What happens when the MOSFET is turned off?
As the drain-source voltage increases, the drain-gate and drain-body capacitance must be charged, and this means the gate driver must sink that charging current. If it can’t, then the gate voltage may rise enough to turn the MOSFET back on, and depending on your circuit, something bad could happen.
Why does MOSFET have a D V D t limit?
Also, keep in mind that if you succeed at achieving a very fast turn-off, you may bump into your MOSFET’s d v d t limit. As the drain-source voltage increases, the drain-gate and drain-body capacitance must be charged, and this means the gate driver must sink that charging current.
Why do MOSFETs have a long reverse recovery time?
A problem can arise because of the slow turn-off (or reverse recovery) of the internal body diode when the opposing MOSFET tries to turn on. MOSFET body diodes generally have a long reverse recovery time compared to the performance of the MOSFETs themselves.
Where does the charging current go in MOSFET?
Remember, the gate charging current must flow through the gate and the source, and back to the driver.
Why does a FET turn off so slowly?
The FET is turning off slowly because the only thing driving the gate at that time is 10 kΩ impedance. That forms a rather large time constant with the effective total gate capacitance, which makes the turn-off slow. The gate of a FET looks capacitive to the driving circuitry.
How does the inductance of a MOSFET affect the switching speed?
The inductance of this loop is proportional to the area it encircles, and at high frequencies, this inductance will limit the switching speed much more than the resistance R G. Common layout practice is to have a solid ground plane under the MOSFET and the driver, with the gate trace as short as possible running over this.
Why do MOSFETs have a lower your D S?
More expensive MOSFETs can deliver less gate capacitance for a lower R D S ( o n) or maximum current handling ability. Also, don’t use a MOSFET with more current handling ability than necessary; you will pay for it in increased gate capacitance. Most gate driver designs can also benefit from driving the gate to a negative voltage.
Where does the capacitance of a MOSFET come from?
A MOSFET gate and driver look something like this: C G is mostly the gate capacitance of the MOSFET itself. The driver may add some capacitance of its own, but it’s usually negligible. L G and R G mostly come from the gate driver circuit.