Mix-and-match dc-to-dc converter design

12/3/2010 8:28 AM EST

In the early 1970s I was trying to develop a line of low-volume, but low-cost "custom" dc-dc converters for a small company in Vancouver, Canada. We were getting a fair number of RFQs (request for quotation) for voltage converters to operate 12 V mobile radios from other dc voltages (such as 24 V, 36 V, 48 V, 72 V and 120 V) in trucks, trains and boats. Sometimes other outputs were required, and sometimes from a 12 Vdc input. The only commonality was about a 100 to 200 W output power. The problem was that volumes ranged from one to a few dozen, which made the NRE (non-recurring engineering) and design time prohibitive.

There was a significant recession on at the time, and our struggling company could use all the business it could get to stay afloat. I had been laid off from Boeing in Seattle in late 1969 (when billboards were put up with "Will the last person out of town please turn off the lights"), and I needed the job too. However, I was already working 10-12 hours a day, 7 days a week.

My bright idea was to do a one-time design for a transistor "chopper" and control circuit for each input voltage in the 12 to 120 Vdc range, and a rectifier-filter circuit for each output in the same range; the circuit schematics would be identical except for component values and part numbers. The isolation transformer primary winding was part of the input circuit design, and the secondary winding was part of the output circuit definition; the same transformer core would be used for all. The same printed circuit board was also used, designed to carry the currents of 12 Vdc input or output, and tolerate the voltages involved with 120 Vdc input or output. Thus the sales department could complete the design largely by pulling the files for the input and output voltage and sending them to production.

Costs were also reduced by using the same minimalist box case for all converters, which had only input and output terminals and fuses; no switches, no meters, no indicator lamps, just an option for mounting bracket orientation. I had some heated arguments with our salesman over this, as sometimes customers wanted switches or meters. I pointed out that most of the applications were mobile, and the converter was mounted out of sight anyway (in the trunk of a car, behind the seat of a truck, under the floor of a caboose, or behind the "dashboard" of a boat). Lamps and expensive meters would not be seen, and an already 'switched' input voltage was generally available.

In the rare case where meters, switches, lights and/or other options were required, rack mounting was also usually wanted. Now the basic converter was simply mounted on the back of a rack panel, and the rest quickly designed by a junior engineer. I had originally wanted to mount the fuses inboard on clips to save a little more, and besides, when a converter failed and blew the fuse, installing a new fuse would only increase the damage.

I had to concede on this one, though, due to an earlier won battle. It was not uncommon for our customers to hook the dc input up backwards, which of course blew the converter beyond repair. Our small company did not want to argue with our large customers when they denied mis-wiring the input, so we 'ate' it as an expensive warranty repair, and I was blamed for a poor design.

An input series diode would have solved the problem, but at a non-trivial expense, and a significant loss of efficiency at low voltages. As an alternative I wanted to use an MR750 series diode (essentially a 30 Amp diode in a 6A axial leaded package) across the input after the fuse. The surge current rating of the diode was sufficient to blow a conventional fast fuse, and avoid other circuit damage. Even this was considered too expensive, so I snuck a cheap 1N4005 1A rectifier into my designs as a "reverse polarity indicator" instead. When nearly all "warranty returns" had this 600 V diode blown to smithereens, it could only be due to a reverse polarity (or > 600 V) input, and I was able to make the case for the larger diode, which saved us a lot in the long run.

The mix-and-match concept worked out quite well, and we were able to ship new "designs" with a short turn-around at a reasonable price. There were some requirements, like the 24 V to 12 V option, that were so common that I tried to convince the company to build a good stock of them to have on the shelf for immediate delivery, which could often clinch a sale. Trepidation and caution were the watchwords, however, so they would only build 1 or 2 extras at a time. For several years, at least until I left the company, these would invariably sell out before the next production run, and we lost some sales when the customer did not want to wait a couple weeks or months.

I found one of these converters years later in a surplus store, and I bought it for $5 as a memento. It still works to this day.

About author Bruce Carsten: "I got my BSEE from Sacramento State in 1965 and went to work for Boeing in the 737 flight test program. I was laid off with 5,000 other engineers in late 1969 and found a job in Canada, where I soon became specialized in switchmode power conversion; I worked there for 23 years before returning to Oregon. I have been self employed as a consultant since 1983, and am in the process of retiring (more or less)."