BRUSHLESS DC MOTOR CONTROLLER
FSP33035
APPLICATION
Three Phase Motor Commutation
The three phase application shown in Figure below is a full–featured open loop motor controller with full wave, six
step drive. The upper power switch transistors are Darlingtons while the lower devices are power MOSFETs. Each
of these devices contains an internal parasitic catch diode that is used to return the stator inductive energy back to
the power supply. The outputs are capable of driving a delta or wye connected stator, and a grounded neutral wye if
split supplies are used. At any given rotor position, only one top and one bottom power switch (of different totem
poles) is enabled. This configuration switches both ends of the stator winding from supply to ground which causes
the current flow to be bidirectional or full wave. A leading edge spike is usually present on the current waveform and
can cause a current–limit instability. The spike can be eliminated by adding an RC filter in series with the Current
Sense Input. Using a low inductance type resistor for RS will also aid in spike reduction. Care must be taken in the
selection of the bottom power switch transistors so that the current during braking does not exceed the device rating.
During braking, the peak current generated is limited only by the series resistance of the conducting bottom switch
and winding.
If the motor is running at maximum speed with no load, the generated back EMF can be as high as the supply
voltage, and at the onset of braking, the peak current may approach twice the motor stall current. The next figure
shows the commutation waveforms over two electrical cycles. The first cycle (0° to 360°) depicts motor operation at
full speed while the second cycle (360° to 720°) shows a reduced speed with about 50% pulse width modulation.
The current waveforms reflect a constant torque load and are shown synchronous to the commutation frequency for
clarity.
Three Phase, Six Step, Full Wave Motor Controller
14/28
2007-3-16
FOSLINK [ FOSLINK SEMICONDUCTOR CO.,LTD ]
