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  • Distributed Power Control Architecture With Multiple MCUs Over FSI

    • SPRACR6 April   2020 F29H850TU , F29H859TU-Q1 , TMS320F280023-Q1 , TMS320F280023C , TMS320F280025C , TMS320F280025C-Q1 , TMS320F280041-Q1 , TMS320F280041C , TMS320F280041C-Q1 , TMS320F280048-Q1 , TMS320F280048C-Q1 , TMS320F280049-Q1 , TMS320F280049C , TMS320F280049C-Q1 , TMS320F28384D , TMS320F28384D-Q1 , TMS320F28384S , TMS320F28384S-Q1 , TMS320F28386D , TMS320F28386D-Q1 , TMS320F28386S , TMS320F28386S-Q1 , TMS320F28388D , TMS320F28388S , TMS320F28P650DH , TMS320F28P650DK , TMS320F28P650SH , TMS320F28P650SK , TMS320F28P659DH-Q1 , TMS320F28P659DK-Q1 , TMS320F28P659SH-Q1

       

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  • Distributed Power Control Architecture With Multiple MCUs Over FSI
  1.   Distributed Power Control Architecture With Multiple MCUs Over FSI
    1.     Trademarks
    2. 1 Introduction
    3. 2 Distributed Power Control Architecture
      1. 2.1 Distributed Power Control Architecture – DC/AC System
      2. 2.2 Distributed Power Control Architecture – AC/DC System
      3. 2.3 Distributed Power Control Architecture – DC/DC System
    4. 3 DC/AC System - Power Topologies
      1. 3.1 MPPT DC/DC Stage
      2. 3.2 DC/AC Inverter Stage
    5. 4 C2000 Controller Configuration for DPCA DC/AC System
    6. 5 Communication Interface – FSI for DPCA DC/AC System
      1. 5.1 FSI Star Connection
      2. 5.2 FSI Daisy Chain Connection
      3. 5.3 FSI Frame
      4. 5.4 FSI Transmission Latencies
    7. 6 Summary
    8. 7 References
  2. IMPORTANT NOTICE
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TECHNICAL WHITE PAPER

Distributed Power Control Architecture With Multiple MCUs Over FSI

Distributed Power Control Architecture With Multiple MCUs Over FSI

Distributed power architecture (DPA) has been widely used in many complex systems that require multiple power and voltage levels. DPA approach allows modular power converter design and, hence, enables general economies of scale, allowing manufactures quickly implement power conversion solutions with a wide range of output power and output voltages. This white paper discusses the application of C2000 microcontrollers (MCUs) in implementing control of such DPA systems. The control approach discussed here essentially implements a very flexible distributed power control architecture (DPCA) using multiple C2000 MCUs, taking full advantage of its control optimized high speed C28x CPU, flexible control peripherals and a new communication module, called the fast serial interface (FSI). Such system level solution can be advantageous in applications like solar string inverters, off-board EV chargers and any other distributed DC/DC or AC/DC applications. First, this document provides an overview of C2000 MCU-based distributed control of such power conversion systems and then presents an example case study of DPCA using the solar string inverter application.

Trademarks

All other trademarks are the property of their respective owners.

1 Introduction

The design advantages of distributed power [1] have helped it spread to industrial controls, data communication, telecom and every application that uses complex systems with a requirement for multiple power and voltage levels. There was a time when the predominant power supply architecture consisted of a centralized power unit that distributed power throughout the system via a network of cables and supply bus bars, but as power demands have changed, so have system power topologies. Driven by increasing power consumption and a need for better supply performance, in conjunction with the availability of wide bandgap GaN and Sic products, more sophisticated power distribution architectures are being implemented today. The modern distributed power architecture (DPA) can spread the concentration of heat throughout the system, support high currents at different voltage levels and provide excellent transient response to rapidly changing loads. DPA architecture is widely used in AC/DC rectifier application where the front-end AC/DC power supply covers the power needs of the entire system. Then the required DC/DC stages are controlled locally in order to achieve high efficiency and improved static and dynamic performance. This way all the subsystems DC/DC connect to a single DC bus making it easier to design and upgrade the sub-systems.

