PCB Tech - PCB layout of power supply circuit

Power supply circuit is an important part of electronic products, power supply printed circuit board design is directly related to the product performance. The power circuit of our electronic products mainly includes linear power supply and high frequency switching power supply. Theoretically speaking, the linear power supply is the user needs how much current, the input should provide how much current; Switching power supply is the user needs how much power, the input provides how much power.

Linear power supply

Linear power supply power devices work in a linear state, such as our commonly used regulator chips LM7805, LM317, SPX1117 and so on. Figure 1 below is the schematic diagram of LM7805 regulated voltage power supply circuit.

PCB layout of power supply circuit

FIG. 1 Schematic diagram of linear power supply

As can be seen from the figure, linear power supply is composed of rectifier, filter, voltage regulator, energy storage and other functional components. At the same time, the general linear power supply is series voltage regulator, and the output current is equal to the input current, I1=I2+I3, I3 is the reference end, 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 set randomly, but determined according to the current between component nodes in the schematic diagram (please refer to "PCB Design Copper and Platinum Thickness, Line Width and Current Relationship Table"). Current size, current flow should be clear, do plate just right.

In PCB design, the layout of components should be compact, all lines should be as short as possible, and components and wiring should be arranged according to the functional relationship of components in the schematic diagram. In this power supply diagram, the rectifier is first, and then the filter, and then the voltage regulator is after the filtering, and then the energy storage capacitor is after the voltage regulator, and only after the capacitor flows through the circuit behind the electricity. Figure 2 is the PCB diagram of the above schematic diagram, and the two diagrams are similar. The left picture and the right picture are a little different. The power supply in the left picture is directly to the input pin of the voltage regulator chip after rectification, and then the voltage regulator capacitor. The filtering effect of the capacitor is much worse, and the output is also problematic. The picture on the right is a better one. We must consider not only the flow direction of the positive power supply, but also the backflow problem. Generally speaking, the positive power line and

the backflow line should be as close to each other as possible.

PCB layout of power supply circuit

FIG. 2 PCB of linear power supply

When designing linear power PCB, we should also pay attention to the heat dissipation problem of the power regulator chip of linear power supply, how the heat comes, if the voltage regulator chip front is 10V, the output is 5V, the output current is 500mA, then there is a voltage drop of 5V on the regulator chip, the heat generated is 2.5W; If the input voltage is 15V, the voltage drop is 10V, the heat generated is 5W, therefore, our board 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 pressure difference is relatively small, the current is relatively small occasions, otherwise, please use switch power supply circuit.

High frequency switching power supply

Switching power supply is used to control the switching tube through the circuit for high-speed conduction and cut-off, generate PWM waveform, through inductance and diode continuation, the use of electromagnetic voltage conversion. Switching power supply power, high efficiency, small heat, we generally use the circuit: LM2575, MC34063, SP6659 and so on. Switching power supply is theoretically equal power at both ends of the circuit, the voltage is inversely proportional, the current is inversely proportional.

Switching power supply PCB design, need to pay attention to the place is: the introduction of the feedback line, diode is to whom the current. As can be seen from Figure 3, current I2 enters the inductor L1 when U1 is on. The characteristic of inductance is that the current flowing through the inductor cannot suddenly be generated or disappear. The change of current in the inductor has a time process. Under the effect of pulse current I2 flow through the inductance, 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, The continuous current I3 starts from the negative end of C3, flows into the positive end of C3 through D1 and L1, which is equivalent to a pump. It uses the energy of the inductor to increase the voltage of the capacitor C3.

There is also the problem of the input point of the feedback line of voltage detection, which should be fed back to the place after filtering, otherwise the output voltage ripple will be larger. These two points are often ignored by many PCB designers, who think that the same network is not the same where it is connected, but in fact, it is different where it is connected, which has a great impact on performance. Figure 4 is THE PCB diagram of LM2575 switching power supply, we see where the wrong picture is wrong.

Why we need to 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 component pins, the current size of node network, etc., see the schematic diagram clearly, PCB design is not a problem. LM7805 and LM2575 circuit respectively represent the typical layout circuit of linear power supply and switching power supply, do PCB, directly according to the two PCB layout and wiring, but the product is different, the circuit board is also different, according to the actual situation adjustment.

The principle of all the power supply circuit and the way of layout are so, and every electronic product is inseparable from the power supply and its circuit, therefore, learn through these two circuits, the other is also clear in the chest.