Power Electronics

Next-Gen Digital Signal Controller ICs Offer 4X More Memory

Potential applications for a new generation of 40 MIPS Digital Signal Controllers include uninterruptible power supplies, inverters, battery chargers, power factor correction, ac-dc and dc-dc power converters.

MICROCHIP TECHNOLOGY'S 16-bit Digital Signal Controller (DSC) ICs were hailed as the next generation of digitally controlled power-supply ICs in 2006 when they were first introduced. Fig. 1 shows a block diagram of the initial offering of DSCs. Now, there is a new generation of DSCs with improved performance that has the block diagram shown in Fig. 2.

The 2010 version of the DSCs provides up to four times the memory, compared to the company's previous SMPS & digital power conversion families. Additionally, you can configure the new DSCs for various topologies, giving power-supply designers freedom to optimize them for specific applications.

The best way to understand the functions of these DSCs is to examine the simplified version in Fig. 3., which shows the basic internal digital control configuration for a switch-mode power supply. These devices are configured within the supply's feedback loop, so speed is critical because the A/D converter (ADC) and processor must sample the output and react as quickly as possible to any changes. In operation, the ADC samples the power supply's output voltage and sends the digital result to the processor that controls a pulse width modulator (PWM). Then, an external power stage rectifies the PWM output and produces a filtered dc output.

Compared with analog supplies, DSC ICs provide extensive fault monitoring, better transient response, and lower-cost redundancy options. In addition, the digital approach eliminates a power supply's drift and need for temperature compensation. This digital technique can program operational settings, which eliminates the manual tweaking of power-supply adjustments. DSC ICs perform these tasks primarily using firmware.

The eight new DSCs shown in the table enable several completely independent digital control loops. These dsPIC33F GS series digital-power DSCs enable digital control loops with 12 to 18 high-speed, 1-ns resolution PWMs and one or two 10-bit, on-chip ADCs, providing 2 to 4 million samples per second (MSPS) for low latency and high-resolution control. They range from 64 to 100 pins and 32 to 64 KB Flash memory. These DSCs feature interactive peripherals that both minimize the intervention of the processor and are able to handle the real-time needs of high-speed current-mode control.

Implementing high-speed, precision digital control loops for digital power conversion applications requires a high-performance DSP engine, along with specialized digital power peripherals. Microchip's 16-bit dsPIC33F GS Series DSCs provide on-chip peripherals specifically designed for high-performance, digital power supplies. These peripherals include high-speed PWMs, ADCs, and analog comparators. Thus, the newly expanded dsPIC33F GS family supports applications such as induction cooking, uninterruptible power supplies, solar and pure sine-wave inverters, intelligent battery chargers, power factor correction, HID lighting, fluorescent lighting, LED lighting, and ac-dc and dc-dc power converters.

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Digital signal control requires fewer hardware platforms because firmware can control power-supply performance while using the same internal hardware. Plus, this type of control can change topologies and configurations on-the-fly: from buck to boost or boost to buck, as well as from continuous to discontinuous. About the only necessary change might be to use different external power output stages. External digital control can change a power supply's operating parameters, but it cannot change topologies on-the-fly. A unique aspect of these DSCs is that their analog comparators can terminate the pulse-width modulation (PWM) signal early. This allows cycle-by-cycle current limiting, which is required for current-mode power supplies.

Additionally, digital signal control enables the implementation of power factor correction (PFC) by adding appropriate firmware and some external hardware. An analog-based supply would require considerably more hardware. And, external digital control cannot provide this capability.

Digital signal control opens the door to extensive creativity, limited only by the available features, the designer s imagination, and the amount of available program memory. For example, this controller can monitor the performance of an individual component within the power supply.

Furthermore, the same basic circuit could be used for uninterruptible power sources (UPSs), power inverters, and digital lighting. This degree of flexibility isn't available with an analog-based supply or external digital control supply.

To be economically feasible, the DSC IC must cost-effectively provide the necessary high-speed power-supply functions. Besides the IC's price, there are other cost-related issues, such as learning the digital control design philosophy and developing the necessary firmware. But once this learning curve is mastered, a similar design approach can be used for all supplies.

Designers should ask themselves two necessary questions. First, what applications fit DSC-based power supplies? The obvious answer is ac-dc and dc-dc converters. The second question is what power levels make the most economic sense. It all depends on the price-performance characteristics of the DSC IC.

Today, the cost of this controller IC could become a small percentage of the overall power-supply cost at 100 W and above. This could include front-end power supplies, high-power dc-dc converters, and bus converters. In the future, economies of scale could permit use of the DSC IC for power supplies below 100 W.

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These devices are able to coordinate the analog-to-digital sampling point relative to the PWM cycle. Configurable control also allows the analog comparators to react to random events and prevent adverse effects on power-supply performance. On-chip multiple PWM circuits enable these devices to function independently without adding to processor overhead. Also, multiple PWM circuits permit their parallel use for different functions, such as voltage regulation and PFC control. The devices analog comparator can be used to terminate the PWM pulse early, enabling cycle-by-cycle current limiting. The multiple-channel ADC permits the parallel sampling of voltages and current throughout a power supply.

