A typical application using the L6235
device is shown in Fig. 1. A high quality ceramic capacitor (C2) in the range
of 100 nF to 200 nF should be placed between the power pins VSA and VSB and
ground near the L6235 device to improve the high frequency filtering on the
power supply and reduce high frequency transients generated by the switching.
The capacitor (CEN) connected from the EN input to ground sets the shutdown
time when an overcurrent is detected. The two current sensing inputs (SENSEA
and SENSEB) should be connected to the sensing resistor RSENSE with a trace
length as short as possible in the layout. The sense resistor should be
non-inductive resistor to minimize the di/dt transients across the resistor. To
increase noise immunity, unused logic pins are best connected to 5 V (high
logic level) or GND (low logic level). It is recommended to keep power ground
and signal ground separated on the PCB.
Fig 1 Typical application
ZERO CROSSING DETECTION
For block commutation, the polarity
of the BEMF changes within the coil that is perpendicularly oriented to the
rotor. Hall sensors are mounted on those positions that their polarity changes
in phase with the BEMF of the associated coils. In other words, Hall sensor
signals represent the polarities of the BEMF of their associated coils. BEMF
polarities together represent the actual position of the rotor. For normal
operation, the switching to the next commutation position is done with each
zero crossing. Direct measurement of the BEMF is possible if there is direct
access to the null terminal N of the BLDC motor. Although the N terminal is
available at some Y connected BLDC motors, it is not available for DELTA
connected motors. So for a flexible implementation, the zero crossing detection
has to be realized without the N terminal. Other solutions reconstruct the
voltage of the N terminal. The sensorless commutation is also possible without
reconstruction of the N terminal voltage. Additionally, other solutions
strongly focus on post processing of signals that are noisy due to PWM
switching and self-induction of coils. With the right signal conditioning, the
sensorless commutation becomes as simple as commutation based on Hall sensors.
Fig 2 Zero Cross detection
The designed open-loop
control BLDC motor for domestic fan application with sensorless control is
realized. Fig. 3 shows the complete drive electronics of the BLDC controlled
fan. The speed of the fan motor is varied through the variable resistor whose
input 230V is stepped down to 24V and then rectified to produce a DC output fed
to the 3-arm inverter bridge as shown in the Fig. 3
Fig 3 Hardware diagram
Fig 4 BEMF along with ZC output
Fig 5 All 3 phase BEMF along with Phase A ZC output
