07 Jun 2011

Digital Control For Power Converters Redefines High Performance

Patrick Le Fèvre, Marketing and Communications Director, Ericsson Power Modules describes how digital power control for power converts significantly improves performance

The adoption of digital control in board-mounted dc-dc converters is pushing these products well beyond the performance limits of conventional power converter designs based on analog control techniques. A new generation of digital power products offers higher efficiency, greater power density, and a level of design flexibility that was previously unavailable to most engineers.

Since their inception, board-mounted dc-dc converters have relied on analog control methods to generate regulated supply voltages. Consider for example, a simple buck converter. Within its inner control loop, the buck converter uses a series of analog functions to generate the pulse-width modulation (PWM) signal that drives its power switches. The analog PWM controller contains an error amplifier, voltage reference, ramp generator and comparator as shown in top section of the diagram below.

Although this analog approach has been the norm for many years, advances in mixed-signal silicon process technology have made it feasible to implement this inner control loop using digital functions. This approach is commonly known as digital power control. In the case of a digitally controlled buck converter, the error amplifier is replaced by an analog-to-digital converter, while the voltage reference, ramp generator and comparator are replaced by logic circuits that perform digital signal processing techniques to generate the PWM signal as depicted in bottom section of the diagram below.

The main difference between an analog-controlled buck converter (top diagram) and a digitally-controlled buck converter (bottom diagram) is the way the PWM signal is generated.

Why Go Digital?

Adopting digital power control offers several benefits. First, it allows supervisory and housekeeping functions to be integrated on the same chip as the PWM controller rather than requiring the use of external components. Features such as output voltage setting, power sequencing, voltage margining, and fault handling are performed by logic circuitry easily added to the digital PWM controller.

This approach allows functions that might otherwise be hardwired to become digitally programmable via a standard interface. For example, many digital PWM controllers incorporate the SMBus interface, which then allows these ICs to be controlled and configured using the industry-standard PMBus communications protocol.

By eliminating external components, this programmable functionality frees up space on the converter’s PCB, which shrink’s the power converter’s footprint while also enhancing its reliability and reducing its cost. One aspect of improved reliability is that the digital values that control the converter’s operation do not drift with time or temperature—unlike the resistors and capacitors used to configure analog PWM controllers.

The ability to digitally configure and monitor a power converter is often referred to as “digital power management.” Although sometimes the terms “digital power management” and “digital power control” are used interchangeably, strictly speaking they refer to different, but related concepts.

A power converter does not have to incorporate digital power control (i.e. digital control of the inner loop) to include digital power management. However, designing a controller based on digital power control allows for an easier and more seamless integration of digital power management functions in the controller chip. Put another way, when you implement digital power control, you get digital power management capability essentially for free, since it adds negligible silicon cost to the PWM controller.

Lifecycle Benefits

Digital power techniques extend the programmable logic model to the power conversion industry, enhancing a power converter’s usability and performance over the full product lifecycle. When the digital power converter is initially designed, it can be easily configured and reconfigured by the power system designer to meet performance goals. It can be reconfigured in production to optimize performance and minimize unit-to-unit variation. The product can be reconfigured again by the distributor to meet different customer requirements and yet again by the customer either during the manufacture of their end equipment or when the end product is operating in the field.

Consider for example, some of the user programmable parameters offered by Ericsson’s 3E family of digital power converters. Users can program output voltage selection; turn-on/off delay times to implement power sequencing for multi-rail loads; slew rate control for inrush current protection; voltage margining for system testing; and multiple thresholds for warning and fault conditions for overcurrent, over-temperature, and under- and overvoltage. With this family, users can even optimize the transient response of the converter’s control loop for the particular load and bulk output capacitance conditions seen in the application as shown below.

A digital power converter enables the end user to reprogram control loop constants to optimize the converter’s performance for operating conditions in the application.

Improved Efficiency Across Operating Conditions

Analog-controlled power converters are generally inefficient when operating under light loads. For example, most analog dc-dc converters only begin to operate efficiently at 15% to 20% of their rated loads, typically achieving their peak efficiency at about 50% to 70% of their rated output power. For a long time, this type of performance was considered acceptable because most systems had relatively stable loads.

But more recently, the operating characteristics of the end systems have changed. Today, systems . . . . . . . . . . .

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