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The Rise of Digital Control for DC/DC Regulation


Traditionally, switching DC/DC converters (voltage regulators) were controlled using analog techniques because these were simple to implement and helped to maximize the power supply’s efficiency. Digital components were too slow and power greedy to challenge analog dominance.

However, over the past several years the introduction of high-speed, low power consumption and inexpensive silicon has rekindled interest in “digital power management.”

Armed with these new chips, designers are keen to take advantage of features such as the ease with which digital control enables optimization of the power supply by adaptation of a closed-loop response – even allowing adjustment “on the fly” to suit changes in the operating environment or to compensate for factors such as capacitive loading and component aging.

This article considers the benefits that digital power brings to voltage regulators by comparing an analog power supply with its digital equivalent. The article then continues by describing some of the latest digital regulators released by major vendors that take advantage of the control techniques discussed.

Control vs. efficiency

Efficiency is a critical design parameter of power supply engineering. In a simple switching step-down (“buck”) converter for a portable device, for example, the designer wants to lower the battery voltage to a level suitable for sensitive silicon but not waste any valuable millivolts in the process. Lossless conversion is impossible, but careful design can see modern switching buck regulators with better than 90 percent efficiency.

Linear Technology, for example, offers the LTC3549, a 2.25 MHz buck regulator for portable applications. The device provides 250 mA output current (with an input voltage of 1.8 V and an output voltage of 1.2 V) and claims an efficiency of up to 93 percent.

Another example of a high efficiency buck regulator is Intersil’s ISL8009A. This chip is a synchronous device that can deliver up to 1.5 A output current. The company says the chip is optimized for generating low output voltages down to 0.8 V from a supply range of 2.7 to 5.5 V with an efficiency of up to 95 percent.

It is this relentless drive for efficiency that has led power supply design to remain entrenched in the analog domain for years. While digital components offered features that enabled enhanced power management, the additional overhead – and drain on efficiency – that came from potential substitute digital components was a compromise too great for many engineers.

Recently, improvements in the power consumption of digital components – both as a result of improvements in CMOS manufacturing techniques and new, enhanced digital architectures – have narrowed the trade-off between control and loss of efficiency. Today, engineers can reap the benefits of digital power supply design without undermining system efficiency.¹ Better yet, digital control can actually improve the efficiency of a previously all-analog design.

Digital power management

How does replacing analog components with digital chips enhance power supply design? The general answer to this question is that digital chips allow the engineer to finely tune the power management of his/her circuit – even down to controlling the energy dissipation during every cycle of the switching power supply’s operation.

A typical voltage regulator is shown in Figure 1. The analog feedback loop regulates the voltage output by comparing a scaled representation of the output to a reference voltage and amplifying the difference (or “error voltage”). The error voltage forms one input to an analog PWM chip that outputs a variable width drive pulse that has an average value tending towards the desired output voltage. Amplified in voltage and current (power) by the power switching stage, drive pulses are averaged by the filter to yield the desired DC output voltage.



Figure 1: Schematic of an analog buck voltage regulator. (Courtesy of Emerson Network Power.)

A control mechanism, typically incorporated into the error amplifier circuitry, compensates for gain variation and phase shift over a range of frequencies, stabilizing the power supply over the anticipated range of operating conditions.

With modern digital components, it is relatively simple to replace this analog feedback loop (error signal, compensator, and PWM) with a digital equivalent. Figure 2 shows this alternative circuit. The power switching, scaling, and filter function are identical to the circuit in Figure 1.



Figure 2: Schematic of a digital buck voltage regulator. (Courtesy of Emerson Network Power.)

In the digital regulator, an analog-to-digital converter (ADC) transforms the difference between the measured (scaled) output voltage and the reference voltage (the error voltage) into a digital value. Part of the increased precision of the digital controller is derived from the resolution at which the ADC converts the error voltage to a binary value. Higher resolution improves the control of the feedback loop. In addition, the precision of the reference voltage and the update frequency of the ADC output to the proportional-integral-derivative (PID) processor significantly affect the stability and accuracy of the output voltage.

The PID processor of the digital regulator replaces the compensator network of the analog device. PID processors are clever devices used on many closed-loop control processes. As the name suggests, a PID controller combines present error (P) with the accumulation of past errors (I) and a prediction of future errors (D) to correct the regulator output by constantly adjusting the duty cycle of the train generated by the digital PWM. The PID processor must also take on the job of compensating for gain and phase-shift factors around the control loop, just as the compensator network does in the analog version.²

(Note that in digital systems, phase shift factors arise from time delays in processing the control data stream. These phase-shift factors are in addition to those present in an analog system, mostly from the output filter).

The PID control algorithm is tuned by altering the coefficients (K) of the three terms to solicit the required response. The relative values of these coefficients in the algorithm determine the system frequency response, for example, the loop gain and phase shift (Figure 3). As with analog loops, a PID control system must be adjusted to create whatever system response is desired and guarantee stability. Usually initial values for the coefficients are calculated or transformed mathematically from an analog design and then methodically altered to obtain the desired response. Although this is not a trivial process, the use of a digital processor does make each change simple to implement.

Figure 3: Schematic of a PID controller algorithm. Coefficients (K) can be adjusted to produce required system response.

The final element in the digital control loop, the digital PWM, generates the same variable width drive pulse as its analog counterpart, but it does so by “calculating” and then “timing” the desired duration of ON and OFF periods of the output signal that forms the square wave. (In contrast, the analog PWM operates by triggering ON at a clock transition and triggering OFF when a fixed voltage “ramp” reaches a preset trip voltage.)

