Digital Power

What is DIGITAL POWER?
Digital Power is the feedback control of a power converter via digital methods as opposed to analog means. In addition, digital power also refers to the digital communication that takes place between the host controller and the power converters in a system. The latter is sometimes referred to as digital power management.

Digital Power ARCHITECTURE

The block diagram of a simplified dc/dc converter is shown in the figure. The block titled DC/DC Switching Converter is the dc/dc circuit. This is a buck circuit with transistors, diodes and passive components. The blocks that are of interest for digital control are the a/d converter, Control Law Processor (typically implemented using a DSP or MCU) and the Digital PWM.

The a/d converter takes the output voltage of the dc/dc converter in some form (an analog signal) and creates an equivalent digital signal. The digital signal is then run through the Control Law Processor, which performs a compensation algorithm on the converted signal. In essence, a reference voltage is compared to the result of the a/d converter within the Control Law Processor. The compensation information is then forwarded to the DPWM (digital pulse width modulator), which adjusts the voltage sent to the dc/dc converter circuit in order to maintain the desired voltage.

A Control Law Processor is typically a PID controller that translates the digital representation of the output voltage from the a/d converter into pulse duration information that the DPWM will use to make adjustments, if necessary. The PID, or proportional integral derivative, contains factors that determine the dc level and dynamic response parameters of the control system. The factors are coefficients of a mathematical equation that can be adjusted to affect system performance. A simplistic view, but it provides a basic understanding of digital feedback control in a power converter.

Digital COMMUNICATION
In addition to the digital control via feedback, there is the possibility of configuring, controlling, and monitoring the digital power converters. During manufacturing, test and in-system use, the power converters, through the changing of registers, can make modifications to output voltage as well as other characteristics. One important advantage is that manufacturers can produce one product and then via configuration create a complete portfolio of products without changes in board manufacturing.

Traditionally, different configurations were accomplished through the selection of different resistor and capacitor values. This added to manufacturing costs. The configuring, controlling and monitoring of power rails can also be achieved with a simple digital bus on the power converter.

Semiconductor companies have introduced parts to accomplish power-up sequencing, margining and other configuration chores. Until recently, these parts relied on proprietary interface protocols and the use of the physical layer of popular buses such as I²C and SMBus. But selecting a part with a proprietary protocol forces the designer to stay with a particular manufacturer for the entire design. A standard, on the other hand, allows for multiple manufacturers to be used within a single design.

PMBus (www.pmbus.com) is an open-standard digital power-management protocol with a fully defined command language and transport and physical interface. PMBus is used for host systems to communicate with power converters (see the figure). The protocol was founded and is being maintained by power supply and semiconductor manufacturers. The first release of the protocol was in March 2005.

The PMBus transport layer is based on the SMBus specification (System Management Bus) (www.smbus.org), which was defined by Intel Corporation for system management communications in PCs and servers. The main difference between SMBus and I²C is a signal line called SMBALERT, which in the PMBus case, allows power converters to interrupt the host of the system. This results in efficient and flexible power control designs.

One thing to note is that the PMBus protocol dictates that a power converter must power-up and work even if there is no digital communication. The configuration is stored in non-volatile memory or is pin-programmed. The physical address of each device is set with pins on the power converter. In addition to the SMBus' clock, data and interrupt lines, the PMBus protocol also specifies two hardwired signals for use with power converter devices. One is a control signal to turn individual power converters on and off and the other is an optional write-protect signal that can be used to prevent any changes to memory-held data. PMBus has the advantage that the host device is not based on a proprietary digital interface. The host device can be a processor or a microcontroller as well as any other intelligent device (i.e. laptop computer). The laptop can be used in manufacturing and test for pre-production calibration.

PMBus communicates via a simple set of commands. Every packet contains an address byte, followed by a command byte, then, if needed, data bytes and an optional packet error code byte. The host uses single start-and-stop conditions to indicate the start and end of the process, and the addressed power converter uses a single bit to acknowledge reception of each byte. Power converters not meeting the manufacturer's specification can be repaired, replaced or recalibrated as part of the test process. PMBus is being backed by converter manufacturers, such as Artesyn Technologies and Astec Power and by semiconductor manufacturers such as Silicon Labs, Ziker Labs and Texas Instruments, to name a few.

DIGITAL POWER SEMICONDUCTOR MANUFACTURERS
Semiconductor manufacturers have recently introduced products to address the digital power market. Silicon Laboratories, Texas Instruments, and Zilker Labs among others have released semiconductor products that can be incorporated into power converters of different types as well as products that can be used in conjunction with the power converters.

