SiC Power Device｜Application

Behavior of Gate-Source Voltages when in a Bridge Configuration: Behavior at Turn-on

2024.04.03

Points of this article

・There are differences in the behavior of the gate-source voltages of SiC MOSFETs in TO-247-4L and T-263-7L packages, which have driver source terminals, and devices in TO-247N packages, which do not have driver source terminals.

・In order to precisely implement gate-source voltage surge countermeasures, the behaviors of both kinds of devices must be understood.

The most widespread application of power switching devices is in bridge configurations similar to those of double pulse test circuits such as that presented in the previous article. The behavior of gate-source voltages in bridge configurations, relating to mutual interaction, has been explained in a Tech Web Basic Knowledge article on SiC power devices, “SiC MOSFETs: Behavior of Gate-Source Voltage in a Bridge Configuration“, and in an application note that is the basis for the former article, “Gate-Source Voltage Behavior in a Bridge Configuration“.

However, there are differences in the behavior of SiC MOSFETs in the TO-247-4L and TO-263-7L packages having driver source terminals and SiC MOSFETs in the TO-247N package without a driver source terminal. In order to precisely implement measures to deal with gate-source voltage surges, these differences in behavior must be understood.

In this and the next article, the behavior of the gate-source voltages in a SiC MOSFET bridge configuration using devices in the TO-247-4L package with a driver source terminal is explained, considering separately the behavior upon LS (low-side) MOSFET turn-on and the behavior at LS MOSFET turn-off.

Behavior of Gate-Source Voltages when in a Bridge Configuration: Behavior at Turn-on

The behavior of LS (low-side) MOSFETs in a bridge configuration at turn-on is here explained, focusing mainly on the differences with devices in TO-247N packages not having a driver source terminal.

The following waveform diagrams show switching waveforms at turn-on; 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=7, 8, and 1-3) 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 (I) to (III) in both the waveform diagrams and the circuit diagram. The event (III) occurs immediately after the interval T2 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-on of LS SiC MOSFETs in a bridge configuration

＜ Definitions of time intervals Tk ＞

T7： Interval during which HS is on (synchronous rectification interval)
T8： Dead time interval in which HS is off, until LS turns on
T1： Interval in which LS is on and the MOSFET currents are changing (at the same time, event (I) occurs)
T2：Interval in which LS is on and the MOSFET voltages are changing (at the same time, event (II) occurs)
T3：Interval in which LS is on

TO-247-4L： Currents in gate terminals at LS turn-on

When comparing the waveform diagrams, the event (I) seen in the TO-247-4L devices differs markedly from that in TO-247N devices: a positive surge in V_GS for non-switching side (HS) devices is observed (for the TO-247N devices, the surge is negative). This occurs because a current I_CGD (HS, green line) during event (I) flows, indicated in the circuit diagram showing the gate terminal currents. This current flows through the gate-drain capacitor C_GD.

The reason for this current is that prior to switching operation, a commutation current I_{D_HS} is flowing in the body diodes of the HS SiC MOSFETs from the source toward the drain, but when switching operation then begins, the switching-side (LS) current I_{D_LS} first increases, and so I_{D_HS} decreases. On the other hand, in general the forward voltage V_{F_HS} of the body diodes of SiC MOSFETs (the dashed circle part in the TO-247-4L waveform diagrams) has a large current dependence, and so when switching is faster dI_{D_HS}/dt rises and dV_{F_HS}/dt also increases. Because dV_{F_HS} ultimately is also the dV_{DS_HS} of the commutation-side SiC MOSFETs, the current I_CGD flows from the drain terminal through C_GD to the gate terminal, inducing a rise in the gate-source voltage. In devices in conventional TO-247N packages, the change in I_{D_LS} is slow, and it may be that there is almost no flowing of I_CGD in event (I).

For details of turn-on 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, “SiC MOSFETs: Behavior of Gate-Source Voltage in a Bridge Configuration“, or the Application Note item on “Behavior of Gate Signals at Turn-on“.

The V_DS waveforms shown on the right are a comparison of TO-247N and TO-247-4L devices. On examining the V_{DS_HS} waveform for the commutation-side SiC MOSFETs, we see that immediately after switching operation has begun, V_{DS_HS} for the TO-247-4L device rises sharply. This is, as explained in the previous article, the result of faster operation due to the provision of a driver source terminal.

Moreover, operation is also faster in event (II), so that the current flowing from HS to LS in the above circuit diagram and charging the HS C_DS is also larger, and therefore there are cases in which measures are needed to deal with drain-source surges not only on the switching side, but also on the non-switching side.

Comparison of V_DS waveforms at turn-on of TO-247N and TO-247-4L devices

Shown next are the V_GS waveforms for 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 V_GS surges are suppressed.

In order to suppress such surges, it is essential that the behavior of the gate-source voltage explained above be understood, and that a surge suppression circuit be connected in proximity to the relevant SiC MOSFETs.

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.

V_GS waveforms at turn-on of TO-247-4L devices
(With and without countermeasures)

The next article will explain behavior at turn-off.

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Behavior of Gate-Source Voltages when in a Bridge Configuration: Behavior at Turn-on

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