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.

APPLICATION NOTES

N-CHANNEL GATES (Q2,Q4,Q6)

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.

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 3003. 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.

P-CHANNEL GATES (Q1,Q3,Q5)

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.

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 1

Figure 2

Rev. G 6/00

3

Figure 3