4kW LLC Converter Secondary Rectifier Diode Selection Guide: From SiC Advantages to Levane Power HLF4K Practice

I. Secondary Rectification: A Critical Efficiency Barrier in 4kW LLC Converters

In the previous two articles of this series, we discussed transformer design (covering high‑voltage insulation, core selection, winding structure, loss estimation, etc.) and 1200V SiC MOSFET selection (including vendor comparison, loss calculation, gate drive design, etc.). This follow‑up article focuses on the output rectifier devices on the secondary side of the LLC resonant converter – a key component that determines the overall conversion efficiency and long‑term reliability of a 4kW high‑voltage DC‑DC power supply.

Recall the key parameters of the Levane Power HLF4K series airborne power supply: input 680‑800 Vdc, output 60 Vdc (adjustable), rated output power 4000 W, conversion efficiency ≥97%, dimensions 187 × 95 × 70 mm, operating temperature -20°C to +55°C. At 4000 W output and 60 V output, the rated output current is approximately 66.7 A. In a full‑bridge LLC topology, the secondary side typically uses a full‑bridge rectifier configuration with four rectifier diodes, each carrying an average current of about 16.7 A under ideal current sharing.

The secondary rectifier diodes in an LLC resonant converter operate in a soft‑switching environment – the primary switches achieve ZVS (zero‑voltage switching), while the secondary diodes naturally achieve ZCS (zero‑current switching). However, under light‑load conditions, during start‑up transients, or during dynamic load changes, the LLC may temporarily deviate from the ideal resonant operating point. At these times, the diodes experience hard‑commutation stress. The reverse recovery current generated by conventional silicon fast‑recovery diodes (FRD) during hard commutation not only causes significant turn‑off losses in the diodes themselves but also induces voltage spikes and severe EMI noise in the primary MOSFETs and the transformer, becoming a key bottleneck that limits further efficiency improvement.

This is exactly where silicon carbide (SiC) Schottky diodes excel. SiC has a bandgap three times that of silicon, a critical electric field ten times higher, and a thermal conductivity more than three times higher. These inherent properties enable excellent performance in high‑voltage, high‑temperature, and high‑frequency applications. SiC devices feature near‑zero reverse recovery charge (Qrr), eliminating voltage spikes in high‑frequency hard‑switching scenarios and allowing the size of the EMI filter on the rectifier stage to be reduced. For high‑power LLC converters that pursue conversion efficiency above 97% and need to operate over wide load and temperature ranges, using SiC Schottky diodes on the secondary rectifier stage has become the mainstream industry choice.

II. SiC Schottky Diode Technology Architectures: From SBD to JBS to MPS

1200V SiC Schottky diodes have evolved through three main technology generations: SBD (Schottky Barrier Diode), JBS (Junction Barrier Schottky), and MPS (Merged PiN Schottky).

Wolfspeed’s 1200V SiC Schottky diode series uses the MPS (Merged PiN Schottky) design, which is more rugged and reliable than standard Schottky barrier diodes. When used together with SiC MOSFETs, they further improve system efficiency. Invent Chip’s SiC diodes also use an MPS structure; the PiN junction turns on more easily at high currents, injecting minority carriers into the high‑resistance drift region to reduce on‑resistance via conductivity modulation. Toshiba has released its 3rd‑generation SiC Schottky barrier diodes, integrating the MPS structure into a JBS architecture to lower forward voltage at high currents while reducing leakage current.

Selection advice: For 4kW LLC applications, JBS devices are sufficient for most operating conditions. If surge tolerance is critical (e.g., harsh outdoor environments, locations with frequent grid fluctuations), MPS devices are preferred. Wolfspeed’s MPS‑based ruggedness and reliability make it a reference benchmark for industrial applications. For cost‑performance balance, JBS remains the mainstream choice.

III. Detailed Selection Parameters

3.1 Reverse Voltage (VRRM)

The voltage stress on the secondary rectifier in an LLC is approximately twice the output voltage (for a full‑bridge rectifier). For a 60 V output, the ideal stress is about 120 V; when the output is adjusted to 400 V, the ideal stress rises to about 800 V. However, the selection must consider possible voltage spikes in the LLC, overshoots during start‑up and dynamic response, and design margin. The final recommendation is to choose a 1200V rated SiC Schottky diode – the engineering standard for 4kW LLC converter secondary rectification. This rating matches existing high‑voltage bus architectures while leaving room for future evolution to 800V platforms.

3.2 Forward Current Capability

At 4kW output and 60 V output, the rated output current is about 66.7 A. With a full‑bridge rectifier using four diodes, each diode carries an average current of about 16.7 A. Considering high‑temperature derating (current capability at 175 °C typically drops to 60‑70% of the 25 °C value), current margin (recommended >30%), and surge current capability (IFSM of SiC SBDs is typically 5‑10 times the rated DC current), it is recommended that each diode be rated for at least 20 A, with 30 A to 40 A devices preferred to balance reliability and cost.

