SiC Power Device｜Application
Behavior of Gate-Source Voltages when in a Bridge Configuration: Behavior at Turn-off
2024.04.24
Points of this article
・The behavior of the gate-source voltage of SiC MOSFETs in TO-247-4L and TO-263-7L packages, which have driver source terminals, is different at both turn-on and turn-off from that of SiC MOSFETs in TO-247N packages, which lack driver source terminals.
・In order to precisely implement gate-source voltage surge countermeasures, the behaviors of both kinds of devices must be understood.
The behavior of the gate-source voltage of SiC MOSFETs in packages with driver source terminals and SiC MOSFETs in packages without such terminals, when used in a bridge configuration, is different. Regarding this difference, in the previous article the behavior at turn-on of LS (low side) SiC MOSFETs was explained. In this article, the behavior at turn-off of LS SiC MOSFETs is addressed.
Behavior of Gate-Source Voltages when in a Bridge Configuration: Behavior at Turn-off
Below is explained the behavior at turn-off of LS SiC MOSFETs in a bridge configuration, in packages having a driver source terminal. As in the previous article, the focus is on differences with devices in TO-247N packages, which do not have driver source terminals.
The following waveform diagrams show switching waveforms at turn-off; the diagrams on the left are for devices in TO-247N packages which do not have driver source terminals, while the diagrams on the right are for devices in TO-247-4L packages having driver source terminals. Both horizontal axes denote time, and the time intervals Tk (k=3 to 7) are defined as indicated. The circuit diagram on the lower right indicates the currents in the gate terminals of TO-247-4L package devices in a bridge circuit. Events occurring in the different time intervals are labeled (IV) to (VII) in both the waveform diagrams and the circuit diagram. The event (VII) occurs immediately after the interval T5 has ended.
Devices in TO-247N packages not having driver source terminals
Devices in TO-247-4L packages having driver source terminals
Switching waveforms at turn-off of LS SiC MOSFETs in a bridge configuration
<Definitions of time intervals Tk>
- T3： Interval during which LS is on
- T4： Interval in which LS is off and the MOSFET voltages are changing (at the same time, event (IV) occurs)
- T5：Interval in which LS is off and the MOSFET currents are changing (at the same time, event (VI) occurs)
- T4 to T6： Dead time interval until HS turns on
- T7：Interval in which HS is on (synchronous rectification interval)
TO-247-4L： Currents in gate terminals at LS turn-off
Upon comparing the waveform diagrams, the events (VI) and (VII) for the TO-247-4L devices differ from those for the TO-247N devices.
The event (VI) is, similarly to during turn-on, the timing with which ID changes; because the HS current ID_HS increases rapidly, the body diode VF_HS rises sharply (dashed circle part in the previous waveform diagram). Consequently the current ICGD again flows due to dVF_HS/dt, and a negative surge occurs.
The diagram on the right shows VDS waveforms at turn-off on the switching side (LS) and the commutation side (HS). Similarly to the case of turn-on, the HS VDS_HS becomes negative when ID changes (interval T5) after completion of the normal dVDS_HS/dt change (interval T4), indicating the occurrence of a nonzero dVF_HS/dt.
Comparison of VDS waveforms at turn-off of TO-247N and TO-247-4L devices
In event (VII), when interval T5 is completed and the change in ID_HS is gone, dVF_HS/dt goes to zero and the current ICGD flowing into the gate terminal stops flowing, the emf due to the energy accumulated in the wiring inductance that exists in the ICGD current path is observed as a positive surge between gate and source. This positive surge is hardly observed at all in TO-247N devices.
For details of turn-off operation of devices using the TO-247N package, please see either the SiC power device article in Tech Web Basic Knowledge mentioned at the outset, “Behavior of the Gate-Source Voltage During Low-side Switch Turn-off“, or the Application Note item on “Behavior of Gate Signals at Turn-off“.
Shown next are the VGS waveforms at turn-off of TO-247-4L devices, comparing the behavior when surge countermeasures are present and not present. When there are no surge countermeasures present (“non-protected”), surges occur, as explained above. When surge countermeasures are implemented (“protected”), we see that VGS surges are suppressed.
In order to suppress these surges, it is essential that the gate-source voltage behavior explained in this and the preceding article be understood, and that a surge suppression circuit be connected near the SiC MOSFETs as a countermeasure.
VGS waveforms at turn-off of TO-247-4L devices
(With and without countermeasures)
For further details, please refer to either the Application Note on “Gate-Source Voltage Surge Suppression Methods“, or to ” SiC MOSFETs: Methods for Suppressing Gate-Source Voltage Surges” in the section of Tech Web Basic Knowledge on SiC power devices.
【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
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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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