introduction
Ever needed a simple low-power isolated housekeeping power supply, and didn't want to buy off-the-shelf brick power supplies or modules? Making or purchasing decisions depends on many factors, but simplicity, solution size, cost, and performance have a significant impact on which path to take.
Achieving ground separation from the noise source voltage is one of the reasons for the need to use an isolated power supply. For example, medical examination cameras, dental instruments, sleep and vital signs monitors, etc., all use displays, which are adversely affected by the noise source voltage. The isolated power supply provides ground separation, which eliminates the noise that causes the display to be abnormal.
Because of the multiple PC boards used to implement a variety of different functions, larger medical systems (eg, CT scans, blood-gas electrolyte analyzers, and some ultrasound systems) typically employ a distributed power architecture and are generally oriented The entire system is assigned a 24V or 48V bus voltage. For reliability reasons and for safety reasons, most distributed power architectures require isolated DC/DC conversion of the bus-to-subsystem operating voltage because such bus voltages can provide large currents.
Flyback converters are widely used in isolated DC/DC applications. Power supply designers are reluctant to choose flyback converters because of the need to meet low power isolation requirements, not because such converters are easy to design! The flyback converter has stability problems due to the well-known right half plane zero in the control loop, and this problem is further complicated by the necessity of driving an optocoupler for output voltage feedback. In addition, flyback converters require a significant amount of time to design transformers, and the choice of commercially available transformers is often limited, and it is likely that custom transformers will be required, which will make things more complicated. Recent advances in power converter technology have made lower power isolated converters easier to design. Linear Technology's recently introduced LT8300 isolated flyback converter solves many of these flyback design problems.
Simple flyback IC design
The LT8300 eliminates the need for an optocoupler, a secondary reference, and a power transformer to add a third winding, while maintaining the primary and secondary isolation with only one power transformer, which must cross the isolation barrier. The LT8300 utilizes a master-side detection scheme that detects the output voltage through the waveform of the flyback transformer's primary switching node. During the off period of the switch, the output diode is responsible for supplying current to the output, and the output voltage is reflected to the primary side of the flyback transformer. The value of the switching node voltage is the sum of the input voltage and the reflected output voltage (the LT8300 can be reconfigured and adjusted). Figure 1 shows the schematic of a flyback converter using the LT8300 and only seven external components.
Figure 1 LT8300 Flyback Converter with Primary Output Voltage Detection
The LT8300 is available in a small 5-lead SOT-23 package that accepts input voltages from 5V to 100V, which can be directly applied to the IC without the need for a series step-down resistor. Thanks to its high voltage built-in LDO and the inherent extra spacing of the VIN and SW pins on the SOT-23 package, the device operates reliably with high input voltage. In addition, its built-in 260mA, 150V internal DMOS power switch allows the device to deliver up to 2W of output power.
Main terminal output voltage detection
By detecting the output voltage on the transformer's main terminal, the LT8300 eliminates the need for additional optocouplers or additional transformer windings. When the power transistor is turned off, the output voltage can be accurately measured on the main switch node (SW pin) waveform, as shown in Figure 2, where N is the turns ratio of the transformer, VIN is the input voltage, and VC is the highest clamp. Bit voltage.
Figure 2 Typical Switch Node Waveform
When operating in boundary mode, load regulation is excellent because the reflected output voltage is always sampled when the diode current crosses zero. The LT8300 typically provides better than ±2% load regulation, as shown in Figure 3.
Figure 3 Figure 1 shows the load and voltage regulation curve of the schematic
Transformer selection and design considerations
Transformer specifications and design are likely to be the most critical part of the successful application of the LT8300. Linear Technology has been working with leading magnetic component manufacturers to produce pre-designed flyback transformers for use with the LT8300 from Wurth Electronik, Pulse Engineering and BH Electronics. All recommended transformers are listed in the LT8300 data sheet.
in conclusion
In medical systems, power isolation can be used to get rid of noisy supply voltages, and power isolation is also required in distributed power architectures. The LT8300-based circuit eliminates the need for an optocoupler, which simplifies the design of isolated flyback converters and makes it easy to purchase off-the-shelf transformers without the need for custom transformers. Operating over an input voltage range of 5V to 100V, the LT8300 delivers up to 2W of output power, making it ideal for a wide range of medical, industrial, telecom, and datacom applications.
Author: Linear Technology Power Products Division, senior product marketing engineer Bruce Haug
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