Adding Intelligence to Control
Joseph Julicher, Microchip Technology
There are few applications addressed by integrated electronics that don’t require some modest form of power regulation, be that a simple current/voltage supply or a more sophisticated and optimised solution. However, with an increased focus on efficiency at every level it is becoming common to extend the simple to include some form of intelligence.
Even a relatively unsophisticated control algorithm can deliver performance benefits such as reduced power consumption, while with a little more effort any device can now include more advanced features such as maximum power tracking, refined battery charging, environmentally aware settings and improved fault tolerance.
This is a perfect scenario for low-cost microcontrollers, even the smallest device is now powerful enough to execute complex algorithms, while providing machine- and human-interfaces. The ability to accommodate some form of standardised communication interface, such as SMbus, LIN or Ethernet adds another level of value.
Risk & Reward
A "low risk" method of making power smarter is to simply monitor the supply using an MCU and relay the parameters it measures to a back-system over some form of communication interface. This approach needs minimal additional components and design effort; typically restricted to adding some method for sensing the vol-tage, current and perhaps temperature. Monitoring other parameters, such as duty cycle or supply frequency, will allow more sophisticated control over power-specific features including voltage levels.
There exist a number of Switched Mode Power Supply ASSPs that offer a method of modifying its functionality based on decisions made through measuring critical parameters. It offers a simple way to increase efficiency and is a task that can be handled by a wide range of microcontrollers; serial interfaces effectively enable an MCU to modify the functionality of the power supply based on monitored parameters.
A further benefit of this approach is that the power supply itself remains under the control of the SMPS ASSP, which means the design team need not have any specific power supply design knowledge beyond understanding the parameters available for modification, and the affect of that; the key control theory remains the domain of the SMPS engineer.
The next step along the implementation curve - and one that offers potential cost savings - is to integrate the SMPS ASSP and MCU functionality in to a single device. It is increasingly possible to achieve this in a high-performance MCU that is closely integrated to a fast sampling ADC; an approach that affords a fully digital, all-software implantation. Of course, this approach incurs the need for greater SMPS design expertise, and the overall performance will be related to how much processing power the MCU can deliver (often limited by system-level power requirements).
The Hybrid Approach
The middle-ground between the two scenarios described above is what some term the Hybrid Approach. In this case, a mixed-signal controller which integrates the necessary analogue peripherals are, again, fully integrated and one such device is the PIC16F753. This device features an operational amplifier, slope compensator, DAC, comparators and PWM controller in a single 14-pin MCU.
Each of the peripherals is programmable, allowing them to be combined in a variety of ways to create a large number of current mode power supplies. As they are software-controlled, configuration is dynamic, allowing it to adapt to changing power supply conditions. This could see the device configured to operate as a hysteretic controller with a simple firmware feed-forward regulator when in standby mode, but allowing the supply to be quickly reconfigured for continuous current mode at a different operating frequency when more power is demanded.
Because the supply’s control resides entirely within the MCU there is no need for additional components to be added later in the design cycle, which has the added benefit of both simplifying the design and lowering the component count. Furthermore, as the solution is fully integrated the firmware has total visibility of the power supply parameters without significantly changing the design process, while the communications and intelligence interface can be developed and verified by the power supply engineering team.
A typical power supply based on the PIC16F753 is shown in Figure 1; most implementations are minor variations of a common SMPS configuration. The Complementary Output Generator produces a complementary output with a programmable deadband from rising and falling inputs, while the CCP is configured to produce a programmable frequency rising edge. The comparator, C1, produces the falling edge when the current exceeds the output of the slope compensator.
The CCP and C1 can be combined to create a maximum duty cycle, which is needed by some topologies such as boost, flyback and SEPIC. The operational amplifier, OPA, is used to provide feedback and compensation, while the DAC is used to provide the Op Amp’s reference (although the Fixed Voltage Reference, FVR, can also be used if programmable levels are not required). The slope compensator can be reset by the comparators or the COG, and it uses a programmable current sink to decay a pre-charged capacitor (in this case, the charge level is set by the OPA).
This is a relatively simple configuration and Figure 2 shows an example of it operating as a boost supply regulating current in a LED string. The flowcharts in Figure 3a through 3c depict the levels of intelligence that can be added once it has been configured, allowing it to target a range of applications.
The addition of intelligence to a power supply has far-reaching benefits and can either be achieved simply, by adding an MCU, or more comprehensively by using a more capable, fully integrated solution such as a high-performance dsPIC or mixed-signal MCU that integrates all the performance and peripherals needed to realise a sophisticated single-chip SMPS.
However it is delivered, smart power has the potential to impact significantly on electrical device operation and - more importantly - efficiency. With today’s highly capable, low-cost and fully integrated solutions, adding intelligence has never made more sense.
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