Using XC866 8-bit microcontroller to realize air conditioner outdoor unit fan control

Application Background

Split type air conditioners usually consist of an outdoor unit and several indoor fan coils. The outdoor unit contains a compressor responsible for heat exchange. To ensure that the heat exchange capacity of the outdoor unit can be adjusted according to the changes in outdoor temperature , a fan is required. Usually, the speed range of the outdoor unit fan is 250rpm to 1000rpm, and the power consumption does not exceed 100W, with a typical value of 70W. The speed command is issued in analog format and the FG signal is fed back to the main control board.

Outdoor units The fan control has special requirements. It needs to start and operate reliably in different climatic conditions. In case of strong outdoor winds, the fan blades may rotate in the opposite direction. In strong typhoon conditions, the motor may only reach half of its maximum speed. There are also other requirements for production convenience. Therefore, it is best to choose a control method that is not affected by motor parameters and is not sensitive to component tolerances or production variations.

Traditional outdoor fans use brushless DC motors with trapezoidal commutation. Trapezoidal control offers several advantages, such as easy control, high torque, and reliable performance. However, trapezoidal commutation can cause audible noise, especially at low motor speeds, due to the inherent torque ripple. To reduce the audible noise while meeting all application requirements, this article proposes a simplified sinusoidal control scheme using Infineon’s 8-bit microcontroller XC866 .

Infineon Outdoor unit Fan solution

The XC866 is a member of the high-performance XC800 8-bit microcontroller family, based on the XC800 core that is compatible with the industry-standard 8051 processor. The XC866 features a dedicated three-phase motor control unit, the Capture Compare Unit 6 (CCU6), and a 10-bit analog-to-digital converter (ADC) with a variety of extended functions. These features make the XC866 ideal for low-end three-phase motor control, such as brushless DC motors and induction motors. Other features of the XC866 include a UART, an SPI interface, and three 16-bit timers. Figure 2 shows a block diagram of the XC866 8-bit microcontroller .

Figure 3 shows the system block diagram of the fan inverter. The microcontroller , gate driver and six IGBTs are mounted on the same circuit board. This circuit board is usually installed in the motor housing. The 310V DC voltage is directly connected to the inverter circuit board, so no rectifier stage is required. Infineon discrete IGBT IKD04N60R and gate driver 6ED003L06-F were selected for this application due to their suitable features and reliable quality.

In actual operation, the fan motor may be in the following states: STOP, CHECK_DIR, BRAKE, RAMP and SINU. After power-on, a program (CHECK_DIR state) will be called to check the rotation status of the fan motor. If it is stationary, the motor will be started using the trapezoidal commutation (RAMP state) method, because this method can provide stronger starting torque. After successful startup, the control method is converted to sinusoidal modulation (SINU state) to reduce audio noise. However, if the motor rotates in the opposite direction after power-on, the software will brake the motor until the motor stops (BRAKE state). The braking force is calculated based on the initial rotation speed. For example, if the fan blade rotates at 400rpm, then the motor braking requires a larger current than the fan blade rotating at 200rpm.

The motor speed and direction information is provided by three Hall sensors. These three sensors are 60° apart. XC866 is integrated with hardware Hall input logic (inside CCU6 module) to avoid software polling of Hall signals and reduce CPU overhead. The motor speed is calculated to calculate the rotor angle. In each PWM interrupt, the rotor angle is updated and used as an index to the sine lookup table. The speed command can be sampled through the ADC channel and the conversion result is provided as a voltage specification. Finally, the CCU6 output 6 PWM signal is generated based on the sine lookup table value and the voltage specification. Figure 4 shows the control block diagram for stable operation. To reduce the switching loss of modulation, a special method is required. There are 6 states in one power cycle, and each state lasts 60°. Two of the states do not implement modulation, such as S5 and S6 of U phase. This is similar to trapezoidal control, while the other 4 states implement sinusoidal modulation, such as S1 and S2 of U phase (using sin( )) and S3 and S4 (using sin(120- )).

The author has tested the product on site and obtained satisfactory results in terms of noise reduction, reverse braking and starting, and efficiency. In addition, this method is not affected by motor parameters and is therefore applicable to fan blades of various models. This is beneficial for shortening product development cycles and improving production convenience.

References:

[1] Infineon's 8-bit Flash Industrial MCU adds XC866 HOT[R/OL]. (2007-5-8). http://www.eepw.com.cn/article/56620.htm

[2] Infineon Application Development Kit accelerates its 8 and 16-bit MCU motor drive design [R/OL]. (2009-3-19). http://www.eepw.com.cn/article/92609.htm

[3] Zhong Xiankun. Single chip microcomputer control technology for air conditioner power-on delay start protector [J]. Electronic Products World, 2008 (8): 107

[4] Leong A, Huang Zhihong, Bannatyne R, et al. Embedded technology market and its characteristics [J]. Electronic Products World, 2009(11):14

[5] Wang Ying. Discussion on the characteristics and development strategy of MCU[J]. Electronic Products World, 2009(9):63