AN1627 Freescale Semiconductor / Motorola, AN1627 Datasheet - Page 10

• Manufacturer Part Number: AN1627
• Description: Low Cost High Efficiency Sensorless Drive for Brushless dc Motor Using MC68HC (7)05MC4
• Manufacturer: Freescale Semiconductor / Motorola

4
The concept below was chosen. The cost target, especially, forced the design to be as simple as
possible.
The 68HC705MC4 is an HC05-based MCU designed for three-phase Brushless DC motor (permanent
magnet) drive applications. General features include 3.5 kb of EPROM, 176 bytes of RAM, a 16-bit timer
including an output compare and two input captures, 4 general-purpose I/O pins, and an SCI (UART)
port in a 28-pin SOIC or DIP package. In addition the MC4 has specific features that target Brushless
DC motor control, including a 2-channel, 8-bit PWM module, a high current source port, and a 6-
channel, 8-bit A/D module.
Key features of the 6-pin, 2-channel PWM module include 16 PWM rates between 122Hz and 23.4kHz
for each PWM channel, buffered data registers with an interlocking mechanism for coherent updates of
the pulse width outputs on each PWM channel, and a 3-output commutation MUX on each PWM
channel for easy control of outputs to the motor drive. The six PWM outputs of the commutation MUX
are connected to an output port with 10 mA current source capability per pin, thus reducing the cost of
external components required for building motor drives.
The Back-EMF zero crossing detection enables position recognition, as explained in previous sections.
The resistor network is used to divide sensed voltages down to a 0-15V voltage level. Simple filtering
prevents the comparators being disturbed by high voltage spikes produced by the switching of the
IGBT’s. The multiplexer selects the phase comparator output which corresponds to the current
commutation stage. This signal is transferred to the microcontroller Input Capture pin (TCAP2).
The voltage drop resistor (0,6Ohm/2W) is used to measure the DC-bus current which is chopped by the
PWM. The obtained signal is rectified and amplified (0-5V). The internal microcontroller A/D converter
is synchronized with the PWM signal. This synchronization avoids spikes when the IGBT’s are switching
and simplifies the electric circuit.
The A/D converter is also used to sense the DC-Bus Voltage and speed setting. The DC-Bus voltage is
divided down to a 5V signal level by a resistor network.
The six IGBT’s (copack with built-in fly back diode) and high voltage 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. PWM technique is used to the control motor phase voltage.
A Simple power supply (rectified mains) contains a Line and a RF filter in order to fulfil the European
Community Regulation (IEC555-1).
As apparent, this concept uses a common ground (no isolation is used).
MOTOROLA
10
Speed Setting
SYSTEM CONCEPT
230V~
Current & Voltage
~
Sensing
Freescale Semiconductor, Inc.
=
For More Information On This Product,
+ Line and RF filters
Power Supply
Go to: www.freescale.com
I-sense
Control Board
MC68HC705MC4
Filter
Figure 4-1.
+
U-adapt
System
(3xdriver & 6xIGBT)
Power Stage
Recognition
Position
Filter
M
REV 0.2
AN1627