Ever need a simple low power isolated housekeeping power supply and did not want to buy an off-the-shelf brick or module? A make-or-buy decision hinges on many factors, but simplicity, solution size, cost and performance weigh heavily on which way to go. A power supply with isolation from input to output is required for several types of applications including some medical systems.
Ground separation from a noisy source voltage is one reason for needing an isolated power supply especially in medical equipment. Exam cameras, dental instruments, sleep and vital sign monitors, to name a few, all use displays that can be adversely affected by a noisy source voltage. An isolated power supply provides ground separation that can eliminate noise causing display irregularities.
Larger medical systems like a CT scan, blood gas electrolyte analyzer and some ultrasound systems typically use a distributed power architecture due to multiple PC boards for various functions and normally distribute a 24V or 48V bus voltage throughout the system. Distributed power architectures usually require isolated DC/DC conversion from the bus to the subsystem operating voltages to enhance the reliability and for safety reasons. This type of bus voltage has the capability to supply a large amount of current and isolation is required to prevent a hazard that can be caused during a short-circuit fault condition.
Flyback converters have been widely used in isolated DC/DC applications for many years. However, they are not necessarily a designer’s first choice. Power supply designers unwillingly select a flyback converter out of necessity for lower power isolated requirements, not because they are easier to design. The flyback converter has stability issues due to the well-known right-half-plane zero in the control loop which is further complicated by the propagation delay, aging and gain variation of an optocoupler.
Furthermore, a flyback converter requires a significant amount of time devoted to the design of the transformer, a task further complicated by the normally limited selection of off-the-shelf transformers and the possible necessity for a custom transformer. Recent advances in power conversion technology have made lower power isolated converters much easier to design. Linear Technology’s recently released LT8300 isolated flyback converter solves many of these flyback design obstacles.