December, 2012
www.us-tech.com Measuring Losses in Switch-Mode Power Supplies By David Maliniak, Technical Marketing Communications Specialist, Teledyne LeCroy, Chestnut Ridge, NY
range, a far cry from the 50-60 per- cent efficiencies one may expect from the older technology of linear power supplies. Power-supply efficiency is a
S
measure of how much energy is wast- ed between the unit’s input and out- put. There are other gains to be expected from switch-mode supplies as well, such as higher power densi- ties. If a given application can toler- ate the increased noise that switch- mode supplies typically generate and/or does not require a fast tran- sient response, then the switch-mode supply is often the better bet. Switch-mode power supply effi-
ciency is important for two key rea- sons. For one, a more efficient unit means more potential size and weight savings, which can be critical in the design of portable systems. For another, turn-on, turn-off, and conduction losses within the supply are converted into heat, which can compromise the health and longevity of the power supply itself as well as that of the system it powers. Thus, regardless of a power supply’s effi- ciency, it’s important to know how much energy is being lost in the transition as the output transistor turns on and off, as well as how much is lost in conduction. In virtually all bench scenarios,
measurements of power-supply loss- es are made with an oscilloscope. At the outset, there are some important considerations with regard to the test equipment itself. Most general- purpose oscilloscopes are useful for ground-referenced measurements, which mean that the oscilloscope probe’s ground lead is connected to the instrument’s case. The case is connected to earth ground through the ground lead in the oscilloscope’s power cable. The resulting single-ended
probe is prone to ground-loop effects that can cause the voltages at the oscilloscope’s BNC input connector to be unequal to the voltages at the probe tip. This becomes a problem particularly when measuring low- amplitude signals such as power- supply losses, because the noise and voltage gradients in the ground-dis- tribution system can be as large, or even larger, than the signal being measured.
No Ground Reference Then there’s the problem of
measuring signals that are not refer- enced to ground, or “floating.” One solution that users have employed is to simply cut the ground lead in the power cord, allowing the case to “float” to the voltage present on the probe’s ground lead and breaking any ground loop with the circuit under test. This is a potentially dan- gerous practice, exposing the user to shock hazards, not to mention incor- rect measurements. A better approach is a probing
system incorporating a true differen- tial amplifier, which will give users the best measurement quality for signals which are not referenced to
Power test setup with HDO oscilloscope.
witch-mode power supplies are known for efficiencies that can run into the high 90 percent
ground. A differential amplifier’s output is grounded, which means that the oscilloscope also is ground- ed, making for a safer situation for users. Most importantly, differential amplifiers have a large common- mode range, permitting measure- ment of small differential signals rel- ative to large common-mode volt- ages. In a true differential system, the two input paths to the amplifier are precisely matched, which further
improves common-mode rejection and noise reduction. An example of such a true differential amplifier is Teledyne LeCroy’s DA1855A, which offers a common-mode rejection ratio of 100,000:1.
Eliminating Error Sources Another equipment-related con-
sideration when measuring power- supply losses is to carefully eliminate
Continued on page 53
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