Useful Information | PCB layout design tips for switching power supplies - How to reduce EMI?

Latest update time：2021-03-16

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Switching power supply PCB layout is an important process in the development of power supply products. In many cases, a power supply designed perfectly on paper may not work properly during the first commissioning because there are many problems with the PCB layout of the power supply.

In order to adapt to the rapid pace of electronic product replacement, product design engineers prefer to choose AC/DC adapters that are easily available on the market, and install multiple sets of DC power supplies directly on the system's circuit board. Since the electromagnetic interference generated by the switching power supply will affect the normal operation of its electronic products, the correct power supply PCB layout becomes very important. The switching power supply PCB layout is completely different from the digital circuit PCB layout. In the digital circuit layout, many digital chips can be automatically arranged by PCB software, and the connection lines between chips can be automatically connected by PCB software. The switching power supply arranged by automatic layout will definitely not work properly. Therefore, designers 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 switching power supply PCB layout

1.1 Capacitor high frequency filtering characteristics

Figure 1 shows the basic structure of a capacitor and its high-frequency equivalent model.

The basic formula for capacitance is

Equation (1) shows that decreasing the distance between the capacitor plates (d) and increasing the cross-sectional area (A) of the plates will increase the capacitance of the capacitor.

Capacitors usually have two parasitic parameters: equivalent series resistance (ESR) and equivalent series inductance (ESL). Figure 2 shows the impedance (Zc) of a capacitor at different operating frequencies.

The resonant frequency (fo) of a capacitor can be obtained from its own capacitance (C) and equivalent series inductance (LESL), that is,

When a capacitor operates at a frequency below fo, its impedance decreases as the frequency increases, that is,

When the capacitor operates at a frequency above fo, its impedance increases with the frequency, that is,

When the capacitor operating frequency is close to fo, the capacitor impedance is equal to its equivalent series resistance (RESR).

Electrolytic capacitors generally have large capacitance and large equivalent series inductance. Since its resonant frequency is very low, it can only be used in low-frequency filtering. Tantalum capacitors generally have large capacitance and small equivalent series inductance, so its resonant frequency will be 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 its resonant frequency is much higher than that of electrolytic capacitors and tantalum capacitors, so it 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-capacitance ceramic capacitors, when selecting bypass capacitors, ceramic capacitors with too high capacitance values cannot be selected. In order to improve the high-frequency characteristics of the capacitor, multiple capacitors with different characteristics can be used in parallel. Figure 3 shows the effect of impedance improvement after multiple capacitors with different characteristics are connected in parallel.

1. Basic points of power supply layout 1: The capacitance of the bypass ceramic capacitor cannot be too large, and its parasitic series inductance should be as small as possible. Connecting multiple capacitors in parallel can improve the high-frequency impedance characteristics of the capacitor.

Figure 4 shows different routing methods from input power (Vin) to load (RL) on a PCB. To reduce the ESL of the filter capacitor (C), its lead length should be as short as possible; and the traces from the positive terminal of Vin to RL and the negative terminal of Vin to R1 should be as close as possible.

1.2 Inductor high-frequency filtering characteristics

The current loop in Figure 5 is similar to the inductance of a coil. The electromagnetic field R(t) generated by the high-frequency AC current will surround the outside and inside of this loop. If the high-frequency current loop area (Ac) is large, it will generate a lot of electromagnetic interference inside and outside the loop.

The basic formula for inductance is

From equation (5), we can see that reducing the loop area (Ac) and increasing the loop circumference (lm) can reduce L.

Inductors usually have two parasitic parameters: equivalent parallel resistance (EPR) and equivalent parallel capacitance (Cp). Figure 6 shows the impedance (ZL) of an inductor at different operating frequencies.

The resonant frequency (fo) can be obtained from the inductance of the inductor itself (L) and its equivalent parallel capacitance (Cp), that is,

When the operating frequency of an inductor is below fo, the inductor impedance increases with the increase of frequency, that is,

When the inductor operating frequency is above fo, the inductor impedance decreases as the frequency increases, that is,

When the inductor operating frequency is close to fo, the inductor impedance is equal to its equivalent parallel resistance (REPR).

