The long-standing debate between digital and analog has recently expanded into the power supply field, drawing attention and responses within the analog community. While numbers remain popular, the real world is inherently analog. Power supplies are no different from other real-world systems: their output—voltage, current, power, etc.—is analog, much like how digital devices such as TVs, cameras, and cellphones produce analog outputs (video, images, sound). Beyond the well-known communication aspects, digital power supplies are now being integrated into key processing units of power conversion systems, which are undergoing digitization. However, the interface to the analog world—the outer layer—remains analog.
Analog circuits represent continuously variable physical quantities, such as length, width, voltage, and pressure. In the analog market, we see data conversion and interface products that handle digital data and circuit components. In fact, in the analog world, mixed-signal applications span from purely analog to fully digital, covering everything from power to signal. Simulation plays a crucial role in generating new circuits, structures, and solutions across technological advancements. According to market intelligence reports, simulation has become a significant mixed-signal domain, spanning all analog circuits from pure analog to digital.
Despite the ongoing trend of digitization, simulation continues to grow. This is because the innovation areas discussed earlier remain unparalleled. For instance, capacitive regulators (charge pumps) were not available 10 years ago, and LED drivers were not present five years ago. These innovations will continue. As a result, the digitization of pure analog circuits maintains growth in a broadly defined analog market, confirming the leading innovation capabilities of simulation. It is 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. The elements in the block diagram (power reference, D/A, driver, filter) are features that connect the "external" analog world, and for the reasons mentioned above, they will remain analog. The communication unit in the block diagram is digital (serial or parallel communication bus). The control unit, traditionally implemented using analog methods, has changed 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. In the next few years, these structures are predicted to replace the analog counterparts.
Power supplies operate in a tough environment, placing significant stress on semiconductor devices. Inductors in switching regulators or coils in motors periodically energize the electronic circuit with voltage spikes higher than the VCC supply and below it. Such overvoltage and undervoltage offsets can activate parasitic transistors in semiconductor devices, which can negatively impact the system. How to prevent these harmful effects from affecting the outside world is beyond the scope of digital electronics. This remains a challenging 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 the "black magic" in the hands of a few skilled designers.
Analog and Digital Structure
Figure 2 shows a block diagram of 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, and 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 is the same as 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, Cc compensation networks.
Figure 4 shows a digital control structure, wherein the input error signal (Vfb-Vref) by the analog/digital converter (ADC) into a digital signal, and thereafter PID compensator digital modulation (DPWM).
At the heart of the digital power conversion control loop is a digital modulator. Figure 5 shows a digital modulator method implemented with a ring oscillator, which is a simple and efficient method. In this example, the ring oscillator operates at 1MHz (T = 1ms), which is also the clock frequency of the digital PWM system. The ring oscillator consists of 255 circuits (in the simplest implementation of ring oscillation, 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 4ns.
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, which means that the mode of operation is continuous or stationary, then analog is usually the method used. 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 not stable, which means that the working mode is discontinuous and changing, then it is better to choose the digital method.
For example, in a notebook or mobile phone voltage regulator application, it is better to choose a digital method. 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 is a mode that adjusts the frequency with the load, thus 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 obvious advantages of digital power management are ease of communication, programming, status reporting, and more. A typical application 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 then reports its value to the microcontroller.
In power-conversion applications, microcontroller-based digital structures have many useful uses, especially in applications that require more than just programmability and current and voltage shaping.
Current Shaping Application
Current shaping is required in light-duty ballast applications, and the intensity and period of the current and the three operating phases (preheat, ignition, and dimming) can be flexibly set for different lamps. Current shaping is also required in PFC applications, and the current must have the same shape as the line voltage.
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