Plasma color TV DC-DC linear regulator circuit principle

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The installation position of the DC-DC converter is relatively scattered. The DC-DC converter can be found on the PDP color TV power board, main board, logic control board and other circuit boards. Because the DC-DC converter shoulders the burden of powering the small signal processing circuit of the color TV, it is more prone to malfunction in the work. As a maintenance person, it is necessary to quickly find the position of the DC-DC converter, correctly measure the input/output voltage of the DC-DC converter, and quickly judge whether the DC-DC converter is good or bad.

1. The basic working principle of linear regulator The linear regulator is controlled by the output voltage feedback, error amplifier and other control circuit to control the tube voltage drop VDO (ie differential pressure) of the regulating tube to achieve the purpose of voltage regulation. l is shown. It is characterized by the fact that VIN must be greater than VOUT and the regulator operates in the linear region (the linear regulator is named from here). When the output voltage fluctuates due to fluctuations in the input voltage or changes in the load current, the VDO is changed by the feedback and control circuit so that the output voltage VOUT is substantially unchanged.
Whether it is a normal linear regulator or an LDO, the principle of operation is the same. The difference is that the adjustment tube structure used by the two is different, so that the LDO has smaller voltage difference and lower power consumption than the ordinary linear regulator.
Some linear regulators used in PDP color TVs have an output control terminal, which means that the output voltage of this regulator is controlled by the control terminal. Figure 2 shows the internal block diagram of the controllable regulator. The EN in the figure (sometimes also represented by the symbol SHDN) is the output control. The low level (or high level) is typically applied by the microprocessor to turn the LDO off (or operating) and the current is approximately 1 μA when the power is turned off.



Figure 1 Linear stabilizer block diagram



Figure 2 Internal block diagram of the controllable regulator

2. Characteristics of Linear Regulators Linear regulators are characterized by low cost, small package size, low peripheral components, and low noise. Linear regulators come in many package types and are ideal for use in PDP color TVs. For the use of fixed voltage output, only two or three small capacitors are needed on the periphery to form the whole solution.
Ultra-low output voltage noise is the biggest advantage of linear regulators. The output voltage has a ripple of less than 35 μV and a very high signal-to-noise rejection ratio. It is ideal for powering small, noise-sensitive signal processing circuits. At the same time, in large linear power supplies, there is no large current change when switching. Electromagnetic interference (EMI) is induced, so it is easy to design.
The disadvantage of linear regulators is that they are not efficient and can only be used in buck applications. The efficiency of a linear regulator depends on the ratio of the output voltage to the input voltage η = Vo: Vi. For example, for a typical linear regulator, at an input voltage of 5 V, the output voltage is only 2.5% at 2.5 V. It can be seen that for ordinary linear regulators, about 50% of the electrical energy is converted into heat loss, which is the main reason why ordinary linear regulators are prone to heat when working. For LDOs, the efficiency is much higher due to the low dropout. For example, with an input voltage of 3.3 V, the efficiency is 76% at an output voltage of 2.5 V.
Therefore, in the PDP color TV, in order to improve the utilization of electric energy, there are fewer ordinary linear regulators, and more LDOs are used.