The long-standing debate between digital and analog has recently extended into the power supply field, drawing attention and responses from the analog community. While numbers remain popular, the real world is inherently analog. Power supplies are no different from other real-world systems—they deliver analog outputs such as voltage, current, and power. This is similar to how digital devices like TVs, cameras, and cell phones also rely on analog signals for functions like video, images, and sound.
Beyond the well-known communication aspects, digital power supplies involve processing units within the power conversion system, which are now being digitized. However, the interface to the analog world—the outer layer—remains analog. Analog circuits represent continuously variable physical quantities like length, voltage, and pressure. In the analog market, we see data conversion and interface products that handle digital data and circuit components.
In reality, the analog world includes a wide range of mixed-signal applications, spanning from pure analog to digital, and from power to signal. Simulation remains a fundamental element in developing new circuits and solutions across all technological advancements. According to market intelligence reports, simulation has become a major mixed-signal domain, covering all analog circuits from pure analog to digital.
Despite the ongoing trend of digitization, simulation continues to grow. This is because the reasons and innovations described above remain an unmatched area for innovation. For example, capacitive regulators (charge pumps) were not available 10 years ago, and LED drivers were not present 5 years ago—this innovation will continue. The digitization of pure analog circuits maintains growth in the broader analog market, confirming the leading innovation capabilities of simulation. It's important to note that no simulation can be digitized without simulation itself.
Digital Limitations
Figure 1 shows a block diagram of a general power conversion and management system. Elements like power reference, D/A, driver, and filter connect the "external" analog world and will remain analog for the reasons mentioned above. The communication unit in the block diagram is digital, using serial or parallel communication buses. Traditionally implemented with analog methods, the control unit has transitioned to digital implementation in the last five years.
Today's industry trends indicate that digital control structures for power conversion (servo control algorithms) and power management (new serial or parallel bus protocol communications, sequencing circuits, etc.) are maturing. These structures are expected to replace their analog counterparts in the coming years.
Power supplies operate in a challenging environment, putting significant stress on semiconductor devices. Inductors in switching regulators or coils in motors periodically energize the electronic circuit with voltage spikes higher than VCC and below it. Such overvoltage and undervoltage offsets can activate parasitic transistors in semiconductor devices, negatively affecting the system. How to prevent these harmful effects from impacting the outside world is beyond the scope of digital electronics. This is a complex problem, even for experienced analog designers. In fact, parasitic parametric issues turn power/analog design into an art rather than a science. There are no SPICE simulators that can simulate the three-dimensional effects of parasitic transistors, and as long as this continues, simulation will remain a "black magic" in the hands of skilled designers.
Analog and Digital Structure
Figure 2 illustrates a typical analog control implementation of a voltage regulator that builds a pulse width modulation (PWM) switching regulator around the modulator. The analog modulator consists of a comparator (the modulation waveform is shown in Figure 3), and the comparator input is a periodic piecewise linear (triangular or sawtooth) modulation waveform VST of period T. The other input is the error signal Vε. If the quasi-steady-state error signal Vε is between the minimum and maximum of the modulation waveform, the intersection of the two waveforms determines the period Ton of the 'on' pulse. Therefore, the comparator output produces a square wave Vsw whose average value matches the DC output voltage Vo. In this method, the PID (proportional-integral-derivative) unit can be implemented with an op amp and external passive components (compensation resistor Rc and capacitor Cc) or with a single chip integrated with Rc and Cc compensation networks.
Figure 4 shows a digital control structure, where the input error signal (Vfb-Vref) is converted into a digital signal by an analog/digital converter (ADC), and then processed by a digital PID compensator (DPWM).
At the heart of the digital power conversion control loop is a digital modulator. Figure 5 shows a digital modulator implemented with a ring oscillator, a simple and efficient method. In this example, the ring oscillator operates at 1 MHz (T = 1 μs), which is also the clock frequency of the digital PWM system. The ring oscillator consists of 255 circuits (in the simplest implementation, the number of gates must be odd) to correspond to 8-bit resolution. Each gate output is delayed by 1/255 clock cycles from the previous gate, approximately 4 ns.
By properly selecting the time delay between the gates, an 'on' pulse at the output of the digital modulator can be generated, which can be made by a digital selector driven by the digital error signal voltage DVε.
Selection Control Algorithm
If the system being regulated is truly linear, meaning the mode of operation is continuous or stationary, analog methods are usually preferred. This is the case with desktop CPU voltage regulators, where the regulator output must be continuously controlled using the same algorithm from no load to full load. If the system is unstable, meaning the working mode is discontinuous and changing, digital methods are more suitable.
For example, in a notebook or mobile phone voltage regulator application, a digital method is preferable. Because power should be saved at light loads, mode changes are required at this time. This usually happens from the PWM algorithm to the PFM (Pulse Frequency Modulation). PFM adjusts the frequency with the load, producing a lower frequency at lighter loads and thus lower switching losses.
In analog systems, such mode changes require a sudden transition from one control loop (such as PWM) to another (such as PFM), where the load is changing. This algorithm is not continuous and must result in a temporary decrease in output stability.
Instead, digital controls inherently configure to handle discontinuities. Therefore, digital control has the ability to handle mode changes in a single control algorithm.
Power Management and Conversion Applications
The clear advantages of digital power management include ease of communication, programming, status reporting, and more. A typical example of such digital control is a smart battery system, a smart battery charger that powers a notebook computer. This system includes a smart charger, a smart battery, and a main microcontroller. In this system, the smart charger slave receives commands from the "master" controller through the system management bus (SMBus). The smart charger then adjusts its parameters to provide the required current, voltage, and power to the smart battery and reports its values to the microcontroller.
In power-conversion applications, microcontroller-based digital structures have many useful applications, especially in scenarios requiring more than just programmability and current/voltage shaping.
Current Shaping Application
Current shaping is necessary in low-duty ballast applications, where the intensity and duration of the current and the three operating phases (preheat, ignition, and dimming) can be flexibly set for different lamps. Current shaping is also essential in PFC applications, where the current must match the shape of the line voltage.
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