MSK3020 Datasheet, PDF (3/6 Pages) M.S. Kennedy Corporation – H-BRIDGE MOSFET POWER MODULE

APPLICATION NOTES
N-CHANNEL GATES (Q2, Q3):
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. 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 3020. 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 iductances
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 (Q1, Q4):
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 fo3r high bandwidth operation.
Rev. A 7/00