BY CLIFF ELLISON
Microchip Technology
Chandler, AZ
http://www.microchip.com
The many choices of MOSFET technologies and silicon processes available today make selection of the appropriate MOSFET driver a challenging process. Functionally, MOSFET drivers convert logic signals to higher voltage and current levels to drive MOSFET gates on and off with fast response times.
For example, MOSFET drivers can be used to convert a 5-V low-current microcontroller output signal to an 18-V several-ampere drive signal for a power-MOSFET input. Selecting the right MOSFET driver for the application requires a thorough understanding of power dissipation in relation to the MOSFET's gate charge and operating frequencies. For example, charging and discharging a MOSFET's gate requires the same amount of energy, regardless of how fast or slow the gate voltage transitions are.
Power handling
A MOSFET driver's power dissipation capabilities are determined by three key elements:
* Power dissipation due to charging and discharging the MOSFET's gate capacitance
* Power dissipation due to the MOSFET driver's quiescent-current draw
* Power dissipation due to cross-conduction (shoot-through) current in the MOSFET driver
Of these three elements, power dissipation due to the charging and discharging of the MOSFET's gate capacitance is most important, especially at lower switching frequencies. This is given by:
Pc = Cg x Vdd2 x F (1)
where Cg = MOSFET gate capacitance, Vdd = supply voltage of MOSFET driver (V), and F = switching frequency.
Peak drive current
In addition to power dissipation, designers must understand the peak drive current required from the MOSFET driver and the associated turn-on and -off times. Matching the MOSFET driver to the MOSFET in an application depends on how fast the application requires the power MOSFET to be switched on and off.
The optimum rise or fall time in any application is based on many requirements, such as EMI, switching losses, lead/circuit inductance, and switching frequency. The relationship between gate capacitance, transition times, and the MOSFET driver current rating is given by:
dT = [dV x C]/I (2)
where dT= turn-on/turn-off time, dV = gate voltage, C = gate capacitance, and I = MOSFET peak drive current.
The total MOSFET gate capacitance can be properly determined by looking at the total gate charge (QG). Gate charge QG is given by:
QG = C x V (3)
Then I = QG/dT.
This method assumes a constant current. A good rule of thumb is that the average value found is half of the MOSFET driver's peak current rating. MOSFET drivers are rated by the driver output peak current drive capability.
The peak current rating is typically stated for the part's maximum bias voltage. This means that, if the MOSFET driver is being used with a lower bias voltage, its peak current drive capability will be reduced.
Another method designers can use to selecting the appropriate MOSFET driver is to use a time-constant approach. In this approach, the MOSFET driver resistance, any external gate resistance, and the lumped capacitance are used.
Tcharge = ((Rdriver+Rgate) x Ctotal) x TC (4)
where Rdriver = RDS(on) of the output driver stage, Rgate = any external gate resistance between the driver and MOSFET gate, Ctotal = total gate capacitance, and TC = number of time constants.
For example:
Qtotal = 68 nC, Vgate = 10 V, Tcharge = 50 ns,
TC = 3, Rgate = 0 W
Rdriver = (Tcharge/(TC x Ctotal)) – Rgate Rdriver, therefore = 2.45 W
As this equation represents an R-C time constant, using a TC of 3 means that the capacitance will be charged to 95% of the charging voltage after the Tcharge time. Most MOSFETs are fully "on" by the time the gate voltage reaches 6 V. Based on this, a TC value of 1 (represents 63% of charging voltage) may be more useful for the application and allow a lower-current driver IC to be used.
Motor control apps
Let us workout an example on selecting a MOSFET driver for a motor-control application where the motor's speed and direction of rotation vary. This application requires the voltage applied to the motor to be modulated. The type of motor, power-switching topology and power-switching element will generally dictate the necessary gate-drive scheme for doing so.
The first step is to select the correct power-switching element for the application, which in turn depends on the ratings of the motor being driven. An important parameter to consider is the start-up current value, which can be up to three times the value of the steady-state operating current.
There are two main choices for the power-switching elements in motor drives�MOSFETs and IGBTs. Assuming the choice is a MOSFET, the MOSFET driver rating for the gate-drive application can be determined.
As shown in Fig. 1, the input stage of the device converts the incoming low-voltage signal to a full range (GND to VDD) signal that turns a cascaded chain of increasingly stronger drive stages on and off. MOSFETs Q1 and Q2 represent the MOSFET driver's pull-up and pull-down output driver stages.
Fig. 1. When selecting a MOSFET driver for a motor-control application, the type of motor, power-switching topology, and power-switching element generally dictate the necessary gate-drive scheme for doing so.
In viewing the MOSFET driver's output stage as a push-pull pair of MOSFETs, it is easy to understand the MOSFET driver's operation. For a noninverting driver, when the input signal goes to a high state, the common gate signal of Q1 and Q2 is pulled low.
The transition of this gate node from a voltage VDD to GND typically occurs in less than 10 ns. This fast transition limits cross-conduction time between Q1 and Q2, and brings Q1 to its fully enhanced state quickly in order to reach peak current as soon as possible. Other MOSFET driver configurations exist.
When the designer knows the motor being driven, power-switching elements, and gate-drive scheme, the MOSFET driver can be chosen based on either Equation 3 or 4.
When a MOSFET has been selected, a vendor-supplied spreadsheet can be used to choose an appropriate MOSFET driver, such as Microchip Technology's Power MOSFET Driver Calculator, located at:
http://www.microchip.com/MOSFETDriverCalculator
With many tools available today, the designer can quickly determine the MOSFET driver's required peak current. Once determined, the most appropriate and cost-effective MOSFET driver is identified. After arriving at the chosen MOSFET driver, the tool calculates the device's power dissipation and the maximum ambient operating temperature allowed, assuming no heat sinking.
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