3.22EjemploDRM007 Co..

Hardware Design Considerations
As a protection for power transitions of the microcontroller’s power
supply, there are pull-down resistors R58 and R59. So, the MOSFETs
are not triggered during transitions.
The gate drive circuit has two different impedance output values, one for
turn-on time and other for turn-off time for each of the power transistors,
TOP and BOTTOM in each phase. This is possible using D7 and D5 for
the turn-off impedances of the transistors per phase. The turn-on
impedance is given by R20 and R21 respectively, and the turn-off
impedance is given by the parallel connection of R44||R21 and R45||R20
respectively. With the values displayed in the schematic, the turn-on
time is 800 ns, and the turn-off time is 600 ns with the IRFB17N50L
MOSFET.
In the software for this reference design, deadtime is fixed to 2 µs. This
gives enough time for the transistors to change their state of
conductance with no short circuit of the phase output.
The bootstrap capacitor C10 is used to turn-on the TOP transistor
without a charge pump circuitry. Turning on the lower transistors first is
recommended in order to charge this bootstrap capacitor each time the
motor is initially energized.
4.10 Current Feedback and Cycle-by-Cycle Limiting
The 3-phase current is sensed by resistor R19 in Figure 4-9, and
amplified by a differential amplifier shown in Figure 4-10. The circuit
provides an amplified voltage of the chopped current of the inverter.
The output of the amplifier represents 0.5 volts per ampere in the shunt
resistor (R19). The MC33502 OPAMP was used for this amplifier circuit.
At this point, for current sensing within the microcontroller the ADC
conversion must be synchronized with the PWM module. That is why a
peak detector circuit was implemented to have a suitable current
waveform for sensing.
DRM007
BLDC Motor Control Board for Industrial and Appliance Applications
64 Hardware Design Considerations MOTOROLA