PCB Blog - The basic points of PCB Board layout of switching power supply

Switching power supply PCB board layout is an important process in the development of power supply products. In many cases, a power supply that is perfectly designed on paper may not work properly during initial commissioning due to many problems with the layout of the power supply's PCB. The basic points of switching power supply PCB layout are discussed in detail, and some practical PCB layout examples are described. In order to adapt to the rapid replacement rhythm of electronic products, product design engineers are more inclined to choose AC/DC adapters that are easy to purchase in the market and install multiple sets of DC power supplies directly on the circuit board of the system. Since the electromagnetic interference generated by the switching power supply will affect the normal operation of its electronic products, the correct layout of the power supply PCB becomes very important. The layout of the switching power supply PCB is completely different from the layout of the digital circuit PCB. In digital circuit layout, many digital chips can be automatically arranged through PCB board software, and the connecting lines between chips can be automatically connected through PCB board software. The switching power supply discharged by automatic typesetting will definitely not work properly. Therefore, planners need to have a certain understanding of the basic rules of switching power supply PCB layout and the working principle of switching power supply.

1. Basic points of PCB Board layout of switching power supply

1.1 Capacitor high-frequency filtering characteristics

Electrolytic capacitors generally have large capacitance and large equivalent series inductance. Because its resonant frequency is very low, it can only be used for low-frequency filtering. Tantalum capacitors generally have larger capacitance and smaller equivalent series inductance, so their resonance frequency is higher than that of electrolytic capacitors and can be used in medium and high-frequency filtering. The capacitance and equivalent series inductance of ceramic capacitors are generally very small, so their resonance frequency is much higher than that of electrolytic capacitors and tantalum capacitors, so they can be used in high-frequency filtering and bypass circuits. Since the resonant frequency of small-capacitance ceramic capacitors is higher than that of large-capacity ceramic capacitors, when selecting bypass capacitors, ceramic capacitors with too high capacitance cannot be selected. In order to improve the high-frequency characteristics of capacitors, multiple capacitors with different characteristics can be used in parallel. Figure 3 shows the effect of impedance improvement after a number of capacitors with different characteristics are connected in parallel. Basic points of power layout 1 The capacitance of the bypass ceramic capacitor should not be too large, and its parasitic series inductance should be as small as possible. Multiple capacitors in parallel can improve the high-frequency impedance characteristics of the capacitor.

1.2 Inductor high-frequency filtering characteristics

In the switching power supply, the Cp of the inductor should be controlled as small as possible. At the same time, it must be noted that the inductance of the same inductance will have different Cp values due to different coil structures, and the inductance of the same inductance will have different Cp values under two different coil structures. The 5 turns of the inductor are wound sequentially. The Cp value of this coiled structure is 1/5 of the equivalent parallel capacitance (C) of the turn coil. The 5-turn winding of the inductor is wound in a crossover sequence. Where windings 4 and 5 are placed between windings 1, 2, and 3, and windings 1 and 5 are very close together, the Cp produced by this coiled structure is twice the C value of a 1-turn coil. It can be seen that the Cp values of two inductors with the same inductance are actually several times different. In high-frequency filtering, if the Cp value of an inductor is too large, high-frequency noise will be easily coupled directly to the load through Cp. Such an inductor also loses its high-frequency filtering function. On a PCB board, Vin is routed through L to load (RL) in different ways. In order to reduce the Cp of the inductor, the two pins of the inductor should be kept as far away as possible. The traces from the positive pole of Vin to RL and the negative pole of Vin to RL should be as close as possible, the parasitic parallel capacitance of the inductor should be as small as possible, and the farther the distance between the pads of the inductor pins, the better.

1.3 Mirror surface

The concept of mirror surface in electromagnetic theory will be very helpful for designers to master the PCB layout of switching power supply. Scenario when DC current flows over a ground plane. The return DC current on the formation is now very evenly distributed over the entire formation. Scenario when high-frequency current flows over the same formation. At this time, the return alternating current on the ground can only flow in the middle of the ground and there is no current on both sides of the ground. Ground plane designers should try to avoid placing any power or signal traces on the ground plane. Once the wiring on the ground layer destroys the entire high-frequency loop, the circuit will generate strong electromagnetic wave radiation and destroy the normal operation of the surrounding electronic devices. Avoid placing any power or signal traces on the ground plane.

