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Posted on 01 July 2019

Digital Power

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Hope or Hype?

By Dr.– Ing. Artur Seibt, Vienna

 

Where it makes sense

Before the term "Digital Power" was ever coined, it has been used since decades whenever it made sense, i.e. in large power supplies, especially in those destined for complex installations. However, the term DP as it is used and advocated now applies to the replacement of analog control by digital control using DSP’s resp. mixed chips in all SMPS. This means to exchange something perfectly and reliably functioning at low cost by something much more expensive, less reliable and without any tangible advantage. This is exemplified by the quality and tenor of the "pushers‘“ arguments.

Quality of arguments for Digital Power (DP)

The abuse of the term "digital“ as a synonym for superiority like "digital is better than analog“ is marketing hype; scrutiny of the claims of the DP pushers yields a list of major disadvantages for the vast majority of SMPS. The pushers overlooked that all SMPS in the past 60 years functioned perfectly without DP which says enough about the need for it. "Naturally, the digital loop concept is driven by the start-ups in the power industry.“ In other words by people with zero or rudimentary knowhow and experience in power design. After conceding: "System makers push the market“, an analyst states bluntly: "Analog technology cannot avoid becoming obsolete“, "Many in the analog community do not appreciate the impact of Moore’s Law and do not understand that the fate of analog power technology is already determined.“ He concludes that DP ic’s will fast become less costly than analogue ones and this would force total conversion. Obviously, he assumes that SMPS consist of circuit boards full of ic’s.

Another analyst : "The analog designers’ identities seem to be tied up with their black-art experience in analog; it’s just too scary or painful to imagine that such knowledge might not be valuable in the near future. However, the analog suppliers find themselves very much in the same position as mom-and-pop grocery stores in the US...“ A DP pusher: "Before the advent of DP, hardware engineers were forever tweaking and experimenting with different design approaches to solve anomalies...“ Arguments of such quality are none but insults, and it remains the secret of these authors why they consider insults the most effective method of convincing prospective customers. Calling analog design a "black art“ denounces professional engineering, not understanding that analog design is immensely more difficult than digital design, the latter being nothing else but a primitive reduced form of analog design!

Claim 1: DP reduces costs, also SMPS can now be designed within minutes

Starting from the indisputable fact that present SMPS function perfectly well, the only salient argument for introducing DP would be lower cost. Costs rule, and as long as a DP ic replacing a TL 431 costs more than this, i.e. a few cents, this claim remains false, even if DP ICs would be given away, this would not materially affect the BOM of a SMPS; it is the inductive and capacitive components, the power semiconductors, the e.c. board, housing, connectors which head the bill, the cost of the control chips is absolutely zilch. The claim that with DP "a SMPS can now be designed within minutes“ is utterly absurd: most of the design effort is and will remain devoted to the design of the major components like transformers, the placement of components, the conductor layout and testing; the design work to fulfil the emi norms may take up a sizeable portion of the whole design costs and time. The design of the control loops is not even worthwhile mentioning. Costs rule, this fact will prevent widespread use of DP, the cost squeeze being extreme. It is astonishing why DP proponents still wonder why the "breakthrough“ they predicted just doesn’t happen, they just forgot about cost, underestimated the intelligence of SMPS designers, also they never mentioned the disadvantages and dangers!

Claim 2: DP is more reliable

In all articles this author encountered, the serious dangers of using today’s digital ic‘s inside a SMPS were omitted. In fact, there is no environment more hostile to digital circuitry! There are extremely strong magnetic and electric hf fields beyond 100 MHz emanating especially from inductive components but also from others and the conductors. And it is just the continuing shrinking of digital ic’s with ever lower voltages down to below 1 V and higher speed which makes them increasingly vulnerable to interference and hence unfit for use inside a SMPS. Most SMPS are extremely densely packed. "Obsolete“ fairly "slow“ 12 V CMOS ic’s with their 6 V threshold are quite safe even close to 800 V signals with rise times of under 10 ns and have been used for decades; there is nothing new about digital circuitry in SMPS wherever it was appropriate, e.g. for generating multiphase signals.

