SiC Power Device｜Application
Non-Discharge RCD Snubber Circuit Design
2024.10.23
Points of this article
・In a non-discharge RCD snubber circuit, the power dissipated by RSNB is only a fraction of the surge power, so that the allowable power dissipation for RSNB can be comparatively small.
・As a result, the capacitance value of CSNB can be made large, so that the clamping effect can be enhanced, and the switching frequency fSW can be raised.
・In general, a circuit with non-discharge RCD snubber circuits added has lower efficiency under light loading but higher efficiency under heavier loads. This is because of the surge suppression effect of the snubber circuits under heavy loading, as a result of which switching losses are reduced.
Here the design of non-discharge RCD snubber circuits, which are the last of the four kinds of snubber circuit, is explained.
- Surges occurring between drain and source
- Types and selection of snubber circuits
- C snubber circuit design
- RC snubber circuit design
- Discharge RCD snubber circuit design
- Non-discharge RCD snubber circuit design
- Difference in surge occurrence depending on package
SiC MOSFET: Non-Discharge RCD Snubber Circuit Design
In a non-discharge RCD snubber circuit, in contrast with discharge RCD snubber circuits, the power dissipated in RSNB is only a part of the surge power, so that the allowable dissipation for RSNB can be comparatively small. Hence more options are available when selecting RSNB, and the capacitance value of CSNB can be increased, so that the clamping effect can be improved.
CSNB is determined by equation (2) which was presented in the article “C Snubber Circuit Design“, and RSNB is determined by equation (3), which appears in “RC Snubber Circuit Design“. However, the RSNB power dissipation is determined by equation (6) below. This equation does not have the second term with CSNB and fSW in equation (4) appearing in “RC Snubber Circuit Design“, and so there is no increase in the dissipated power due to either CSNB or fSW. Hence the capacitance of CSNB can be set high, and a snubber circuit with a prominent clamping effect can be obtained. This circuit can also accommodate high fSW frequencies.
Fig. 8 shows the discharge path after operation of the non-discharge RCD snubber circuit. Because the upper arm surge flows to PGND and the lower arm discharge current flows to HVdc through RSNB, the wiring inductance does not have much of an effect. On the other hand, due to the large change in current, the wiring inductance LSNB connected between the MOSFET drain and source must be set as small as possible.
Fig. 9 shows waveforms, obtained using the P02SCT3040KR-EVK-001 evaluation board, to verify the effect of a non-discharge RCD snubber circuit using a SCT3080KR SiC MOSFET. Here (a) is the measurement circuit, and (b) shows waveforms measured with and without the snubber circuits; in these turn-off waveforms, RG_EXT=3.3 Ω, HVdc=800 V, and the drain current ID was about 70 A.
When the snubber circuits were not connected, a surge of 1210 V occurred at turn-off, but with the snubber circuits added the surge was 1069 V, for a decrease of approximately 12%. Also, voltage ringing that had been occurring together with the surge was eliminated by the snubber circuits, enabling a substantial EMI reduction.
Fig. 10 is a graph comparing conversion efficiencies of a buck converter (step-down converter). The input voltage was set to 400 V, the output voltage to 200 V, and RG_EXT to 6.8 Ω; the graph shows efficiencies at a switching frequency of fSW=100 kHz.
When the load power was varied between 1 kW and 4.8 kW, at power values of about 4 kW or lower the efficiency was at most 0.4% better without the snubber circuit, but above 4 kW the efficiency was 0.15% better with the snubber circuit present. This is because at high load power levels the power loss due to surges (losses caused by equivalent series resistances of capacitors and other components due to resonant currents and the like) became large, and surge suppression by a snubber circuit resulted in lowered switching losses.
【Download Documents】 Basics of SiC Power Devices
This handbook explains the physical properties and advantages of SiC, the differences in characteristics and usage of SiC Schottky barrier diodes and SiC MOSFETs with a comparison to Si devices, and includes a description of full SiC modules with various advantages.
SiC Power Device
Basic
- What are SiC Schottky barrier diodes? ? Introduction
- What are SiC-MOSFETs? – SiC-MOSFET Features
- What are Full-SiC Power Modules?
- Summary
- Introduction
- What is silicon carbide?
Application
-
Introduction
- SiC MOSFET Bridge Configuration
- SiC MOSFET Gate Driving Circuit and Turn-On/Turn-Off Operation
- Currents and Voltages Occurring Due to Switching in Bridge Circuits
- Behavior of the Gate-Source Voltage During Low-side Switch Turn-on
- Behavior of the Gate-Source Voltage During Low-side Switch Turn-off
- Summary
- SiC MOSFETs: Method for Determining Losses from Switching Waveforms
-
SiC MOSFETs： Snubber Circuit Designs ーIntroductionー
- Non-Discharge RCD Snubber Circuit Design
- Surges Occurring between Drain and Source
- Types and Selection of Snubber Circuits
- C Snubber Circuit Design
- RC Snubber Circuit Design
- Discharge RCD Snubber Circuit Design
- Non-Discharge RCD Snubber Circuit Design
- Differences in Surge Occurrence Depending on Package
- SiC MOSFETs： Snubber Circuit Designs ーSummaryー
- Points to Note When Measuring SiC MOSFET Gate-Source Voltages: General Measurement Methods
-
Conventional MOSFET Driving Method
- Packages Provided with Driver Source Terminals
- Differences Made by and Benefits of a Driver Source Pin
- Benefits of a Driver Source Terminal: Comparisons Using Double Pulse Tests
- Behavior of Gate-Source Voltages when in a Bridge Configuration: Behavior at Turn-on
- Behavior of Gate-Source Voltages when in a Bridge Configuration: Behavior at Turn-off
- Points to be Noted Relating to Board Wiring Layout Key Points of This Article
- Verification of Loss Reduction Using Latest-Generation SiC MOSFETs
- About Surges in Gate-Source Voltages
Product Information
- SiC Schottky Barrier Diodes
- SiC MOSFET
- SiC Power Modules
- SiC Schottky barrier diode Bare Die
- SiC MOSFET Bare Die
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