How Fast Does a MOSFET Gate Driver Need to Be?

MOSFETs can be driven in either switching configuration or as toggle switches that pass DC current. When using switching elements, it is well known that a MOSFET can experience losses, which leads to heating of the component and a risk of failure.There is also the possibility of access drain Source voltage leading to failure, particularly when driving inductive loads. but a much more common instance where a mosfet experiences thermal failure is the case of week driving of the gate terminal.

When the gate terminal in a MOSFET is driven, the channel resistance transfers from off to on. If the wrong gate driver is used for switching, then one can expect a circuit to experience loss and possibly failure of the component. As we will discuss in this article, this is primarily related to the speed at which the gate driver can toggle the MOSFET.

Specifications for MOSFET Gate Drivers

MOSFET drivers are essentially square-wave generators with some defined duty cycle. They also include a programming or feedback mechanism that sets or adjusts the duty cycle of the output PWM signal. This allows them to switch MOSFET gates at a desired frequency and set the power delivery to a load.

Like any component, gate drivers have a range of specifications, but the most important specifications for toggling the gate on a MOSFET are:

• Drive voltage range

• Drive current and sink current

• Rise/fall time

Gate drivers attempt to source as much current as possible as fast as possible in order to fully toggle a MOSFET into its ON state. Conversely, they also try to sink current as fast as possible in order for a MOSFET to drop into its OFF state. Faster rise times are considered better for toggling a MOSFET, but there are factors that limit the turn-on time, as discussed below.

Rise time and fall time specifications given for an example gate driver.

Before calculating the actual turn-on time of the MOSFET, one should note that there are many gate drivers supporting specific power supply topologies. For example, there are half-bridge, full-bridge, push–pull, and flyback gate drivers designed for specific power regulators. They include feedback sensing and provide PWM generation to drive the MOSFETs.

With these features, MOSFETs can be used in common switching regulator applications without excessive additional circuitry. In some cases, a push–pull amplifier stage may be needed to bring the gate drive output from a MOSFET gate driver up to the correct voltage and current. Check your gate driver specifications in the data sheet for details.

Gate Driver Rise Time vs. Actual Rise Time

Gate Drive components have a rise time specification, but this is not the actual time it takes for the mosfet to turn on. MOSFETs have their own rise time specification, and this also is not the actual time required for the mosfet to turn on. Both the gate drive rise time and the MOSFET’s rise time values are lower limits; the larger of these two values is the fastest time at which a MOSFET can possibly turn on.

What limits the actual turn-on time is the total gate capacitance. The total capacitance of the gate needs to charge up to the full voltage provided by the gate driver. During this transition, the gate driver eventually accumulates some total amount of charge, just like a charging capacitor. This total amount of charge is also a function of the gate drive voltage and drain-source current.

Because the gate behaves like a charging/discharging capacitor, the time required to charge up the gate can be estimated using a linear model:

Charging: T = (gate charge)/(drive current)

Discharging: T = (gate charge)/(sink current)

This will be a lower limit on the time required to charge up the gate. This is because the toptal gate charge time is nonlinearly related to the gate drive voltage, and the gate drive voltage is varying during the gate pulse rise time.

Example

Suppose we have a MOSFET with an 8 ns minimum turn-on time driven with a gate driver. The gate driver is sourcing 2 A of current into a gate with 100 nC total charge, and the gate driver voltage rise time is 22 ns.

Using the above charging equation, the charging time is:

T = (100 nC)/(2 A) = 50 ns

Because the charging time is larger than the MOSFET’s minimum turn-on time (8 ns) and larger than the gate driver’s rise time (22 ns), the actual amount of time the MOSFET will take to turn on is 50 ns.

Now suppose the gate driver sources with much higher current of 15 A; the charging time would now be:

T = (100 nC)/(15 A) = 6.7 ns

This rise time is much faster than the actual rise time of the gate driver, so the turn-on time in this case is 22 ns.

In both cases, the rise time is much higher than the minimum possible turn-on time, which puts the component at risk of switching losses. When rise times are longer, the potential switching losses are larger, and a faster gate driver would be needed.

To better understand behavior of switching circuits with MOSFETs, designers can build and run highly accurate circuit simulations with the complete set of analysis tools in PSpice from Cadence. PSpice users can access a powerful SPICE simulator as well as specialty design capabilities like model creation, graphing and analysis tools, and much more.

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