1.4 High-frequency loop

There are many high-frequency loops composed of power devices in the switching power supply. If the △ loop is not properly handled, it will have a great impact on the normal operation of the power supply. In order to reduce the electromagnetic wave noise generated by the high-frequency loop, the area of the loop should be controlled very small. The high-frequency current loop has a large area, which will generate strong electromagnetic interference inside and outside the loop. For the same high-frequency current, when the loop area is designed to be very small, the internal and external electromagnetic fields of the loop cancel each other out, and the entire circuit will become very quiet. The area of the high-frequency loop should be minimized as much as possible.

1.5 Via and Pad Placement

Many designers like to place many vias under multilayer PCB boards. However, it is necessary to avoid placing too many passes on the high-frequency current return path. Otherwise, the high-frequency current traces on the ground will be damaged. If some vias must be placed on the high-frequency current path, a space can be left between the vias for the high-frequency current to pass smoothly. Via placement should not disrupt the flow of high-frequency currents on the ground plane, and designers should also be aware that different pad shapes will produce different series inductances. . The placement of the bypass capacitor also takes into account its series inductance value. Bypass capacitors must be ceramic capacitors with low impedance and low ESL. But if a high-quality ceramic capacitor is placed in the wrong way on the PCB, its high-frequency filtering function will disappear.

1.6 Power DC output

Many switching power supplies have loads far away from the output ports of the power supply. In order to avoid the electromagnetic down disturbance caused by the power supply itself or the surrounding electronic devices to the output wiring, the output power wiring must be very close to minimize the area of the output current loop.

1.7 Separation of ground layers on the system board

The system board of a new generation of electronic products will have analog circuits, digital circuits, and switching power supply circuits at the same time. To reduce the effect of switching power supply noise on sensitive analog and digital circuits, it is often necessary to separate the ground planes of the different circuits. If a multi-layer PCB is used, the ground layers of different circuits can be separated by different PCB layers. If the entire product has only one ground layer, whether it is ground layer separation on a multi-layer PCB board or a ground layer separation on a single-layer PCB board, the ground layers of different circuits should be connected to the ground layer of the switching power supply through a single point. Seven different circuits on the system board require different ground planes, and the ground planes of different PCB board are connected to the power ground plane through a single point.

2. Example of PCB layout of switching power supply

The designer should be able to distinguish the components in the power circuit and the components in the control signal circuit on this circuit diagram. If the designer treats all the components in the power supply as if they were in a digital circuit, the problem is quite serious. Usually, it is first necessary to know the path of the high-frequency current of the power supply and to distinguish between the small-signal control circuit and the power circuit components and their traces. Generally speaking, the power circuit of the power supply mainly includes input filter capacitors, output filter capacitors, filter inductors, and upper and lower power FETs. The control circuit mainly includes a PWM control chip, a bypass capacitor, a bootstrap circuit, a feedback voltage divider resistor, and a feedback compensation circuit.

