The basic working principle and characteristics of plasma color TV DC-DC linear regulator

The DC-DC converter in PDP color TVs is often located on various boards such as the power board, main board, and logic control board. Since it's responsible for powering the low-voltage signal processing circuits, it tends to be more susceptible to failure. As a technician, being able to quickly locate the DC-DC converter and accurately measure its input and output voltages is essential for efficient troubleshooting and diagnosis.

1. Basic Working Principle of Linear Regulators

A linear regulator regulates the voltage drop across the regulating transistor (VDO) by using feedback from the output voltage and an error amplifier, as shown in Figure 1. It requires that the input voltage (VIN) must be higher than the output voltage (VOUT), and it operates in the linear region—hence the name "linear regulator." When the output voltage fluctuates due to changes in input voltage or load current, the feedback and control circuit adjusts VDO to maintain a stable VOUT.

Whether it’s a standard linear regulator or an LDO, the fundamental principle remains the same. The difference lies in the design of the adjustment transistor, which allows LDOs to operate with a smaller voltage dropout and lower power consumption compared to conventional linear regulators.

Some linear regulators used in PDP TVs have an output control pin, meaning their output voltage can be controlled externally. Figure 2 shows the internal block diagram of such a controllable regulator. The EN pin (sometimes labeled SHDN) is typically used by the microcontroller to enable or disable the regulator. When disabled, the current consumption drops to around 1 μA.

Figure 1: Linear stabilizer block diagram

Figure 2: Internal block diagram of the controllable regulator

2. Characteristics of Linear Regulators

Linear regulators are known for their low cost, compact size, minimal external components, and low noise performance. They come in various package types and are well-suited for use in PDP TVs. For fixed-output applications, only a few small capacitors are needed on the periphery to complete the circuit.

One of the biggest advantages of linear regulators is their ultra-low output voltage noise. The ripple is typically less than 35 μV, and they offer excellent signal-to-noise ratio. This makes them ideal for powering sensitive signal processing circuits. Additionally, large linear power supplies do not produce large current changes during operation, resulting in minimal electromagnetic interference (EMI), which simplifies the design process.

However, linear regulators are inefficient and can only be used in step-down configurations. Their efficiency depends on the ratio of the output voltage to the input voltage, η = Vo / Vi. For example, if the input is 5 V and the output is 2.5 V, the efficiency is about 50%, meaning half the energy is lost as heat. This is why traditional linear regulators tend to overheat during operation. LDOs, on the other hand, are much more efficient due to their low dropout voltage. At an input of 3.3 V and an output of 2.5 V, the efficiency can reach up to 76%.

In PDP TVs, where energy efficiency is important, LDOs are increasingly preferred over traditional linear regulators to reduce power loss and improve system reliability.