DEVELOPMENT TOOLS

The dsPIC33F GS series DSCs are supported by the MPLAB® Integrated Development Environment, MPLAB C Compiler for dsPIC DSCs, MPLAB SIM 30 Software Simulator, MPLAB ICD 3 In-Circuit Debugger, and MPLAB REAL ICE™ In-Circuit Emulation System.

For advanced development, Microchip's Explorer 16 Development Board (DM240001). The Explorer 16 is a low-cost, efficient development board to evaluate the features and performance of Microchip's new PIC24 Microcontroller, the dsPIC33 DSC families, and the new 32-bit PIC32MX devices. Coupled with the MPLAB ICD 3 In-Circuit Debugger or MPLAB REAL ICE, real-time emulation and debug facilities speed evaluation and prototyping of application circuitry. The Explorer 16 features two interchangeable plug-in modules (PIMs) that support the specific processor family of interest.

The Explorer 16 can be used with the buck/boost converter PICtail™ Plus daughter board. (AC164133). The Buck/Boost converter PICtail™ Plus Daughter Board provides an easy and economical development platform for the dsPIC® SMPS and digital power conversion GS family. These DSCs are designed to provide low-cost, efficient control for wide range of power supply topologies and power conversion applications.

Buck/Boost Converter PICtail™ Plus Board (Fig. 4) consists of two independent dc-dc synchronous buck converters and one independent dc-dc boost converter. The board operates from a +9V to +15V dc input. This board can be controlled either by interfacing to 28-pin Starter Development board or to Explorer 16 Development Board. The control boards provide closed-loop Proportional-Integral-Derivative (PID) control in the software to maintain the desired output voltage. The dsPIC® SMPS and Power Conversion family devices provide necessary memory and power supply peripherals which enables to build the control loops in software without the need for an external circuit.

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A new dsPIC33F GS series Plug-in Module (MA330024) is available for the Explorer 16, which enables development with this new DSC family; specifically, the 100-pin dsPIC33FJ64GS610.

APPLICATIONS

Power Factor Correction (PFC) by average current mode control uses a DSC, as shown in Fig. 5. Many power control applications demand a stable, regulated DC power source with reduced input current harmonic content and better power factor, so power factor correction is now a requirement and no longer a matter of choice. The dsPIC devices enable implementation of PFC, in addition to the integration of this algorithm with other functions of the system such as primary side control and communication on a single chip.

Implementing advanced software digital control loops for power conversion applications requires a high-performance DSP engine along with specialized peripherals. The high-performance CPU and rich peripherals of the dsPIC DSC devices enable solutions that require a minimum of external support chips. Besides the space and cost-saving benefits of the dsPIC DSC solutions, special features enable advanced power conversion. Devices that are well suited for PFC applications include devices from the motor control and power conversion family, such as the dsPIC33FJ12MC201/202. For higher performance PFC applications, the dsPIC33FJ06GS101 is a good choice.

DIGITAL 300 WATT AC/DC POWER SUPPLY WITH PFC

SMPS and digital power conversion families include peripherals designed specifically for power conversion applications such as 1-ns resolution PWM, 4 MSPS ADC, and on-chip, high-speed analog comparators with integrated programmable reference voltages.

In this typical application, the ac-dc design unit works with universal input voltage range [85 - 65 VAC], and produces multiple dc outputs (Fig. 6). The design is based on a modular structure, which features three major power stages. The input stage is a PFC Boost Converter, the intermediate stage is a phase-shifted zero voltage transition (ZVT) converter, which includes ZVT full bridge converter and synchronous rectification. The third stage is single-phase and multi-phase buck converters.

Implementing advanced software digital control loops for power applications requires a high-performance DSP engine along with specialized peripherals. The high-performance CPU and rich peripherals of the dsPIC DSC devices enable these solutions. These devices include peripherals specifically designed for power conversion. Peripherals such as a high speed PWM, ADC and analog comparators can be tied together using an internal configurable control fabric that enables them to interact directly with one another, resulting in performance gains in digital power applications.

Isolated dc-dc power supplies are used in a wide variety of applications ranging from telecommunication equipment, industrial equipment, digital televisions, and server racks. As shown in Fig. 7, this application accepts 36V to 75V on the input and supplies a stable 12V output. The design uses a highly efficient phased shift full bridge converter and synchronous current doubler rectification on the secondary side. The single dsPIC device supports the complete power conversion control as well as intelligent power management and system communications. Devices that are well suited for dc-dc converter applications include ones from the SMPS and digital power conversion family, such as the dsPIC33FJ16GS502.

An uninterruptible power supply (UPS) application (Fig. 8) utilizes high-frequency switching techniques to implement three optimized digital power-conversion stages. The dsPIC33F GS series of digital-power DSCs from Microchip is the heart of this offline UPS application. The dsPIC33F GS controls all critical operations of the system, as well as the housekeeping operations. The dsPIC33F GS controller IC ensures that fast and reliable switchover from the power mains to the inverter, and from the inverter back to the mains, is done intelligently to ensure that power to the load is transferred without surges and sags that can interrupt operation.

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