Silicon Laboratories (www.silabs.com) has introduced their Si825x series of digital power controllers (see the figure). These controllers combine a digital signal processor with a system management controller. This architecture provides the digital power control and power management functions needed for various power supply and converter topologies. The Silicon Labs architecture allows for smaller size and lower power consumption compared with other DSP-based power converter designs. The feedback control section incorporates an a/d converter, programmable DSP filter engine, six-channel DPWM and programmable over current protection (OCP) hardware detector.

Separate from the feedback section is the system management controller, which handles fault detection and recovery, and optimizes the feedback control. An item to note is that the system management controller handles PMBus communication and external device management via the SMBus interface on-board. The system management controller is based on a 50-MIPS 8051 CPU with 32 kB of flash with additional peripherals for development. Si825x designs are supported with a complete toolset. The toolset consists of a real-time firmware kernel, loop compensation and timing simulator/designer tools, system controller, and configuration wizards. An integrated development environment is also available.

Zilker Labs (www.zilkerlabs.com), a start-up company, is offering a power management and conversion IC called the ZL2005. This IC incorporates a synchronous buck controller and flexible gate drivers with system management functions. The ZL2005 does not require programming, which makes for easier and quicker circuit designs. The chip is fully configurable using simple pin-strap connections,-resistor selection or industry-standard PMBus commands over the chip's SMBus interface. The ZL2005 performs soft-start and soft-stop, "power good" and enable, output voltage and current monitoring, input voltage monitoring, output voltage tracking, sequencing and margining, and thermal monitoring and shutdown. By connecting two or more ZL2005s via the digital interface on-board, it is possible to support multiphase designs up to 25A. Unlike other digital power controllers, the Ziker Labs devices integrate the PMBus code into the IC.

Texas Instruments (www.ti.com) has introduced digital power semiconductors designed to improve on performance, power consumption and also to limit complexity. The Fusion Digital Power solutions consist of three overlapping series of chips, the UCD9K, UCD8K and UCD7K.

The UCD9K family consists of digital power controllers with specific power peripherals onchip. The family includes the DSP engine, DPWM, 12-bit a/d converter and communication-port for PMBus interface. The UCD9K would need other power stages and drivers to create a complete circuit.

The UCD7K devices are MOSFET drivers that interface the digital power controller, possibly the UCD9K, to the power switching stage. The UCD8K controllers integrate the UCD7K driver functionality with a digitally controlled analog PWM controller to close the feedback control system. The Fusion family offers a modular approach to digital power. This allows the designer choices in digital and mixing analog and digital for the best performance for a particular application.

POWER CONVERTER MANUFACTURERS
Astec (www.astec.com) and Artesyn (www.artesyn.com) are two power converter manufacturers that have announced digital power converters with PMBus support. Astecs DTX42K48 shown in the photo is the first product to be introduced in their family of digital dc/dc converters. The converter is isolated and can accept dc inputs from 36V to 75V and has a dc output capability from .96 to 1.44V. Output power can also reach up to 50W. The DTX42K48 has the capability of communicating via the PMBus, but as per the PMBus specification, it does not need to communicate via the bus in order to function as per the specification. Customers can configure the converter via Astec support tools. Preprogrammed, off the shelf versions will also be available.

See Associated Figure

Artesyn Technologies has released the DPL20C Point of Load Converter. It is a 20A output converter that features an extensive set of digital configuration, monitoring and diagnostic capabilities via the PMBus interface. The DPL20C stores all configuration and set-up data in onboard non-volatile memory, and powers-up with these default settings, thus eliminating the need for external power controllers. The DPL20C has an input voltage range from 4.5V to 13.8V. The output voltage is programmable with a range from 0.6V to 5.5V. With all digital power converters, the DPL20C has programmable sequencing, tracking and margining. Artesyn also allows for real-time monitoring of voltage, current, and temperature, with automatic warning of fault condition warnings. Also, the DPL20C can use 45 executable PMBus commands. The DPL20C was scheduled to start shipping in Q4 2005.

WRAPUP
In summary, Digital Power offers the following:

1. Less complex, smaller and lower power consumption designs that perform feedback control on power converters.
2. On the fly" configuration as well as control and monitoring of power converters in systems.
3. A standard communication protocol, PMBus.
4. The setting of registers as opposed to reworking components on the board to make configuration changes.

Andrew Leone is a regular contributor to EEPN. He earned his BEE from Johns Hopkins University.