Digital Block with a dsPIC® DSC
Contents
What is Digital Power?
Digital power conversion applications use digital algorithms, which are integrated and available in the dsPIC® Digital Signal Controllers (DSCs). The digital algorithms monitor, control, and supervise power conversion functions, configurations, and extending to full-loop control. In the following sections. We'll take a look at the basic implementation and components of the dsPIC DSC in power conversion applications.
Digital Power Applications
Digital power can be used in many popular power conversion applications.
- DC-AC Power Inverters
- Uninterruptible Power Supplies (UPS)
- Battery chargers
- AC-DC Power Supplies
- DC-DC Converter, Point of Load (POL)
- HID Lighting
- Solid State Lighting, such as LED
Digital Power Implementation Using a dsPIC® DSC
With digital power conversion, the power stage is unchanged from its analog counterpart. The dsPIC replaces three operating blocks from the analog buck topology: the summing node which compares the “real” output voltage and the reference voltage, the compensator, and the PWM generator (see accompanying image). There are a number of advantages to using the dsPIC in place of these three blocks. First, it reduces the component count. While this reduces overall system cost, it also means reduced complexity leading to improved reliability of the entire system. Second, the compensator design is more versatile because it's now represented by an algorithm in the dsPIC DSC. Using the dsPIC also means that embedded designers can implement additional functions such as fault management, temperature monitoring, soft start, and sequencing. In the next several sections we'll investigate how the digital implementation is derived from the original analog blocks.
PWM Generator
The PWM generator is an on-chip peripheral of the processor. This means that after the processor initializes it at start-up it will continuously perform its task without further intervention. The details of how the output signal is generated will be addressed later. As the dsPIC processes the input values, it provides either a duty cycle or frequency parameter to the PWM generator, which in turn produces a square wave output to the field (in this case the switch's gates).
Controller Algorithm
The controller algorithm replaces the compensator from the analog version. It replicates in the digital domain the same transfer function that is implemented in the analog design. The digital solution enables designers to employ techniques that are not possible with the analog solution, including proprietary digital compensation algorithms and non-linear control techniques. The full digital solution enables a customized response to power input change or load change events, which can lower system cost and increase system efficiency.
Additionally, a digital compensator's topology and behavior is much easier to change compared to its analog counterpart. No circuit redesign or hardware changes are required. Only a few constant values in the software need to be modified.
Analog to Digital Converter
The Analog to Digital Converter (ADC) reads a voltage and converts it into a number. This is the key in digital design. Everything is reduced to a number. The input and output voltages, currents flowing into the inductors, transformers and the output connections have to be converted into numbers so that they can be processed by the system. The dsPIC, an enhanced 16-bit PIC® microcontroller is used as the core for power conversion applications due to its fast and efficient numeric processing capabilities.
Hardware capable of performing fast multiply and add operations (multiply-accumulate) is required to guarantee real-time results. The dsPIC satisfies this requirement with its integrated DSP engine, which operates at very high speed and resolution on the numbers generated and used by the system.
Note:
The dsPIC family of Digital Signal Controllers (DSCs) features a fully implemented digital signal processor (DSP) engine, with up to 70 MIPS performance, C compiler friendly design and a familiar microcontroller architecture and design environment. The dsPIC 16-bit Flash DSCs provide the industry’s highest performance, and have features supporting motor control, speech and audio, intelligent sensing applications, in addition to digital power conversion. For more information, please visit the "dsPIC33C Digital Signal Controllers" page. The accompanying image shows the DSP engine block diagram of the dsPIC33FJ32GS406.
The accompanying image demonstrates the logical design and implementation of a digital system.
A digital design requires information about the circuit topology. A set of equations and numbers describe the input and output relationships and a set of parameters describe the system. The system works with these parameters and equations in real time.
When running, the dsPIC will gather numeric values from the analog to digital converter, though it could also use the comparator and/or serial communication ports. The numeric values represent the system's input and output voltages and currents. The DSP core then uses these numbers to solve the equations that describe the system and feeds the results back to the system, closing the control loop. The accompanying image shows an application example of a synchronous buck converter using dsPIC33FJ32GS606.
