Behavior of gate-source voltage in bridge configuration: when turned on

“TO-247-4L and TO-263-7L package SiC MOSFETs with driver source pins, SiC MOSFET gate-source voltage behavior is different compared to TO-247N package SiC MOSFET products without driver source pins .

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Key takeaways from this article:

TO-247-4L and TO-263-7L package SiC MOSFETs with driver source pins, SiC MOSFET gate-source voltage behavior is different compared to TO-247N package SiC MOSFET products without driver source pins .

To properly implement SiC MOSFET gate-source voltage surge countermeasures, it is necessary to understand the SiC MOSFET voltage behavior on a case-by-case basis.

Among the most common applications for power switching devices includes the same bridge configuration as the double-pulse test circuit mentioned in the previous article. For the behavior of the gate-source voltage in the case of a bridge structure, in Tech Web Basics SiC Power components “SiC MOSFETs: Behavior of the gate-source voltage in a bridge structure” and the application on which this article is based In the guide “behavior of gate-source voltage in bridge structure”, the behavior of mutual influence is described.

However, the TO-247-4L and TO-263-7L packaged SiC MOSFETs with driver source pins behave differently than the TO-247N packaged products without driver source pins, requiring To properly implement gate-to-source voltage surge countermeasures, it is necessary to understand the behavior of voltages.

Starting from this article, the gate-source voltage behavior of TO-247-4L packaged SiC MOSFETs with driver source pins in the case of bridge configuration will be divided into LS-side (low-side) MOSFET turn-on and turn-off The two cases are introduced separately in two sections.

Behavior of gate-source voltage in bridge structure: when turned on

The following describes the operation of the LS-side (low-side) MOSFET in the bridge structure when it is turned on, focusing on the differences from the TO-247N package MOSFET that does not have a driver source pin.

The figure below shows the switching waveforms during turn-on. The left side is the TO-247N package product without driver source pins, and the right side is the TO-247-4L package product with driver source pins. Each horizontal axis represents time, and the definition of the time range Tk (k=7, 8, 1-3) is described below the waveform graph. The circuit diagram at the bottom right shows the gate pin current for the TO-247-4L packaged product in a bridge circuit. In the waveform and circuit diagrams, use (I) to (III) to represent the events that occur in each time range. Event (III) occurs immediately after the end of the T2 period.

Each switching waveform when the LS-side SiC MOSFET is turned on in a bridge structure

TO-247-4L: Gate pin current when LS is on
T7: HS is the ON period (synchronous rectification period)
T8: Dead time before HS turns off and LS turns on
T1: During LS conduction and MOSFET current change[event (I) occurs at the same time]T2: During LS conduction and MOSFET voltage change[event (II) occurs simultaneously]T3: LS ON period

In the waveform comparison, the event (I) of TO-247-4L is significantly different from the event (I) of TO-247N, a positive surge is observed on the VGS of the non-switching side (HS) (TO-247N is a negative surge ). This is caused by the current ICGD in the gate pin current diagram (I) (HS side, green line). This current will flow through the gate-drain capacitance CGD.

This current flows because the commutation current ID_HS flows from the source to the drain in the body Diode of the HS-side SiC MOSFET before the switching operation, but when the subsequent switching operation starts, the switching-side (LS) The current ID_LS first gradually increases, so ID_HS gradually decreases. On the other hand, the forward voltage VF_HS of the body diode of SiC MOSFET (the dotted circled part of the TO-247-4L waveform) has a large current dependence, so as the switching speed increases, dID_HS/dt increases , dVF_HS/dt will increase, and dVF_HS/dt is ultimately also the dVDS_HS/dt of the commutator-side SiC MOSFET, so ICGD flows from the drain pin to the gate pin through CGD, causing the gate-source voltage to rise. In the conventional TO-247N package, ID_LS changes slowly, and it can be considered that ICGD of event (I) hardly flows.

For a detailed introduction to the turn-on action of the TO-247N, please refer to the article “Gate-Source Voltage Action when the Low-Side Switch is Turned Off” in the Tech Web SiC Power Components Basics mentioned at the beginning of this article or in the application note. “Operation of gate signal at turn-on”.

Comparison of VDS waveforms when TO-247-4 and TO-247-4L are turned on

The VDS waveform shown above is a comparison of TO-247N and TO-247-4L. As can be seen from the figure, regarding the VDS_HS of the commutation-side SiC MOSFET, the VDS_HS of the TO-247-4L rises sharply immediately after the switching operation starts. As mentioned in the previous article, this is the speedup effect due to having the driver source pins.

In addition, since the event (II) is also in a high-speed state, the current flowing from the HS side to the LS side and charging the CDS on the HS side as shown in the previous circuit diagram also becomes large, so not only the switching side, but also the non-switching side. The side also needs to take countermeasures against the surge between the drain and source.

Below is the VGS waveform of TO-247-4L. This waveform graph compares the results with or without surge countermeasures. As can be seen from the figure, the aforementioned surge occurs without taking a surge countermeasure (Non-Protected). After implementing the surge countermeasure (Protected), the VGS surge was well suppressed.

VGS waveform when TO-247-4L is on (with or without countermeasures)

In order to suppress these surges, it is necessary to understand the behavior of the aforementioned gate-source voltage, and to connect a surge suppression circuit in close proximity to the SiC MOSFET as a countermeasure.

For more detailed information, please refer to the application note “Gate-Source Voltage Surge Suppression Methods” or Tech Web Basics SiC Power Components “SiC MOSFETs: Gate-Source Voltage Surge Suppression” method” (in serial).

In the next article, we will describe the behavior of SiC MOSFET gate-source voltage when the low-side SiC MOSFET is turned off.