Application of single chip microcomputer in power supply design

Power supply designers often face conflicting requirements. On the one hand, they need to reduce size and cost, while on the other hand, they need to provide more functions and increase output power. Due to the limitation of principle, the functions of analog power supply itself are limited, and the design of analog power supply controller is becoming more and more complicated. For this reason, some designers turn to pure digital power supply design. However, for many designers, it is not easy to turn to unfamiliar fields so quickly. A more feasible compromise is to use traditional analog power supply, but add a digital microcontroller as the front end.

The advantage of this design is that the control of the power supply itself is still implemented using analog technology. Therefore, the power supply designer can add new functions to an existing design without having to start from scratch with a fully digital design. With this approach, the familiar error amplifier, current sensing, and voltage sensing circuits are still used in the design. Of course, although some design elements (such as the compensation network) are still implemented with discrete components, the rest is controlled by the microcontroller.

The functions that a microcontroller can provide can be divided into four categories: control, monitoring, judgment functions and communication. We will discuss these functions in detail below.

The first type of control functions is related to the hardware interface between the microcontroller and the power supply. In analog design, it is very important to leave an interface for connecting to the microcontroller. Some power supply controllers generate control signals (such as reference voltages) internally. Such controllers provide few external connection points to the microcontroller. Microchip's MC P1630 power supply controller is designed to provide a rich connection point for the microcontroller. For the purpose of this article, we assume that the power supply controller provides two control points - a shutdown input and the ability to set the reference voltage, as shown in Figure 1. Although these two connection points may not seem like many, they can provide very powerful control functions and complex functions.

Currently, the role of microcontrollers in many power supply designs is mainly monitoring. Many microcontrollers have on-chip analog-to-digital converters (ADCs) and analog comparators. Therefore, microcontrollers are ideal for monitoring signals such as input voltage, input current, output voltage, output voltage, and temperature.

The ability to monitor such a wide range of signals allows the microcontroller to perform more functions, such as intelligent fault detection. The versatility of the microcontroller comes from its programmability, which can be easily customized to meet the design requirements. In this way, fault conditions can be classified and handled. Brief overcurrent and other non-critical faults may only require setting a flag. Faults such as overheating may require shutting down the power supply until the fault is corrected. Faults that require restarting the power supply can also be more tightly controlled. If there are too many faults in a certain period of time, the microcontroller can permanently shut down the power supply.

The powerful processing power of the microcontroller can also realize complex calculations and measurements, such as real-time calculation of power. Determining the power value in an analog system requires complex analog calculations. But for a microcontroller, it is just a piece of cake. Parameters such as input power, output power, efficiency, and power loss can all be calculated.

Finally, the microcontroller’s monitoring capabilities can also support more advanced features such as fault prediction. By comparing the operating current in real time with historical data, the power supply designer can determine the conditions that will cause the power supply to fail. The ability of the power supply to predict its own failure can save costs and provide higher reliability.

Monitoring data is not just for fault detection. There are many other actions that can be taken based on the data. These tasks fall into the category of decision functions. Decision functions allow power designers to add greater flexibility, functionality, and protection to their designs. Let's consider the case of soft start or undervoltage lockout. By using a microcontroller to perform these tasks, the lockout voltage and soft start ramp rate are both programmable and do not rely on analog devices.

Deterministic functions can also perform more complex tasks. For example, a power-up sequence can be programmed to monitor another voltage and not start until the monitored voltage reaches a set value. There may also be situations where two voltages must rise in proportion or follow each other. All of these functions can be implemented by modifying the software without making any changes to the hardware.

Another possible application of the deterministic function is to adjust the current limit based on temperature. This allows the power supply designer to use the temperature derating parameters of the device to ensure reliable operation.

The use of judgment functions can also achieve device compensation, thereby improving its accuracy. Many data sheets give the change of parameters with temperature. In this case, the microcontroller can be used to implement temperature compensation. In this way, designers can use lower-cost components and compensate the results according to temperature. Microchip application note AN1001 (DS01001) describes how to achieve ±0.1°C temperature sensing accuracy using a ±6°C temperature sensor through compensation.

The MCU's diagnostic capabilities can also be used to self-calibrate the power supply, providing a known voltage at the output, which is detected and stored by the voltage feedback circuit. In this way, any errors in the voltage feedback resistors can be eliminated, allowing low-cost resistors to be used without sacrificing accuracy. Furthermore, the hardware for both the 5V and 3.3V power supplies is identical, with the only difference being the calibration process.

These are just a few examples of how the microcontroller's judgmental functions can be used. They are given only to show how powerful the microcontroller is. As you can see, a large number of power supply parameters can be monitored and controlled by a small and inexpensive microcontroller. But we haven't discussed the storage and retrieval of information yet. This is where power supply communication comes in.

There are many ways to communicate with a power supply, from the simplest jumper or switch settings to complex protocols such as Ethernet. Simple communication methods can be used to set parameters such as output voltage or operating mode. More complex protocols can support more complex and comprehensive control and monitoring of the power supply.

The real value lies in remote communication. This is extremely important for telecom and server power supplies that are located remotely. This remote monitoring capability also allows operators to improve system reliability.

In addition, remote communications allow operators to adjust voltage and current limits based on anticipated load conditions. At the same time, using redundant power supplies can further improve reliability and uptime. Once a power supply receives a signal indicating a fault has occurred, it can notify the operator, shut down the faulty power supply, and activate the backup power supply. This process can also be automated, with the faulty power supply automatically activated and switched to the backup power supply based on set conditions.

Power supply communication is not just for monitoring and setting operating parameters. Many microcontrollers have on-chip EEPROMs to store data such as production information. Once a device failure occurs, the equipment operator can easily determine which power supplies are affected. At the same time, maintenance history can also be stored. This ensures that the power supply's production data, maintenance history, and operating information are always at hand, saving the latest information.

There is a common misconception about the rich tasks that can be implemented using microcontrollers as listed above. Designers may think that these tasks must be implemented using high-end microcontrollers or digital signal processors. In fact, all the tasks described in this article can be easily implemented using low-cost 8-bit microcontrollers. In addition, this design using microcontrollers is not intended to replace existing analog functions, but rather to complement analog systems, providing the entire power system with the flexibility and processing capabilities that only digital microcontrollers can provide.