PCB design of power supply module

Power circuit is an important part of an electronic product. The design of power circuit directly affects the performance of the product. The power supply circuit of our electronic products mainly includes linear power supply and high frequency switching power supply. Theoretically speaking, linear power supply means that the user needs as much current as the input needs to provide; switching power supply means that the user needs as much power as the input needs to provide.

Linear power supply

Linear power supply power devices work in linear state, such as lm7805, LM317, spx1117 and so on. The following figure is the schematic diagram of lm7805 regulated power supply circuit. Figure 1 Schematic diagram of linear power supply

It can be seen from the figure that the linear power supply consists of rectifying, filtering, voltage stabilizing, energy storage and other functional elements. At the same time, the linear power supply generally used is a series voltage stabilizing power supply, the output current is equal to the input current, I1 = I2 + I3, i3 is the reference terminal, the current is very small, so I1 ≈ I3. The reason why we talk about current is that in PCB design, the width of each line is not arbitrarily set, but is determined according to the current between component nodes in the schematic diagram (please refer to the relationship table of PCB design copper platinum thickness, line width and current). The size and direction of the current should be made clear so that the board can be made just right.

In PCB design, the layout of components should be compact, all the wiring should be as short as possible, and the components and wiring should be arranged according to the functional relationship of components in the schematic diagram. In this power supply diagram, it is the first rectification, then filtering, after filtering, then voltage stabilization, after voltage stabilization, then energy storage capacitor, after flowing through the capacitor, then power supply to the following circuit. Figure 2 is the PCB diagram of the above schematic diagram. The two diagrams are similar. The left figure and the right figure are a little different. After rectification, the power supply in the left figure goes directly to the input pin of the voltage stabilizing chip, and then comes the voltage stabilizing capacitor. The filtering effect of the capacitor here is much worse, and the output is also problematic. The picture on the right is a better one. We should not only consider the flow direction of the positive power supply, but also the earth return. Generally speaking, the positive power supply line and the earth return line should go in and out together as far as possible, and be close to each other. Figure 2 PCB diagram of linear power supply

When designing the PCB of linear power supply, we should also pay attention to the heat dissipation of the power regulator chip of linear power supply and how the heat comes from. If the front-end voltage of the regulator chip is 10V, the output terminal is 5V, and the output current is 500mA, then there is a 5V voltage drop on the regulator chip, and the heat generated is 2.5W; if the input terminal voltage is 15V, the voltage drop is 10V, and the heat generated is 5W, Our layout is to set aside enough heat dissipation space or reasonable heat sink according to the heat dissipation power. Linear power supply is generally used in the situation of small voltage difference and small current, otherwise, switch power supply circuit should be used.

High frequency switching power supply

Switching power supply is to use the circuit to control the switch tube for high-speed conduction and cut-off, generate PWM waveform, through the inductor and freewheeling diode, use the way of electromagnetic electric conversion to adjust the voltage. Switching power supply power, high efficiency, low heat, we generally use the circuit: LM2575, mc34063, sp6659, etc. In theory, the power of both ends of the circuit is equal, the voltage is inversely proportional, and the current is inversely proportional. Figure 3 Schematic diagram of LM2575 switching power supply circuit

When designing the switch power supply PCB, we should pay attention to the following aspects: the lead-in point of the feedback line and the freewheeling diode. It can be seen from Figure 3 that when U1 is on, the current I2 enters the inductor L1. The characteristic of the inductor is that the current can not be generated or disappear suddenly when it flows through the inductor, and there is a time process when the current changes in the inductor. Under the action of the pulse current I2 flowing through the inductor, some electric energy is converted into magnetic energy, and the current increases gradually. At a certain time, the control circuit U1 turns off I2. Due to the characteristics of the inductor, the current can not disappear suddenly. At this time, the diode works. It replaces the current I2, so it is called freewheeling diode. It can be seen that the freewheeling diode is used for the inductor, and the freewheeling current I3 is generated from the inductor Starting from the negative end of C3, it flows into the positive end of C3 through D1 and L1, which is equivalent to a water pump. By using the energy of inductance, the voltage of capacitor C3 is increased. There is also the problem of the introduction point of the feedback line of voltage detection, which should be fed back after filtering, otherwise the output voltage ripple will be larger. These two points are often ignored by many PCB designers. They think that the same network is not connected there the same way. In fact, the connection places are different and the performance is greatly affected. Figure 4 is the PCB diagram of LM2575 switching power supply. Let’s take a look at the wrong picture. Figure 4 PCB diagram of LM2575 switching power supply

Why should we talk about the principle of the schematic diagram in detail, because the schematic diagram contains a lot of PCB drawing information, such as the access point of the component pin, the current size of the node network, and so on. If we see the schematic diagram clearly, the PCB design will not be a problem. Lm7805 and LM2575 represent the typical layout circuit of linear power supply and switching power supply respectively. When making PCB, it is OK to directly lay out and route according to these two kinds of PCB diagram, but the product is different, the circuit board is also different, and it is adjusted according to the actual situation.

The principle and layout of all power circuits are the same, and every electronic product is inseparable from the power supply and its circuits. Therefore, after learning these two circuits, the rest will be clear.