Small microcontrollers add big functionality

Power-supply designers are often charged with conflicting tasks: they are required to reduce size and cost while simultaneously adding features and increasing output power. Despite the ever-increasing complexity of analogue power-supply controllers, their features are limited when the supply is created. To this end, some designers have turned to purely digital power supplies. However, for many designers, this is a radical departure into unfamiliar territory. A worthy compromise comes in the form of a traditional analogue power supply with a digital microcontroller as a front end.

The beauty of this design lies in the fact that the power supply is still controlled using analogue techniques. This allows power-supply designers to bring added functionality to existing designs without the need to start from scratch with a digital design. The familiar error amplifier, current sense and voltage sense are all still present. While some design elements, such as the compensation network, are still implemented using discrete components, others are now controlled by the microcontroller.

The features added by a microcontroller can be grouped into four categories: control, monitoring, deterministic functions and communications. These categories will each be discussed in more detail.

 

Powerful control

The first group of features falls into the control category and is dependent on the hardware interface between the microcontroller and the power supply. It is important to determine the points in the analogue design when the microcontroller can be connected. Some power-supply controllers generate control signals, such as voltage references, internally. These controllers provide very few external connection points for a microcontroller.

Other power-supply controllers, such as Microchip’s MCP1630, are designed to be microcontroller friendly and provide a wealth of connection points. For the scope of this article, it will be assumed that the supply controller provides us with two control points; a shutdown input, and the ability to set the reference voltage, as shown in Figure 1. While these two connection points may seem insignificant, they can provide very powerful control and complex functionality.

 

 

Parameter and fault monitoring

Microcontrollers currently find their way into many power-supply designs with a role in monitoring. Many microcontrollers have on-board ADCs and analogue comparators. This makes them ideally suited for monitoring signals, such as input voltage, input current, output voltage, output current and temperature.

Monitoring this wide variety of signals enables the microcontroller to perform functions, such as intelligent fault detection. The versatility of the microcontroller lies in its ability to be programmed and customised to suit the design’s needs. To this end, not all fault conditions need be treated in the same way. Brief over-current situations and other non-critical fault conditions may only set flags. An over-temperature condition may require the supply to shut down until the fault condition is clear. Faults that restart the supply can be tallied. Too many faults in a given timeframe may permanently disable the power supply.

The processing power of the microcontroller is also utilised through its ability to acquire complex metrics, such as power. Determining power in an analogue system requires an intricate analogue calculation. For the microcontroller, however, this is no problem. Parameters such as input power, output power, efficiency and power losses can all be calculated.

Finally, the microcontroller’s monitoring abilities enable more advanced tasks, such as failure prediction. By comparing current operating conditions to previously collected data, power-supply designers can determine conditions that lead to supply failure.

In these cases, the power supply’s ability to predict its own failure can provide for cost savings and higher reliability.

 

Deterministic functions

Fault detection is not the only reason for monitoring data. There are many other actions that can be taken based on this data. These tasks fall into the category of deterministic functions.

Deterministic functions allow power-supply designers to add flexibility, functionality and protection to their designs. Consider the basic tasks of soft start or under-voltage lockout. By allowing the microcontroller to perform these tasks, the lockout voltage and soft-start ramp rates are programmable and do not rely on analogue components.

Tasks that are more complex can also be executed. Consider power-up sequencing: the power supply can be programmed to monitor another voltage, waiting until the voltage rises fully before starting up. Perhaps two voltages must rise proportionally or track one another. Any of these possibilities can be implemented through changes in software, with no hardware changes required.

Another possibility for a deterministic function may be scaling the current limit based on temperature. This allows power-supply designers to use component temperature derating parameters to ensure reliable operation.

Deterministic functions also make it possible to compensate components to increase their accuracy. Many datasheets show how parameters vary with temperature. In these cases, the microcontroller can be used to perform temperature compensation. This allows the designer to use lower-cost components and compensate their results based on temperature. Microchip application note AN1001 (DS01001) describes how a ±6°C temperature sensor can be compensated and perform as a ±0.1°C temperature sensor.

As a final deterministic function, consider a self-calibrating power supply, where known voltage is supplied at the output, sensed through the voltage-feedback circuit and stored. Using this method, any tolerance in voltage-feedback resistors is eliminated, and lower-cost resistors can be used without sacrificing accuracy. Furthermore, the hardware for a 5V and 3.3V power supply is the same: only the calibration process changes.

These examples of deterministic functions are by no means an exhaustive list, but show the power, and potential, of the microcontroller. It is clear that a number of power-supply parameters can be monitored and controlled via small, inexpensive microcontrollers. However, the means for storing and retrieving this information has not been covered. This is where power-supply communications become important.

 

Power-supply communications

The real value here is that this communication can happen remotely. This is especially important for remotely-located telecommunication and server power supplies. This remote monitoring also allows operators to increase the reliability of their systems.

Additionally, remote communication may allow operators to adjust their voltage and current limits based on anticipated loading. In the same way, the use of redundant power supplies can increase reliability and up-time. Should a power supply receive a signal indicating that conditions for failure are present, the operator can be notified, disable the failing power supply and activate the spare supply, or the process can be automated and the failing power supply can activate its own spare.

Power-supply communications are not just used for monitoring and setting operating parameters. Many microcontrollers contain on-board EEPROM that can be used to store data, such as production information. Should a component failure arise, an equipment operator can easily determine which supplies are affected. It is also possible to store repair history. Doing so ensures that the power supply’s production data, repair history and operating information will always be present and up to date.

Given the extensive list of microcontroller possibilities, there is a common misconception about the processor required to perform these tasks. Designers may believe that only high-end microcontrollers or digital signal processors can be used. All of the tasks described in this Design Note can easily be performed with low-cost, 8-bit microcontrollers. Again, the goal is not to replace the existing analogue functionality, but merely to supplement it with the flexibility and processing power that only a digital microcontroller can provide.