BRUSHLESS DC MOTOR CONTROLLER

2007-3-16

14/28

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