User Manual

PWM Voltage Generation for BLDC
3-Phase BLDC Motor Control with Sensorless Back-EMF, ADC, Zero Crossing, Rev. 3
Freescale Semiconductor 15
Preliminary
A current shunt is used to measure the DC-Bus current. The obtained signal is rectified and amplified (0-3.3V
with 1.65V offset). The controller’s A/D converter as well as Zero Crossing detection is synchronized with the
PWM signal. This synchronization avoids spikes when the IGBTs (or MOSFETs) are switching and simplifies
the electric circuit.
The A/D converter is also used to sense the DC-Bus Voltage and drive Temperature. The DC-Bus voltage is
divided down to a 3.3V signal level by a resistor network.
The six IGBTs (copack with built-in fly back diode) or MOSFETs and gate drivers create a compact power
stage. The drivers provide the level shifting that is required to drive high side bridge circuits commonly used in
motor drives. The PWM technique is applied to the control motor phase voltage.
5. Control Technique
5.1 Control Technique - General Overview
The general overview of used control technique is shown in Figure 4-1. It will be described in following
subsections:
• PWM voltage generation for BLDC
• Back-EMF Zero Crossing sensing
• Sensorless Commutation Control
• Speed Control
The implementation of the control technique with all the software processes are shown in Flow Chart, State
diagrams and Data Flow. Refer to
Section 7.1, Section 7.2, and Section 7.3.
5.2 PWM Voltage Generation for BLDC
A 3-phase voltage system (see Figure 3-2.) needs to be created to run the BLDC motor. It is provided by
3-phase power stage with 6 IGBTs (MOSFET) power switches controlled by the device on-chip PWM module
(see
Figure 5-1).
The PWM signals, with their current state, are shown in Figure 5-2 and Figure 5-3.
Figure 5-2 shows that both the Bottom and Top power switches of the non-fed phase must be switched off.