Even before the threat of terrorism caught the attention of the world, many public and private institutions have begun installing video security systems in important infrastructure. Video security provides the visual images necessary to improve situational awareness, prevent vandalism, theft or other crimes, speed up response and management decision making, and improve staff and public safety.
From airport to bridge, from refinery to oil/gas pipelines, from ports to highways, etc., incorporating video security equipment into their preparation plans has benefited them a lot.
The first line of defense for airport security is fences, levels and walls. In particular, sensitive areas such as oil depots, terminals, and even baggage handling areas have a higher level of security and are equipped with security checkpoints. Most of these areas, as well as most of the locations inside the airport, are usually monitored using surveillance cameras.
The most demanding installation of video security systems is the casino, where various tricks and fast-moving actions must be captured, inspected and archived to meet strict regulations. The strategic placement of the “Eye of the Sky†camera provides some complement to the security personnel's physical equipment resources, helping them to better manage crowded people and deploy personnel in a timely and efficient manner. High-quality video images from gambling areas provide security professionals with the detailed information they need to prevent cheating and capture “thousands of peopleâ€.
Video surveillance systems play an important role in retail stores as a visual precaution against potential criminal behavior and provide operators and security professionals with responsibility for liability claims, employee theft or other management issues. tool. Therefore, when the perpetrator is caught on the video, the handling of shoplifting and employee theft can be made easier. In addition, with video recording, the understanding and handling of events such as customer slips, falls and liability claims will be more accurate.
In the management of today's correctional institutions (such as prisons, correctional institutions, etc.), there are many problems that must be routinely disposed of. Smuggling, violence, prisoners and police officers are only a small part of them. As a result, video security systems have played an unprecedented role in maintaining order and ensuring a safe working environment.
As healthcare organizations grow in size and begin to provide around-the-clock care, they become more vulnerable to a wide range of security risks. While industry guidelines place mandatory requirements for continuous improvement of safety precautions in their “care environment†standards, they are choosing the right tools to meet their needs to protect patients, visitors, employees and intellectual property (IP). The people responsible for decision-making are hospitals and health care institutions.
Deployment of surveillance cameras <br> In most locations, hundreds of cameras may be required to achieve complete surveillance coverage. One of the most common methods of monitoring and managing multiple location facilities is to use Power over Ethernet (PoE) capable Internet Protocol (IP) cameras. Such an implementation can significantly simplify camera installation and reduce associated costs because data and power are transmitted using the same cable, and the camera does not have to be installed near the power source. However, the maximum amount of power that can be drawn from an Ethernet cable needs to be treated with care. For more information on the maximum allowable power that can be drawn from a PoE interface, please consult the IEEE 802.3af (PoE) or IEEE 802.3at (PoE+) specifications.
Surveillance cameras typically use a 24VDC or 12VDC operating supply voltage with a power stage of up to 25W. The PoE line will provide 48V, which needs to be stepped down and isolated from its input to output for safety and fault conditions, resulting in a more complex power supply. However, recent advances in power conversion technology have made the design of low to medium power isolated converters much simpler and easier.
Flyback converters have been widely used in isolated DC/DC applications for many years, however, they are not necessarily the designer's first choice. Power supply designers use flyback converters not because they reduce the design difficulty, but because of the pressure required to isolate the lower power, it is reluctant. Due to the well-known right half-plane zero in the control loop, the flyback converter has stability problems, and the propagation delay, aging, and gain variations of the optocoupler can further complicate the problem. In addition, the flyback converter also requires a significant amount of time to design the transformer, and this design work becomes more complicated due to the limited variety of commercially available transformers available and the possibility of custom transformers. However, Linear Technology's recently released LT3748 isolated flyback controller solves many of the challenges of this type of flyback converter design.
New Flyback IC Simplifies Design <br> First, the LT3748 eliminates the need for an optocoupler, a secondary reference, and a power transformer to add a third winding, while maintaining isolation between the primary and secondary (only a portion) Must cross the barrier barrier). The LT3748 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 LT3748 can be reconstructed). This output voltage feedback technology achieves better than ±5% overall adjustment accuracy over the entire line voltage input range, temperature range, and load range. Figure 1 shows a schematic of a flyback converter using the LT3748.
