SiC Power Device｜Application
SiC MOSFETs： Snubber Circuit DesignsSurges Occurring between Drain and Source
2024.09.25
Points of this article
・Surges that occur between drain and source are caused by resonances due to inductive components and the MOSFET parasitic capacitance.
・In many cases it is not realistic to use a layout that minimizes wiring inductance; hence it is important to locate a snubber circuit as close as possible to switching devices to reduce wiring inductance.
As our initial topic, we explain drain-source surges that occur in SiC MOSFET power conversion circuits.
- 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
Surges Occurring between Drain and Source of SiC MOSFETs
Surges between drain and source occur because the energy of the current that flows at turn-on remain stored in inductances in the wiring and board pattern, and this energy causes resonance with parasitic capacitances of switching devices. Fig. 1 explains the paths of ringing currents during surge occurrence. Fig. 1 shows the case of a bridge circuit in which switching devices are connected on the high side (hereafter “HS”) and the low side (“LS”), in which the LS is turned on and the switching current IMAIN flows. This current IMAIN normally flows in from VSW and passes through the wiring inductance LMAIN.
Next, when the LS is turned off, the current IMAIN that had been flowing through LMAIN normally passes through the HS and LS parasitic capacitances via the bulk capacitor CDCLINK connected between the input power supply HVdc and PGND, as indicated by the dotted line. At this time, a resonance phenomenon due to LMAIN and the SiC MOSFET parasitic capacitance COSS (CDS+CDG) occurs between the LS drain and source, so that a drain-source surge appears. The maximum value VDS_SURGE of this surge is expressed by the following equation, where the voltage applied to the HVdc pin is VHVDC and the resistance when the MOSFET is turned off is ROFF (*1).
![V_(DS_SURGE)=(V_A∙e^(-(a/ω)[tan^(-1)〖(a/ω)+ϕ〗 ] ))/(1+(a/ω)^2 )+V_HVDC](https://techweb.rohm.com/wp-content/uploads/2024/09/sic_ap_5_f02.webp){kind=small}
Here,
Fig. 2 shows the turn-off waveform with surge when using the SCT2080KE SiC MOSFET. We see that when 800 V is applied to HVdc, VDS_SURGE is 961 V, and the ringing frequency is about 33 MHz. Using equation (1) to calculate LMAIN from this waveform yields a value of about 110 nH.
Next, a snubber circuit CSNB is added as shown in Fig.3. The waveform of the surge voltage at turn-off after LMAIN is virtually removed is shown in Fig.4.
By the addition of this CSNB, the surge is reduced by 50 V or more (to about 901 V), and the ringing frequency is higher as well at 44.6 MHz. Thus we see that LMAIN for the circuit network including CSNB, is smaller.
Upon similarly using equation (1) to calculate LMAIN, we find that the value is decreased from about 110 nH to approximately 71 nH. Ordinarily, it would be desirable to use a pattern design that minimizes this wiring inductance; but because the device’s heat dissipation design is typically prioritized, the ideal wiring design may not be feasible.
As one means of addressing this issue, by positioning the snubber circuit as close to the switching device as possible and forming a bypass circuit, the wiring inductance that is the origin of surges can be minimized, and the energy accumulated in the minimized wiring inductance can be absorbed. The voltage of the switching device can then be clamped, and turn-off surges can be diminished.
*1: Basics of Switching Converters, K. Harada, T. Ninomiya, F. Koshi, Corona, February 1992, pages 95-107
【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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