The gossip: Analog and digital will soon battle for control of power-supply
regulation. The reality: When it comes to feedback-loop control, both approaches
seem to happily coexist. (See "True Digital," p. 46, and "Looks Like Analog,
Designs Like Digital," p. 48.)
Indeed, many vendors offer a choice. Some of digital control's initial programmability
advantages are now available even in controllers and regulators that use analog
feedback. Still, digital power has some appeal.
"Power supplies have become a part of the overall system, and they're expected to handle power management," says Mikhail Guz, Power-One's director of strategic marketing and applications. "That last point is particularly important as more and more system engineers want to be able to talk to the power supply in real time for monitoring and diagnostics." Many power-management engineers agree.
"An engineer can use a single digital product in many different modes and applications," Guz continues. "They can relatively easily implement adaptive control and nonlinear control algorithms. Unlike analog, digital controllers are not subject to component tolerances and aging effects. Moreover, most of the digital control loop is built on CMOS processes that scale much more readily than analog processes, leading to cost savings in future digital generations."
WHAT'S IT ALL ABOUT?
To set the parameters for discussion, we're talking about pulse-width-modulated
(PWM), pulse-density-modulated (PDM), and pulse-frequency-modulated (PFM) switching
regulator and controller ICs. Some integrate drivers for the transistor or transistors
that do the actual switching. Others don't. Some even include the switching
FETs, if they supply modest loads. Consequently, the digital-versus-analog issue
depends on how the regulator's control loop is closed.
Figure 1 shows two of the most common variations
of PWM switching topology—the buck and the boost converter—boiled
down to their basics. In a synchronous configuration, a second transistor would
replace the diode. In a sense, the use of pulse-width modulation makes these
converters quasi-digital, at least compared to 723-style linear regulators based
on a series-pass element. In fact, PWM makes it possible to use digital control
loops. However, the converters in Figure 1 lack the circuitry that controls
the duty cycle of the switch or switches, which can be implemented in either
the analog or digital domain.
VOLTAGE-AND CURRENT-MODE CONTROL
Whether analog or digital, there are two ways to implement the feedback loop:
voltage mode and current mode. For simplicity, first consider how they're implemented
in the analog domain. A thorough discussion is available in Maxim's Application
Note AN2031 (http://pdfserv.maxim-ic.com/en/an/AN2031.pdf).
In a voltage-mode topology, a sample of the output voltage is subtracted from
a reference voltage to establish a small error signal that's compared to an
oscillator ramp signal (Fig. 2). When
the circuit output voltage changes, the error voltage also changes, which in
turn alters the comparator threshold. In turn, this will vary the output pulse
width. These pulses control the on-time of the regulator switching transistor.
Pulse width decreases as output voltage tries to increase.
One advantage of current-mode control is its ability to manage the inductor current. A regulator using current-mode control has a current loop nested within a slower voltage loop. That inner loop senses peak currents in the switching transistors and keeps those currents constant, pulse-by pulse, by controlling each transistor's on-time.
Meanwhile, the outer loop senses dc output voltage and supplies a control voltage
to the inner loop. In the circuit, the slope of the inductor current generates
a ramp that's compared with the error signal. When the output voltage sags,
the controller supplies more current to the load (Fig.
3).
In these control topologies, the gain around the control loop must not exceed unity at any frequency where phase shift around the loop reaches 360°. Phase shift combines the intrinsic 180° that results from feeding the control signal into the inverting input of the feedback operational amplifier, the additional delays in the amplifiers and other active elements, and the delays introduced by capacitors or inductors ( especially the output filter's large capacitors).
Stabilizing the loop requires compensation for gain variation and phase shift
over a range of frequencies. Historically, stabilizing a power supply with analog
PWM generally required an empirical approach: Try out different combinations
of passive components on an actual circuit board with the same layout as the
production board, and observe the circuit's time-domain response to changes
in supply voltage and load demand. Recently, this has become easier. Analog
controller companies now implement their own versions of various "plug a value
into a register" capabilities first introduced for digital controllers.
DIGITAL CONTROL LOOPS
One of the best explanations of switching-supply digital control is in the white
paper "An Introduction to Digital Control of Switching Power Converters" by
Artesyn's Geof Potter. It can be found at www.astecpower.com/whitepaper/dcdc/.
(This URL reflects Artesyn's acquisition by Emerson Network Power in February.
Astec is part of Emerson.)
The paper describes a voltage-mode control topology, but the discussion can
be extended to current mode. Most digital implementations of voltage-mode control
comprise an analog-to-digital converter (ADC), a microcontroller or DSP implementing
some control law, and a digital pulse-width modulator (DPWM) that takes the
controller's output and generates the signals necessary to drive the transistor
or transistors that perform the switching (Fig.
4).
First, the ADC produces a succession of digital representations of the output
voltage that are fed to the controller. The control law is either the familiar
proportional-integrating (PI) or proportional integrating/differentiating (PID)
algorithm.
In a PID controller (the more complex example), an algorithm that's based on
a series of coefficients operates on each ADC input. The proportional coefficient
is a gain factor that's related to sensitivity. The integral coefficient adjusts
the PWM duty cycle according to how long an error is present. The derivative
coefficient compensates for time (effectively phase) delays around the loop.
Taken together, the coefficients in the PID algorithm determine the system frequency
response.
Using its control law, the controller translates the ADC's representation of
output voltage into the pulse-duration (duty-cycle) information necessary to
maintain the desired output voltage. This is then sent to a DPWM that performs
the same drive-signal generation function as its analog counterpart.
Note the difference in how the switching transistors are managed in analog and digital schemes. The analog controller triggers the switching transistor ON at a clock transition and triggers it OFF when a voltage ramp reaches a preset trip voltage, while the PID controller calculates the desired duration of the switching transistors' ON and OFF periods.
In theory, analog control offers continuous resolution of the output voltage. But the interaction of the ADC resolution and sampling rate, plus the DPWM switching rate, make things a little tricky.