With outstanding material properties, Silicon Carbide (SiC) is triggering technological transformation in core sectors including servers and industrial power supplies. This tutorial focuses on SiC devices, especially the SiC JFET product family. Starting with how SiC restructures power supply design logic, it analyzes its application value in industrial and server power supply scenarios. We have previously released: How SiC Revolutionizes Power Design for Industrial & Server Power Supplies, Three Alternatives to Si & SiC MOSFETs, In-depth Analysis of SiC Cascode JFET & SiC Combo JFET, Replacing Super Junction MOSFETs with SiC CJFET. This article explains the mandatory requirement for snubber circuits in typical CJFET implementations.
(1) What Is a Snubber Circuit?
Snubber circuits deliver switching speed regulation and oscillation suppression for power circuits adopting onsemi SiC cascode JFET (CJFET). For conventional discrete power devices such as silicon/SiC MOSFETs and IGBTs, external gate resistors RG(on) and RG(off) are standard configurations. Tuning these resistor values charges and discharges the gate-drain capacitance CGD, which effectively adjusts the device voltage slew rate (ΔVDS/Δt) and current slew rate (IDS/Δt), while limiting voltage overshoot during device turn-off.
The cascode structure of CJFET (a portmanteau of "cascade" and "cathode", first proposed by R.W.Hickman and F.V.Hunt in 1939 to describe series-connected triode regulator topologies) consists of two series-connected components. For CJFET, the overall equivalent CGD is formed by two series capacitances: the CGD of the low-voltage silicon MOSFET, and the CDS of the SiC JFET. Since the SiC JFET features near-zero CDS, the total equivalent CGD of the cascode structure approaches zero. As a result, the conventional method of tuning CGD to control switching speed becomes nearly ineffective for CJFET.
The optimal approach to control CJFET switching speed, voltage overshoot and parasitic oscillation is to connect a capacitor-type (C-type) or resistor-capacitor-type (RC-type) snubber across the device drain and source terminals; the selection depends on the converter topology. When deployed in half-bridge configuration, RC snubber circuits bring the following advantages:
- Significantly reduced turn-off switching loss
- Minimized gate turn-off resistor RG(off) for further turn-off loss reduction
- In soft-switching applications such as Zero Voltage Switching (ZVS), RC snubbers produce smoother output waveforms without extra turn-on loss, because recycled energy replaces dissipated power loss
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(2) Why Capacitor-Type (C-Type) Snubber Reduces Switching Loss
The circuit diagram below shows a half-bridge power circuit with inductive load. The turn-off waveform diagram at the bottom right includes the blue curve for freewheeling device displacement current Idisp and the red curve for Device Under Test (DUT) total current ID, which combines snubber capacitor CS current and device intrinsic output capacitance COSS current.
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At the initial state, the device channel stays conductive. A decoupling capacitor Cd clamps the bus voltage to a constant level. At the instant of turn-off, the low-side DUT generates a voltage dv/dt, and the high-side device generates reverse dv/dt accordingly. The high-side displacement current Idisp reduces the total ID as illustrated in the waveform. The magnitude of displacement current can be estimated via the dedicated formula.
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Adding a snubber capacitor to this half-bridge circuit cuts down total turn-off current during the dv/dt transition phase. The orange ID and VDS curves only reflect device current (excluding displacement current) with a much faster falling speed. The underlying principle is clear: device channel impedance rises sharply the moment turn-off initiates; meanwhile, the snubber capacitor CS provides an alternative current path with slowly rising impedance. As channel impedance surges, load current diverts to the snubber capacitor branch. Therefore, the current flowing through the main power device drops drastically, achieving dramatic reduction of total turn-off switching loss.
(3) Recommended Layout for C-Type and RC-Type Snubber Circuits
The schematic below demonstrates two valid snubber configuration schemes. Bus snubber circuits leverage decoupling capacitor Cd, which must be physically placed as close as possible to half-bridge switching devices to minimize parasitic inductance in high-speed switching loops.
RC snubber circuits are mandatory for all hard-switching conversion stages (e.g., the first stage of Totem-Pole PFC). For LLC topologies, C-type snubbers are recommended on the primary side. C-type snubbers are also preferred in synchronous rectification, especially for the slow leg of Totem-Pole PFC or secondary side of conventional high-voltage DC LLC converters. Phase-shift full-bridge topologies withstand higher turn-off current than standard LLC circuits, so RC snubbers are the better choice.
For RC snubber implementation, set resistance to the minimum feasible value to retain low switching loss and high efficiency. Ensure the resistor is soldered to wide PCB copper traces, which act as heat dissipation paths; narrow traces fail to dissipate heat generated by the snubber resistor.
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(4) The Empowerment Wave of Silicon Carbide
As demonstrated above, migrating soft-switching power supply designs from traditional silicon MOSFETs to SiC JFETs delivers remarkable efficiency and reliability improvements across the entire system ecosystem.
Take modern data centers as a typical application case. Adopting onsemi EliteSiC CJFET in power designs greatly reduces thermal dissipation demand and boosts switching speed. Power Supply Unit (PSU) engineers can adopt cost-effective, high-efficiency topologies such as full ZVS-capable Totem-Pole PFC (TPPFC). This upgrade saves critical cabinet space for server racks: airflow channels are optimized, and more power modules can be integrated per cabinet. Power Usage Effectiveness (PUE) is pushed closer to the ideal value of 1.0 to lift overall power efficiency, which is critical for generative AI and other high-compute, high-power-consumption scenarios.
Improved cabinet power quality simplifies power distribution hardware, cutting footprint and energy consumption simultaneously. Optimized space utilization allows data center operators to refine existing site layouts instead of constructing new facilities, saving millions to tens of millions of dollars in capital expenditure. All these benefits originate from silicon carbide transistors, which lower resistive and capacitive power loss and heat generation.
This is the core empowerment wave. Optimizing power management workflows from the start with superior materials, mature manufacturing processes, stable supply chains and enhanced performance brings natural performance gains. This is the transformative value onsemi delivers to the industrial power supply industry.