Introduction to LCD backlight, I/O port and power protection circuit

As LCD TV screens become larger and brighter, they require more operating voltage and higher operating current, and the need for more stable and reliable circuit protection technology becomes more urgent.

There are many types of circuit protection devices that can be used to help protect LCD displays from damage caused by excessive current or excessive voltage transients. Overcurrent protection can take the form of fuses or resettable PPTC (polymer positive temperature coefficient) devices. Which solution is chosen depends on the design of the LCD, the type of hazard the product may be exposed to, and the associated safety requirements.

PPTC devices are generally suitable for circuits that experience large inrush currents at startup. PPTC devices can withstand the impact of inrush currents and save the trouble of replacing burned fuses. Fuses are suitable for circuit designs that do not require self-recovery capabilities, or circuits that only cause overcurrent impacts when a system error occurs. Fuses are also a practical solution when soft-start circuits are used to limit inrush currents.

To improve the safety and reliability of equipment, a variety of overvoltage and overcurrent protection devices can be used in conjunction as discussed in this article. The overvoltage protection devices include metal oxide varistors (MOVs), multilayer varistors (MLVs), and electrostatic discharge (ESD) surge protection devices.

LED backlight protection

LED backlighting enhances the viewing experience, provides more flexible backlighting architectures, and enables thinner display designs than traditional cold cathode fluorescent lamp (CCFL) technology. Other benefits of LED backlighting include: higher efficiency, lower power consumption, longer life, better durability, and greater contrast for higher definition.

LEDs require precise power and thermal management systems because much of the electrical energy supplied to an LED is converted into heat rather than light. Without proper thermal management measures, this heat will negatively impact the life and color output of the LED.

Transient and surge voltages coupled from the power line can also reduce LED life, and many LED drivers can be easily damaged by incorrect DC voltage levels and polarity, while the output of the LED driver can be damaged by a short circuit. Most LED drivers for LCD TV applications include built-in safety features such as thermal shutdown and LED open and short circuit detection. However, additional overcurrent protection devices may be needed to help protect ICs and other sensitive electronic components.

PPTC devices can also be used to prevent thermal runaway, which can occur if the cooling channels of an LCD monitor are blocked. The PPTC device has the ability to detect and respond to overtemperature events, so if it is installed in the right place, it can cut off the current when the LED is operated without proper heat dissipation.

Figure 1 shows how to connect a PolySwitch PPTC device in series with an LED to provide overcurrent protection. To fully utilize the PolySwitch device, it can be thermally bonded to a metal core circuit board or LED heat sink. If the LED does not have built-in ESD protection circuitry, a PESD protection device can be connected in parallel with the LED, which helps protect the LED from damage caused by ESD surge voltage.

Figure 1: Overheat protection scheme for LED backlight

[page] Overcurrent and overvoltage protection for I/O ports

I/O port protection helps protect components from damage caused by short circuits, improves reliability and protects customer safety. To meet regulatory agency requirements, I/O ports must provide a method to shut down or limit current in the event of an overload or short circuit.

As data rates increase and circuits become smaller and more sensitive, it becomes more important to protect devices from damage caused by transient voltages in circuits. HDMI, USB, and DisplayPort specifications require that live connectors accessible to end users must have overcurrent protection. Overcurrent protection devices must automatically recover without user intervention, and their preset trigger thresholds must be higher than the allowed transient current to prevent false protection action.

PPTC devices have proven their effectiveness in a variety of high-speed interface applications. Like traditional fuses, they can limit current after a specified threshold is exceeded. But unlike fuses, PPTC devices have the ability to self-recover after the fault is cleared and power is reapplied. The low impedance, fast response time and small form factor of PPTC devices have made it the preferred method of overcurrent protection in many bus-powered architecture applications.

Powered ports are also susceptible to damage from overvoltage transients including ESD pulses. Figure 2 shows a typical circuit protection design using PolySwitch devices for overcurrent protection, where the PESD device and varistor used help protect the port from damage caused by overvoltage.

Figure 2: Typical DisplayPort circuit protection design using MLV devices, PESD suppressors, and PolySwitch overcurrent protection devices

MLV devices provide low capacitance shunt protection, high current handling and energy absorption overvoltage protection required in LCD TV applications. PESD arrays are installed on data lines to help shunt ESD away from static sensitive circuits. The low capacitance of PESD devices helps prevent degradation of high data rate signals.

Protection of multiple output power supplies

Switching mode power supplies (SMPS) offer the size, weight and energy saving advantages required by consumer electronics and continue to replace linear regulators in many applications including LCD TV displays. However, because SMPS lack the inherent impedance of previous designs, they often require more robust circuit protection.

PPTC overcurrent protection devices can help manufacturers meet UL60950-1/LPS (Limited Power Source) requirements for SMPS and help improve the safety and reliability of equipment.

Under normal operating current, PPTC devices have very low resistance values. In overcurrent conditions, PPTC devices "jump" to a high-impedance state. The added resistance helps protect the equipment in the circuit by reducing the current that may flow under fault conditions to a low steady-state level. The PPTC device will remain locked in a high-impedance state until the fault is cleared. Once power is restored, the PPTC device will reset itself and allow current to flow, allowing the circuit to resume normal operation.

Although PPTC devices cannot prevent faults from occurring, they can react quickly to limit current to safe levels, thus helping to prevent the fault from damaging downstream devices. In addition, the small form factor of PPTC devices makes them easy to use in space-constrained applications.

Figure 3: Typical circuit protection design for a switch-mode power supply

As shown in Figure 3, PolySwitch devices can be used in series with the power input to help protect components within the system from damage caused by short circuits, circuit overloads, or customer misoperation. In addition, MOV devices installed at the input can help provide overvoltage protection for LED modules.

The PolySwitch device can also be placed after the MOV. Many equipment manufacturers prefer to use a protection circuit that combines a resettable PPTC device with an upstream fail-safe protection means. In the example of Figure 3, R1 is a ballast resistor used in combination with the protection circuit.

There are many standards for LCD-TV interface fault protection on the market, each of which meets different fault protection requirements. It is always wise to place the protection device as close to the chipset I/O and Vcc pins as possible, because the circuit board traces are easily affected by the conducted transient signals.

Good grounding practices, coupled with reliable circuit protection devices, can enhance transient voltage or current protection, help reduce repair rates and repair costs, and simplify the work required to meet applicable standards.