2021.05.12 SiC Power Device
In a series of six articles with the title "SiC MOSFET: Behavior of Gate-Source Voltage in Bridge Configuration", we have explained the behavior of the gate voltage that can occur in a bridge configuration in which rapid switching of large currents is performed.
MOSFET gate voltages in a bridge configuration are extremely complicated, due to the fact that the MOSFETs operate in relation to each other. Moreover, the behavior changes dramatically depending on the conditions of the gate driving circuit. For example, even when the circuit design and the MOSFETs and other components are the same, the behavior can be completely different if the PCB and the wiring layout are different. In fact, it should be considered that the behavior changes greatly with the PCB and wiring; if one assumes that there is no need for diligent evaluations when the PCB and layout are different because the circuit design has a proven track record, all manner of problems will result.
An understanding of the basic operation and the mechanisms underlying waveform behavior will make possible countermeasures when problems occur in actual design. The example used in these articles was a boost (step-up) circuit with LS MOSFET switching, but in a buck (step-down) circuit with HS MOSFET switching, the same operation occurs simply by interchanging the LS and HS operation. Hence the same basic approach can be applied for various circuit topologies using hard switching.
The following provides links to the series articles and summarizes the key points in each; we hope they will prove useful.
SiC MOSFET: Behavior of Gate-Source Voltage in Bridge Configuration
・Power switching devices are used as switching elements in various power supply applications and power lines.
・Circuit types used vary greatly, and there are many methods of use as well.
・In a bridge configuration in which switching elements are series-connected on high and low sides, the elements affect each other through their alternating on-off operation.
・Detailed understanding of switching operation is necessary for successful fast switching conversion of large amounts of power.
・In examining" SiC MOSFET: Behavior of Gate-Source Voltage in Bridge Configuration", the simplest synchronous boost circuit, using MOSFETs in a bridge configuration, is used as an example.
・The configuration and operation of the example circuit, and the voltage and current waveforms, should be understood.
・The manners of changes in VDS and ID are different during turn-on and during turn-off.
・The effects of these changes on VGS are considered based on an equivalent circuit that includes the parasitic components of the gate driving circuit.
・Due to changes in the VDS and ID of MOSFETs in switching operation in gate driving circuits, parasitic capacitances and inductances cause currents and voltages to appear in the circuits.
・Because dVDS/dt and dID/dt assume both positive and negative values, the polarities of the resulting currents and voltages are different during turn-on and turn-off.
・In a bridge configuration, due to MOSFET gate capacitances and changes in VDS and ID resulting from switching, in some cases HS self turn-on may occur when the LS switch turns on.
・One method to deal with self turn-on involves reducing the values of external gate resistors, but in order to avoid effects on other operation, measures are necessary to reduce the gate resistance only during HS turn-off.
・When the LS switch has turned off also, the same behavior as during turn-on occurs.
・Minus surges occurring on the HS may exceed the rated value, and in such cases circuit modification is necessary.