MOSFET Gate Drive
MOSFET Gate Drive:
- MOSFET, being a voltage controlled device, does not require a continuous gate current to keep it in the ON state. However, it is required to charge and discharge the gate-source and the gate-drain capacitors in each switching operation.
- The switching times of a MOSFET essentially depends on the charging and discharging rate of these capacitors. Therefore, if fast charging and discharging of a MOSFET is desired at fast switching frequency the gate drive power requirement may become significant.
- To turn the MOSFET on the logic level input to the inverting buffer is set to high state so that transistor Q3 turns off and Q1 turns on.
- Note that, during turn on Q1 remains in the active region. The effective gate resistance is RG R1 / (β1 1). Where, β1 is the dc current gain of Q1. To turn off the MOSFET the logic level input is set to low state. Q3 and Q2 turns on whole Q1 turn off.
- The switching time of the MOSFET can be adjusted by choosing a proper value of RG. Reducing RG will incase the switching speed of the MOSFET.
- However, caution should be exercised while increasing the switching speed of the MOSFET in order not to turn on the parasitic BJT in the MOSFET structure inadvertently.
- The drain-source capacitance (CDS) is actually connected to the base of the parasitic BJT at the p type body region. The body source short has some nonzero resistance. A very fast rising drain-source voltage will send sufficient displacement current through CDS and RB as shown in Fig 6.10 (c). The voltage drop across RB may become sufficient to turn on the parasitic BJT.
- This problem is largely avoided in a modern MOSFET design by increasing the effectiveness of the body-source short. The devices are now capable of dvDS/dt in excess to 10,000 V/μs. Of course, this problem can also be avoided by slowing down the MOSFET switching speed.
- Since MOSFET on state resistance has positive temperature coefficient they can be paralleled without taking any special precaution for equal current sharing. To parallel two MOSFETs the drain and source terminals are connected together as shown in Fig (d).
- However, small resistances (R) are connected to individual gates before joining them together. This is because the gate inputs are highly capacitive with almost no losses. Some stray inductance of wiring may however be present.
- This stray inductance and the MOSFET capacitance can give rise to unwanted high frequency oscillation of the gate voltage that can result in puncture of the gate oxide layer due to voltage increase during oscillations. This is avoided by the damping resistance R.