The boost diode in a power factor correction stage lives a hard life: it blocks 400–800 V, switches at the converter frequency, and carries the full output current. Silicon fast-recovery diodes have historically survived the job, but their reverse-recovery charge grows with temperature and di/dt, creating losses, ringing, and EMI that limit the whole converter. Silicon carbide Schottky barrier diodes (SiC SBDs) remove the problem at its root. This article explains the physics of that difference, works through a PFC efficiency example, and gives a selection checklist for engineers deciding between silicon and SiC.
The Problem Silicon Diodes Can’t Escape
A silicon PN or fast-recovery diode is a minority-carrier device. When it conducts, carriers are stored in the drift region; when the voltage reverses, those carriers must be swept out before the diode blocks. That sweep-out is the reverse-recovery event: a reverse current spike, defined by its charge Qrr and its waveform, that flows through the diode and the switching transistor.
The consequences in a continuous-conduction-mode (CCM) PFC boost converter:
- Switching loss. The MOSFET turns on while the diode is still recovering, adding the recovery charge to the turn-on loss.
- Ringing and EMI. The abrupt end of recovery, combined with parasitic inductance, produces high-frequency oscillation at the diode node—the classic source of conducted and radiated emissions.
- Temperature dependence. Silicon recovery gets worse as the diode heats, so the loss and EMI degrade exactly when the converter is hottest.
Designers have fought this with softer-recovery diodes, snubbers, and careful layout. SiC SBDs attack the mechanism instead.
What SiC Changes at the Device Level
A Schottky diode is a majority-carrier device: conduction relies on majority carriers across a metal-semiconductor junction, with no minority-carrier storage. Silicon Schottky diodes work this way, but silicon’s low critical field limits them to low blocking voltages—a silicon Schottky at 600–1200 V would have unacceptable resistance and leakage. Silicon carbide’s critical field is roughly ten times higher, so a SiC Schottky can block 650–1200 V with a thin, low-resistance drift layer.
The result is a diode that combines:
- High blocking voltage (650 V to 1200 V and beyond);
- Negligible reverse-recovery charge—effectively zero Qrr, independent of temperature and di/dt;
- Low forward voltage with a negative temperature coefficient at moderate current;
- High-temperature operation, with leakage that rises but remains manageable.
The “zero recovery” property is the headline: it removes the diode’s contribution to the MOSFET turn-on loss and eliminates the recovery-induced ringing.
The PFC Efficiency Example
Compare a 650 V silicon fast-recovery diode and a 650 V SiC SBD in a 1 kW CCM PFC boost stage at 100 kHz with a 400 V bus and roughly 1 A average diode current. This is an illustrative case for method; real values depend on the exact parts, waveform, and temperature.
| Item | Silicon fast-recovery diode | SiC Schottky diode |
|---|---|---|
| Conduction loss (1 A avg, 1.5 V vs 1.3 V) | ~1.5 W | ~1.3 W |
| Recovery charge Qrr | ~1–2 µC, rising with temperature | ~0 |
| Diode-induced MOSFET turn-on loss | ~2–3 W at 100 kHz | ~0.2–0.5 W (Coss-related only) |
| Ringing at the diode node | Significant | Minimal |
| Loss at 125 °C junction | Rises (recovery worsens) | Stable |
The numbers are illustrative and depend on the exact parts and operating point, but the pattern is consistent: silicon recovery adds several watts at the MOSFET, plus snubber and filter burden. SiC removes them. In a 1 kW supply, that is often the difference between needing a bigger heatsink and staying in the same envelope.
Two practical side effects follow:
- The MOSFET runs cooler, because its turn-on loss drops—sometimes the reason a design can use a smaller, cheaper switch.
- The EMI filter gets easier, because the high-frequency ringing source disappears; measured spectra typically show lower noise at the recovery-frequency bands.
Reliability and the Things to Verify
SiC SBDs are robust, but they are not magic, and three parameters deserve attention:
- Surge current. The forward surge rating (IFSM) and the surge curve define behavior during inrush, load steps, and fault events. A SiC SBD’s surge capability differs from silicon diodes of the same current class; verify it against your worst-case event.
- Leakage at high temperature. Leakage current rises exponentially with temperature. In high-ambient designs, the leakage power can become a real loss term and, in parallel strings, can cause current imbalance. Check the leakage curve at the operating junction temperature.
- Avalanche capability. Many SiC Schottky devices have limited avalanche energy compared with silicon PN diodes; if the circuit can impose unclamped spikes, verify the datasheet’s avalanche rating or add protection.
Also confirm the package and thermal path: the diode’s power loss must be removed through the PCB or heatsink, and the datasheet thermal resistance assumes a defined copper area. The Documents section of the Good-Ark site carries datasheets and package outlines for the device families, and the SMPS application section shows SiC Schottky diodes working beside fast-recovery and bridge families in supply designs.
Where SiC SBDs Earn Their Cost
The device costs more than silicon, so the design question is where the premium pays back:
- CCM PFC at high power. Every watt of recovery loss removed scales with power and frequency.
- High-frequency converters. The recovery penalty grows with frequency; SiC’s advantage grows with it.
- High-temperature applications. Silicon recovery worsens with heat; SiC stays stable.