Figure 1: LT3748 Flyback Converter with Primary Output Voltage Detection
The LT3748 accepts an input voltage from 5V to 100V, and the input voltage within this range can be applied directly to the IC. With a high-voltage on-board LDO and an MSOP-16 package (with four pins removed for additional high-voltage pin spacing), the device operates reliably at high input voltages. In addition, the device's on-board gate driver powers an external N-channel power MOSFET, allowing it to deliver up to 50W of power (its maximum output power depends on external component selection, input voltage range, and output voltage).
In addition, the boundary mode operation used by the LT3748 further simplifies system design and reduces the overall converter footprint and footprint. The LT3748 flyback converter turns its internal switch on immediately after the secondary current drops to zero and turns off when the switch current reaches the predetermined current limit. Thus, it always operates in the transition of continuous conduction mode (CCM) and discontinuous conduction mode (DCM), which is commonly referred to as boundary mode or critical conduction mode.
Primary Output Voltage Detection <br> The output voltage detection of an isolated converter typically requires an optocoupler, secondary reference voltage, and optical driver. An optocoupler is used to transmit an output voltage feedback signal over an optical link while maintaining an isolation barrier. However, the optocoupler transmission ratio changes with temperature and aging, which reduces its accuracy.
The LT3748 eliminates the need to add an optocoupler, reference voltage, and optical driver by detecting the output voltage on the transformer's primary side. As shown in Figure 2, the output voltage can be accurately measured on the main switch node waveform during the power transistor turn-off, where N is the transformer turns ratio, VIN is the input voltage, and VC is the maximum clamp voltage. The LT3748 sets the output voltage by selecting the RFB and RREF and the turns ratio of the transformer.
Figure 2: Typical Switch Node Waveform
Since the reflected output voltage is always sampled at the diode current zero crossing point, the load regulation performance is greatly improved in the boundary mode operation. The LT3748 typically provides ±3% load regulation.
Transformer Selection and Design Considerations <br> As far as the successful application of the LT3748 is concerned, the specification and design of the transformer may be the most critical part. In addition to the common considerations for designing high frequency isolated power transformers (achieving low leakage inductance and tight coupling), the transformer turns ratio must also be tightly controlled. Since the voltage on the secondary side of the transformer is inferred from the sampled voltage on the primary, the turns ratio must be tightly controlled to ensure a consistent output voltage. A ±5% turns ratio tolerance between transformers is likely to produce more than ±5% variation in the output voltage. Fortunately, most magnetic component manufacturers are able to guarantee ±1% or better turns ratio tolerance.
Linear Technology has partnered with major magnetic component manufacturers to produce pre-designed flyback transformers for use with the LT3748. Table 1 is an abbreviated list of some of the recommended commercial transformers, manufactured by Wurth Lectronics and Pulse Engineering. See the product manual for the LT3748 for a complete list. These transformers are typically capable of withstanding a primary to secondary breakdown voltage of 1,500 VAC (for a duration of one minute). Transformers with higher breakdown voltages and custom transformers can also be used.
Table 1: Commercial transformers available for the LT3748
The LT3748 can be modeled with any of the transformers listed in Table 1 using LTspice software (free download). The simulation circuit contains information on how the circuit is started, its response to the load step, and the voltage waveform at different points inside the circuit. With this software, design changes can be easily made and the impact of related changes on their circuit performance can be understood.
Figure 3 shows a demo board using the LT3748. The circuit accepts an input voltage ranging from 22V to 75V and produces an isolated 12V output at currents up to 2.5A.
Figure 3: LT374830W application circuit (size: 38mm x 19mm x 9.5mm)
Conclusion <br> Although the design of isolated flyback converters is not simple; in addition to modules or complex discrete implementations, we now have an alternative. The LT3748-based circuit simplifies the design of isolated flyback converters without the need for optocouplers, secondary reference voltages, and optical drivers. The device maintains a primary-to-secondary isolation (only a portion spans the isolation barrier). Commercially available transformers that are easy to purchase avoid the need for custom transformers. With a 5V to 100V operating input voltage range and the ability to deliver up to 50W of continuous output power, the LT3748 is the right choice for a wide range of security IP camera applications.
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