Switching power supply PCB layout, basic points of analysis

1 Basic points of PCB layout of switching power supply

1.1 Capacitance high Frequency filter characteristics

Figure 1 is the basic structure of the capacitor and the high-frequency equivalent model.

The basic formula for capacitance is

Formula (1) shows that reducing the distance between the capacitor plates (d) and increasing the cross-sectional area (A) of the plates increases the capacitance of the capacitor.

Capacitance typically has two parasitic parameters equivalent series resistance (ESR) and equivalent series inductance (ESL). Figure 2 is the impedance (Zc) of the capacitor at different operating frequencies.

The resonant frequency (FO) of a capacitor can be obtained from its own capacitance (C) and the equivalent series inductance (LESL), i.e.

When a capacitor is operating at a frequency below the FO, its impedance decreases with increasing frequency, i.e.

When the capacitor is operating at more than FO, its impedance increases with frequency, i.e.

When the capacitor operating frequency is close to FO, the capacitance impedance equals its equivalent series resistance (RESR).

Electrolytic capacitors generally have a large capacitance and a large equivalent series inductance. Because of its low resonant frequency, it can only be used on low-frequency filtering. Tantalum capacitors generally have a larger capacitance and a smaller equivalent series inductance, so its resonant frequency is higher than the electrolytic capacitor, and can be used in high-frequency filtering. porcelain capacitor capacitance and the equivalent series inductance is generally small, so its resonant frequency is much higher than electrolytic capacitors and tantalum capacitors, so can be used in high-frequency filter and bypass circuit. Because the resonant frequency of the capacitor is higher than that of the large capacitance porcelain capacitor, therefore, the capacitor value of the ceramic chip capacitor is not chosen when the bypass capacitor is selected. In order to improve the high frequency characteristics of the capacitor, a number of different characteristics of the capacitor can be used in parallel. Figure 3 is a number of different characteristics of capacitors in parallel after the effect of impedance improvement

Power typesetting basic points 1 the capacitance of the bypass porcelain capacitor can 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.

Figure 4 shows the different traces of the input power (Vin) to the load (RL) on a single PCB. In order to reduce the ESL of the filter Capacitor (C), the lead length should be shortened as much as possible, while Vin. The positive to RL and VIN negative to R1 should be as close as possible to the route.

1.2 Inductance high frequency filter characteristic

The current loop in Figure 5 resembles the inductance of a one-turn 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, there will be a large electromagnetic interference in the inner and outer parts of this loop.

The basic formula for inductance is

From the formula (5), reduce the area of the loop (Ac) and increase the loop circumference (LM) can be reduced L.

Inductance usually has two parasitic parameters of equivalent shunt resistance (EPR) and equivalent shunt capacitance (Cp). Figure 6 is the impedance (ZL) of the inductor at different operating frequencies.

The resonant frequency (FO) can be obtained from the inductance Self inductance value (L) and its equivalent shunt capacitance value (Cp), i.e.

When an inductor is operating at a frequency below the FO, the inductance impedance increases with frequency, i.e.

When the inductor operating frequency is above FO, the inductance impedance decreases with the increase of frequency, i.e.

When the inductor operating frequency is close to FO, the inductance impedance equals its equivalent shunt resistor (repr).

The CP of the inductor in the switching power supply should be controlled as small as possible. It must also be noted that the inductance of the same inductor will produce different CP values due to the different coil structure. Figure 7 shows the inductance of the same inductor in two different coil structures under different CP values. Figure 7 (a) The 5-turn windings of the inductor are wound sequentially. The CP value of this coil structure is 1/5 of the equivalent shunt capacitance value (C) of the L-turns coil. Figure 7 (b) The 5-turn windings of the inductor are wound in a cross-order. wherein the windings 4 and 5 are placed between the windings 1, 2, 3, while the windings L and 5 are very close. The CP of this coil structure is twice times the value of the 1-turn coil C.

As you can see, the CP values of the 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 is easily coupled directly to the load via the CP. This inductance also loses its high-frequency filtering function.

