutation loop.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to select devices
- Verify voltage rating, pulsed and continuous current, avalanche or short-circuit capability and thermal resistance.
- Compare RDS(on) at the actual gate voltage and hot temperature, not only the room-temperature headline value.
- Record Qrr, trr, Irr, body-diode forward voltage, Coss, Crss, Ciss and gate charge.
- Check the test current, voltage, di/dt, gate resistance, gate voltage and temperature behind every data-sheet number; Toshiba’s example reports recovery time at specified forward current and negative current slew, not as a device-independent constant (Toshiba).
- Use typical curves for screening, then validate worst-case behavior in the intended layout.
Low Qrr alone is insufficient: a device may still lose more through Coss, gate drive, channel resistance, thermal limits or package inductance. A SiC study measured a 116.7% increase in recovery energy from 25 °C to 100 °C in one device and setup; it demonstrates temperature sensitivity, not a universal percentage (study).
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Measure the real commutation event
Double-pulse test
- Use the intended MOSFETs, driver, gate resistors, bus decoupling and commutation layout in a controlled half-bridge fixture with an inductive load.
- Set the target current with the first pulse.
- Turn the conducting device off, apply the intended dead time and turn on the complementary device.
- Capture gate-to-source voltage, switch-node voltage, VDS and bridge current.
- Compute instantaneous power, p(t) = v(t)i(t), and integrate over commutation to obtain energy.
- Repeat at minimum, nominal and maximum bus voltage; several currents; cold and hot temperatures; multiple dead times and gate resistances.
- Record peak voltage, peak current, ringing, gate disturbance and protection behavior, not only average efficiency.
Tektronix describes double-pulse testing and direct waveform-based recovery-energy measurement (Tektronix application note). Use a correctly rated differential probe and current probe or low-inductance shunt. Keep connections short; a long oscilloscope ground lead can create false ringing and is hazardous on a floating bridge node.
Validate in the amplifier
After the fixture passes, measure the actual amplifier with its LC filter, speaker or reactive load, modulation and protection circuits. Check high current, light load, discontinuous operation and the region around current zero. At light load, Coss, gate-drive or circulating-current loss may dominate instead of recovery. Soft-switching modulation can also greatly reduce conventional hard-switching recovery loss.
Layout and driver remedies
- Minimize the high-current commutation loop and place ceramic decoupling directly across the half-bridge supply path.
- Keep gate-drive loops short, separate them from power-current loops and use a Kelvin-source connection where available.
- Control common-source inductance; use suitable gate resistance, including separate turn-on and turn-off values when needed.
- Use a gate-to-source pull-down and Miller clamp when appropriate for the driver and device.
- Measure ringing frequency and damping before fitting an RC or RCD snubber. A snubber should not conceal a poor layout.
- Place any parallel Schottky immediately beside the MOSFET terminals.
Troubleshooting guide
| Symptom | Likely contributors | First checks |
|---|---|---|
| MOSFETs hot with no audio output | Dead-time conduction, circulating current or switching loss | Measure gate timing, bridge current and temperature |
| Large turn-on current spike | Body-diode recovery, shoot-through or parasitic inductance | Compare current spike with gate overlap and diode conduction |
| Switch-node ringing | Recovery di/dt, parasitic L, Coss resonance or probe artifact | Shorten the loop and remeasure with a proper differential probe |
| High THD near zero crossing | Dead-time nonlinearity or diode conduction | Sweep dead time and measure THD versus current |
| Failures only when hot | Temperature-dependent recovery, timing drift or higher RDS(on) | Repeat double-pulse tests at elevated junction temperature |
| GaN stage overheats during dead time | Reverse-conduction voltage and excessive dead time | Reduce dead time only while retaining verified shoot-through margin |
Design rule
Minimize body-diode conduction and recovery, but never reduce dead time below the worst-case nonoverlap required by the actual MOSFETs or GaN devices, driver, temperature, load and PCB. Device data-sheet values are useful for screening; measured voltage, current and energy waveforms in the real commutation loop determine whether the design is efficient, quiet and reliable.
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