Some thoughts on DC/DC converters, part five
The following is Chapter 4, part five of a five part series "Some thoughts on DC/DC converters" by the late Jim Williams and Brian Huffman from Linear Technology's Volume II book entitled, "Analog Circuit Design-- Immersion in the Black Art of analog design" by Bob Dobkin and the late-Jim Williams published by Elsevier/Newnes.
The 5V to ±15V converter—a special case
Five volt logic supplies have been standard since the introduction of DTL logic over twenty years ago. Preceding and during DTL’s infancy the modular amplifier houses standardized on ±15V rails. As such, popular early monolithic amplifiers also ran from ±15V rails (additional historical perspective on amplifier power supplies appears in AN11’s appended section, “Linear Power Supplies—Past, Present and Future”). The 5V supply offered process, speed and density advantages to digital ICs. The ±15V rails provided a wide signal processing range to the analog components. These disparate needs defined power supply requirements for mixed analog-digital systems at 5V and ±15V. In systems with large analog component populations the ±15V supply was and still is usually derived from the AC line. Such line derived ±15V power becomes distinctly undesirable in predominantly digital systems. The inconvenience, difficulty and cost of distributing analog rails in heavily digital systems makes local generation attractive. 5V to ±15V DC/DC converters were developed to fill this need and have been with us for about as long as 5V logic.
Figure A1 is a conceptual schematic of a typical converter. The 5V input is applied to a self-oscillating configuration composed of transistors, a transformer and a biasing network. The transistors conduct out of phase, switching (Figure A2, Traces A and C are Q1’s collector and base, while Traces B and D are Q2’s collector and base) each time the transformer saturates1.
Figure A1 • Conceptual Schematic of a Typical 5V to ±15V Converter
Figure A2 • Typical 5V to ±15V Saturating Converters Waveforms
Note 1: This type of converter was originally described by Royer, et al. See References.
Transformer saturation causes a quickly rising, high current to flow (Trace E). This current spike, picked up by the base drive winding, switches the transistors. Transformer current abruptly drops and then slowly rises until saturation again forces switching. This alternating operation sets transistor duty cycle at 50%. The transformers secondary is rectified, filtered and regulated to produce the output.
This configuration has a number of desirable features. The complementary high frequency (typically 20kHz) square wave drive makes efficient use of the transformer and allows relatively small filter capacitors. The self-oscillating primary drive tends to collapse under overload, providing desirable short-circuit characteristics. The transistors switch in saturated mode, aiding efficiency. This hard switching, combined with the transformer’s deliberate saturation does, however, have a drawback. During the saturation interval a significant, high frequency current spike is generated (again, Trace E). This spike causes noise to appear at the converter outputs (Trace F is the AC-coupled 15V output). Additionally, it pulls significant current from the 5V supply. The converters input filter partially smooths the transient, but the 5V supply is usually so noisy the disturbance is acceptable. The spike at the output, typically 20mV high, is a more serious problem. Figure A3 is a time and amplitude expansion of Figure A2’s Traces B, E and F. It clearly shows the relationship between transformer current (Trace B, Figure A3), transistor collector voltage (Trace A, Figure A3) and the output spike (Trace C, Figure A3). As transformer current rises, the transistor starts coming out of saturation. When current rises high enough the circuit switches, causing the characteristic noise spike. This condition is exacerbated by the other transistors concurrent switching, causing both ends of the transformer to simultaneously conduct current to ground.
Figure A3 • Switching Details of Saturating Converter