Electronic Design - PCB design of power module

The power circuit is an important part of an electronic product. The design of the power circuit directly affects the performance of the product.

Classification of power supply circuits

PCB factory's power supply circuit mainly includes linear power supply and high frequency switching power supply. In theory, the linear power supply is how much current the user needs, how much current the input end must provide; the switching power supply is how much power the user needs, how much power the input end provides.

Linear power circuit schematic diagram example

Linear power supply power devices work in a linear state, such as our commonly used voltage regulator chips LM7805, LM317, SPX1117, etc. The linear power supply is composed of functional components such as rectification, filtering, voltage stabilization, and energy storage. At the same time, the generally used linear power supply is a series stabilized power supply, the output current is equal to the input current, I1=I2+I3, I3 is the reference terminal, and the current is very small, So I1≈I3. The reason why we talk about current is that the width of each line is not randomly set during PCB design, but is determined according to the current between the component nodes in the schematic (please check "PCB Design Copper and Platinum Thickness, Line Width and Current relationship table"). The magnitude and direction of the current must be clarified so that the board is just right.

Linear power supply PCB diagram

When designing the PCB, the layout of the components should be compact, all the connections should be as short as possible, and the components and traces should be laid out according to the functional relationship of the schematic components. In this power supply diagram, it is rectified, filtered, and filtered before the voltage is stabilized. After the voltage is stabilized, the energy storage capacitor is used. After the capacitor flows through the capacitor, power is used for the subsequent circuits.

When designing a linear power supply PCB, you should also pay attention to the heat dissipation problem of the power regulator chip of the linear power supply. How does the heat come from? If the front-end voltage of the regulator chip is 10V, the output terminal is 5V, and the output current is 500mA, then it is in the regulator chip. There is a voltage drop of 5V, and the heat generated is 2.5W; if the input voltage is 15V, the voltage drop is 10V, and the heat generated is 5W. Therefore, our layout is to reserve enough according to the heat dissipation power Cooling space or reasonable heat sink. Linear power supply is generally used in the occasions where the voltage difference is relatively small and the current is relatively small, otherwise, please switch to the switching power supply circuit.

Examples of schematic diagram of high frequency switching power supply circuit

The switching power supply is to use the circuit to control the switching tube to conduct high-speed turn-on and turn-off, to generate the PWM waveform, through the inductor and the freewheeling diode, and use the electromagnetic power conversion method to adjust the voltage. The switching power supply has high power, high efficiency and low heat. The circuits we generally use are: LM2575, MC34063, SP6659, etc. In theory, the switching power supply has the same power at both ends of the circuit, the voltage is inversely proportional, and the current is inversely proportional.

When designing a switching power supply PCB, the points that need to be paid attention to are: the lead-in point of the feedback line and who is the freewheeling diode for freewheeling. It can be seen from Figure 3 that when U1 is turned on, the current I2 enters the inductor L1. The characteristic of the inductor is that the current cannot suddenly occur or disappear when the current flows through the inductor. There is a time course for the current change in the inductor. Under the action of the pulse current I2 flowing through the inductor, part of the electrical energy is converted into magnetic energy, and the current gradually increases. At a certain time, the control circuit U1 turns off I2. Due to the characteristics of the inductance, the current cannot suddenly disappear, and the diode works at this time It takes over the current I2, so it is called a freewheeling diode. It can be seen that the freewheeling diode is for the inductor. The freewheeling current I3 starts from the negative terminal of C3 and flows into the positive terminal of C3 after D1 and L1. This is equivalent to a water pump, using the energy of the inductance to increase the voltage of the capacitor C3.

There is also the problem of the introduction of points in the feedback line of the voltage detection, which should be fed back after filtering, otherwise the output voltage ripple will be greater. These two points are often overlooked by many of our PCB designers, thinking that the same network is not the same where it is connected. In fact, the connection place is different, and the performance impact is great. Figure 4 is the PCB diagram of the LM2575 switching power supply. Let's take a look at the wrong picture.

Why do we need to explain the principle of the schematic diagram in detail, because the schematic diagram contains a lot of information about drawing the PCB, such as the access point of the component pin, the current size of the node network, etc., after seeing the schematic diagram clearly, the PCB design is not a problem. LM7805 and LM2575 circuits represent the typical layout circuit of linear power supply and switching power supply respectively. When making a PCB, you can directly follow the layout and wiring of these two PCB diagrams, but the product is different and the circuit board is also different. Adjust according to the actual situation.

The principle of the power supply circuit and the layout method are the same, and every electronic product is inseparable from the power supply and its circuit. Therefore, after learning these two circuits, the others are also clear. Chest out.