# PCB design of power module

- Oct 17, 2018-

Power supply circuit is an important component of electronic products.

Classification of power circuit

The power circuit of our electronic products mainly includes linear power supply and high frequency switching power supply.In theory, a linear power supply is how much current the user needs and how much current the input has to provide;Switching power supply is the user needs as much power, the input is provided as much power.

An example of a schematic diagram of a linear power supply circuit

The power components of linear power supply work in linear state, such as LM7805, LM317, SPX1117, etc.The schematic diagram of LM7805 stabilized voltage supply circuit is shown in figure 1 below.

Figure 1. Linear power supply schematic diagram

It can be seen from the figure that the linear power supply is composed of rectification, filtering, stabilized voltage, energy storage and other functional elements. Meanwhile, the linear power supply commonly used is a series stabilized voltage power supply, and the output current is equal to the input current. I1=I2+I3.The reason why we talk about current is that when PCB is designed, the width of each line is not arbitrary, but depends on the current between the component nodes in the schematic diagram (please refer to the PCB design copper-platinum thickness, line width and current relationship table).Current size, current flow should be clear, make the plate just right.

Linear power PCB diagram

When designing PCB, the layout of the components should be compact, and all the wires should be as short as possible.In this power supply diagram, the voltage is stabilized after rectification, filtering and filtering, and the energy storage capacitance is achieved after stabilization, and the power is used for the circuit behind.

Figure 2 is the PCB diagram of the above schematic diagram. The two diagrams are similar.The left figure and the right figure are slightly different from each other. The power supply in the left figure directly reaches the input pin of the voltage stabilizing chip after the rectification, and then the voltage stabilizing capacitor is generated.The right picture is a better one.We should consider not only the direction of the positive power supply, but also the ground return problem. Generally, the positive power line and the ground return line should be as close to each other as possible.

Figure 2. Linear power supply PCB

When designing the linear power supply PCB, we should also pay attention to the heat dissipation problem of the power stabilized chip of the linear power supply. If the front voltage of the stabilized chip is 10V, the output voltage is 5V, and the output current is 500mA, then there is a voltage drop of 5V on the stabilized chip, and the heat generated is 2.5w.If the input terminal voltage is 15V and the voltage drop is 10V, the heat generated will be 5W. Therefore, according to the heat dissipation power, sufficient heat dissipation space or reasonable heat sink should be set aside.Linear power supply is generally used in the case of small pressure difference and small current. Otherwise, switch to switching power supply circuit.

Schematic diagram of high frequency switching power supply circuit

The switching power supply is used to control the switching tube through the circuit to conduct and cut off at high speed, to generate PWM waveform, through the inductance and continuous-current diode, the use of electromagnetic electrical conversion of pressure regulation.Switching power supply has high power, high efficiency and low heat. The common circuits we use are LM2575, MC34063, SP6659, etc.Switching power supply theoretically means that the power at 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

Switch power PCB

In the design of switching power supply PCB, attention should be paid to the introduction point of the feedback line and to whom the afterflow diode is used.As can be seen from figure 3, when U1 leads, current I2 enters inductance L1. The inductance characteristic is that when current flows through the inductance, it cannot suddenly appear or suddenly disappear. When current changes in the inductance, there is a time process.Under the effect of pulse electric current flows through the inductance of the I2, a portion of the electrical energy into magnetic energy, a current increases gradually, at some point, the control circuit U1 shut off the I2, due to the characteristics of inductance current can't suddenly disappeared, diode work at this time, it replace current I2, so called fly-wheel diode, it can be seen that the fly-wheel diode is used to inductance, stream current I3 is from the negative side of C3, via D1, L1 after into C3 is end, here is the equivalent of water pump, using inductance energy, increasing the voltage of capacitor C3.

There is also the voltage detection feedback line introduction point problem, should be filtered after the feedback back, otherwise will make the output voltage ripple larger.These two points are often overlooked by many of our PCB designers, who think that the same network is not the same where it is connected, but it is different where it is connected, and the performance impact is significant.Figure 4 is the PCB diagram of LM2575 switch power supply. Look at the wrong picture.

Figure 4. PCB of LM2575 switching power supply

Why do we need to talk about the principle diagram in detail? The reason is that the principle diagram contains a lot of information about PCB, such as the access point of component pin, the current size of node network, etc. After seeing the principle diagram, PCB design is not a problem.LM7805 and LM2575 circuits respectively represent typical cloth circuit of linear power supply and switching power supply. When making PCB, you can directly arrange the PCB layout and wiring according to the two PCB diagrams.

The principle of the power circuit and the way of the cloth plate are all the same, and each electronic product cannot be separated from the power source and its circuit. Therefore, the two circuits have been learned, and the rest have been well understood.