DPA concept is also being increasingly used today in fast DC charging application because it enables modular design and allows adjusting to a wide range of output voltages. Here again a front-end power factor correction (PFC) stage, such as a three phase Vienna PFC, provides the high voltage DC bus which is then used to power multiple DC/DC stages that finally charge the battery. The modular approach makes it possible to achieve economies of scale for manufacturers, enabling them to reuse existing sub-units and design blocks when addressing new customers. In the event of a failure, the modular approach also simplifies maintenance and repairs. With the push towards shorter charging times the total power that is needed in a DC charger increases significantly. Consequently, the power delivered by each sub-unit and design block in a DC charger goes higher to provide a balance between performance, power, and ease of use. The sub-units themselves are based upon efficient multi-level, multi-phase topologies allowing heat generation to be spread across the available volume, as well as enabling scalability. The modular approach also enables general economies of scale, enabling manufactures to quickly implement a wide array of charger output powers as market demands develop.

Similar DPA approach is also being used today in solar inverter and energy storage systems (ESS), specifically, in solar string inverters with ESS. For the solar string inverter with or without ESS capability, the load-end DC/AC inverter supply covers the power needs of the entire system while achieving high efficiency and improved static and dynamic performance. Then the front-end DC/DC stages are operated locally in order to achieve high efficiency, optimized maximum power point tracking (MPPT) and other features, such as, rapid shutdown control. Common factors that impact energy production in a solar inverter include shading, different module types and orientations or large temperature fluctuations. In this case a distributed architecture can increase energy production largely due to having the maximum power point tracking (MPPT) capability at each dc/dc stage. The DPA architecture also provides obvious advantage of increased uptime for string inverters. Since string inverters are converting less power for fewer panels, if one string fails, the whole array's energy is not lost, just the power from that string. The same applies when inverters need to be disconnected for preventive maintenance of system components. Also, during scheduled maintenance of system components (excluding the inverter stage), the array can be turned off in smaller sections, which further increases uptime.

Thus, with overall goals of cost effectiveness, scalability, modular design, high efficiency, optimize operation and fast static and dynamic performance DPA provides obvious advantages in many AC/DC, DC/DC and DC/AC power conversion applications.

C2000 microcontrollers (MCUs) with its control optimized high speed CPU, flexible control peripherals and fast serial interface (FSI) is a perfect fit for implementing distributed digital control of such DPA systems. Multiple C2000 MCUs can be used to control multiple power conversion stages and then all the MCUs can establish a very fast communication link between them using their FSI ports. This approach essentially implements digitally controlled flexible distributed power control architecture (DPCA) for many DPA systems. The new FSI module is a serial communication peripheral capable of reliable high-speed communication across isolation devices. It provides a higher speed, lower latency, noise tolerant signaling across isolation with built in error detection and correction. It provides variable packet size (2 to 32 bytes, not meant for large data transfer) and so ideal for bytes of data transfer between control loops in a DPCA system. With embedded data robustness checks, data-link integrity checks, skew compensation and integration with control peripherals, the FSI module can enable high-speed, robust communication in any DPA system and thus allows implementation of distributed power control architecture (DPCA).

2 Distributed Power Control Architecture

This section discusses different applications where the C2000-based distributed digital power control architecture (DPCA) can be used to achieve modular and flexible power designs.

