A variety of circuit protection devices can be used to help protect LCD screens from damage caused by excessive current or voltage transients. Overcurrent protection may take the form of a fuse or a resettable polymeric positive temperature coefficient (PPTC) device.
Deciding whether to specify a PPTC device or a single-use fuse depends on the LCD design and the types of threats the device may be exposed to. Overvoltage protection devices that are frequently used include multi-layer varistors (MLVs) and electrostatic-discharge (ESD) protection devices.
CCFL BACKLIGHTING PROTECTION
Cold-cathode fluorescent lamps (CCFLs) provide high-efficiency backlighting for LCDs. The lamp operates on high-voltage ac power and requires an efficient, high-voltage dc-ac converter. Power is derived from the 5-V and 12-V buses (Fig. 1). The LCD controller itself and the surrounding controller logic are powered from the 5-V bus. The LCD inverter and other electronics on the board are powered from the 12-V bus.
Misconnections and mishandling can cause large overloads and short circuits to the system. In addition, component failures on the board can have devastating effects. Isolating critical circuits with fuses or PPTC devices helps prevent component damage during this type of fault. It also helps manufacturers meet UL-limited current requirements.
Many inverters incorporate “soft start” circuitry to limit inrush currents. But, as the inverter ages, start currents may rise over time. This increases the in-rush current and, without adequate protection, may result in the destruction or degradation of the primary power supply.
Slow-blow surface-mount fuses help provide overcurrent protection on systems that experience large and frequent current surges as part of their normal operation. The small form-factor fuses enhance electronic system reliability without compromising protection against high-current fault conditions. They also provide a time-lag design that helps prevent nuisance openings during pulsed- and high-inrush-current conditions.
LED BACKLIGHTING PROTECTION
LED backlighting enhances the viewing experience, offers more flexible backlight architectures, and enables thinner display designs than conventional CCFL technology. Other benefits include higher efficiency, reduced power consumption, and a longer lifespan.
LEDs require precise power and heat-management systems, since most of the electrical energy supplied to an LED is converted to heat rather than light. Without adequate thermal management, this heat will have a negative effect on both the LED’s lifespan and colour output.
Power-line coupled transients and surges can also reduce LED lifespan, and many LED drivers are susceptible to damage resulting from improper dc voltage levels and polarity. Short circuits also may damage or destroy LED driver outputs. Most LED drivers for LCD TV applications include built-in safety features, including thermal shutdown, as well as open and short LED detection. However, additional overcurrent protection devices may be needed to help protect ICs and other sensitive components.
PPTC devices can also be used to help prevent thermal runaway, which may occur if the monitor’s cooling vent is blocked. Due to its ability to detect and respond to over-temperature events, a PPTC device, mounted in the appropriate location, can interrupt current in the event that LCDs are operating without adequate ventilation.
Tyco Electronics’ PolySwitch PPTC device (Fig. 2) is connected in series with the LED to help provide overcurrent protection. To fully leverage the PolySwitch device, it can be thermally bonded to the metal core circuit board or LED heatsink. If the LED is not equipped with built-in ESD protection, a low-capacitance personnel ESD (PESD) protection device placed in parallel with the LED can help protect against damage caused by ESD surges.
OVERCURRENT AND OVERVOLTAGE PROTECTION FOR I/O PORTS
I/O port protection helps protect components from short-circuit damage and improves reliability and customer safety. To meet regulatory agency requirements, I/O ports must supply a method of interrupting or limiting current in the event of an overload or short circuit.
As data rates increase and circuitry becomes smaller and more sensitive, protecting equipment from damage caused by circuit transients becomes even more critical. The HDMI, USB, and DisplayPort specifications require end-user-accessible, powered connectors to implement overcurrent protection. The overcurrent protection device must be resettable without requiring user mechanical intervention, and its preset trip limit must be above allowable current transients to prevent false trips.
PPTC devices have demonstrated their effectiveness in a variety of high-speed interface applications. Like traditional fuses, they limit current after specified limits are exceeded. However, unlike a fuse, PPTC devices can reset after the fault is cleared and the power is cycled. Their low resistance, fast time-to-trip, and small form factor have made them the preferred method of overcurrent protection in many powered bus architectures.
Powered ports are also susceptible to damaging overvoltage transients, including ESD pulses. Figure 3 shows a typical circuit protection design utilizing a PolySwitch device for overcurrent protection, as well as PESD devices and MLVs to help protect against damage caused by overvoltage conditions.
The MLV provides low-capacitance shunt protection and offers the high current-handling and energy-absorption overvoltage protection required for LCD TV applications. The PESD arrays are installed over the data lines to help shunt ESD away from sensitive circuitry. The PESD device’s low capacitance helps prevent degradation of high data rate signals.
LCD TVs host an array of accessory ports, including but not limited to VGA, DVI, S-Video, composite video, and audio I/O ports. Tyco Electronics’ GDT, MLV, PESD and PolySwitch devices can be used in a coordinated circuit protection solution (Fig. 4).
The VGA ports on LCD TVs allow for efficient connection from a computer video card. ESD or cable discharge events can contribute to VGA port failures. IEC 61000-4-2 requirements are applicable here, and low-capacitance protection devices, such as PESD and 3-pF MLV devices (Fig. 5), can help manufacturers comply with that standard. Note that here the signal line voltages are less than 1 V p-p on the RGB signal lines and 5 V p-p or less for the control lines.
It’s always good practice to apply protection devices as close as possible to the chip sets’ I/O and VCC pins, as board traces may be susceptible to conducted transients. Good grounding practices coupled with robust circuit protection devices can enhance transient protection, help reduce warranty returns and repair costs, and facilitate compliance with applicable standards.