Figure 8 shows the different traces of Vin via L to load (RL) on a PCB. To reduce the inductance of the CP, the inductance of the two pins should be as far away from. And the line of VIN positive to RL and VIN negative to RL should be as close as possible.

Power typesetting basic points 2 the parasitic shunt capacitance of inductance should be as small as possible, the distance between the inductor pin pads is better.

1.3 Mirrored faces

The concept of mirror surface in electromagnetic theory is very helpful for designers to master the PCB layout of switching power supply. Figure 9 is the basic concept of a mirrored surface.

Figure 9 (a) is a scenario where the DC current flows over a ground layer. At this point the return DC current on the formation is very evenly distributed across the ground level. Figure 9 (h) shows the scenario when the high-frequency current flows over the same stratum. At this point, the return AC current on the stratum can only flow in the middle of the ground plane and on both sides of the ground level there is no current at all. A day. Understanding the mirror surface concept, it is easy to see the problem of line on the ground level in Figure 10. Ground plane (Ground Plane), no juice personnel should try to avoid placing any power or signal line on the stratum. Once the formation of the line to destroy the entire high-frequency loop, the circuit will produce strong electromagnetic radiation of the ox and destroy the normal work of the peripheral electronic devices.

Power typesetting basic points 3 avoid placing any power or signal lines on the stratum.

1.4 HF Loop

Switching power supply In a number of high-frequency circuit composed of power devices, if the ring is not good, it will be the power of the normal work of a great impact. In order to reduce the electromagnetic noise generated by the high frequency loop, the area of the loop should be controlled very little. As shown in L1 (a), high-frequency current loop area is very large, it will be in the inner and outer loop to generate a strong electromagnetic disturbance. The same high-frequency current, when the loop area is designed to be very small, as shown in one (b), the loop inside and outside the electromagnetic field offset each other, the entire circuit will become very quiet.

Power Typesetting Essentials 4 The area of the HF loop should be minimized.

1.5 over hole and pad placement

Many designers like to place many Vias (VIAS) on multi-layer PCBs. However, you must avoid placing too much on the high-frequency current return path. Otherwise, the high-frequency current line on the formation will be destroyed. If some vias must be placed on the high-frequency current path, a space between the vias can be set aside to allow the high-frequency current to pass smoothly, figure 12 shows how the hole is placed.

Power typesetting basic points 5 the hole placement should not destroy the high frequency current flowing through the stratum.

Designers should also pay attention to the different shape of the pads will produce different series inductance. Figure 13 shows the series inductance values for several pad shapes.

The bypass capacitor (decouple) is placed to take into account its series inductance value. The bypass capacitor must be a low-impedance and low-ESL planchette ceramic chip capacitor. But if a high-quality ceramic capacitor is placed on the PCB in the wrong way, its high-frequency filtering function disappears. Figure 14 shows the correct and incorrect placement of the bypass capacitor.

1.6 Power supply DC output

Many switching power supplies are loaded away from the power supply's output port. In order to avoid the output line by the power itself or the surrounding electronic devices generated by the electromagnetic disturbance, the output power line must be as close as Figure L5 (b), so that the output current loop area as far as possible to reduce.

Separation of L.7 strata on the system board

The next generation of electronic products system board Seven will also have analog circuit, digital circuit, switching power supply circuit. In order to reduce the impact of power-supply noise on sensitive analog and digital circuits, it is often necessary to separate the ground layers of different circuits. If multilayer PCB is used, the ground layer of different circuit can be separated by different PCB layer. If the entire product has only one layer of ground layer, it must be separated in a single layer as in Figure 16. Whether the stratigraphic separation on the multilayer PCB or the single-layer PCB on the formation of separation, the different layers of the circuit should be a single point and the switching power supply of the ground layer connected.

Power Typesetting Essentials 6 system board seven different circuits require different ground layers, and the ground layer of different circuits is connected by a single point to the power ground layer.