In a 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 inductor with the same inductance will produce different Cp values due to different coil structures. Figure 7 shows the different Cp values of the inductor with the same inductance under two different coil structures. Figure 7 (a) The 5-turn winding of the inductor is wound in sequence. The Cp value of this coil structure is 1/5 of the equivalent parallel capacitance value (C) of the 1-turn coil. Figure 7 (b) The 5-turn winding of the inductor is wound in a cross sequence. Among them, windings 4 and 5 are placed between windings 1, 2, and 3, and windings 1 and 5 are very close. The Cp produced by this coil structure is twice the C value of the 1-turn coil.

It can be seen that the Cp values of two inductors with the same inductance differ by several times. 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 will lose its high-frequency filtering function.

Figure 8 shows different routing methods for Vin through L to load (RL) on a PCB. To reduce the Cp of the inductor, the two pins of the inductor should be as far away as possible. The routing from the positive electrode of Vin to RL and the negative electrode of Vin to RL should be as close as possible.

2. Basic points of power supply layout 2: The parasitic parallel capacitance of the inductor should be as small as possible, and the distance between the inductor pin pads should be as far as possible.

1.3 Mirror surface

The concept of mirror plane in electromagnetic theory will be of great help to designers in mastering the PCB layout of switching power supplies. Figure 9 shows the basic concept of mirror plane.

Figure 9(a) shows the situation when a DC current flows over a ground plane. At this time, the return DC current on the ground plane is very evenly distributed over the entire ground plane. Figure 9(h) shows the situation when a high-frequency current flows over the same ground plane. At this time, the return AC current on the ground plane can only flow in the middle of the ground plane, and there is no current on both sides of the ground plane. 1. Understanding the concept of mirror plane, we can easily see the problem of routing on the ground plane in Figure 10. Ground plane, technicians should try to avoid placing any power or signal routing on the ground plane. Once the routing on the ground plane destroys the entire high-frequency loop, the circuit will generate strong electromagnetic wave radiation and destroy the normal operation of surrounding electronic devices.

3. Basic points of power layout 3: Avoid placing any power or signal traces on the ground layer. Ensure the integrity of the ground layer.

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 to be very small. As shown in Figure 11(a), if the area of the high-frequency current loop is large, it 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, as shown in Figure 11(b), the electromagnetic fields inside and outside the loop cancel each other out, and the entire circuit will become very quiet.

4. Basic points of power supply layout 4: The area of high-frequency loops should be reduced as much as possible.

1.5 Via and Pad Placement

Many designers like to place a lot of vias (VIAS) on multilayer PCBs. However, it is necessary to avoid placing too many vias on the high-frequency current return path. Otherwise, the high-frequency current routing on the ground layer will be destroyed. If some vias must be placed on the high-frequency current path, a space can be left between the vias to allow the high-frequency current to pass smoothly. Figure 12 shows the via placement method.

5. Basic points of power supply layout 5: Via placement should not disrupt the flow of high-frequency current in the ground formation.

Designers should also note that different pad shapes will produce different series inductance. Figure 13 shows the series inductance values for several pad shapes.

The placement of the bypass capacitor (Decouple) must also take into account its series inductance value. The bypass capacitor must be a ceramic capacitor with low impedance and low ESL. However, if a high-quality ceramic capacitor is not placed correctly on the PCB, its high-frequency filtering function will disappear. Figure 14 shows the correct and incorrect placement of the bypass capacitor.

1.6 Power DC output

The load of many switching power supplies is far away from the output port of the power supply. In order to avoid electromagnetic interference generated by the power supply itself or surrounding electronic devices, the output power supply line must be very close as shown in Figure 15 (b) to minimize the area of the output current loop.

