Design changes brought by digital power supply

Digital power is a hot topic today. Power supplies that use digital technology are rapidly changing power conversion and power control. Even though digital power is hot, engineers seem to be slow to accept it. The two biggest obstacles facing digital power are visible cost and intellectual property (patent infringement). Of course, cost is a problem for very simple power designs. But in more complex designs that require power management , fault management, and telemetry, digital power has a cost advantage because most products are highly integrated. The launch of new products such as Intersil Zilker Labs' ZL2008 (Figure 1) provides flexible processing principles for solving intellectual property problems, and intellectual property and licensing transactions have been resolved. Once these obstacles are eliminated, digital power will become the development direction of power conversion.

This article will answer the question, “Why should you switch to digital power?” To answer this question, first look at two related questions: Why change at all? Why switch from analog technology to other technologies?

Why comprehensive change?

The short answer to this question is that there really is no other choice. Technology is constantly advancing, and so are the demands of power conversion. Along with these changes come changes in government regulations and changing economic factors.

From a power supply perspective, the load is changing. The evolution of semiconductor technology is driving the load change in the direction of smaller and smaller process dimensions. Microprocessors and ASICs that once required several volts now require less than 1 volt. This means that the power supply must drive a much lower impedance than the previous power supply. New applications have emerged, creating new load types (such as high-brightness LEDs). Circuit boards have become more crowded, so the integration of components is also increasing. Power conversion requires power supplies to meet more than just static requirements. In fact, the change in requirements is accelerating. To meet the requirements of technology changes, power engineers need to think outside the box and find solutions that can meet the design challenges of today and tomorrow.

Government regulations are not static. From restrictions on the use of hazardous substances to requirements for power quality to requirements for efficiency, regulatory factors have pushed our designs beyond simple power conversion and have required more comprehensive considerations. Design changes must keep pace with regulatory changes. In addition, in the era of economic globalization, not only local regulations, but also the countries where customers are located will formulate relevant regulations on power supplies.

Finally, there is increasing pressure from economic factors to design innovative products that provide more value to customers. This requires not only that the product is cheap, but also that the cost of achieving a certain performance is concerned. A bicycle may be cheaper than a car, but if you need to get from Beijing to Tianjin in two hours, no matter how cheap the bicycle is, it will not meet this requirement. You need more advanced technology. The pressure of pricing competition, fuel costs, labor costs and warranty costs are just a few of the economic factors that drive changes in power supply design.

Economic factors are forcing companies to hire fewer power engineers. As a result, a design that once took several engineers and nearly a year to complete must now be completed in a few months by a single part-time engineer.

Why move from analog to other technologies?

If analog power conversion were perfect, there would obviously be no need to move from analog to digital control. Analog power control has its limitations, and one key limitation is the lack of flexibility of analog controllers, which affects their ability to change with the environment. In particular, the functions of analog controllers are fixed, and the analog components used in the controller are limited and have fixed values.

The characteristics of analog controllers are set by the silicon chip and the parameter controllers of external components. If the internal characteristics need to be changed, there are two choices: change the silicon chip or add external active devices to obsolete, improve, imitate or replace the controller. The time to change the silicon chip ranges from 3 months to more than 1 year, and the silicon chip design changes often cannot keep up with the market changes. Adding external active devices increases cost, delays schedule, reduces power density, and generally has a negative impact on reliability.

The external components used to set the controller's variable operating parameters have fixed values ​​and limited available values. For example, if a resistor is selected and soldered to the circuit board, the resistor value can only be changed by removing it and replacing it with another resistor. Also, the external components have only one value and cannot adapt to changing environments. Of course, nonlinear capacitors , resistors, and inductors are occasionally used in power supply designs, but the range of variation of these components is still fixed. If you use a positive temperature coefficient (PTC) resistor, it cannot behave like a negative temperature coefficient resistor. Therefore, analog power control designs are still limited in their ability to be optimized or adapted to changing load or environmental conditions.

In addition, we are limited to the resistor, capacitor, and inductor values ​​in the supplier's product catalog, and the component values ​​are also restricted by physical properties. Resistors, capacitors, and inductors only have positive values. This numerical limitation restricts the parameter space of analog solutions. Although most analog power designers understand that only positive components can be used, they are not very clear about the limitations of the circuit's operating state. When making design changes, if the analog components you use are not limited, your solution selection will be much less restricted.

A relevant example arises in compensation for voltage -mode control. In voltage-mode control, the output filter inductor and capacitor form a double pole in the closed-loop transfer function. In an efficient circuit, this double pole may become a complex conjugate pair of poles. A Type III analog compensation network (Figure 2) is often used to compensate voltage-mode controllers. Unfortunately, a typical Type III compensation network implemented with resistors and capacitors has only one real zero to compensate the pole of the controlled device. The real zero is only marginally effective if the complex pole is not undercompensated. Complex conjugate zeros are not possible with positive-valued resistors and capacitors in a Type III compensation network. Engineers try to compensate voltage-mode controllers with limited analog compensation networks, but in the cases mentioned above, they cannot achieve adequate compensation and waste a lot of time trying to do so.

Why switch to digital power?

Assuming we must move from analog control to other technologies, why is digital control the solution? Digital control can solve the problem because it has better performance than analog control, is more flexible, and is easier to use in complex designs. Digital control takes advantage of the advantages of analog control and goes beyond it.