2.1 PCB layout of the power supply circuit

The correct placement and routing of power components on the PCB will determine whether the entire power supply works properly. Designers must first have a certain understanding of the voltage and current waveforms on switching power devices. Current and voltage waveforms of a step-down switching power supply power circuit component. Since the current flows from the input filter capacitor (Cin), the upper-end FET (S1), and the F-end FET (S2) an alternating current with high frequency and high peak values, the Cin-S1-S2 The loop area formed should be minimized. At the same time, the loop area formed by S2, L, and the output filter capacitor (Cout) should also be minimized. If the designer does not make the power circuit PCB according to the points described in this book, it is likely to make the power supply PCB shown in net 19. There are many errors in the layout of the PCB: because Cin has a large ESL, the high value of Cin The frequency filtering ability basically disappears; second, the area of the Cin-S1-S2 and S1-LCout loops is too large, and the generated electromagnetic noise will cause a great disturbance to the power supply itself and peripheral circuits; third, the L pad If it is too close, the Cp will be too large and its high-frequency filtering function will be reduced; fourth, the Cout pad lead is too long, causing the FSL to be too large and losing the high-frequency filtering line. The area of the Cin-S1-S2 and S2-L-Cout loops has been controlled. The connection point between the source of S1, the drain of S2, and L is a single piece of a copper pad. Since the voltage at this junction is high frequency, S1, S2 and L need to be very close together. Although there is no high peak high-frequency current on the trace between L and Cout, the wider trace can reduce the loss of DC impedance and improve the efficiency of the power supply. If the cost allows, the power supply can be used on a double-sided PCB with a ground plane on one side, but attention must be paid to avoiding power and signal lines on the ground plane. A ceramic capacitor is added to the input and output ports of the power supply to improve the high-frequency filtering performance of the power supply.

2.2 PCB layout of the power control circuit

The layout of the power control circuit PCB board is also very important. An unreasonable layout will cause drift and oscillation of the output voltage of the power supply. The control circuit should be placed on the side of the power circuit, not in the middle of the high-frequency AC loop. The bypass capacitor should be as close as possible to the Vcc and ground pins (GND) of the chip. Feedback divider resistors are also placed near the chip. The loop from the chip driving to the FET should also be shortened as much as possible, and the driving circuit loop from the control chip to the upper and lower FETs should be as short as possible.

2.3 Switching power supply PCB layout example 1

A low-cost PWM controller (Semtech model SCIIO4A) is used in this power supply. The lower layer of the PCB board is a complete ground plane. There is no separation between the power plane and the control plane on this PCB. It can be seen that the power circuit of the power supply consists of the input socket (the upper left end of the PCB board) through the input filter capacitors (C1, C2,), S1, S2, L1, and the output filter capacitors (C10, C11, C12, C13), until the output Socket (lower right end of PCB board). SC1104A is placed on the lower left end of the PCB board. Since the power circuit current does not pass through the control circuit on the ground plane, it is not necessary to separate the control circuit ground plane from the power circuit ground plane. If the input socket is placed on the lower left end of the PCB board, the power circuit current will directly pass through the control circuit on the ground plane, and it is necessary to separate the two.

2.4 Switching power supply PCB layout example 2

Another step-down switching power supply, the power supply can convert 12V input voltage into 3.3V output voltage, and the output current can reach 3A. An integrated power controller (Semtech model SC4519) is used on this power supply. This kind of controller integrates a power tube in the power controller chip. Such a power supply is very simple and is especially suitable for consumer electronic products such as portable DVD players, ADSL, and set-top boxes. As in the previous example, for this simple switching power supply, the following points should also be paid attention to in the layout of the PCB.

1) The loop area enclosed by the input filter capacitor (C3), the grounding pin (GND) of SC4519, and D2 must be small. This means that C3 and D2 must be very close to the SC4519.

2) Separate power circuit ground planes and control circuit ground planes may be used. The components connected to the power ground layer include input socket (VIN), output socket (VOUT), input filter capacitor (C3), and output filter capacitor (C2), D2, SC4519. The components connected to the control ground include output voltage divider resistors (R1, R2), feedback compensation circuits (R3, C4, C3,), enable sockets (EN), and synchronization sockets (SYNC).

3) Add a hole near the grounding pin of SC4519 to connect the grounding layer of the power circuit with the grounding layer of the control signal circuit at a single point. The layout diagram of the upper layer of the power supply PCB board. In order to facilitate the reader's understanding, the power ground plane and the control signal ground plane are represented by different colors. Here the input sockets are placed on the top of the PCB, and the output sockets are placed on the bottom of the PCB. The filter inductor (L1) is placed on the left side of the PCB board and close to the power ground plane, while the feedback compensation circuit (R3, C4, C5), which is more sensitive to noise, is placed on the right side of the PCB board and close to the control signal ground plane. D2 is very close to pins 3 and 4 of SC4519. The layout diagram of the lower layer of the power PCB board.