1.7 Layer separation on the system board

The new generation of electronic products will have analog circuits, digital circuits, and switching power supply circuits on the system board. In order to reduce the impact of switching power supply noise on sensitive analog and digital circuits, it is usually necessary to separate the ground planes of different circuits. If a multi-layer PCB is used, the ground planes of different circuits can be separated by different PCB layers. If the entire product has only one ground plane, it must be separated in a single layer as shown in Figure 16. Whether the ground plane is separated on a multi-layer PCB or on a single-layer PCB, the ground planes of different circuits should be connected to the ground plane of the switching power supply through a single point.

6. Basic points of power supply layout 6: Different circuits on the system board require different ground layers, and the ground layers of different circuits are connected to the power supply ground layer through a single point.

2. Switching power supply PCB layout example

Designers should be able to distinguish between components in the power circuit and components in the control signal circuit on this circuit diagram. If the designer treats all components in the power supply as components in the digital circuit, the problem will be quite serious. Usually, you need to know the path of the high-frequency current of the power supply first, and distinguish between the small signal control circuit and the power circuit components and their routing. Generally speaking, the power circuit of the power supply mainly includes input filter capacitors, output filter capacitors, filter inductors, and upper and lower power field effect transistors. The control circuit mainly includes PWM control chip, bypass capacitor, bootstrap circuit, feedback voltage divider resistor, and feedback compensation circuit.

2.1 Power circuit PCB layout

The correct placement and routing of power devices 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 the switching power device.

Figure 18 shows the current and voltage waveforms on the power circuit components of a step-down switching power supply. Since the current flowing through the input filter capacitor (Cin), the upper field effect transistor (S1) and the F-end field effect transistor (S2) is an AC current with a high frequency and a high peak value, the loop area formed by Cin-S1-S2 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 key points described in this article, it is very likely that the power supply PCB shown in Figure 19 will be produced. There are many errors in the PCB layout of Figure 19: First, since Cin has a large ESL, the high-frequency filtering ability of Cin is basically eliminated; second, the area of the Cin-S1-S2 and S1-LCout loops is too large, and the electromagnetic noise generated will cause great interference to the power supply itself and the surrounding circuits; third, the pads of L are too close, resulting in Cp being too large and reducing its high-frequency filtering function; fourth, the Cout pad lead is too long, resulting in FSL being too large and losing the high-frequency filtering line.

The area of the Cin-S1-S2 and S2-L-Cout loops has been minimized. The connection point between the source of S1, the drain of S2 and L is a whole copper pad. Since the voltage at this connection point is high frequency, S1, S2 and L need to be very close. Although there is no high-frequency current with a high peak value 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 use a double-sided PCB with one side completely grounded, but it must be noted that power and signal lines should be avoided as much as possible on the ground layer. A ceramic capacitor is also added to the input and output ports of the power supply to improve the high-frequency filtering performance of the power supply.

2.2 Power Control Circuit PCB Layout

The layout of the power control circuit PCB is also very important. Improper layout will cause drift and oscillation of the power output voltage. The control circuit should be placed on the edge of the power circuit and never in the middle of the high-frequency AC loop. The bypass capacitor should be as close to the Vcc and ground pins (GND) of the chip as possible. The feedback voltage divider resistor should also be placed near the chip. The loop from the chip drive to the field effect tube should also be shortened as much as possible.

7. Basic points of power supply layout 7: The driving circuit loop from the control chip to the upper and lower field effect transistors should be as short as possible.

2.3 Switching power supply PCB layout example 1

Figure 21 is the component wiring diagram of the PCB in Figure 17. A low-cost PWM controller (Semtech model SCIIO4A) is used in this power supply. The lower layer of the PCB is a complete ground layer. There is no separation between the power ground layer and the control ground layer of this PCB. It can be seen that the power circuit of this power supply is from the input socket (upper left end of the PCB) through the input filter capacitor (C1, C2,), S1, S2, L1, output filter capacitor (C10, C11, C12, C13), all the way to the output socket (lower right end of the PCB). SCll04A is placed at the lower left end of the PCB. Because the power circuit current does not pass through the control circuit on the ground layer, it is not necessary to separate the control circuit ground layer from the power circuit ground layer. If the input socket is placed at the lower left end of the PCB, then the power circuit current will directly pass through the control circuit on the ground layer, and it is necessary to separate the two.