3.3 Conduction Loss and Forward Voltage Drop (VF)

Conduction loss is calculated as Pcond =VF ×IF,rms ×D. VF is highly dependent on junction temperature (Tj). SiC diodes have a positive temperature coefficient – VF increases as Tj rises. This is a crucial difference from silicon FRDs, which have a negative temperature coefficient. In parallel operation, silicon FRDs tend to suffer from current imbalance and thermal runaway because VF decreases with temperature. In contrast, the positive temperature coefficient of SiC devices means that in a parallel configuration, the hotter device will automatically have a higher VF, reducing its current, achieving natural current sharing and thermal balance. This makes SiC diodes well‑suited for multi‑device parallel applications.

Typical 1200V/20A SiC Schottky diodes have a VF of about 1.5‑1.6 V at 25 °C, rising to about 1.7‑1.8 V at 125 °C. The ROHM SCS240KE2C has a typical VF of 1.8 V at I_F=20 A, Tj=150 °C; onsemi NDSH40120C has a typical VF of about 1.75 V at 40 A; Wolfspeed’s 1200V series has a typical VF of 1.5 V at 20 A. Choosing a device with lower VF directly reduces conduction losses, providing a clear direction for designers pursuing higher system efficiency.

3.4 Switching Loss and Capacitance Charge (Qc)

Switching loss is calculated as Psw = fsw ×Ctotal ×VR × VR / 2. This is where SiC Schottky diodes most clearly outperform silicon FRDs – their near‑zero reverse recovery charge (Qrr) eliminates voltage spikes in high‑frequency hard‑switching scenarios. The ROHM SCS240KE2C has a total capacitance charge Qc of 66 nC and a reverse recovery time of 18 ns, with switching characteristics almost unaffected by temperature. Vishay’s 3rd‑generation SiC diodes have a Qc as low as 28 nC, greatly reducing capacitive switching losses.

Selection tip: When other parameters are similar, prefer devices with lower Qc/Qrr. This will significantly reduce switching losses during hard‑commutation transients and lower system EMI.

IV. Mainstream 1200V SiC Schottky Diode Product Matrix

Infineon’s CoolSiC Schottky 1200V G5 diode series offers current ratings of 10A, 15A, 20A, 30A, and 40A. A 10A device can directly replace a 30A silicon diode, significantly improving system efficiency. Nexperia’s 1200V/20A SiC Schottky diode (PSC20120J/L) is designed for ultra‑low‑loss rectification in industrial applications, playing a key role in high‑efficiency scenarios such as AI servers, telecom equipment, and solar inverters.

V. Practical Selection for Levane Power HLF4K Series Secondary Rectifier

Based on the HLF4K‑D60‑800’s specifications: output 60 Vdc, 4000 W rated power, efficiency ≥97%, operating temperature -20°C to +55°C, and compact dimensions 187 × 95 × 70 mm (which limits heatsink volume), the key points for secondary rectifier selection are:

1. Stress calculation: Output peak current ≈66.7 A → per‑diode average current in full‑bridge rectifier ≈16.7 A.

2. Derating: Consider current derating (≈80‑85%) at 55°C ambient, plus temperature safety margin → per‑diode rating ≥20 A → 30 A to 40 A recommended. ROHM SCS240KE2C’s 40 A rating provides ample current margin.

3. VF vs. loss trade‑off: Each 0.1 V reduction in VF reduces conduction loss per diode by about 1.7 W, totalling about 6.8 W for four diodes, increasing efficiency by about 0.17 percentage points. The difference between Wolfspeed’s 1.5 V, onsemi’s 1.45 V, and ROHM’s 1.8 V can affect the efficiency window by up to 0.6 percentage points.

4. Surge capability: A high IFSM rating (typically 5‑10 times the rated current) is an important redundancy metric to ensure reliable operation during grid fluctuations and load transients. The ROHM SCS240KE2C has IFSM ratings of 83 A / 160 A.

5. Package and thermal management: Given the compact dimensions of the HLF4K (187 × 95 × 70 mm), the TO‑247 package combined with a heatsink and forced air cooling is recommended for efficient integrated heat dissipation.

6. Local sourcing path: InventChip’s IV1D12030U2 (30 A/1200 V) improves overall efficiency by about 0.5% in 30 kW charging modules, with IFSM=224 A and VF=1.56 V@25 °C. Local alternatives are becoming increasingly mature.

VI. Loss Calculation and Thermal Design

6.1 Conduction Loss Pcond

Take the ROHM SCS240KE2C as an example. Assume VF $1.8V$ at IF $20A$, Tj = $150 °C$. The total average output current is 66.7 A. With a full‑bridge rectifier using four diodes, the average current per diode is about 16.7 A. Conduction loss per diode is approximately 1.8 V×16.7 A×0.5≈$15\mathbf{W}$. Total conduction loss for four diodes is about 60 W, accounting for about 35‑40% of the total converter losses (assuming 97% efficiency corresponds to about 120 W total loss). Choosing a device with lower VF will further reduce the total conduction loss.