Intelligent manipulation

A key advantage of digital control for voltage regulation is that the components can be easily connected to the product’s bus, facilitating communication and data storage. This allows measurements such as the regulator’s temperature, input voltage, and output current to be continuously monitored. The information is then used to modify the PID algorithm coefficients ‘on-the-fly’ to optimize the regulation for the operating environment. Such customizable control is beyond the capability of analog systems.

The benefits of digital power management extend beyond closed-loop control. Significantly, digital control can even be used to address the key challenge faced by power supply designers: efficiency.

Consider the top and bottom metal oxide semiconductor field effect transistors (MOSFET) in a switching voltage regulator. For the supply to operate properly, the circuit must be designed such that the two MOSFETS never conduct simultaneously. Designers add “dead time” to the switching function (facilitated by the PWM device) to guarantee simultaneous conduction cannot occur.

Ideally, the dead time should be as small as possible to maximize efficiency. However, it must be long enough to encompass the variability of component tolerances, resulting in a fair degree of margin in conventional analog products. Worse yet, the effect of dead time varies depending on the regulator load. At lower loads it has a greater impact on efficiency.

Digital power management allows the manufacturer to set a unique dead time for each product, optimized to suit the performance of the actual components used to assemble the device. This eliminates the need for a built-in margin to compensate for component-to-component variability and creates a net increase in efficiency. Figure 4 illustrates the efficiency improvement of the digital regulator over an equivalent analog product.³

Figure 4: Efficiency improvement of digital regulator with custom dead time compared with analog product with standard dead time.

Ready-made solutions

Designers can take advantage of digital power management without the hassle of designing the circuits from scratch. Texas Instruments (TI), for example, offers a range of modules that can be used to build a complete digital power supply. The heart of such a system is the Digital Powertrain Module, which acts as the power conversion section of a digital power system and works in tandem with a digital power controller.

One example is the company’s PTD08A020W, is a 20 A rated, non-isolated digital PowerTrain module. The unit operates from an input voltage range of 4.75 to 14 V, and provides step-down power conversion across a wide range of output voltages from 0.7 to 3.6 V. The company says that the wide input voltage range makes the PTD08A020W particularly suitable for computing and server applications that utilize a loosely regulated 8, 9.6, or 12 V intermediate distribution bus. Additionally, the wide input voltage range increases design flexibility by supporting operation with tightly regulated 5 or 12 V intermediate bus architectures.

A separate digital power controller such as TI’s UCD9240 supervises the PTD08A020W. The PTD08A020W receives control signals from the digital controller and provides parametric and status information back to the digital controller. The UCD9240 is a multi-rail, multiphase synchronous buck digital PWM controller for non-isolated dc/dc regulation. The chip integrates the dedicated circuitry required for closed-loop control plus a serial interface to support configurability and monitoring.

The UCD9240 allows the engineer to adjust sequencing, margining, tracking, and phase management to optimize for the power system’s efficiency. Loop compensation and calibration are supported without the need to add external components.

Other manufacturers offer complete digital voltage regulators; for example, CUI Inc’s NQB-D Series is a family of fully regulated quarter-brick intermediate bus DC/DC regulators (see the TechZone article “An Introduction to Board-Mounted DC/DC Converter Bricks”) with advanced digital control (Figure 5).

Figure 5: CUI’s NQB-D Series brings digital control to the quarter-brick form factor.

The converters incorporate a 32-bit ARM-based microcontroller with power-optimizing firmware that the company claims will maximize and flatten the efficiency curve, providing peak performance across a much wider range of loading conditions. In addition, the NQB-D series provides users a range of configurable power management features via PMBus, including output voltage, voltage margining, fault management, precision delay ramp-up, and start/stop.

For its part, GE Power Electronics offers the 3 A Digital PicoDLynx power modules. These are non-isolated DC/DC converters that can – as the name suggests – deliver up to 3 A of output current. The modules operate over a range of input voltages (3 to 14.4 V) and provide a regulated output voltage from 0.45 to 5.5 V, programmable via an external resistor and PMBus control. Features include a digital interface using the PMBus protocol, remote ON/OFF, adjustable output voltage, overcurrent, and overtemperature protection.

The PMBus interface supports a range of commands to control and monitor the module. The module also includes the “Tunable Loop” feature that allows the user to optimize the dynamic response of the converter to match the load allowing a reduction in output capacitance which the company says leads to savings on cost and board real estate.

The rise of digital power

Analog DC/DC voltage regulators are remarkable devices offering stable output voltages and high efficiency performance. However, in the case of the integrated modules typically used for point-of-load (PoL) applications, the engineer is pretty much stuck – with the exception of some minor changes to loop response by changing input and output filter components¬¬ – with the operating parameters selected by the supplier. While those parameters work well for most anticipated operating conditions, there are inevitably some trade-offs.

Digitally-controlled voltage regulators give the engineer back the control to optimize his power supply to suit the needs of a specific application. This is a benefit in many designs; for example, in a portable product where the engineer is tasked with maximizing battery life, or where precision control over output voltage is critical such as when powering DSPs or ASICs.

There are some drawbacks – for example, increased cost and greater board area – but these are becoming much less significant as more vendors embrace digital power and enhance the technology. Digital power is clearly set to increase its share of the voltage regulation sector.

References:

  1. “Digital Power Helps Get Products to Market More Quickly,” CUI Inc.
  2. “An Introduction to Digital Control of Switching Power Converters,” Geof Potter, Emerson Network Power
  3. “Performance Improvements for OEM System Designers - A Digital Control Case Study,” Ericsson.
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