2.1 Distributed Power Control Architecture – DC/AC System

The first DPCA application is illustrated in Figure 1. This is the C2000 MCU-based solar string inverter DPCA system where the photovoltaic (PV) string output is processed through a MPPT DC/DC converter and then applied to the DC/AC inverter inputs. The inverter finally delivers power to the grid. As indicated in Figure 1, all DC/DC and DC/AC power conversion stages are controlled by multiple C2000 MCUs. A key feature of such PV system is the efficiency of conversion at which the power converter stage can extract the energy from the PV arrays and deliver to the load. The maximum power point tracking (MPPT) of the PV output for all sunshine condition allows reduction of the cost of installation and maximizes the power output from the PV panel. Therefore, the DC-DC converter employs some MPPT algorithm to efficiently extract the PV output power and convert the PV output voltage to a high voltage DC bus. The converter draws dc current from the PV panel such that the panel operates at its maximum power transfer point. This requires maintaining the panel output, such that, the DC-DC converter input at a level determined by the MPPT algorithm. The DC-DC converter, depending on the system requirement, can use either an isolated power stage or a non-isolated stage. This is most frequently implemented using an interleaved boost or multi-level boost converter stage. The high voltage bus from the DC-DC converter is then used to power the DC-AC inverter that eventually supplies the load and connects to the grid. Depending on the power level of the string inverter the MPPT DC/DC could consists of multiple power stages controlled by multiple C2000 MCUs. Then all of the MCUs, as indicated in Figure 1, could be connected through the high-speed FSI ports for exchange of information between the DC/DC and the DC/AC stages. This exchange of information could be anything related to closed loop control, system parameter monitoring and other system level management functions needed to support the proper operation of this solar inverter DPCA system.

solar-string-inverter-in-a-dpca-configuration-spracr6.gifFigure 1. Solar String Inverter in a DPCA Configuration

Figure 2 shows the same solar string inverter DPCA system as in Figure 1 with the added energy storage feature. This makes it an energy storage system (ESS). All DC/DC and DC/AC power conversion stages are controlled by multiple C2000 MCUs. For the ESS with string inverter, the grid-tie converter indicated in Figure 2 needs to be bi-directional. Also a second bi-directional DC/DC is needed between the energy storage device, such that, the battery, and the high voltage DC bus. Here again, based on the power level of the ESS, the DC/DC could consists of multiple power stages controlled by multiple C2000 MCUs and then all the MCUs could be connected through the FSI ports for overall system control and management.

solar-string-inverter-with-energy-storage-in-a-dpca-configuration-spracr6.gifFigure 2. Solar String Inverter With Energy Storage in a DPCA Configuration

2.2 Distributed Power Control Architecture – AC/DC System

Figure 3 illustrates the DPCA configuration for a fast DC charging application for electric vehicle (EV). Most of the modern off-board fast chargers have a power rating of 50 kW to 135 kW which allows battery recharging in less than 30 minutes. New 150 kW fast chargers have also been announced with 350 kW chargers as a part of long-term plan. These commercially available fast chargers are powered from 208/480 V three phase input. This is implemented with a 3 phase PFC, such as a Vienna PFC, followed by a DC/DC as illustrated in Figure 3. The DC/DC converter, usually a resonant topology, provides an efficient means of interfacing the output DC bus voltage from a three phase Vienna rectifier/T-type rectifier to the battery of an electric vehicle. The phase-shifted full-bridge topology with the SiC devices also constitutes an alternative solution for the DC/DC stage. A single C2000 MCU could be used to control the Vienna PFC and single or multiple C2000 MCUs could control multiple DC/DC stages. Then, all the MCUs could be connected through the FSI ports for overall system management. For more details on C2000 MCU-based Vienna PFC and EV charging DC/DC control, see the Vienna Rectifier-Based, Three-Phase Power Factor Correction (PFC) Reference Design Using C2000™ MCU and the Bidirectional CLLLC Resonant Dual Active Bridge (DAB) Reference Design for HEV/EV Onboard Charger, respectively.

fast-dc-charger-for-in-a-dpca-configuration-spracr6.gifFigure 3. Fast DC Charger for EV in a DPCA Configuration

The fast DC charging system illustrated in Figure 3 shows some limitation in high power EV charging applications. When implemented this way the overall system efficiency, including the service transformer (that provides the 208/480Vac input), is very low (about 93%). In addition to this, the overall size, weight and the installation cost of such a system is very high when the power rating is high to enable fast charging.

