MIC4607 Datasheet, PDF (23/42 Pages) Microchip Technology – 85V, Three-Phase MOSFET Driver with Adaptive Dead-Time, Anti-Shoot-Through and Overcurrent Protection

6.0 APPLICATION INFORMATION
6.1 Adaptive Dead Time
For each phase, it is important that both MOSFETs of
the same phase branch are not conducting at the same
time or V_IN will be shorted to ground and current will
“shoot through” the MOSFETs. Excessive
shoot-through causes higher power dissipation in the
MOSFETs, voltage spikes and ringing. The high switch-
ing current and voltage ringing generate conducted and
radiated EMI.
Minimizing shoot-through can be done passively,
actively or through a combination of both. Passive
shoot-through protection can be achieved by imple-
menting delays between the high and low gate drivers
to prevent both MOSFETs from being on at the same
time. These delays can be adjusted for different appli-
cations. Although simple, the disadvantage of this
approach is that it requires long delays to account for
process and temperature variations in the MOSFET
and MOSFET driver.
Adaptive Dead Time monitors voltages on the gate
drive outputs and switch node to determine when to
switch the MOSFETs on and off. This active approach
adjusts the delays to account for some of the varia-
tions, but it too has its disadvantages. High currents
and fast switching voltages in the gate drive and return
paths can cause parasitic ringing to turn the MOSFETs
back on even while the gate driver output is low.
Another disadvantage is that the driver cannot monitor
the gate voltage inside the MOSFET. Figure 6-1 shows
an equivalent circuit of the high-side gate drive.
MIC4607
The MIC4607 uses a combination of active sensing
and passive delay to ensure that both MOSFETs are
not on at the same time. Figure 6-2 illustrates how the
adaptive dead-time circuitry works.
FIGURE 6-2:
Diagram.
Adaptive Dead-Time Logic
For the MIC4607-2, a high level on the xPWM pin
causes HI to go low and LI to go high. This causes the
xLO pin to go low. The MIC4607 monitors the xLO pin
voltage and prevents the xHO pin from turning on until
the voltage on the xLO pin reaches the V_LOOFF thresh-
old. After a short delay, the MIC4607 drives the xHO pin
high. Monitoring the xLO voltage eliminates any exces-
sive delay due to the MOSFET driver’s turn-off time
and the short delay accounts for the MOSFET turn-off
delay as well as letting the xLO pin voltage settle out. If
an external resistor is used between the xLO output
and the MOSFET gate, it must be made small enough
to prevent excessive voltage drop across the resistor
during turn-off. Figure 6-3 illustrates using a diode
(D_LS) and resistor (R_LS2) in parallel with the gate resis-
tor to prevent a large voltage drop between the xLO pin
and MOSFET gate voltages during turn-off.
FIGURE 6-1:
MIC4607 Driving an
External MOSFET.
The internal gate resistance (R_{G_FET}) and any external
damping resistor (R_G) and HS pin resistor (R_HS), iso-
late the MOSFET’s gate from the driver output. There
is a delay between when the driver output goes low and
the MOSFET turns off. This turn-off delay is usually
specified in the MOSFET data sheet. This delay
increases when an external damping resistor is used.
 2016 Microchip Technology Inc.
FIGURE 6-3:
Low-Side Drive Gate
Resistor Configuration.
A low on the xPWM pin causes HI to go high and LO to
go low. This causes the xHO pin to go low after a short
delay (t_HOOFF). Before the xLO pin can go high, the
voltage on the switching node (xHS pin) must have
dropped to 2.2V. Monitoring the switch voltage instead
of the xHO pin voltage eliminates timing variations and
excessive delays due to the high side MOSFET
DS20005610A-page 23