In the design, the lower tube is normally open and the upper tube is driven by the PWM strategy to drive the motor. Considering that the magnitude of the PWM duty of the motor drive varies, the sampling strategy can be determined according to the length of the half-bridge upper tube ON time, because if the upper tube on-time is too short, in order to avoid the influence of MOSFET turn-on and turn-off, leave it to The sampling time becomes short, which is not conducive to the accuracy of sampling.

Author: Hardy Zhou

This article mainly refers to the TIDA-010031 reference design to analyze the principle of the ADC sampling and integration square wave non-inductive control, so that everyone can better complete similar program designs.

1.The following is a typical three-phase BLDC motor control block diagram.

Three half-bridges drive BLDC brushless motors to detect low-side bus current

2. Typical BLDC motor phase current and back-EMF waveform analysis

From the waveform point of view, every 60 degree electrical angle, only two half-bridges have drive voltage output, and the upper and lower tubes of the other half-bridge are fully closed, and this phase voltage is in a floating state.

After the BLDC motor is running, the phase wires have back electromotive force.

The back electromotive force of the motor comes from the change of the magnetic flux caused by the rotation of the motor rotor, and the change of the magnetic flux will produce an induced voltage on the stator winding.

For the same motor, the peak value of the back-EMF is almost a fixed ratio to the motor speed.

3. Integral of the back-EMF voltage and time from the zero-crossing point to the peak value of the back-EMF

According to the above description that the peak value of the back EMF is almost fixed proportional to the speed (electrical frequency), set . Vm is the peak value of the back electromotive force. For the same motor, we can consider that Kv is almost constant.

Take the blue area on the left side of the above figure as an example. The electrical angle of this area (the area between the back-EMF zero-crossing point and the next commutation point) is 30 degrees, which is 1/12 of the electrical angle (360 degrees).

Suppose the current electrical frequency of the motor is f, and the unit is Hz.

The peak voltage of the back EMF is , The unit is volts.

Suppose the time from the center tap value to the maximum value of the back EMF is t1, and .

Then the integral value of the blue integral area is equal to the area of ​​the blue triangle:

As you can see, the result of the integration is the Kv value , So the integration result is almost unchanged.

So we can follow the integral value and the fixed thresholdMake a comparison to determine the commutation point.

4. How does ADC sample back-EMF

The square wave non-inductive BLDC ADC sampling integral control, the circuit is designed with a three-phase phase voltage ADC sampling circuit, and the two phases of the motor every 60 degrees electrical angle interval have input voltage and Inductor current at the same time, without adding additional circuits It is difficult to obtain the back-EMF voltage, and the floating phase has no half-bridge voltage input and inductor current, so the actual back-EMF voltage of the phase can be derived from the detection of the phase terminal voltage. For details, you can see the following derivation. The derivation process refers to TI Reference design TIDA-010031 http://www.ti.com/lit/ug/tiduej4/tiduej4.pdf?ts=1588819919326

So if you are interested in this motor control method, you can view the software and hardware development materials of the reference design on TI’s official website.

In the design, the lower tube is normally open and the upper tube is driven by the PWM strategy to drive the motor. Considering that the PWM duty of the motor drive varies, the sampling strategy can be determined according to the length of the half-bridge upper tube ON time, because if the upper tube on-time is too short, in order to avoid the influence of MOSFET turn-on and turn-off, leave it to The sampling time becomes short, which is not conducive to the accuracy of sampling.

When the upper tube is ON for a long time, when the upper tube is ON, the back electromotive force voltage of the floating phase is detected at the moment away from the MOSFET switch (usually sampling in the middle of the ON time).

The driving logic at this time is that the upper tube of phase A is on and the lower tube of phase B is on, so there is .

Ea, Eb, Ec are the three opposite electromotive force voltages of the motor, and va, vb, vc are the midpoint voltages of the three-phase half-bridge, that is, the three-phase input voltage of the motor.

La, Lb, Lc are the three-phase inductance of the motor, ia, ib, ic are the three-phase input current of the motor, Ra, Rb, Rc are the three-phase input resistance of the motor (considering that the three-phase resistance is equal), vn is the midpoint of the three-phase motor Voltage.

It can be obtained that when Ec=0, that is, when the back EMF crosses zero,, Which means that when the ADC detectsWhen, it means that this moment is the zero-crossing point of the C-phase back EMF, then in theory, after 1/12 of the electrical cycle time, the motor needs to be commutated.

When the upper tube ON time is relatively short, detect the back-EMF voltage of the floating phase when the upper tube is OFF (usually sampling in the middle of the OFF time)

The driving logic at this time is that the upper tube of phase A is turned off and the lower tube of phase B is still on. At this time, the body diode of the lower tube of phase A is freewheeling, so there is.

It is obtained that when Ec=0, that is, when the back EMF crosses zero, vc=0, that is, when the ADC detects vc=0, it means that this moment is the zero crossing point of the C-phase back EMF, then theoretically passes 1 /12 electrical cycle time, the motor needs to be commutated.

From the above analysis, we can see that the following two points need to be paid attention to in the design of using ADC sampling integration method for non-inductive BLDC control

The integral threshold is related to the peak value of the back-EMF and the speed ratio of the motor. It may be different for different motors and needs to be adjusted for the motor.

When the upper tube is designed to be driven by PWM, different ADC sampling strategies can be used to deal with the large duty and small duty conditions. At the same time, the judgment of the back-EMF zero-crossing point also needs to be adjusted.

When the motor is at a high speed, the electrical frequency is relatively high, and the ADC integral sampling is based on the PWM switching cycle sampling. Therefore, to obtain a more accurate commutation point, a relatively high switching frequency is required. If the switching frequency is relatively low, it means that the sampling rate is slow. , May cause the commutation delay to be relatively large, thereby affecting the normal control of the motor.