An alternative approach is to use Solid-State Transformer [3] (SST) technology to directly connect the fast charger to the 2.4 kV Medium-Voltage (MV) single-phase line. This avoids the use of service transformer, improves efficiency and significantly reduces the installation costs. However, due to the relatively high voltage (2.4 kV ac) at the converter input, a modular design approach needs to be adopted if commercially available Silicon Carbide (SiC) devices are to be used. One example is the Multi-Cell Boost (MCB) topology with three input-series-output-parallel (ISOP) connected dc/dc converter modules as shown in Figure 4. This is adopted for the fast charger application due to its high power density, high efficiency, low cost, low number of switches and expected high reliability.

input-series-output-parallel-multi-cell-dcdc-for-fast-ev-charger-spracr6.gifFigure 4. Input-Series-Output-Parallel Multi-Cell dc/dc for fast EV Charger

By using this topology, the losses can be reduced significantly compared to the state-of-the-art solution with low-frequency transformer and three-phase PWM rectifier as shown in Figure 3. This topology has additional advantages such as independent control of input and output stages, a relatively simple control strategy for dc bus capacitor voltage balancing and the reduction of the input and output inductors by interleaving the 3-level boost converters at the input side and the NPC converters at the output side. The ISOP configuration of the adopted MCB topology modules enables scalability in both the input voltage and the output power ratings. Here again, based on the power level of the fast charger and its input voltage ratings, the ISOP configuration could consists of multiple power stages controlled by multiple C2000 MCUs and then the MCUs can be connected through the high speed FSI ports to form a fast EV charging DPCA system.

Figure 5 shows the isolated/non-isolated off-line AC/DC DPCA system where the input AC voltage is rectified through a PFC stage and then applied to the high frequency isolated DC/DC converter. The PFC stage draws sinusoidal input current in phase with the input voltage in order to maintain high power factor (PF) and low THD input current and provides a regulated high voltage DC output to the DC/DC stage. The DC/DC stage generates an isolated low voltage DC output with tight regulation and fast transient response against fluctuations in input voltage and output load current. Such AC/DC system is usually controlled by single or dual C2000 MCUs. However for higher power level both the power conversion stages could consist of multiple converters controlled by multiple C2000 MCUs. All the MCUs can then be connected through the high speed FSI port for overall system management.

off-line-acdc-power-supply-in-a-dpca-configuration-spracr6.gifFigure 5. Off-Line AC/DC Power Supply in a DPCA Configuration

2.3 Distributed Power Control Architecture – DC/DC System

Figure 6 illustrates a DC distribution [4] used in many industrial systems. Industrial plants demand highly reliable supply of power. The loads in these plants are sensitive to power supply disturbances. DC distribution addresses some of these challenges. Such dc systems use power converters to convert ac source(s) to dc and distribute the power using dc lines. DC/AC converters are used at the load end when it is required. For the motor type loads, driven by inverter drives, DC voltage is directly applied to drive inputs.

The power converters are the main components of such DC systems. The fast response capability of these converters facilitates use of alternate power sources to provide highly reliable supply of power. The alternate power sources in combination with power converters also provides a very effective method for meeting the ever-increasing high-power loads in certain applications such as, on ships that also need highly reliable and very high-quality power supply. For this kind of applications the conventional ac systems are very hard to maintain. The power converters in the DC system also provide very effective filtering against power disturbances. The fuel-cell technology is another driver for dc distribution, as the fuel cells produce dc power. Thus, the dc distribution becomes attractive for many industrial systems with heavy motor controlled loads and/or sensitive electronic loads.

dc-distribution-system-in-a-dpca-configuration-spracr6.gifFigure 6. DC Distribution System in a DPCA Configuration

Here again, multiple C2000 MCUs could be used to control the power converters, and then, all of the MCUs could be connected through the FSI ports for overall system management.

 

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