Isolated power supplies – Is analog dead?


Analog versus digital

My first switching power supply design was a 150W DC to AC inverter for automotive. It was in 1999; my manager stepped in the lab and gave me a nice looking metal box with a cigarette plug adapter at one end of it and AC outlet at the other end. He told me the output of that box was 120VAC, 60Hz square wave, and my task was to build a better one generating sine wave output.

In my sample box for the prototype, I had PWM controllers, a bunch of comparators, amplifiers and voltage references. That project had stopped 4 months later.

I finally realized that with those devices, there was no way to generate beautiful sine wave with acceptable voltage distortion when plugging an electric screwdriver into the AC socket, not to mention load like a laptop AC adapter.

No question it was a frustrating experience, but that project did make me think about implementing sophisticated algorithms through something “smarter.”

Three years later, I played with digitally controlled power for the first time. I used a Gate Array Logic (GAL) device to convert regular push-pull signal out of a PWM controller into Phase Shift Full Bridge control logic. I have to admit, programing the GAL device was fascinating.

The first time I was amazed by digital control technology was 8 years ago when I spoke to a designer building a kilowatt server power supply. He taught me the term “auto-tuning,” which means the power supply itself can find its best operating point over a range of input and load condition and compensates for component drift over time or variation from lot to lot. I still remember at that moment.

The only word that came out my mind was, “Wow!”

Today, I’m the Marketing Manager of the High Performance Isolated Product Line of Texas Instruments focused on developing the UCD3138 family of digital power controllers.  It has been my pleasure to witness the digital revolution first hand. Together, the UCD3138 Digital PWM Controller and C2000 Microcontroller family represent TI’s flagship solutions for offline and isolated DC/DC power supplies.

With all this hoopla on digital control, it is important to note that state-of-the-art analog controllers for isolated power supplies (i.e. UCC28251, LM5045, etc.) are still relevant and beneficial.

I have opportunities to work with power supply designers from all walks of life, and have a clear picture of how a designer benefits from digital power controllers, as well as pure analog controllers.

There seems to be a perception in the industry that analog control is dead or dying.  As much as I love digital control, Analog meets very different needs than digital control.

To help you determine when to choose one or the other, I made a table:

When to choose Analog

When to choose digital

System requires…

Part

System requires…

Part

Ultra low standby power

Example: tablet / cell phone chargers, AC adapters for laptop, cameras, gaming, etc.

UCC28710

UCC28740

LM5023

Sophisticated control scheme

Example: HV Air Conditioner, UPS, Electro Vehicle, etc.

C2000 family

Ultra low solution cost

Example: AC adapters for set-top-box, networking, etc.

UCC28700

UCC28720

Robust performance under varied input/load/fault conditions

Example: Startup into Pre-Biased load, input DIP transient, etc.

UCD3138

 

Ultra small form factor

Example:  Telecom 1/32 brick power modules

LM5025A

LM5020

Communication, diagnostic, hot-booting, self-calibration / auto-tuning

Example: power supply for RRU of Base Station, Solar Energy system, etc.

UCD3138

UCD3138064

C2000 family

Ultra simple

Example: Power supplies for home appliance with PWM logic and high voltage driver/FET integrated, non-isolated AC/DC buck for powering microcontrollers & LED displays, LED lamp, etc.)

UCC25230

LM5017

LM5018

LM5019

System level efficiency optimization

Example: Power supply for server, high end networking, etc.

UCD3138

UCD3138064

An E2E member started a conversation regarding digital vs. analog control in the microcontroller forum here. It would be worth visiting that source for more information on differentiating analog and digital control.

With isolated power supplies, there will always be a need for analog PWM controllers as the choice between analog and digital control is never black and white.  There are certain cases which require analog control, and there are certain cases which clearly benefit from digital control. 

Fortunately, you have the flexibility to choose based on what is best for your design!

What is your experience with digital versus analog control? Leave me a comment and let me know.

  • My first "moment" was when I discovered the Unitrode UC3842 PWM controller could replace all the discretes in my self oscillating flyback supply.

    My second "moment" was when I realized just what could be achieved if I ventured into the Digital Domain for power control. The power stage, topology, mosfets, diodes, transformers, etc, is still basically the same whether you control in Analog or Digital, so the experience gained in one can be applied to the other.

    What stands out is the versatility of Digital, being able to change variables on the fly. How many times have I paused to change a resistor or capacitor value, pulled out the solder iron and went to task, only to realize the subject board was still powered up.

    When you think about it, a PWM controller like UC3842 is just a single bit A-D converter.

    Then you also have the ability to get out of the S domain, normal R's and C's for compensation, and do 'un-natural' loop response in the Z domain. Common for years in motor control, but WOW, the things you can do in power supply control if you dare.

    It's so exciting to be so close to digital power control development at TI, for this old analog power designer.

  • Yes, those moments are something to an engineer. I envy that you have witnessed the switching power supply industry grew up from baby.

    I’m actually very interested in the topic “a PWM controller like UC3842 is just a single bit A-D converter”, coz I have never looked at analog PWM from there. Can you share me more insight about it?

  • Good to see that I'm not th eonly one who still sees use in analog technology, picking one or the other where it fits best.

    For some applications, such as audio pre-amplification, I still prefer a simple linear/analog µA7805 (or similar) over all the newer, more efficient, digitally controlled regulators.

    However, the forum thread mentioned above (to which I contributed) is about the meaning of separate analog and digital supply pins on some of the mixed-signal MSP430 microcontrollers. It is not related to analog or digitally controlled power supply. Even though thread title, the original question and the first answer, without considering the MSP430 context, seem to fit.

  • Hi Jens-Michael, thank you for clarifying that. I had been searching over the forum trying to locate a topic related to what I was posting. It appeared to me digital vs. analog is not one lot of people had talked here. Digital vs. analog is a huge topic actually, not only for power supply but also fr many many application. Some issues and principles are pretty common cross difference ones. I would be very happy to see people share their thoughts from all view angle :-)