Looking back at the six or seven power conversion companies I have worked in so far, I think it is somewhat intriguing to realize that professionals often learn (and contribute) the most in smaller companies. Just a plain coincidence that these companies are also often the ones engineers are least likely to ever want to admit having worked for, many years later! Call it a 'trial by fire' or 'hardening by heat treatment.' Whatever! I remember Bruce Carsten had written a last-page article in some forgotten publication over a decade ago titled "Why Innovations seem to come from Smaller Companies". A very pointed and thought-provoking article, one that I glanced over several times over the years. I just recently threw it away after cleaning up after completing my book.
This month however, I decided I was finally going to come to terms with my past in a way, and try to force myself to remember in vivid (and somewhat painful) detail how I learned to deal specifically with leakage inductance many years ago, while working in a rather small outfit making innovative integrated switcher ICs for off-line flyback applications. In doing so, it will also be clear to you, what really hampers the flyback topology itself, at higher output power levels and low output voltages.
But before I get into that, I will take stock of some of the interesting correspondence I received in response to last month's column. In particular I had an interesting Email exchange with Paul Tuinenga, co-founder of MicroSim (creators of PSpice, now part of Cadence). I will let Paul state it best in his own words: "I have this recollection of a passage from an out of print book (I think it was) 'Paper Money' by Adam Smith, in which the author tells of a conversation with a Southern stock-broker . 'The computer is like a dog. Very useful. Wouldn't think of hunting without one. They spot birds and retrieve. But you don't give the gun to the dog'!" Paul and I ended up being pretty much on the same page as he also agreed with my basic sentiment that "PSpice essentially includes all the equations...like say Kirchoff's laws. So it does a great job in predicting the final outcome (usually...!). However, in using it, we engineers therefore tend to forget the actual equations ourselves - which is a curse for any good engineer, since he loses the power of 'optimization' so essential to a good designer...he just forgets to think! Not the direct fault of the machine though. In a way, he gets blinded by the luxury of a powerful machine...which 'does' but can't 'think'. I feel he can certainly use it to AID design, not as a substitute for design."
Returning to the Flyback topology, working in this particular switcher IC company, I had been trying to come up with a more accurate set of 'Quick Selection Curves' for their next-gen Switcher IC family. Their older Excel spreadsheet and the corresponding published efficiency curves were really not holding up very well in actual bench verification. Seemed to be finally giving credence to the constant griping by previous customers that the efficiency and max power curves were 'unachievable' and 'how the hell did you come up with these anyway?" Not that the company was initially really looking to correct these apparent inaccuracies, since when it got assigned to me, the decision was based mainly on some rather astute financial and business sentiments (I didn't say 'Machiavellian')
Actually this story itself is interesting enough to merit a slight detour here, since it keenly epitomizes the touch-and-feel of the entire dotcom era in the Silicon Valley area (while it lasted). The previous applications senior engineer (the one who had created the original Excel spreadsheet and thus indirectly ensured only he understood it fully) had quit suddenly. He had then thoughtfully set up a lucrative private consulting agency - but not before taking with him a whole lot of stock options from this flyback company ("high flying adored did you believe in your wildest moments, all this would be yours" - Evita).
The consulting activity was actually was in parallel to a 'full-time' senior position that he also took up in a rather sleepy company (you guessed it, a 'big' one). I just don't know whether this latter company knew/didn't know/didn't care/thought it was perfectly OK, or even admired any human's ability to 'multiplex' so dramatically. I couldn't do it for sure! Oh yesthe talented Mr. Ripley was also managing to teach evening EE classes at the local university in his 'spare time?!' Several years later, after this 'big' company got swallowed, and then re-swallowed successively, by bigger and bigger companies, it apparently just became too big even Mr. Ripley, and so one fine day, along with the whole dot-com era, he too got a pink slip. But, till that transpired, he was more than willing to come back, again and again, to the previous and precious flyback IC company, generating all the efficiency and selection curves for every future product family they desired (essentially by magically reconfiguring the previous spreadsheet).
