Digital power supply design and technology implementation

Several technical issues in digital power design and implementation have always been topics of discussion among engineers and experts in the industry. This article will discuss the following aspects:

1. What is digital power supply? What is the essential difference between digital power supply and analog power supply?

The essence of the so-called digital power supply is that the PWM regulation of the output current/voltage of the power supply is generated by the digital chip according to a certain digital control method and algorithm. This is the most essential feature of the digital power supply. Those power supplies that have expanded 8-bit and 16-bit microcontrollers to provide digital input and output operation interfaces and remote communication interfaces, but the PWM regulation of the power supply still relies on analog power modulation chips, can only be said to have the face of a digital power supply, but not the "core" of a digital power supply.

2. What are the technical bottlenecks in the implementation of digital power supply?

At present, digital power supplies still have problems with high-speed/high-precision ADC technology (digital power supply feedback input); high-speed/high-precision power supply PID regulation or PWM regulation of other algorithms; and high-speed/high-precision PWM output problems (digital power supply DAC output).

Many 32-bit DSP/ARM chips have high-speed 10-bit and 12-bit ADCs that can be used for high-frequency switching power supplies as high-speed ADC acquisition. However, their signal input range is generally 0~3.0/3.3V. The common analog input range of plus or minus 10V in industrial sites cannot be solved by any DSP or ARM chip ADC, so signal conditioning circuits can only be added to the external end. Although the product catalogs of a few well-known analog device manufacturers such as ADI include ADC products that fully meet the high-speed, high-precision (16bit~18bit) requirements and have an input signal range of plus or minus 5V to plus or minus 10V, there are very few successful product application records in mainland China. The problem is probably only known to the engineers who are debugging these devices.

High-precision power supply PID regulation or PWM regulation of other algorithms is not a problem in the current popular 32-bit DSP or ARM processor, but if you add the word "high speed", many software engineers may frown. Take TMS320F2812, the leading star in TI's motion control field, as an example. If the switching frequency of the power supply device reaches 300KHz, at a system frequency of 150MHz, the task left for software engineers is to complete the most demanding tasks such as ADC input data processing and power supply PID function regulation within 500 DSP instruction cycles. If you want to avoid the harmonics caused by the power electronic device when the cycle is turned on/off, the ADC samples at the middle moment of the device turning on, then the counter uses the UP-DOWN method to count and synchronously trigger the ADC sampling at the counting cycle value. At this time, the software engineer has only a pitiful 250 DSP instruction cycles available, and the power supply PWM regulation task is quite arduous!

If the ADC problem can be solved by external high-speed and high-precision devices, and the power supply PWM regulation can be completed by using a higher-speed DSP/ARM/FPGA, then the last high-speed/high-precision PWM output problem, that is, the resolution problem of high-speed digital PWM, can only be solved by international large manufacturers that provide DSP/ARM/FPGA. In fact, the resolution of digital PWM is not a problem in the medium and low frequency range of switching power supplies (this is also an important reason why TI's C28XDSP can be popular in the fields of motor drive, inverter, etc.); but when it comes to high-frequency switching power supplies or high-precision power supplies, this problem immediately becomes very prominent. Why is there still a blank in high-frequency and high-precision digital switching power supplies in China? It will be very clear if you use the calculation formula of digital PWM resolution to calculate.

3. What are the benefits of digitalization? Why do we need digitalization? Is there anything that analog methods cannot achieve?

Many people say that they have very low requirements for power supply and do not need it to have such high indicators and characteristics - such low-requirement applications are currently still a forbidden zone for digital power supply.

Digital power supply cannot be digitized for the sake of digitization. The market where it exists is in some areas where analog power supply is difficult to achieve, such as high-power high-voltage inverters using SVPWM algorithm. It has been more than ten years since the space vector algorithm was proposed. There are many domestic literature and technical reports on its advantages over SPWM algorithm (which can be implemented with analog solutions, and many domestic companies also use DSP to implement it). This is where digital technology exists. However, there are basically no mature products in China in this regard, and the market has been monopolized by large foreign companies such as Siemens and ABB.

What are the advantages of digital power supply over analog power supply? I think it lies in the flexibility of digital devices. The internal parameters of the power supply controlled by digital devices can be adjusted online, which means that the dynamic characteristics of the power supply are variable and can adapt to the load changes in a fairly large range while ensuring certain performance. The communication advantages of digital power supply enable power supply equipment to have a variety of remote control methods, which will bring many benefits to the operation and monitoring of the equipment.

There is another point that I personally think is the most fatal threat of digital power to analog power: the development of digital technology is too fast, so fast that it is a bit breathtaking. A few years ago, the microcontroller was a microcontroller and the DSP was a DSP. The boundaries seemed to be quite clear. Now, whether it is 32-bit ARM or DSP, they are all improved Harvard structures. The difference in architecture is becoming more and more blurred, the performance is getting stronger, and the price is getting lower and lower. A few years later, when a high-performance digital device with a price of less than $1 but integrated with high-speed, high-precision ADC, DAC, and PWM output is placed in front of you, will you still stick to the analog solution?

4. Price Issue

Cost control and the cost performance of power supply equipment are always the principles that designers must abide by. Digital power supply has appeared more than ten years ago, but it has been limited to some special high-end applications because of its high price. Thanks to the rapid progress of electronic technology in recent years, the performance of digital control chips has continued to leap forward, but the price has continued to fall. Digital power supply has begun to slowly penetrate and occupy the application field of traditional analog power supply, and it is developing faster and faster. Some friends said that I would not consider digital chips if the price exceeds 40, and some friends have higher price threshold requirements and would not consider them if they exceed 20. However, I would like to remind you that there are two ways to reduce the price of digital devices. One is the traditional way, that is, the price of a model of device slowly decreases, and the other is that the production company launches a new and cheap alternative product. I think this second disguised price reduction method is unique to digital devices, and the action is particularly large. The price of new products can even shrink to a fraction of the price of old products with the same core. It is recommended that as a designer, you should pay considerable technical attention to digital devices. When to start evaluating the performance of a device, when to consider a certain plan as a technical reserve, and when to use a plan as a formal product production plan... The head of the R&D department of a company that relies on high-tech products must have such technical vision.