6.2 Switching Loss Psw

In a soft‑switching LLC, the diodes operate with ZCS under ideal resonant conditions, making switching loss nearly zero. However, under light load or during dynamic regulation when the LLC deviates from resonance, the diodes may experience hard commutation, and switching loss cannot be neglected. The formula is Psw = fsw ×Ctotal ×VR × VR / 2. Thanks to the ultra‑low Qc/Qrr of SiC Schottky diodes, switching losses during hard commutation are kept under control. Practical measurements show that using SiC Schottky diodes improves the overall efficiency of the AC‑DC stage while allowing higher operating frequencies, thereby reducing the size of magnetic components.

6.3 Junction Temperature Estimate

Junction temperature is estimated as Tj =Tcase +(Ptotal ×RθJC). The ROHM SCS240KE2C has RθJC ≈$1.0K/W$. With good heat sinking, the junction temperature can be kept within a safe range. Wolfspeed’s MPS structure and Infineon’s thin‑wafer technology both offer excellent thermal performance, helping to keep junction temperatures within safe limits in high‑power‑density designs.

6.4 Parallel Operation Considerations

If multiple diodes need to be paralleled for higher output current (e.g., above 9 kW), the positive temperature coefficient of SiC Schottky diodes effectively prevents thermal runaway. However, careful attention must still be paid to symmetric PCB layout and uniform heatsink temperature distribution.

VII. RC Snubber Circuit Design Points

Although SiC Schottky diodes significantly reduce reverse recovery effects, reverse voltage spikes exceeding the 1200 V rating can still appear across the diodes during hard commutation or when parasitic PCB parameters are large. It is recommended to include an RC snubber circuit as a safety margin during the initial design phase. Typical starting values: resistor R=$22 \Omega$ to $47\mathrm{\Omega }$ (1 W), capacitor C=$470pF$ to $1nF$ (voltage rating >1000 V, NP0/C0G dielectric). The snubber components should be placed as close as possible to the diode’s anode‑cathode terminals with very short leads. After debugging, the snubber may be simplified or retained depending on measured spike levels.

VIII. Layout and Thermal Design Points

1. Minimise high‑frequency loops: The high‑frequency current loop formed by the LLC transformer secondary winding, rectifier diodes, and output capacitor should be as short as possible. Use wide, short copper traces to directly connect the diodes to the transformer secondary pads and the output capacitor, minimising parasitic inductance.

2. Thermal management: For TO‑247 packages, apply thermal grease (or a thermal pad) to the metal backplate and firmly attach to a heatsink with screws. If multiple diodes are used in parallel, ensure they are mounted on an isothermal heatsink surface. SiC Schottky diodes have a maximum junction temperature of 175 °C, allowing higher operating temperatures and providing ample safety margin for high‑temperature environments.

3. Kelvin sensing: If synchronous rectification is used, current sense signals should be routed using a Kelvin connection – separate traces from the sense resistor terminals to avoid interference from the high‑current path.

IX. Conclusion

Following the transformer design (first article) and the SiC MOSFET selection (second article), the secondary rectifier diodes represent the final piece of the LLC resonant converter output stage. From international vendors such as ROHM, Wolfspeed, onsemi, and Infineon, to local alternatives like InventChip, the product matrix of 1200 V SiC Schottky diodes is becoming increasingly rich, allowing engineers to choose flexibly based on cost, performance, supply, and certification requirements.

SiC devices are pushing traditional rectifiers and power supplies into a new era of “high voltage, high frequency, high efficiency”. In high‑efficiency rectifier topologies such as LLC, using SiC diodes in the high‑voltage rectifier stage allows switching frequencies to be pushed to 100 kHz–500 kHz while significantly reducing switching losses. This trend aligns perfectly with Levane Power’s technology roadmap – in compact airborne power supplies, higher operating frequencies mean smaller magnetic components and higher power density, precisely the technical foundation that enables the HLF4K series to achieve >97% efficiency and a compact 187 × 95 × 70 mm form factor.

About Levane Power

Levane Power Technology Co., Ltd. is a high‑tech enterprise focusing on DC/DC power products in the new energy field, dedicated to the R&D and manufacturing of high‑performance, high‑efficiency power conversion systems. Leveraging more than twenty years of technical accumulation and automotive‑grade development standards, Levane Power provides customised power solutions characterised by high efficiency, high power density, and high reliability for tethered drones, new energy vehicles, industrial equipment electrification, and other fields.

For more product information, technical specifications, or customisation, please visit the Levane Power official website or contact the technical support team directly.

Series Articles (links to be added as appropriate):

• [4kW LLC Transformer Design Deep Dive: From Insulation Challenges to Levane Power HLF4K Series Airborne Power Supply Practice] – Part 1

• [4kW LLC Converter 1200V SiC MOSFET Selection Guide: From Loss Calculation to Levane Power HLF4K Practice] – Part 2

• [4kW LLC Converter Secondary Rectifier Diode Selection Guide: From SiC Advantages to Levane Power HLF4K Practice] – Part 3 (this article)

Coming soon: Subsequent articles in this series will cover output filter capacitor selection, loop compensation and control design, overall efficiency optimisation and thermal management, and more.