A lot has been said about not reinventing the wheel. But how about not repeating errors? 'Errors' are like wheels that we shouldn't even have tried to invent, leave aside re-invent.. because this wheel was probably square in shape to start with, and there was never any chance of it succeeding.
Yet some may be surprised to know how often this may already have happened. And will happen again. It's just that we don't hear much about it.
If we had inside knowledge, we would often find that such projects usually had impeccable beginnings. Clear design goals, a thoughtful strategy, solid design trade-offs, but then one embarrassing turn to the left. The result was a product that had no known identifiable sire, and in fact no one even remembered ever having worked on it! We all know that marketing, given its nature, always tries to emphasize successes. And no doubt they would be working overtime to gloss this one up. But engineers, with an eye on not repeating known errors in future development, are always keenly interested in what should or could have been done. If only they knew.
Here is a list of some eyebrow raising situations which caught my attention over the years. The examples leave one incredulous:
Example Number 1
A telecom project required a rack of several 3000W Power Factor Corrected ('PFC') hot-pluggable power supplies. Two brilliant teams went about it, one writing C++ code and the other designing the power sections and the backplane. This was to operate off a 3-phase AC mains supply.
The engineers thought the best way out was to parallel three single phase 1000W power-trains; that is, each power-train would be running off a different input Phase and the common Neutral. In doing so, the required minimum voltage rating of the FET switches would be the usual 450V or so, as for any single-phase PFC stage, rather than other techniques which would usually require FETs with roughly twice the voltage rating. The project did get completed.
Then the Marketing guys appeared and informed them that they just couldn't sell it -----!! Because in many countries and areas, a 3-phase incoming supply point does not even include the Neutral wire (which in any case is not even designed to carry that much of return current either). This was a complete dead-end. But couldn't the Marketing guys have got involved a little earlier? Say about 2 years before the project came to a head?
Example Number 2
Users of a control IC meant for a Flyback topology should recognize that the maximum allowable duty cycle should never be set to 100%. A 100% duty cycle basically means the switch is no longer switching, and could just end up staying ON permanently. So if the output voltage is low, and the IC is trying to get it to rise by increasing the duty cycle 'D', and if D is 100%, there is now unfortunately zero available time for the current to freewheel into the output. So how can the output ever rise up try as hard as the controller insists? Yes such an IC has unfortunately been released into the general market. Look around!
Example Number 3
Another product is a fairly popular off-line switcher IC family meant for Flyback applications. In an off-line case, the value of Dmax has several more implications. Here we must recall that the earlier generation of this switcher family had a maximum duty cycle of about 67%. When the next generation was conceived, the one-man product-definition team heuristically assigned a Dmax of 78%. His idea was that by 'allowing' a wider input current pulse, we would automatically get a lower current pedestal, and this would enhance the 'power capability' of his device (since this figure was being based purely on current limit, not on dissipation).
This design strategy could actually have succeeded. But there is a catch. Let us consider what happens if we just remove the input power. By allowing the duty cycle to go up to such a high level, the momentary peak currents actually increase much more now, as compared to a case where the Dmax is set lower. This has severe implications on the transformer, since its size is related to the saturation level. There are also some other subtle issues. For example, it can be shown that the dissipation in the zener clamp can also go up significantly, thus worsening the overall efficiency. So the 'advantage' if any, turned out to be an illusion.
Example Number 4
The popular 3842/3844 series I wrote about earlier, despite its popularity was apparently hastily conceived. Here we actually have a current mode controller that has no built-in slope compensation!! One would think that everything required for a particular topology should be inherent in the design. After all, we don't buy a bicycle from a store and then go out looking for a pair of tires for it! In this case we do just that.
Example Number 5
One of the first semiconductor companies to come up with ICs for implementing a Boost PFC pre-regulator had got it all wrong, and they admitted that to us privately: You can never hope to do proper sine wave shaping with peak current sensing! You need to do average current mode control, because it is the average current drawn that forms the input current waveshape. Their competitor understood this, and so despite having broken in later, they quickly became the market leaders in PFC ICs. The former company changed hands, and is virtually unknown as a separate entity today.
That's all for now. Please do write me at email@example.com.