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MOSFET (Si/SiC) Forum Discussions

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How is the total gate charge determined for CoolMOS?
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srhim1971
Employee
10 replies posted 5 replies posted First question asked
Employee
The below diagram available in CoolMOS™ device datasheet shows the typical variation of the requisite gate charge at the given gate-source and drain-source voltages for turning on a Mosfet with the specified current.

2604.attach

The gate charge comprises the charge QGS, which is required for charging the gate-source capacitance CGS. During this phase, after the gate threshold voltage VGS(th) has been reached, the drain current rises to its specified value, and the drain-source voltage then falls. But until this voltage VDS has fallen to its actual residual value, the gate-drain capacitance (Miller capacitance) has to be discharged. This charge component is defined as the gate-drain charge QGD. The charge Total Gate charge QG = QGS + QGD is not sufficient fully to turn on the transistor since the residual voltage and the drain-source turn-on resistance have not yet been minimized. Only with a charge corresponding to a gate-source voltage of VGS = 10 V are turn-on resistance, and thus static losses, optimized. This whole charge QGtot or total gate charge depends on the drain-source voltage that has to be switched; the magnitude of the drain current that has to be switched has little influence on the requisite overall charge

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1 Reply
srhim1971
Employee
10 replies posted 5 replies posted First question asked
Employee
The below diagram available in CoolMOS™ device datasheet shows the typical variation of the requisite gate charge at the given gate-source and drain-source voltages for turning on a Mosfet with the specified current.

2604.attach

The gate charge comprises the charge QGS, which is required for charging the gate-source capacitance CGS. During this phase, after the gate threshold voltage VGS(th) has been reached, the drain current rises to its specified value, and the drain-source voltage then falls. But until this voltage VDS has fallen to its actual residual value, the gate-drain capacitance (Miller capacitance) has to be discharged. This charge component is defined as the gate-drain charge QGD. The charge Total Gate charge QG = QGS + QGD is not sufficient fully to turn on the transistor since the residual voltage and the drain-source turn-on resistance have not yet been minimized. Only with a charge corresponding to a gate-source voltage of VGS = 10 V are turn-on resistance, and thus static losses, optimized. This whole charge QGtot or total gate charge depends on the drain-source voltage that has to be switched; the magnitude of the drain current that has to be switched has little influence on the requisite overall charge
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