Yes, he was certainly counting on making much more that a 40 hours-per-week exempt employee like me. However, company management may have been on to him, opening the door just a little for him (a few hours per week over a few months), perhaps with the unstated intention of transferring his expert knowledge back in-house, and afterwards dumping him. All this while, with some help from him, I had successfully developed far more elaborate Mathcad models which I then used to put out the quick-selection curves for two new product families. In the process I also showed the company what they needed to do to improve their efficiency estimates, especially in regard to leakage inductance. Job done, then I quit, suddenly as well. I probably left the company with the unenviable choice of either figuring out the older, flawed but simpler Excel program, or running with the more accurate and bench-verified, but horribly complex Mathcad program. I really don't know what they did after that, though I suspect they may have had to play footsie with the other engineer again " at least to make sense of the spreadsheet.
During the bench verification process, I noticed that for 12V outputs, we got a nice fit with the theoretical efficiency estimates on that spreadsheet, but for lower voltage outputs (5V, for example) the bench test verification went way off the efficiency estimate curves, especially for high loads. Why so? We went over every loss term, often with a microscope, both on the bench and in the program, refining the models more and more, almost to the extent of making the entire Mathcad file completely iterative. (It would take roughly 24-36 hours to generate the final efficiency curves from the moment I would hit 'calculate' on a 600MHz PC. The company had to put three PC workstations in my cubical to ensure I would get some work done while the simulation was running!). But no luck getting the actual measured efficiency to match the simulation! We were certainly getting much better with each iteration, but still couldn't explain where the remaining couple of efficiency points were going.
I rechecked my program several times, dotting the i's and crossing the t's (the little German living inside of me ever since my Leipzig days) but it looked solid every way I attacked it. In desperation, I then started poring over some old literature looking for clues, and this one from a very old Philips publication "3C85 Handbook" caught my attention: "Leakage will reflect from secondary side to primary side according to square of turns ratio of the transformer." That was it! We all know it is standard design practice for any universal-input off-line Flyback to keep the reflected output voltage 'VOR' fixed at an ideal of about 105V. The VOR is the output voltage multiplied by the turns ratio. So basically, the required turns ratio for a 5V output is
For a 12V output the turns ratio is
The secondary side leakage (uncoupled) inductance is associated not only with the actual transformer windings, but the lead-out terminations and even the PCB traces leading to and returning from the diode and output capacitor. Assume two inches of total secondary side trace length for example, and remembering the thumb rule of 20nH/inch, we get say 40nH secondary leakage inductance. For a 12V output, this will reflect to the switch side as an effective leakage of 40*(8.75)2=3062 nH or about 3μH. This will be added to the existing primary side leakage (typically about 10μH) to give about 13μH. The associated energy will be dissipated in the zener clamp. However for a 5V output, the same 40nH gets reflected as 40*(21)2= 17640 nH or about 18μH! This will give a total primary side leakage of 10+18=28μH! Triple the first estimate. Enough to inflict trauma on the wimpy zener clamp which probably was just expecting a nice sunny day paddling away on the beach, but got hit by a Tsunami instead.
As soon as I understood this, and learned to correctly estimate and perform an 'in-PCB' measurement of effective leakage (not by just shorting the secondary side pins of the transformer, but by placing thick shorts across the diode and output cap) and then modeling it into the Mathcad file. The fit to bench results was almost too good to be true - within 1% over the entire load range. After much more testing, by several engineers in fact, the company finally acknowledged this to be the missing piece of the Flyback jigsaw puzzle, and then published the new product family quick-selection curves I had generated and verified, and also the guidelines on leakage inductance measurement. Of course they didn't want to 'alarm' previous customers by going back and correcting the curves of the previous family (which they now knew were super-optimistic). Plain marketing sense!
That is all I could manage this month after a rather hearty and long Thanksgiving break, and that coming in just after a tiring 4-hour course on Magnetics I had to prepare for and present at Powersystems World 2004 (in which, incidentally, I had Keith Billings himself in the adjoining room giving a similar 4-hour course on hold-your-breath: Magnetics! What timing! Luckily (for me) we managed to split the attendees almost evenly. I walked in to say Hi to Keith too.
After suitable reflection, do feel free to write me at firstname.lastname@example.org with any free-wheeling comments you may have. Just don't forget to copy Steve at email@example.com so he can keep an eagle eye on where this column is headed!