DC-DC regulated power supply application circuit design strategy

A linear regulator is a type of integrated voltage regulator that uses a transistor or FET operating in its linear region to reduce excess voltage from the input and deliver a stable output voltage. Its main function is to maintain a consistent DC output voltage even when the input voltage or load varies within the allowed range, ensuring safe and reliable long-term operation of the circuit. A commonly used integrated linear regulator is known as a standard or NPN linear regulator. It typically consists of key components such as a reference voltage source, a sampling circuit, an error amplifier, and an adjustment transistor. These elements work together to regulate the output voltage effectively. The basic working principle of a standard linear regulator is illustrated in Figure 1. In this configuration, a series regulator is made up of two NPN transistors, VT2 and VT3, forming a Darlington pair. VT1 acts as a driver transistor, which is a PNP type. The input voltage is labeled as U1, while the output is U0. Resistors R1 and R2 form a voltage divider, creating a sampled voltage UQ that is applied to the non-inverting input of the error amplifier. This voltage is compared with a reference voltage, UREF, connected to the inverting input. The difference between these two voltages is amplified by the error amplifier, producing an error signal Ur, which adjusts the voltage drop across the series regulator to stabilize the output voltage. For example, if the output voltage U0 decreases, both UQ and Ur also decrease. This causes the drive current to increase, reducing the voltage drop across the adjustment transistor and allowing U0 to rise again. Conversely, if U0 increases, the drive current from the error amplifier decreases, increasing the voltage drop across the adjustment transistor and bringing U0 back down. The feedback loop continuously ensures that the voltages at the two inputs of the error amplifier are equal, meaning U0 = UREF. Several important points should be noted: First, the output voltage is controlled via a feedback circuit that requires compensation to ensure stability. Some regulators have built-in compensation, eliminating the need for external components, while others require an external network. Second, the feedback loop measures the output voltage using a resistor divider and sends the sampled voltage to the non-inverting input of the error amplifier, while the reference voltage is connected to the inverting input. The error amplifier then adjusts its output to match the sample voltage with the reference voltage, resulting in an output voltage that is typically several times the reference value. Third, the current through R1 and R2 is negligible compared to the load current, making them ideal for accurate voltage division. Fourth, the driver transistor VT1 must be a PNP type. This is because the base-emitter junction of an NPN transistor has a positive voltage (UB > UE), which would not make sense in this context. In contrast, the base-emitter voltage of a PNP transistor is negative, allowing UB < UE, which is necessary for proper operation. Finally, Figure 1.2.2 represents a simplified version of the circuit. In practice, additional circuits such as a start-up circuit, overcurrent protection, and thermal protection are essential for full functionality and safety.

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