# Question regarding put capacitor in series with the gate of mosfet

#### ir_tronik

Joined Jul 12, 2017
16
Hi, I studied an application from Linear Technology about the battery reverse protection circuit.
But, I do not understand in some parts of the circuit below about the capacitor C1 provides an ultra fast charge pump to drive the gate of MN1 Mosfet down during reverse battery attach. Could someone explain to me on how the C1 works in this circuit?

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#### DickCappels

Joined Aug 21, 2008
10,067
With just R3 switching the MOSFET would take longer than if the gate signal goes through C1.

C1 and the input capacitance of the MOSFET form a capacitive voltage divider with most of the pulse amplitude appearing at the gate of the MOSFET. If C1 was not there you would be driving the gate capacitance through R3, which would cause an RC delay in the switching.

#### ir_tronik

Joined Jul 12, 2017
16
With just R3 switching the MOSFET would take longer than if the gate signal goes through C1.

C1 and the input capacitance of the MOSFET form a capacitive voltage divider with most of the pulse amplitude appearing at the gate of the MOSFET. If C1 was not there you would be driving the gate capacitance through R3, which would cause an RC delay in the switching.
Could you elaborate more on the operation? Let say during the reverse connection, what will happen.
I could not understand the operation during reverse.

#### Tonyr1084

Joined Sep 24, 2015
7,454
I'm going to use some wrong terminology - and probably get yelled at for doing so - but - - -

A capacitor when not charged acts like a conductor. When charged it acts like a VERY high ohm resistor. During the time it's charging it is going from zero ohms to infinite ohms. I've placed a capacitor in series with a relay coil. When power is applied the relay would click in momentarily until the capacitor charged sufficiently to drop the amount of current going to the relay, and the relay would click out. Larger caps would produce a longer click-in / click-out period over that of a smaller cap. DC does not flow through a capacitor. However, during the time it's charging - current IS flowing.

So what I think Dick is saying is that when first connected the MOSFET will turn on nearly instantaneously. But as it charges it hands over the work R3 is doing.

#### ir_tronik

Joined Jul 12, 2017
16
I'm going to use some wrong terminology - and probably get yelled at for doing so - but - - -

A capacitor when not charged acts like a conductor. When charged it acts like a VERY high ohm resistor. During the time it's charging it is going from zero ohms to infinite ohms. I've placed a capacitor in series with a relay coil. When power is applied the relay would click in momentarily until the capacitor charged sufficiently to drop the amount of current going to the relay, and the relay would click out. Larger caps would produce a longer click-in / click-out period over that of a smaller cap. DC does not flow through a capacitor. However, during the time it's charging - current IS flowing.

So what I think Dick is saying is that when first connected the MOSFET will turn on nearly instantaneously. But as it charges it hands over the work R3 is doing.
Then, what will happen if the battery is reversed? What is the meaning of the " capacitor C1 provides an ultra fast charge pump to drive the gate of MN1 Mosfet down during reverse battery attach " What is meant of ultra fast charge pump?

#### Sensacell

Joined Jun 19, 2012
3,257
Capacitors do not pass DC currents, charging a battery is a DC, slow motion affair.

But... connecting the battery backwards is a rapid, transient event, a transient is the same as AC- so the capacitator passes current.
The capacitor serves as a transient current path to rapidly reduce the gate voltage - instantly shutting the MOSFET down.
Current only flows for an instant, while the capacitors charge or discharge to equilibrium.

This scheme provides rapid off switching, without the penalty of low resistor values that would waste power in normal DC operation.