This problem is aggravated by the fact that microcomputers/ DSP’s are basically highly vulnerable to interference: while analog or other digital logic will either ignore an interference pulse or react at most with a short hiccup, a single pulse disturbing any of e.g. 32 address lines will cause the program to fetch from a wrong memory location, so e.g. an add instruction will be replaced by e.g. a branch instruction with catastrophic consequences. If an interference disturbs a data bus or control signal, equally fatal results will ensue. But even without the influence of interference, plain software errors will lead to disastrous failures, especially in high power supplies. Due to the nature of these problems it will be close to impossible to ever reconstruct the cause of the failure! A bit further down into the nitty-gritty: the inputs and outputs of CMOS ICs must be protected against exceeding Vcc or ground levels by more than 0.3 V; if this is not strictly observed, arbitrary malfunctions or total failures may result. Interference pulses are not only present during "normal“ operation, but especially during start-up, overload or short-circuit conditions not to speak of extremely high interference from the mains and the environment. Many SMPS are not shielded, due to the high frequencies of cell phones any conductor of a few centimeters may pick up high levels of hf which would not affect properly designed analog circuits but well microcomputers or DSP’s running at speeds of several hundred MHz, generating by the way also additional highly undesirable emi.

Inside a SMPS temperatures run anywhere from 60 to over 100 degrees; transformers are designed for 100 degree operation. Such ambient temperatures are detrimental to microcomputers/ DSP’s. The usual answer that such chips are also used in cars disregards the fact that the actual operating life of a car is just a few thousand hours while a standard SMPS is expected to operate for 5 to 30 years (not all electronics are pc’s designed for 2 years). Here, another basic severe problem of all present digital ic’s comes in: after a few years they will no longer be available, hence a repair of a defective SMPS will become impossible.

In case of malfunction of a DP ic, the state of its ports may become any; if e.g. the drive signal of a power FET lasts a few microseconds too long, it can destroy the FET. How do the proponents of DP guarantee that any conceivable malfunction will be automatically detected and corrected within at most a few microseconds? The benefits of DP in a cell phone may be real, but 3.6 V DC and 230 V AC with the full power of the mains behind are two different stories. No type of fuse or electronic circuit can prevent some components of a SMPS from burning, e.g. capacitors and inductors. Analog circuitry is by nature extremely reliable, anybody planning to replace it by something inferior should first learn about product safety and manufacturer’s liability and think twice about it! In the EU, the liability is squarely on the manufacturers!

In Germany the highest court (Bundesgerichtshof) ruled in 2004 that a person who suffered damage by a product could not only sue the company but additionally all managers of that company who decided upon product safety! The author has seen many managers' faces fall after they were presented with this court verdict! As long as the "pushers“ of DP could not prove that their products resp. circuitry are of equal reliability, they better abate their marketing hype or give in writing that they will defend their customers in all cases of product safety litigation.

Claim 3: DP offers better performance and higher efficiency

Speaking about "better performance“ resp. "higher efficiency“ the first question is which SMPS was taken as the reference for this claim, an old-fashioned and poorly designed one? The performance and the efficiency of a SMPS are determined entirely by the circuit concept and the proper design resp. selection of the vital components which is indeed an art -  not a black one, but a professional one. Efficiencies of off-line PFC’s run from 96 to 98 % of combined PFC plus main converter SMPS from 87 to to > 90 %, without DP, of course. Any further increase will require better components. Low efficiency has nothing to do with the control loop but with poor design and cheap components; no DSP in the control loop can compensate for a poor transformer. Also, a properly designed SMPS will function over its full line and load ranges down to no load with standard analog control circuitry, a "change of parameters“ claimed as necessary by the DP pushers is superfluous, it may be advantageous in KW SMPS which constitute only a small fraction of the SMPS population.

Summary

Unless DP pushers can prove that their products save costs or offer better performance so badly needed that customers are willing to pay for it, it will fail; no customer will pay a cent more for "digital control".

 

 

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