- EMI-sensitive designs. The ringing reduction can shrink the filter and shorten compliance cycles.
- Solar and EV systems. High-voltage, high-efficiency stages with tight loss budgets.
At low power, low frequency, or with generous thermal budgets, a well-chosen silicon fast-recovery diode with a snubber can still be the economical answer. The selection should compare the full converter—switch, diode, heatsink, filter, and compliance cost—not the two diodes alone. The main-switch side of the same PFC stage is covered in the companion article on super junction MOSFETs in this series.
SiC SBDs in Bridgeless PFC and Resonant Stages
The boost diode’s role changes as topologies evolve, and SiC SBDs follow:
- Bridgeless PFC. The bridgeless boost removes the input bridge diodes to cut conduction loss, which pushes more current through the boost diode and switch. The recovery performance of the boost diode becomes even more critical because there is no bridge to absorb the ringing; SiC SBDs are the standard choice in this topology for exactly that reason.
- Interleaved CCM PFC. Multiple boost stages in parallel reduce input ripple but multiply the diode count. Each diode’s recovery loss adds up; zero-recovery SiC parts keep the total manageable and simplify the interleaving timing.
- LLC and resonant converters. The output rectifier in an LLC converter sees sinusoidal currents and zero-current-switching transitions. Recovery charge matters less at the ZCS instant, but the diode’s forward drop and thermal behavior still set the output loss; SiC SBDs appear where efficiency targets are tight.
In all three, the practical measurement that settles the debate is the efficiency and EMI scan on the real board. The lab results usually show a 0.3–1% efficiency gain from SiC at full load in CCM PFC stages, plus a measurable drop in the conducted-emission peak near the switching frequency—but the exact numbers depend on the power level, frequency, and layout, so prototype measurement beats spreadsheet comparison.
One more point for procurement: SiC SBDs are supplied with a range of current ratings and packages (D2PAK, TO-220, TO-247, and surface-mount). The package outline and the thermal pad design differ between suppliers even for the same current class; verify the outline and thermal resistance against the board footprint before committing, and request the datasheet revision and qualification summary with the sample request.
The Measurement Checklist: Qrr, Turn-on Loss, and EMI
The SiC vs silicon decision is best settled on the bench, with measurements that isolate the diode’s contribution:
- Double-pulse test for recovery and turn-on overlap. Capture the boost-diode recovery waveform and the MOSFET turn-on current overlap at the same bus voltage, inductor current, di/dt, and junction temperature for both diodes. Record Qrr and the peak recovery current; the SiC part should show near-zero recovery while the silicon part’s Qrr grows with temperature.
- Conducted EMI scan at identical conditions. Run the same load, frequency, and input voltage, and compare the quasi-peak and average spectra near the switching frequency and its harmonics. The recovery ringing appears as a broadband peak; removing it changes both the peak value and the filter margin.
- Efficiency at several load points. Measure at 20%, 50%, and 100% load after thermal stabilization. The recovery saving is largest at full load in CCM; the light-load result shows how much of the advantage remains when switching events are fewer.
- Leakage at operating temperature. Measure reverse leakage at the rated voltage and the worst-case junction temperature, and add it to the loss budget—this is the term that decides whether a low-forward-drop Schottky stays stable in a hot design.
- Surge and fault check. Apply the inrush or fault event the design expects and verify the surge rating and the protection response, because recovery-free switching does not remove surge stress.
When interpreting the results, separate the diode’s contribution from the layout’s: the SiC part removes recovery-induced ringing, but the MOSFET’s output capacitance and the commutation-loop inductance still generate edges that must be managed. The SMPS application section shows the diode families in supply designs, and the Documents section carries the datasheets and package outlines for the measurement setup.
Frequently Asked Questions
Is a SiC Schottky diode the same as a SiC MOSFET body diode? No. The SiC MOSFET body diode also has negligible recovery, but a discrete SiC SBD offers its own voltage/current ratings and is often added in parallel when the body diode’s forward drop or thermal behavior is not sufficient.
Why does the EMI improve with SiC SBDs? The recovery current spike and its abrupt termination are the main high-frequency noise sources at the boost diode. With essentially no recovery, the ringing at the diode node largely disappears.
Can a SiC SBD replace a silicon fast-recovery diode one-for-one? The package and ratings may match, but verify surge, leakage, and thermal behavior; the lower loss may also change the MOSFET’s operating point, so re-check the full stage.
Does leakage current limit SiC SBD use at high temperature? Leakage rises exponentially with temperature and can dominate at high junction temperatures. Review the leakage curve; in most PFC designs the junction stays in a range where leakage is a minor term.
Does a SiC SBD improve light-load efficiency too? The recovery saving scales with the number of switching events, so the biggest gain appears at full load in CCM. At light load the efficiency gain is smaller, but the EMI improvement and the stable high-temperature behavior remain.
The Measurement That Settles It
SiC Schottky barrier diodes win where recovery loss, EMI, or high-temperature operation dominate the converter’s cost and complexity. The device removes the problem at the physics level, but the decision still requires reading surge, leakage, and thermal data at operating conditions. When the comparison is done at system level—switch, heatsink, filter, and compliance—the premium for SiC often pays for itself.