Microcontrollers become a part of the most of newly design devices. They can integrate still more and more functionalities on single chip, as they offer complex peripherals which were available only on sophisticated devices few years ago. One of the functionality which can be integrated is switched-mode power conversion unit, which has some advantages comparing to other available solutions.
Switched-mode power conversion units are important part of battery chargers, power factor correction modules and power supplies. They have significant impact on system level parameters such as stand-by power consumption or overall system conversion efficiency. Design concepts of power converters have quite a long history, and well established, specialized integrated circuit solutions for typical use cases. However, when it comes to digitally controlled power converters, the number of design choices is limited.
"Important feature of digitally controlled power converter is possibility of reconfiguration of control algorithm"
The design options are: standard controller with additional interface for digital control, switching regulator with built-in interface, and fully digital controller. The solution based on the first option requires additional, mixed-signal hardware which increases complexity of the design. In the second option, required hardware is integrated with converter controller, however currently there is a limited number of available design options which available controllers offer. The third option gives the biggest freedom for designer, and is the one I’d like to look closer at.
In general, digital power conversion unit includes analogue signal acquisition module, data processing unit, and control signal generation part. There are specialized chips on the market designed for that kind of use, but the same components are also available as a peripherals of popular microcontrollers. The crucial subsystem, which let build cost-efficient, compact power converter with the use of a single microcontroller is composed of DAC (digital-to-analogue converter) connected to analogue comparator, which can control pulse width modulator (PWM).
The combination of these peripherals built-in to microcontroller is very well suited to build switched-mode current source, which then can be used as apart of power converter working in current mode. The power converter of this kind has two control loops – the first one has lower bandwidth and controls output voltage. The output of this loop is a reference signal for the second, inner control loop, which regulates peak current. Implementation of current control loop in microcontroller equipped with mentioned earlier peripherals can be realized in hardware, and can be fully independent of CPU. That kind of solution has a number of advantages. The most important is that it can work with switching frequency of hundreds of kHz without any need of intervention from CPU during operation. Moreover, the resolution of produced control pulse is independent from microcontroller clock (pulse is started synchronously by internal timer module but it is switched off asynchronously by comparator).Resolution of current peak regulation is defined by resolution of DAC producing reference signal for comparator. The outer control loop regulating voltage can be much slower, and can be implemented as a software module. One of the products which can benefit from that solution is single-chip LED current mode power supply. In this case the voltage loop is not needed at all as it can be replaced with simple voltage limiter. Microcontroller can easily regulate output current, and simultaneously handle communication protocols, diagnostics algorithms, derating procedures etc. Important feature of digitally controlled power converter is possibility of reconfiguration of control algorithm. This property can be used to build bidirectional converter, which uses single transformer in buck mode to charge battery or to boost its voltage to supply motor inverter or prepare bus voltage for uninterruptible power supply.
There are numerous advantages of those digitally controlled power converters. They let build compact, highly integrated, cost efficient devices which allow further modifications within the software area. On the other hand, they require additional, low power supply for control unit and need careful design to achieve proper EMC immunity. They are also more difficult to design as multidisciplinary knowledge is required and hardware is tightly connected with control software.
Integration of power conversion unit into microcontroller is an interesting option, worth to consider especially when setpoint for the output voltage, current or power need to be controlled during operation. Reconfiguration of control algorithm gives flexibility unavailable on different approaches. There are also benefits of this solution when additional runtime diagnostic is needed, or more than one power converters are handled by single microcontroller.