Flyback converter simplifies isolated power design
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.
Simple Flyback IC design
The LT8300 eliminates the need for an optocoupler, secondary-side reference voltage and extra third winding off the power transformer, all while maintaining isolation between the primary and secondary-side with only one part, the power transformer having to cross the isolation barrier. The LT8300 employs a primary-side sensing scheme which is capable of detecting the output voltage through the flyback primary-side switching node waveform. During the switch off-period, the output diode delivers the current to the output, and the output voltage is reflected to the primary-side of the flyback transformer. The magnitude of the switch node voltage is the summation of the input voltage and reflected output voltage, which the LT8300 is able to reconstruct. This output voltage feedback technique results in better than ±5 percent total regulation over the full line, load and temperature range. Figure 1 shows a flyback converter schematic using the LT8300.
Figure 1: LT8300 Flyback converter with primary side output voltage sensing
(Click on image to enlarge)
Furthermore, the LT8300 runs in a low-ripple Burst Mode Operation at light load, which reduces the quiescent current to only 330µA, a feature that increases the battery run time during sleep mode. Other features include internal soft-start and undervoltage lockout. The transformer turns ratio and 1 external resistor are all that is needed to set the output voltage.
Primary-side output voltage sensing
Output voltage sensing for an isolated converter normally requires an optocoupler and secondary side reference voltage. An optocoupler transmits the output voltage feedback signal through the optical link while maintaining the isolation barrier. However, an optocoupler transfer ratio changes with temperature and aging, degrading its accuracy. Optocouplers also can be nonlinear from unit to unit which causes different gain/phase characteristics from circuit to circuit. A flyback design employing an extra transformer winding for voltage feedback can also be used to close the feedback loop instead of an optocoupler. However, this extra transformer winding increases the transformer’s size and cost.
The LT8300 eliminates the need for an optocoupler or extra transformer winding by sensing the output voltage on the primary-side of the transformer. The output voltage is accurately measured at the primary-side switching node waveform during the off time of the power transistor as shown in Figure 2, where N is the turns ratio of the transformer, VIN is the input voltage and VC is the maximum clamped voltage.
Figure 2: Typical switch node waveform
Boundary mode operation reduces converter size and improves regulation
A LT8300 flyback converter turns on its internal switch immediately after the secondary side current reduces to zero and turns off when the switch current reaches the pre-defined current limit. Thus, it always operates at the transition of continuous conduction mode and discontinuous conduction mode, commonly referred to as boundary mode or critical conduction mode.
Boundary mode control is a variable frequency current mode switching scheme. When the internal power switch turns on; the transformer current increases until its preset current limit set point is reached. The voltage on the SW pin rises to the output voltage divided by the secondary-to-primary transformer turns ratio plus the input voltage. When the secondary current through the diode falls to zero, the SW pin voltage falls below VIN. The internal DCM comparator detects this event and turns the switch back on, thus repeating the cycle.
Boundary mode returns the secondary current to zero at the end of every cycle, resulting in the parasitic resistive voltage drop not causing load regulation errors. Furthermore, the primary flyback switch is always turned on at zero current and the output diode has no reverse recovery loss. This reduction in power loss allows the flyback converter to operate at a relatively high switching frequency, which in turn reduces the transformer size when compared to lower frequency alternative designs. Figure 3 shows the SW voltage and current along with the current in the output diode.
Figure 3: Flyback converter waveforms in boundary mode
Figure 4: Load & line regulation curves of Figure 1 schematic
The transformer specification and design is probably the most critical part of successfully applying the LT8300. Since the voltage on secondary side of the transformer is inferred by the voltage sampled on the primary-side, the turns ratio must be tightly controlled to ensure a consistent output voltage.
Linear Technology has worked with leading magnetic component manufacturers to produce pre-designed flyback transformers for use with the LT8300. Table 1 shows an abbreviated list of recommended off-the-shelf transformers from Wurth Elektronik, Pulse Engineering and BH Electronics. A complete listing is shown in the LT8300 data sheet. These transformers typically withstand a 1,500VAC breakdown voltage for one minute from the primary to secondary. Higher breakdown voltages and custom transformers can also be utilized.
An LT8300 circuit can be modeled using any of the transformers listed in Table 1 by downloading a free copy of the LTspice software at www.linear.com/LTspice. This simulation produces realistic results to help further ease and confirm the design of such converters. The simulation circuit includes information on how the circuit starts up and its reaction to load steps for different input voltage. It is easy to make changes and see the impact to its circuit performance.
Conclusion
Power supply isolation can be utilized in medical systems to get off a noisy supply voltage and is also needed in distributed power architectures for safe operation. A LT8300 based circuit simplifies the design of an isolated flyback converter by eliminating the need for an optocoupler, secondary-side reference voltage and extra third winding off the power transformer. This device maintains primary to secondary isolation with only one part crossing the isolation barrier. Readily available off-the-shelf transformers prevent the need for a custom transformer. The LT8300 operates from a 5V to 100V input voltage range with up to 2 watts of output power making it well suited for a wide range of medical, industrial, telecom and datacom applications.
About the author:
Bruce Haug is Senior Product Marketing Engineer, Power Products, at Linear Technology Corp.
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