The compact size and cost-effectiveness of Switched-Mode Power Supplies (SMPSs) have made them workhorses of efficient power conversion. Traditional SMPS designs are purely analog, as these are simple to implement and help to maximize power supply efficiency. The past two decades have seen the rise of digital control methods and Digital Signal Controllers (DSCs), based on general-purpose or dedicated microprocessors, Digital Signal Processors (DSPs), or programmable logic devices for SMPS design. The design flexibility, superior performance, and inherent low cost in digital control systems have significantly increased the popularity of digital controllers. This article aims to illustrate the scope, benefits, and performance of digital power supplies vis-a-vis their analog counterpart. The quantitative comparison will also cover an overview of a market-ready digital power starter kit for SMPS devices.
Analog vs. Digitally Controlled DC Power
Any digital power converter employs a combination of analog and digital circuitry. A "digital power control" refers to implementing digital circuitry within the inner control loop of a power converter, instead of analog schemes. Figure 1 illustrates a high-level, generic two-stage analog and digital AC-DC power supply block diagram. The front end converter boosts the Power Factor Correction (PFC) circuit, while the second stage is a DC-DC phase-shifted full-bridge converter. The power train, drive circuits, and feedback circuits are common elements of both analog and digital power supplies. The dedicated analog controller and housekeeping circuitry can be combined into a single dsPIC DSC for the digital version of this power supply.
Figure 1: A high-level block diagram of a two-stage Analog and Digital AC-DC Power Supply (Image Source: Microchip)
A typical analog power supply uses a housekeeping MCU to accomplish its power management requirements. This housekeeping MCU transmits the local system parameters to a master controller or data logger. However, such a configuration requires supplemental hardware interfaces between the housekeeping MCU and the power conversion circuits to measure and generate system parameters data.
Conversely, a digital power supply eliminates the need for any additional circuitry, as the DSC has already measured all system parameters. A DSC is a single-chip that combines a Microcontroller (MCU) control features with the computation and throughput skills of a DSP within a single core. The parameters can be stored in the DSC's memory and transmitted to the remote system, using on-chip communication peripherals. A simple software routine can also make any modifications to the system operation without additional hardware.
The Benefits of Digital Control for Power Conversion
A digital power supply offers significant advantages over its analog counterpart. Digital power supplies offer lower cost, higher performance, higher efficiency, and higher power density. Let us discuss these benefits in this section:
1. Lower Cost: Because multiple analog controller blocks can be integrated into the DSC, system performance can be enhanced with simultaneous reduction of system complexity and bill of materials (BOM) costs. Figure 2 shows the differences between typical analog and digital power supply components. The analog controller (indicated by arrows) needs supplemental connections. A consequence of digitalization is diminished component count. This helps achieve simpler layouts, smaller PCB sizes, reduced PCB fabrication and assembly costs, and improved product quality and reliability.
Figure 2: Typical analog and digital stage circuitry (Image Source: Microchip )
2. Increased Efficiency: The DSC used to build a digital power supply has the advantage of having on-board high-performance peripherals, such as the PWM generator, the ADC and the comparator, and the high-performance DSP engine. The combination of these features allows power supply engineers to implement many digital algorithms into a single DSC. System performance is thus increased, with higher power efficiency.
3. Power Density: One of the leading power conversion market trends is the increase in power density, in the form of a continuous escalation in power generated by components that are getting progressively smaller. Digital controllers provide benefits here, due to their ability to integrate multiple functions. It is possible to implement many control algorithms in the DSC and control several converters with other auxiliary functions.
4. Versatility: An analog solution is hard-wired, whereas a DSC is fully programmable. Code programmability is particularly essential during the production cycle. Post-production tests can be performed by downloading a test program into the chip. This adds the versatility of manifold tests to check for functionality and verify performance. Flash programmability allows users to change the final version of the code, track all possible bugs, or customize last-minute customer requests after the overall design completion.
5. Monitoring and Protection: An essential requirement in power systems is the management of fault conditions. In a power supply system, some typical checks are performed to ensure that the entire system operates normally, not producing any over-temperature, over-current, or over-voltage conditions. The fault management peripheral in the digital power supply is fully programmable, allowing power supply designers to select between the cycle-by-cycle monitoring. A digital controller takes note of all the fault conditions in the entire system and reacts almost instantaneously to the fault, regardless of fault location.
6. Communication: The move to digital also brings benefits in terms of communication. The parameters may be configured and changed in run-time and real-time, either locally or from a remote central unit. The behavior of the converter can also be changed dynamically through code, if needed. New code releases can be locally or remotely uploaded to upgrade the system, fix bugs, or add functionality. In digital power supplies, the parameters stored in the DSC memory can be transmitted to the remote system using on-chip communication peripherals such as SPI, I2C, UART, or CAN. Local or remote data logging can be used in those systems where the quality of the produced power must be monitored and reported. Data can be recorded for operational conditions along with fault events.
The Microchip Technology dsPIC33C Digital Power Starter Kit
The Microchip Technology dsPIC33C Digital Power Starter Kit demonstrates the capabilities and features of the SMPS device family. This kit features onboard dsPIC33CK256MP505 Digital Signal Controllers (DSCs). These consist of one independent DC/DC synchronous Buck Converter, one independent DC/DC Boost Converter, independent resistive loads, LCD, protection circuitry, USB/UART converter/bridge, and onboard programmer/debugger circuitry that eliminates the need for any additional hardware. Figure 3 represents the top view of the dsPIC33C Digital Power Starter Kit, with components list marked for each section on board.
The dsPIC33C device controls both power stages (Buck and Boost). The output voltage of the Buck Converter (Buck Out) can be programmed from 1V to 3.8V output, with a default programmed voltage of 3.3V. The Boost Converter (Boost Out) output voltage can also be programmed from a 10V to 17.8V output, with a default programmed voltage of 15V.
|Number||Figure 3: Part Description|
|1||2x16 Character LCD|
|2||Buck Converter Stage|
|3||Boost Converter Stage|
|6||Push Buttons for Controlling Resistive Loads|
|7||Reset Push Button|
|8||User Push Button|
|9||dsPIC33C Digital Signal Controller (DSC)|
|11||DC Power Input|
|12||Buck Converter Output|
|13||Programming Connection for dsPIC DSC|
|14||Boost Converter Output|
The dsPIC33 DSC device featured in the Digital Power Starter Kit is customized to provide low-cost and efficient control for a wide range of power supply topologies. The dsPIC33 DSC device provides the necessary memory and peripherals for ADC conversion, PWM generation, analog comparison, and general-purpose I/Os, avoiding the need to perform these functions in external circuitry.
In the dsPIC33C Digital Power Starter Kit, an auxiliary PIC24F microcontroller controls loads and protection circuitry. The PIC24F microcontroller and the dsPIC33C device communicate over UART, exchanging information such as load temperature, load status, and protection status. An on-board temperature sensor is located in the area of the resistive loads, enabling the user to program a temperature protection limit and a high-temperature fail-safe shutdown.
The dsPIC33C DSCs series is ideal for a variety of power conversion and monitoring applications, such as UPSs, inverters, and power management units within complex equipment, such as copiers, telecom switches, and routers, which require advanced power management.