Power circuits are crucial in electronic products. Its design quality directly affects the performance and stability of the whole product. Therefore, mastering the design of the power supply circuit is the key to ensure the reliable operation of electronic products.

Classification of power supply circuits
In electronic products, power supply circuit is mainly divided into linear power supply and switching power supply. The characteristic of linear power supply is that the current provided by the input side matches the user’s demand; The switching power supply provides the corresponding input power according to the power required by the user. This difference makes both have advantages and disadvantages in performance and efficiency.

Schematic diagram of linear power supply circuit
The power component of a linear power supply is maintained in a linear state while operating. Common voltage regulator chips, such as the LM7805, LM317 and SPX1117, are typical representatives of linear power supplies. Figure 1 below shows the circuit schematic of the LM7805 regulated power supply.

Figure 1 Schematic diagram of linear power supply

It can be seen from the circuit diagram that the linear power supply is mainly composed of functional components such as rectification, filtering, voltage regulation and energy storage. Usually, the linear power supply uses a series voltage regulator design, and the output current is equal to the input current, that is, I1 = I2 + I3, where I3 is the reference end current, usually very small, so I1 ≈ I2. In PCB design, the current size and flow direction are very important, because the width of each line needs to be determined according to the current between the various nodes in the schematic diagram. Understanding the magnitude and flow of current helps to accurately design the PCB and ensure the performance and stability of the power circuit.

Linear power supply PCB diagram
In the PCB design, the component layout should be as compact as possible, and the connection should be as short as possible, in accordance with the functional relationship of the components in the schematic diagram. Taking the power supply circuit as an example, the design process is usually rectified first, then filtered, then stabilized, and finally added to the energy storage capacitor to ensure that the power supply can stably supply the subsequent circuit.

Figure 2 shows the schematic PCB layout. The layout on the left side sends the rectified power supply directly to the input foot of the voltage regulator chip, and then to the voltage regulator capacitor, which will greatly reduce the filtering effect of the capacitor, resulting in unstable output. The layout on the right is more reasonable, filter first, and then regulate the voltage, and the power line and the ground return line are as close as possible to optimize the stability of the power supply. In the design, we should not only pay attention to the flow direction of the positive power supply, but also consider the return path of the ground line, usually the positive power line and the ground line should be as close as possible to the same direction and improve the overall performance of the circuit.

Figure 2 PCB diagram of linear power supply

When designing PCB of linear power supply, special attention should be paid to the heat dissipation of power regulator chip. The heat is mainly determined by the difference between the input voltage and the output voltage as well as the output current. For example, if the input voltage of the regulator chip is 10V, the output voltage is 5V, and the output current is 500mA, then there will be a 5V voltage drop on the chip and the heat generated will be 2.5W. If the input voltage is increased to 15V, the voltage drop is 10V, and the heat will increase to 5W. Therefore, when distributing the board, it is necessary to reserve enough space for heat dissipation, and consider the reasonable configuration of heat sinks to meet the heat dissipation requirements. Linear power supply is suitable for the occasion of small pressure difference and small current; If the voltage difference is large or the current is high, switching power supplies are recommended to improve efficiency and reduce heat dissipation requirements.

High frequency switching power supply circuit schematic example
The switching power supply generates PWM waveform by controlling the high-speed on-off and on-off of the switching tube, and then realizes electromagnetic conversion through the inductor and the continuous current diode to regulate the voltage. They have a large power output, high efficiency and low heat generation. Common switching power supply chips include LM2575, MC34063 and SP6659.

In theory, the input and output power of the switching power supply are equal, and the voltage is inversely proportional to the current. This means that the relationship between the input voltage and the output voltage is inversely proportional, and the relationship between the input current and the output current is inversely proportional. This design allows switching power supplies to perform well in high-power and high-efficiency applications.

Figure 3 Schematic diagram of LM2575 switching power supply circuit

PCB diagram of switching power supply
In the PCB design of switching power supplies, there are two key points that need special attention: the input point of the feedback line and the role of the continuous current diode.

As can be seen from Figure 3, when U1 is on, the current I2 flows through the inductor L1. The characteristics of inductance determine that the current cannot change instantaneously, but has a gradual change process. As I2 flows through the inductor, part of the electrical energy is converted into magnetic energy, and the current gradually increases. When U1 is turned off, the current in the inductor cannot suddenly disappear, and the current diode D1 takes over the current of I2, so it is called the current diode. The continuous current I3 flows out of the negative end of capacitor C3, through D1 and L1, and into the positive end of C3. In this way, the energy of the inductor is used to raise the voltage of the capacitor C3.

In addition, the point of entry of the feedback line is also very important. The feedback line should be introduced at the filtered position, otherwise it will increase the ripple of the output voltage and affect the performance of the power supply. These two details are often overlooked by PCB designers, but the actual effect varies greatly. Figure 4 shows the PCB layout of the LM2575 switching power supply so you can view and identify problems in the design.

Figure 4 PCB diagram of LM2575 switching power supply

It is important to understand the schematic in detail because it provides critical information needed to design the PCB, including the connection points of the component pins and the current magnitude of the node network. After a thorough understanding of the schematic, PCB design will become more smooth.

The LM7805 and LM2575 circuits represent typical layouts of linear and switching power supplies, respectively. In the actual PCB design, the layout and wiring of the two circuits can be directly referred to, and appropriate adjustments can be made according to the different products.

Although the design of the power circuit may vary, the core principle and the layout method are the same. Master the design of these two circuits, the design of other power circuits will become more clear and understandable.