2.4 Switching power supply PCB layout example 2

Figure 22 shows another step-down switching power supply, which can convert a 12V input voltage into a 3.3V output voltage with an output current of up to 3A. An integrated power controller (Semtech model SC4519) is used on this power supply. This controller integrates a power tube into the power controller chip. Such a power supply is very simple and is particularly suitable for use in 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 noted when PCB layout:

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

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

3) Add a via near the SC4519 ground pin to connect the power circuit ground layer to the control signal circuit ground layer in a single point manner.

Figure 23 is the layout of the upper PCB layer of the power supply. In order to facilitate the reader's understanding, the power ground layer and the control signal ground layer are represented by different colors. Here the input socket is placed on the upper side of the PCB, while the output socket is placed on the lower side of the PCB. The filter inductor (L1) is placed on the left side of the PCB and close to the power ground layer, while the feedback compensation circuit (R3, C4, C5) that is more sensitive to noise is placed on the right side of the PCB and close to the control signal ground layer. D2 is very close to pins 3 and 4 of SC4519. Figure 24 is the layout of the lower PCB layer of the power supply. The input filter capacitor (C3) is placed on the lower layer of the PCB and very close to SC4519 and the power ground layer.

2.5 Switching Power Supply PCB Layout Example 3

Finally, the PCB layout points of a multi-output switching power supply are discussed. This power supply has 3 sets of input voltages (12V, 5V and 3.3V) and 4 sets of output voltages (3.3v, 2.6V, 1.8V, 1.2V). This power supply uses an integrated multi-way switch controller (Serotech model SC2453). SC2453 provides a wide input voltage range of 4.5V to 30V, two synchronous buck converters with switching frequencies up to 700kHz and output currents up to 15A, and output voltages as low as 0.5V. It also provides a dedicated adjustable positive voltage linear regulator and a dedicated adjustable negative voltage linear regulator. The TSSOP-28 package reduces the required circuit board area.

Two out-of-phase buck converters can reduce input current ripple. Figure 25 is the schematic diagram of this multi-channel switching power supply. The 3.3V output is generated by the 5V input, the 1.2V output is generated by the 12V input, and the 2.6V and 1.8V outputs are generated by the 3.3V input. Since all components on this power supply must be placed on a small PCB area, the power ground layer and the control signal ground layer of the power supply must be separated. Referring to the key points discussed in the previous sections, first distinguish the components connected to the power ground layer and the components connected to the control signal ground layer in Figure 25, and then place the control signal components on the signal ground layer and connect them to the power ground layer close to the SC2453 control signal ground layer through a single point. This connection point is usually selected at the ground pin of the control chip (pin 21 in SC2453). Figure 26 describes the layout of this power supply in detail.

8. Basic points of power supply layout 8: The components under the switching power supply power circuit and control signal circuit need to be connected to different ground layers. These two ground layers are generally connected through a single point.

3. Conclusion

8 key points for switching power supply PCB 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. Connecting multiple capacitors in parallel can improve the impedance characteristics of the capacitor;

2. The parasitic parallel capacitance of the inductor should be as small as possible, and the distance between the inductor pin pads should be as far as possible;

3. Avoid placing any power or signal traces on the ground formation;

4. The area of the high-frequency loop should be reduced as much as possible;

5. The placement of vias should not destroy the path of high-frequency current on the formation;

6. A small circuit on the system board requires a different ground layer, and the ground layer of the small circuit is connected to the power ground layer through a single point;

7. The drive circuit loop from the control chip to the upper and lower field effect transistors should be as short as possible;

8. The switching power supply power circuit and control signal circuit components need to be connected to the same ground layer. These two ground layers are generally connected through a single point.

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