Electronic Design - About the bottom line design of switching power supply PCB

The switching power supply PCB is not only a high-frequency, high-current switching circuit (such as switching transistors and high-frequency transformers, etc.), but also a small current measurement and control circuit, the connection between the circuits is intricate.

When designing a PCB, it is necessary to start from the circuit, distinguish the relationship between large and small currents, the relationship between measurement, control and drive circuits, and handle the layout of the ground wire. In the PCB design of the switching power supply, the design of the ground wire is very important. This is because at the moment when the switching transistor of the switching power supply is turned on and off, the voltage and current change drastically, and due to the existence of the common impedance of the ground wire, the drastically changing voltage and current will cause serious interference on the ground wire. In the layout of the ground wire, it is not feasible to just stay on the concept of "DC with the same potential". It is necessary to consider the dynamic process of the circuit and pay attention to the current in the ground wire and its flow direction. Through the flow of current, we can analyze whether the layout of the ground wire is reasonable or not, and judge whether there is interference.

The rationality of the ground wire layout can be judged by the following two conditions.

1. Whether the current in the ground wire flows through other circuits, parts and wires that have nothing to do with this current.

2. Is there any current in other parts and circuits flowing into the ground wire of this part of the circuit? Circuit 1 and circuit 2 form a loop with the power supply through the common ground section AB. It can be known from the previous analysis that the line segment AB can be equivalent to the series connection of a resistor and an inductance, with a certain impedance. Since all the currents of circuit 1 and circuit 2 pass through the line segment AB, the line segment AB becomes a common impedance. When the whole circuit is working, the current change of circuit 1 or circuit 2 will cause the potential change of point A, which will affect the working state of circuit 1 and circuit 2, and form common impedance interference.

Common impedance interference of the ground wire In the PCB design of the switching power supply, the operating frequency of the switching power supply is usually only tens to hundreds of kilohertz, and the use of single-point grounding can already meet the requirements. The single-point grounding method can also separate the noise source from the sensitive circuit.

A switching power supply inverter circuit partially grounded

(1) In order to obtain better electromagnetic compatibility performance and reduce the influence of the ripple of the power frequency rectification on the operation of the SG6840 circuit, the output of the bridge rectifier circuit should first be connected to the capacitor C2, and then connected to the inverter circuit.

(2) In the high-current loop of the inverter part (C2-high-frequency transformer-MOSFET-RS-C2), the connection between RS and C2 should be as short as possible, and no element should be placed between RS and C2. Device.

(3) Separate the ground loop of C1, connect one route to GND of SG6840, and connect the other route to bias winding.

(4) C1 should be as close as possible to VDD and GND of SG6840, in order to obtain the best possible decoupling and filtering effect.

(5) RL, RT and photoelectric coupler ground in SG6840 control circuit should be connected together and close to GND of SG6840.

(6) The GND of SG6840 should be connected to the ground of RS.

PCB design example of TOPSwitch switching power supply

1. TOP204YA1 switching power supply circuit is a switching power supply circuit diagram with TOP204YA1 chip as the core. TOP204YA1 switching power supply circuit diagram

2. Single-point grounding at the source pin of TOP204YA1 As shown in Figure 7-17, the automatic restart/compensation capacitor C5 must be connected to the source of TOP204YA1 by a single-point grounding method. In order to avoid false turn-off or unstable operation during the turn-on period due to excessive source switch current, a reasonable PCB layout must be carried out. The high-voltage return line of the input capacitor C1 must be directly connected to the source pad, and cannot be connected to the line of C5. The return line of the bias/feedback should also be directly connected to the source pad. TOP204YA1 source pin terminal The source pin terminal must be as short as possible, do not bend or extend the source pin. For the drain, if necessary, the drain pin can be bent or extended appropriately. To install TOP204YA1, it must be completely inserted into the PCB pad.

3. PCB layout and wiring of key components PCB layout and wiring of key components.

4. PCB layout and wiring check items The PCB layout and wiring check items are as follows.

1. The primary pins of TOP204YA1 (U1), C1 and transformer T1 should be as close as possible to reduce the wiring length and loop area of the PCB. High-speed switching currents flow through the connecting wires of these components, which usually cause common-mode EMI emission. Pay attention to the installation of TOP204YA1 and radiator.

2. The primary pins of VD1, VR1 and transformer T1 should be as close as possible to reduce the PCB wiring length and loop area. High-speed switching currents flow through the connecting wires of these components, which usually cause common-mode EMI emission.

3. The drain of TOP204YA1 (U1) is connected to the primary pin of T1 and the printed wire of the clamp diode VD1 must be very short. In addition to high-speed currents flowing on the printed wires, there are also high switching voltages, which can also cause additional common-mode EMI emissions.

4. The source of TOP204YA1 (U1) should be directly connected to C1, and there should not be other branch lines connected to this line.

5. Y1 capacitor C7 should be directly connected to the return end of the bias winding of the transformer T1 and the return end of the secondary output winding with thick and short printed wires.

6. The return terminal of the primary bias winding of transformer T1 should be directly connected to the source of TOP204YA1 (U1), and no other components should be connected to this printed wire, because the test voltage of lightning surge will induce noise on it. Voltage. 7. The bias diode VD3 should be as close as possible to the bias winding pin of the transformer T1 to shorten the length of the anode lead (high switching voltage on the anode lead) and increase the length of the relatively "clean" cathode lead.

8. The cathode of VD3 should be directly connected to C4, and no other components should be connected to this printed wire, because the test voltage and rectified current of lightning surge will induce noise voltage on it. Therefore, C4 should be connected to the photoelectric coupling device U2 through the wiring of the PCB and the jumper on the PCB component surface.