Power Tip # 19: Easily create multiple negative output voltages

Interestingly, for each phone line, the loads are mutually exclusive. That is, you can't ring and talk on the phone at the same. However, there may be multiple phone lines in a system giving rise to a number of loading scenarios. These systems are often powered from a 12-volt source that already has been isolated from the input power, so a second level of isolation is usually not needed. Power levels are usually less than 25 watts and regulation requirements are generally in the three to 10 percent range.

The flyback topology seems like the obvious choice for this application. The power level is consistent with a flyback. Multiple high-voltage outputs are easy to generate with a flyback, and the flyback is well understood.

However, there are challenges with the flyback: the topology includes unclamped power switch voltages that can ring excessively; it usually needs a two-stage output filter; and cross regulation over zero to full load will not be three percent.

Figure 1 shows an alternate approach. Do you recognize the topology? Let's look at some of this topology's good aspects, assuming that all the loading is only the –27 volt output.

Figure 1: This unusual topology provides very good cross regulation. What is it?
( Click on image to enlarge; note that this is a high-resolution image and it will enlarge with full readability.)

Turning off the switch causes the C16 to clamp the power switch (Q1). The output rectifier (D2) is clamped by C16 when the power switch is on. Therefore, the ringing typically associated with the flyback is not present. Also, the input and output currents through the coupled inductor can be continuous, which greatly simplifies both input and output filters.

This topology is a C'uk converter. The challenge with implementing this topology is that the typical engineer does not understand it well. This is primarily because the C'uk only finds occasional use or the engineer does not even consider it most of the time.

A power supply for VOIP phones is extremely cost-sensitive, power-loss sensitive, and needs reasonable (5%) cross regulation. These are high-volume systems with large price pressures, and are typically battery backed-up where every watt is at a premium. Each output needs to be well-controlled for cross loading and to protect downstream amplifiers. This combination of requirements is a challenge for a flyback due to the ringing, and requires either pre-loads or additional power regulation circuits. The C'uk converter excels in this application as shown in Table 1.

Table 1: Output accuracy is better than 5% for all load combinations.
(Click on image to enlarge)

Efficiency was two percent better than a flyback, even assuming the flyback had no preload. This was made possible by the use of lower voltage switches and diodes, due to the lack of ringing in the circuit.

To summarize, although not commonly considered, the C'uk converter excels in this application that requires: 1) no isolation; 2) positive-to-negative voltage conversion; 3) multiple outputs; 4) good cross regulation; 5) high efficiency; and 6) low cost/minimal parts count.

For a previous discussion of this topic, see How to Make Residential Phones Regulate their own Power, co-authored by John Betten and me. This was before we discovered how nicely this topology works.

Please join us next month when we will discuss unintended resonant responses.

For more information about this and other power solutions, visit www.ti.com/power-ca.

About the author

Robert Kollman is a Senior Applications Manager and Distinguished Member of Technical Staff at Texas Instruments. He has more than 30 years of experience in the power electronics business and has designed magnetics for power electronics ranging from sub-watt to sub-megawatt with operating frequencies into the megahertz range. Robert earned a BSEE from Texas A&M University, and a MSEE from Southern Methodist University.

The Power Tips! series:
#1, July 2008: Picking the right operating frequency for your power supply
#2, August 2008: Taming a noisy power supply
#3, September 2008: Damping the input filter–Part 1
#4, October 2008: Damping the input filter–Part 2
#5, November 2008: Buck-boost design uses a buck controller
#6, December 2008: Accurately Measuring Power Supply Ripple
#7, January 2009: Efficiently driving LEDs offline
#8, January 2009: Reduce EMI by varying power supply frequency
#9, March 2009: Estimating Surface Mounted Semiconductor Temperature Rise
#10, April 2009: Simply Estimate Load Transient Response
#11, May 2009: Resolve Power Supply Circuit Losses
#12, July 2009: Maximize Power Supply Efficiency
#13, July 2009: Don't get burned by inductor core losses
#14, July 2009: SEPIC converter makes an efficient bias supply
#15, September 2009: Design a low-cost, high-performance LED driver
#16, September 2009: Snubbing the forward converter
#17, November 2009: Snubbing the flyback Converter
#18, December 2009: Your regulator's output-voltage accuracy may not be as bad as you think