SiC Power Device｜Application
Benefits of a Driver Source Terminal: Comparisons Using Double Pulse Tests
2024.03.13
Points of this article
・By providing a driver source terminal, turn-on and turn-off losses can be greatly reduced.
・The increase in switching speed results in larger ID turn-on peaks or VDS turn-off surges, some countermeasure should be employed.
In the previous article, the difference between devices with and without a driver source terminal, and the benefits of the driver source terminal, were confirmed through the principles of operation and governing equations. In a MOSFET provided with a driver source terminal, the effects of the inductance of the source terminal can be eliminated, and consequently switching losses can be decreased. In this article, the benefits of the driver source terminal are confirmed through double pulse tests.
Benefits of a Driver Source Terminal: Comparisons Using Double Pulse Tests
In order to compare actual switching operation of a MOSFET not having a driver source terminal and a MOSFET provided with a driver source terminal, the circuit shown on the right was used to conduct double pulse tests in which the low-side (LS) MOSFET was switched.
The gate terminal and the source terminal or the driver source terminal of the high side (HS) MOSFET are connected via RG_EXT, and only the commutation operation due to the body diode is used. In the circuit diagram, solid lines are connections to the source terminals, and dashed lines represent connections to the driver source terminals.
The waveforms of the drain-source voltages VDS and drain currents ID at turn-on and turn-off, as well as the switching losses, are compared. The devices used in tests were SiC MOSFETs with a maximum rating (VDSS) of 1200 V and an on-resistance (RDS(on)) of 40 mΩ. The TO-247N package (part number: SCT3040KL) has no driver source terminal, while the TO-247-4L package (SCT3040KR) and the TO-263-7L package (SCT3040KW7) have driver source terminals. Driving conditions were an RG_EXT of 10 Ω, and an applied voltage VHVDC of 800 V; the waveforms are for an ID of approximately 50 A.
Compared with the TO-247N device (dashed blue line) without a driver source terminal, the turn-on ID currents of the TO-247-4L device (dashed red line) and the TO-263-7L device (dashed green line), both with driver source terminals, rise sharply. As a result, we see that whereas the switching loss for the TO-247N device (blue line) is 2742 µJ, that of the TO-247-4L device (red line) is about 38% lower at 1690 µJ, and that of the TO-263-7L device (green line) is lower by 24% at 2083 µJ, for dramatic loss reductions.
From the turn-on waveforms, we can confirm that the ID peak value for the TO-247-4L device is large, at 80 A, or 23 A larger than that for the TO-247N device. This is because, the driver source terminal enables high-speed switching, causing charge/discharge times to be shortened, and consequently increasing the charge current peak value, despite the fact that the energy of charging/discharging of COSS accompanying MOSFET switching operation is constant. There is an increase in the peak current due to self turn-on of the high-side MOSFET as well, but the above increase is not due to self turn-on.
The TO-263-7L ID peak value is 60 A, not as large as for the TO-247-4L device. Similarly to the difference in turn-off surges discussed below, this is due to the difference in the package inductance of the commutation side MOSFET (HS). In other words, the emf due to the package inductance total for the switching side (LS) and the commutation side MOSFETs resulting from dID/dt pulls down the switching side MOSFET VDS, causing the energy accumulated in COSS of the switching side MOSFET to be discharged, but for the TO-263-7L device the discharge current is smaller, and turn-on ID peak value is smaller too.
For similar reasons, the switching side MOSFET VDS of the TO-247-4L device is decreased at turn-on, and consequently the turn-on switching loss EON is reduced.
However, if there is no measure to address self turn-on of the TO-247-4L or TO-263-7L devices, there is the concern that when self turn-on has occurred the peak value of the turn-on current may be further increased. Hence it is recommended that some measure be employed without fail to deal with self turn-on, such as a mirror clamp circuit or a nF-order capacitor connected between the gate and the source. For details, please refer to the Application Note “SiC MOSFET: Gate-Source Voltage Surge Suppression Methods”.
Next, turn-off waveforms are shown. Compared with the switching loss for the TO-247N device (solid blue line) of 2093 µJ, the loss for the TO-247-4L device (solid red line) is about 30% lower at 1462 µJ, and that of the TO-263-7L device (solid green line) is reduced by approx. 29% at 1488 µJ; these decreases, while not so great as those at turn-on, are still substantial improvements.
The origin of the turn-off surge observed in VDS upon turn-off is the total parasitic inductance of the main circuit. This is the sum total of the wiring inductance LMAIN of the double pulse test circuit shown above and the package inductances (LDRAIN+LSOURCE) of the switching side and commutation side MOSFETs. Hence in the cases of the TO-247-4L device (solid red line) and the TO-247N device (solid blue line), the package inductances of which are essentially the same, the faster dID/dt becomes, the greater is the surge. In these tests, the result for TO-247-4L is 1009 V, or about 119 V higher than the 890 V result for the TO-247N device, and so it may be necessary to use a snubber circuit or some other surge countermeasure.
The reason for the lower surge observed for the TO-263-7L (solid green line) compared with the TO-247-4L (solid red line), although both have driver source terminals, lies in the difference in package structures. The drain of the TO-263-7L is allotted to a fin on the rear surface of the package, and is directly soldered to the PCB. And by allotting the source terminals to five out of the seven terminals, it may be that the package inductance is smaller than for the TO-247-4L. The smaller the package inductance on the commutation side, rather than the switching side, the smaller are switching side surges.
The table below summarizes the comparison of switching losses.
| Package (Part No.） | Drain source terminal | EON[μJ] | EOFF[μJ] |
|---|---|---|---|
| TO-247N（SCT3040KL） | no | 2742 | 2093 |
| TO-247-4L（SCT3040KR） | yes | 1690 | 1462 |
| TO-263-7L（SCT3040KW7） | yes | 2083 | 1488 |
Conditions: VDS = 800 V, ID = 50 A, RG_EXT = 10 Ω
【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?
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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
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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
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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
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