Convert Power with Digital Signal Controllers

A full digital power conversion system includes digital power control and digital power management. In digital control, feedback or feedforward loops that regulate power-system output are directly controlled by a digital signal controller (DSC), which drives a power switch duty cycle using pulse-width modulation technique.

06/01/2006


AT A GLANCE
 
  • Digital signal controller applications

  • Pulse-width modulation

  • Cut power control system costs

  • Less maintenance, more efficiency

Sidebars:
Where digital signal controllers can be applied

A full digital power conversion system includes digital power control and digital power management. In digital control, feedback or feedforward loops that regulate power-system output are directly controlled by a digital signal controller (DSC), which drives a power switch duty cycle using pulse-width modulation technique.

The DSC also provides the digital power management function—configuration, tracking, monitoring, protection, guiding supply sequence, and communication capabilities. In a more traditional approach, the power control function would be performed using analog circuitry, and power management performed with a microcontroller. The graphic below compares a dc-to-dc switched-mode power supply (SMPS) implemented in a traditional method, with a mix of analog and digital control, with one implemented digitally.

Digital power conversion

Benefits of digital power conversion are:

  • More universal and flexible applications;

  • Improved control algorithms;

  • Intelligent mode management and fault supervision;

  • Real-time control and monitoring of operating status;

  • Lower system and maintenance costs;

  • Ability to lower the cost of isolation;

  • Improved efficiency and performance; and

  • Better protection of intellectual property.

Discrete Control
In addition to cost and performance advantages, an all-digital design (right) can be simpler than an analog system with digital management, explains Freescale Semiconductor.

Example applications with SMPS products help illustrate benefits. The main purpose of a power supply is to provide regulated and stable power to a load regardless of power grid conditions. SMPS is one type of power supply widely used in office equipment, computers, communication systems, and other applications, because of its high efficiency and high energy density.

The digital signal controller used to implement digital power conversion in products should have features suited for power electronics applications. These include high performance digital signal processing (DSP) and microcontroller (MCU) processing from one processor with high-performance pulse-width modulator (PWM) peripheral, 12-bit analog-to-digital converter, high-performance timer, and integrated flash memory.

Inside an ac-to-dc SMPS

In a digitally controlled, high-frequency ac-to-dc SMPS, with two subsystems, the primary side is the ac-dc converter with power-factor correction (PFC). On the secondary side is a full bridge dc-dc converter. The ac-dc part uses an interleaved PFC boost control structure, which includes a full bridge rectifier and two assistant switches to realize the zero voltage switch (ZVS) of the main switches. Implementing a ZVS scheme reduces the components' stress and switching losses and improves efficiency. The dc-dc converter portion uses the control structure of a phase-shifted full bridge (PSFB).

Integrated features of the on-chip PWM module perform the phase shifting. On the secondary side, a current doubler rectification is implemented. This structure eliminates expensive analog isolation, reduces PCB size and heatsink and improves efficiency, enabling higher power density. High-level functional and performance requirements of the design are:

  • Input voltage: 85-265 V ac;

  • Input frequency: 45-65 Hz;

  • PFC switch frequency: 100 kHz;

  • dc bus voltage: 380 V;

  • Input power factor: >0.99;

  • dc-dc switch frequency: 150 kHz;

  • Output voltage: 48 V dc;

Implementing a SMPS digitally is very demanding on the control processor. This is especially true of dc-to-dc SMPS, which are numerous in systems and require a higher-speed PWM and timer channels and smaller devices.

In the second diagram, a board-mounted isolated dc-to-dc SMPS includes a full brick, isolated switch-mode, forward-type dc-to-dc converter controlled by a DSC on the secondary side. Optional functions can include system communication interfaces using SPI, SCI, or I2C. Module input could come from a power supply in the range of 36-75 V dc and output is a regulated 3.3 V dc.

Software-based control

Control and power management functionality can be implemented in software via digital control loops. Control functions implemented within the DSC could include:

  • Monitor input voltage;

  • Power on/off control;

  • 3.3 V output uses average mode current control method;

  • Power system monitoring and communication protocols;

  • Intelligent fault and mode management;

  • Power-up sequence;

  • Overcurrent roll back (when current limit is reached, control is switched to constant current control);

  • Hiccup: when overcurrent, over-voltage or over-temperature is detected, shuts output, then restarts the system.

Full digital control in the SMPS lowers the number of system components, enhances system reliability, and makes it possible to add advanced functions easily, without increasing cost, because the control is performed via software running on the processor. Advanced algorithms can improve performance and power density. With the majority of the IP in software, which can be protected with flash security, it is much more difficult for competitors to replicate the design.

In addition, with control performed in software, it is possible to have a single hardware platform support varied types of end-system requirements by loading different software into the internal flash.

Advances in digital signal controllers are enabling the switch to digital power conversion by making end products technically and economically feasible. Digital power conversion can achieve technical and business benefits.


Online Extra

Inside an ac-to-dc SMPS


An all-digital switched mode power supply (SMPS), says Freescale, can place additional demands on the control processor; a dc-to-dc SMPS might require higher speed pulse-width modulation, timer channels, and smaller devices.

In a digitally controlled, high-frequency ac-to-dc SMPS, with two subsystems, the primary side is the ac-dc converter with power-factor correction (PFC). On the secondary side is a full bridge dc-dc converter. The ac-dc part uses an interleaved PFC boost control structure, which includes a full bridge rectifier and two assistant switches to realize the zero voltage switch (ZVS) of the main switches. Implementing a ZVS scheme reduces the components’ stress and switching losses and improves efficiency. The dc-dc converter portion uses the control structure of a phase-shifted full bridge (PSFB).

Integrated features of the on-chip PWM module perform the phase shifting. On the secondary side, a current doubler rectification is implemented. This structure eliminates expensive analog isolation, reduces PCB size and heatsink and improves efficiency, enabling higher power density. High-level functional and performance requirements of the design are:

  • Input voltage: 85-265 V ac;

  • Input frequency: 45-65 Hz;

  • PFC switch frequency: 100 kHz;

  • dc bus voltage: 380 V;

  • Input power factor: >0.99;

  • dc-dc switch frequency: 150 kHz;

  • Output voltage: 48 V dc;

Implementing a SMPS digitally is very demanding on the control processor. This is especially true of dc-to-dc SMPS, which are numerous in systems and require a higher-speed PWM and timer channels and smaller devices.
In the second diagram, a board-mounted isolated dc-to-dc SMPS includes a full brick, isolated switch-mode, forward-type dc-to-dc converter controlled by a DSC on the secondary side. Optional functions can include system communication interfaces using SPI, SCI, or I2C. Module input could come from a power supply in the range of 36-75 V dc and output is a regulated 3.3 V dc.


Author Information
William Hutchings and Charlie Wu are senior systems/application engineers, Freescale Semiconductor,


Where digital signal controllers can be applied

End products where digital signal controllers could be used include:

Uninterruptible power supply (UPS): online and offline;

Switched mode power supply (SMPS): ac to dc, and isolated dc to dc;

General-purpose inverters: dc to ac, ac to ac.



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