APPLICATION NOTES
N-CHANNEL GATES (Q1,Q4)
For driving the N-Channel gates, it is important to keep in mind that it is essentially like driving a capacitance to a sufficient
voltage to get the channel fully on. Driving the gates to +15 volts with respect to their sources assures that the transistors are on.
This will keep the dissipation down to a minimum level [RDS(ON) specified in the data sheet]. How quickly the gate gets turned ON
and OFF will determine the dissipation of the transistor while it is transitioning from OFF to ON, and vice-versa. Turning the gate
ON and OFF too slow will cause excessive dissipation, while turning it ON and OFF too fast will cause excessive switching noise
in the system. It is important to have as low a driving impedance as practical for the size of the transistor. Many motor drive IC's
have sufficient gate drive capability for the MSK 3004. If not, paralleled CMOS standard gates will usually be sufficient. A series
resistor in the gate circuit slows it down, but also suppresses any ringing caused by stray inductances in the MOSFET circuit. The
selection of the resistor is determined by how fast the MOSFET wants to be switched. See Figure 1 for circuit details.
Figure 1
P-CHANNEL GATES (Q2,Q3)
Most everything applies to driving the P-Channel gates as the N-Channel gates. The only difference is that the P-Channel gate to
source voltage needs to be negative. Most motor drive IC's are set up with an open collector or drain output for directly interfacing
with the P-channel gates. If not, an external common emitter switching transistor configuration (see Figure 2) will turn the P-
Channel MOSFET on. All the other rules of MOSFET gate drive apply here. For high supply voltages, additional circuitry must be
used to protect the P-Channel gate from excessive voltages.
Figure 2
BRIDGE DRIVE CONSIDERATIONS
It is important that the logic used to turn ON and OFF the various transistors allow sufficient "dead time" between a high side
transistor and its low side transistor to make sure that at no time are they both ON. When they are, this is called "shoot-through",
and it places a momentary short across the power supply. This overly stresses the transistors and causes excessive noise as well.
See Figure 3.
Figure 3
This deadtime should allow for the turn on and turn off time of the transistors, especially when slowing them down with gate
resistors. This situation will be present when switching motor direction, or when sophisticated timing schemes are used for servo
systems such as locked antiphase PWM'ing for high bandwidth operation.
Rev. G 10/04
3
MSK [ M.S. KENNEDY CORPORATION ]
