MOSFET Gate Driver | Calculations

keywords: Power MOSFET series gate resistor value, Power MOSFET series resistor power requirement, Power MOSFET Driver calculation

The Objectives

This is a simple, but very useful calculator to estimate:

1. MOSFET gate current, $i_{G}$ [A].
2. Time that the MOSFET will turn on, $t_{ON}$ [us].
3. The resistor ($R_{G2}$) value that must be added to the MOSFET gate and achieve the above two estimations [Ohm]
4. The average power ($P_{ave}$) which must be endured by $R_{G2}$. This value must be employed instead of the pulse power. $R_{G2}$ will limit the power delivered by the MOSFET driver which can be tuned to limit $t_{ON}$.

1 Metal Oxide Semiconductor Field Effect Transistors (MOSFET)

MOSFETs need to be matched to the power requirements with a factor of safety (FoS) of at least 1.2 for both currents and voltages. You will encounter two types of channels, N and P{channel; P-channel MOSFETs are rarely used and thus, you will have to go with the N-channels. N-channel MOSFETs have the advantage of having less conduction losses owing to the fact that they employ electron-based minority carriers which are much more efficient than the hole-based minority carriers of the P-channel ones.

1.1 Important MOSFET parameters

The important parameters which will be employed in later sections are:

$V_{GS}$	Normally found at the maximum ratings section, this is the gate-source voltage range; it may range from 10 to 20V.
$Q_{G}$	A dynamic characteristic which is the total charge of the MOSFET’s gate [nC]. It can be employed to estimate the instantaneous current needed to turn on a MOSFET in a specied time.
$t_{on}$	This is a design parameter to maximize or minimize the worst case input gate current [ns].
$R_{G}$	This is the internal gate resistance of the MOSFET. We will use this fixed resistance to calculate an additional resistor to limit the gate current, $i_{G}$.

1.2 Gate current equations

  The gate current, $i_G$, is a parameter that is essential to choose a MOSFET driver (more on this on the next section). The gate current can be calculated as follows:

$\begin{equation}\begin{split} i_G &= C (dV/dt) \\ i_G &= Q_G/t_{on} \end{split} \end{equation} \qquad (1)$

which is a convenient way to estimate the maximum current that the MOSFET driver will have to supply during the $t_{on}$. The $i_G$ is necessary in order to calculate the wire diameters/material or PCB widths.

1.3 MOSFET driver | $R_{G2}$ equation

The $R_{G2}$ value can be calculated with the simple formula:

$\begin{equation} R_{G2} = V_{GS}/i_G – R_G \end{equation} \qquad (2)$

1.4 Power determination of the $R_{G2}$

   The underlying pulse and average power ($P_{pulse}$ and $P_{ave}$, respectively) relationships of the MOSFET are presented as follows:

$\begin{equation} \begin{split} P_{pulse} &= i_G V_{GS}\\ P_{ave} &= P_{pulse}\frac{t_{on}}{T_{PWM}} \end{split} \end{equation} \qquad(3)$

where either one of $i_G$ or $t_{on}$ is a design parameter while the other is calculated with equation (1) and $T_{PWM}$ is the PWM period. $P_{pulse}$ is the maximum instantaneous power that the MOSFET driver will provide which is limited by $R_{G2}$ (refer to equation (2)). However, $P_{pulse}$ will only be under effect during $t_{on}$. Therefore, it is advisable to buy one that can endure 1 or 2 times the $P_{ave}$ instead of buying a full power $P_{pulse}$ rated resistor.

  Note: $P_{ave}$ is only dependent on $V_{GS}$, $Q_G$ and $T_{PWM}$ and is easily found after some equation rearrangements.

2 Putting all the equations to work! MOSFET $i_G$, $R_{G2}$ and power calculation example

In this section,the following design parameters are employed:

$\begin{equation} \begin{split} V_{GS} &= 12 [V]\\ T_{PWM} &= 10 [\mu s] \end{split} \end{equation}$

From the IPB083N10N G N-channel MOSFET from Infineon:

$\begin{equation} \begin{split} Q_G &= 55 [nC]\\ R_G &= 1 [\Omega] \end{split} \end{equation}$

where the maximum $Q_G$ was taken. If a $t_{on}$ of 500 [ns] is defined, then an $i_{G}$ can be calculated with equation (1) as follows:

$\begin{equation} i_G = \frac{55 [nC]}{500 [ns]} = 0.11 [A] \end{equation}$

Please note that this current will only be applied to the first 500[ns] needed to turn on the MOSFET, after that, a very small current will be needed to maintain the MOSFET ON because MOSFETs are voltage-driven. Of course, smaller $t_{on}$ will require much higher $i_G$.

With the previous $i_G$ of 0.11 [A], using equation (2), an $R_{G2}$ of 108 [latex options:inline]\Omega[/latex] is to be bought and soldered to the final PCB. Of course, you would rather choose a lower standard resistance over a higher one in order to turn on the gate a little faster than calculated.

In this example, we will use a 100 [ohm] resistor. Therefore, the new gate current is calculated by rearranging eqn. (2):

$\begin{equation} \begin{split} i_{G}& = V_{GS}/(R_{G2} + R_{G}) \\  i_{G} &= 0.1212 [A] \end{split} \end{equation}$

And the new time to turn on the MOSFET is, after rearranging eqn. (1):

$\begin{equation}\begin{split}t_{ON} &= Q_G/i_{G}\\ t_{ON} &= 453.8 [ns] \end{split} \end{equation}$

The rated power of the $R_{G2}$ is calculated using eqn. (3) and given as:

$\begin{equation} \begin{split} P_{pulse} &= 1.45 [W]\\ P_{ave} &= 0.066[W] \end{split} \end{equation}$

In this case, a resistor with a rated power of 1/4 [W] will be chosen, which is well above the average power.

Edit: VBS will be employed instead of VGS if you are using a bootstrap IC. VBS is the MOSFET driver’s effective voltage to turn on the MOSFET. VBS is equal to the driving voltage of the bootstrap IC minus the voltage drop of the Bootstrap diode.