Imagine using the same power components, including the same FETs, inductors, and capacitors, and comparing the performance of a system using an analog controller versus a system using digital control. At first, you would naturally think that since performance is determined by the components, it would be hard to tell the difference. But then you realize that the controller affects many aspects of performance. Here are some examples.

1. Transient response: The control mechanism greatly affects the transient response of the system. For example, the transient response of a hysteretic controller will be very different compared to a current mode. Each control mode has both advantages and disadvantages. Digital solutions allow you to seamlessly switch from one mode to another to provide the best transient response. While analog solutions can provide good point solutions, there are rarely enough static operating conditions to allow you to achieve the desired point solution.

2. Regulation accuracy: Generally speaking, regulation accuracy is defined based on line voltage, load, and temperature, because each of these conditions affects regulation accuracy. Digital controllers can monitor these conditions and take control measures to optimize over the entire range of operating conditions.

3. Stability: Digital control can provide better compensation (better calling of poles and zeros) than analog solutions, so the control in terms of stability is much better. In addition, the compensation can change with the changing conditions, so that the system can achieve the best stability under a wide range of conditions. The compensation of analog controllers is fixed, while digital control can provide adjustable or even adaptive compensation.

4. Fault response: Digital power controllers provide a large number of fault response options. Each fault has a unique response characteristic that can be adjusted according to user needs. Analog controllers generally have only one fixed fault response (such as power failure/intermittent/overload), and users can only choose to use it or not. Digital control can also provide a filter function to reduce the possibility of false faults.

5. Efficiency: Many control factors affect efficiency, including dead time, switching frequency, gate drive level, diode emulation, phase addition and phase loss, etc. In view of these factors, the digital control algorithms provided by current digital control are optimized over the entire range of operating conditions. Therefore, at a certain operating point, you may be able to adjust the analog controller to a very high efficiency, but the digital controller can optimize all operating points.

6. Reliability: Reducing the number of components and lowering operating temperature (through efficiency optimization) are two ways for digital power supplies to improve system reliability. In addition, flexible fault response and the ability to detect small changes in component parameters can significantly reduce downtime.

The reality is that digital control may be overkill for most simple designs and basic requirements. Of course, digital power control is flexible enough for these simple applications, and may have more functionality than is actually needed. On the other hand, the most complex designs require a full feature set, and it is difficult to find an analog controller that can do the job without adding a lot of circuitry . At this point, digital controllers are clearly the favored solution.

Digital power controllers are far more flexible than traditional analog technology in the sense that they can be used in a wide variety of applications without the need for additional circuitry.

Digital power control generally has a higher level of integration than analog controllers. However, the level of integration is not enough to meet the requirements of design reuse and flexibility. Component values ​​also need to be flexible. Imagine that the compensation components ( resistors and capacitors ) of a typical analog compensator are integrated into the controller, and the values ​​of the resistors and capacitors are fixed. Integrating these components actually reduces the flexibility of the controller unless some method is taken to adjust the values ​​of the components to suit the application requirements. For example, in a digital controller, the compensator is integrated into the controller, and the compensation parameters are stored in digital registers in the controller memory. To change the compensation parameters, simply change the values ​​in the registers.

Digital power controllers have an advantage over analog controllers in terms of ease of use. First, due to the high level of integration of digital power control, there are far fewer components to identify, purchase, and track, making digital power controllers very easy to use. Second, the values ​​of the integrated components are defined by digital registers, and the values ​​in the registers can be easily modified through the device's pins or through digital communication interfaces and graphical user interfaces. As will be shown in later examples, configuring the design is as simple as clicking a mouse. This is much easier than analog solutions, which still require soldering irons and boxes of components. As you redesign and optimize, every change to the value of a component increases the risk of the design.

Finally, digital controllers are easier to use because you can get the design done with just a few numbers, and it is easier to design digitally. Here are a few examples to illustrate this point.

Analog resistors and capacitors have only positive values. Integrating these functions/values ​​digitally removes this limitation, making it easier to implement solutions that were previously difficult to implement in the analog domain.

Compensation is a good example. Digital compensation can do much more than analog compensation, for example, voltage mode control of a high-Q circuit is easy to implement with a digital controller but almost impossible to implement with an analog controller.

Optimization algorithms to improve performance. Analog designs tend to be point solutions, but loads, voltage sources, and environmental conditions are rarely fixed. Therefore, optimization algorithms can be used to optimize performance under these changing conditions. These algorithms are easily implemented in digital controls using embedded microcontrollers and non-volatile memory.

Self-discovery algorithms in digital controllers free designers from time-consuming system identification. For example, auto-compensation is a new feature on many digital controllers released this year. The controller determines the characteristics of the controlled device and adopts a configuration appropriate for that specific device.

Since the values ​​of components, operating status, and environmental conditions are stored in digital registers, telemetry is possible and easy to use. The system can quickly diagnose faults and change operating parameters with very short instructions to get the system up and running.

To borrow a Chinese proverb: "If you don't change your ways, you will repeat the same mistakes." If we don't find a way to meet the challenges of power conversion, the future is not good. The emergence of digital power has come at the right time and provided us with the diamond drill that we much needed. Now, let's change!