AVR single chip microcomputer control method of switching power supply

The microcontroller controls the switching power supply. There are several control methods for the power output alone.

One is that the MCU outputs a voltage (via DA chip or PWM) which is used as the reference voltage of the power supply. This method only replaces the original reference voltage with the MCU, and the output voltage value of the power supply can be input by key. The MCU is not added to the feedback loop of the power supply, and the power supply circuit is not changed. This method is the simplest.

The second is to expand the AD of the single-chip microcomputer, continuously detect the output voltage of the power supply, adjust the output of the DA according to the difference between the output voltage of the power supply and the set value, control the PWM chip, and indirectly control the operation of the power supply. In this way, the single-chip microcomputer has been added to the feedback loop of the power supply, replacing the original comparison and amplification link. The program of the single-chip microcomputer must use a more complex PID algorithm.

The third is to expand the AD of the single-chip microcomputer, continuously detect the output voltage of the power supply, and output PWM waves according to the difference between the output voltage of the power supply and the set value, so as to directly control the operation of the power supply. In this way, the single-chip microcomputer intervenes in the operation of the power supply the most.

The third method is the most thorough single-chip microcomputer control of the switching power supply, but it also has the highest requirements for the single-chip microcomputer. The single-chip microcomputer is required to have a fast computing speed and be able to output a sufficiently high-frequency PWM wave. Such a single-chip microcomputer is obviously also expensive.

DSP-type microcontrollers are fast enough, but their prices are also very high. From a cost perspective, they account for too large a proportion of the power supply cost and are not suitable for use.

Among the cheap microcontrollers, the AVR series is the fastest and has PWM output, so it can be considered for use. However, the operating frequency of the AVR microcontroller is still not high enough, so it can only be used reluctantly. Let's calculate the level at which the AVR microcontroller can directly control the switching power supply.

In AVR microcontrollers, the maximum clock frequency is 16MHz. If the PWM resolution is 10 bits, then the frequency of the PWM wave, that is, the operating frequency of the switching power supply, is 16000000/1024=15625 (Hz). Obviously, the switching power supply is not enough to operate at this frequency (within the audio range). Then take the PWM resolution as 9 bits, and the operating frequency of the switching power supply is 16000000/512=32768 (Hz). It is outside the audio range and can be used, but it is still a certain distance from the operating frequency of modern switching power supplies.

However, it must be noted that the 9-bit resolution means that the power tube conduction-off cycle can be divided into 512 parts. As for conduction alone, assuming the duty cycle is 0.5, it can only be divided into 256 parts. Considering that the pulse width is not linearly related to the output of the power supply, it is necessary to at least halve it, that is, the power supply output can only be controlled to 1/128 at most. Regardless of the load changes or the network power supply voltage changes, the degree of control can only go so far.

Also note that there is only one PWM wave in the above description, which is single-ended operation. If push-pull operation (including half-bridge) is required, two PWM waves are required, and the above control accuracy is halved, which can only be controlled to about 1/64. For power supplies with low requirements such as battery charging, it can meet the use requirements, but for power supplies with higher output accuracy, this is not enough.

To sum up, AVR microcontrollers can only be used reluctantly in direct PWM control.

However, the second control method listed above, that is, the microcontroller adjusts the output of the DA, controls the PWM chip, and indirectly controls the power supply, does not have such high requirements for the microcontroller, and the 51 series microcontroller is already competent. The price of the 51 series microcontroller is still lower than that of the AVR.

Netizen coocle once expressed his opinion: "The disadvantage of the single-chip microcomputer controlled switching power supply is that the dynamic response is not enough, and the advantage is that the design is flexible, such as protection and communication. My idea is to combine the single-chip microcomputer with the pwm chip. The frequency of the pwm output of the general single-chip microcomputer is generally not too high. If the frequency is too high, it is difficult to achieve single-cycle control. So I think the single-chip microcomputer can complete some flexible analog settings, and the pwm chip can complete some work behind it."

Coincidentally, there is an original article "Research on DPWM Circuit" in the Electronic Power Comprehensive Area, which also uses digital circuits to output PWM waves to directly control the operation of the switching power supply. He uses CPLD plus a single-chip microcomputer for control. It is well known that the price and development difficulty of CPLD are by no means comparable to single-chip microcomputers, so why does he do this? The reason is as the author said, because the PWM width of the single-chip microcomputer is small, resulting in low precision and cannot meet the requirements of the system. The author also said that in these cases, the application of off-chip PWM circuits is undoubtedly an ideal choice. He chose CPLD chips to implement PWM. I suggest: use the original control chip of the switching power supply to implement it. Not only is the price low, but it is also easy to implement protection functions such as single-cycle current detection. We don't have to do digital control for digital control.