2 switching power supply PCB Layout Example

Pressure switch power supply schematic diagram. The staff 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 a component in a digital circuit, the problem can be quite serious. It is usually necessary to know the path of the high frequency current of the power supply, and distinguish the small signal control circuit and power circuit components and their routes. Generally speaking, power supply circuit mainly includes input filter capacitance, output filter capacitance, filter inductor, Upper and lower end power field effect tube. The control circuit mainly includes PWM control chip, bypass capacitance, bootstrap circuit, feedback voltage divider resistor and feedback compensation circuit.

2.L Power circuit PCB Layout

The correct placement and trace of the power supply device on the PCB will determine whether the entire power supply is working properly. The designer first has to have a certain understanding of the voltage and current waveform on the power device of the switching power supply.

Figure 18 shows the current and voltage waveforms for a buck switch power circuit component. Since the input filter capacitance (Cin), the upper field effect Tube (S1) and F-end FET (S2) flow through the current is a high frequency and peak value of AC current, so the loop area formed by the cin-s1-s2 to minimize. The loop area composed of s2,l and output filter capacitors (Cout) is also minimized.

If the juice is not in accordance with the main points described in the production of power circuit PCB, it is possible to make a network 19 of the power PCB, figure 19 of the PCB layout there are many errors: first, because CIN has a large esl,cin of high-frequency filtering ability basically disappeared; Cin-s1-s2 and S1-lcout Loop area is too large, the electromagnetic noise generated by the power itself and the surrounding circuit caused much greater than disturbance; Thirdly, the pad of L is too close, causing the CP to be too large to reduce its high-frequency filtering function; IV, the cout solder disc leads are too long, caused the FSL to be too large and lost the high-frequency filter line. The area of the CIN-S1-S2 and S2-l-cout loops is controlled to a minimum. The source of the S1, the connection point of the S2 drain and L is a whole piece of copper plate pad. Because the voltage on the connection point is high frequency, S1, S2, and L need to be very close. Although there is no high-frequency current on the line between L and cout, a relatively wide path can reduce the loss of DC impedance and improve the efficiency of the power supply. If the cost is allowed, the power supply can be fully grounded on both sides of the PCB, but it must be noted in the formation of the best to avoid the power and signal lines. A ceramic chip 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 power control circuit PCB layout is also very important. Unreasonable typesetting can cause the power output voltage to drift and oscillate. The control line should be placed on the edge of the power circuit and must not be placed in the middle of the high frequency AC loop. The bypass capacitor should be as close as possible to the chip's VCC and Ground foot (GND). Feedback voltage divider resistors are best placed near the chip. The loop that drives the chip to the FET also needs to be reduced as short as possible.

Power typesetting basic points 7 control chip to the upper and lower field effect tube drive circuit to be as short as possible.

2.3 Switching power supply PCB Layout Example 1

Figure 21 is the figure of the PCB component surface trace. A low-cost PWM controller (Semtech model SCIIO4A) is used in this power supply. The lower layer of the PCB is a complete ground plane. There is no separation between this PCB power stratum and the controlled strata. You can see that the power circuit of the supply is provided by the input socket (upper left of the PCB) through the input filter capacitor (C1,C2,), S1,S2,L1, Output filter Capacitor (c10,c11,c12,c13), until the output socket (PCB right bottom). The scll04a is placed on the lower left side of the PCB. Because the power circuit current does not pass through the control circuit in the stratum, it is unnecessary to separate the ground layer of the control circuit from the ground layer of the power circuit. If the input socket is placed on the left end of the PCB, then the power circuit current in the stratum will pass through the control circuit directly, it is necessary to separate the two.

2.4 Switching power supply PCB Layout Example 2

Figure 22 is another buck switching power supply that enables the 12V input voltage to be converted to a 3.3V output voltage with an output current 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 supply controller chip. Such a power supply is very simple, especially suitable for use in portable DVD, ADSL, set-top boxes and other consumer electronics products.

As in the previous example, for this simple switching power supply, PCB layout should also pay attention to the following points.

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

2) The separated power circuit grounding layer and control circuit can be used to connect the stratum. Components connected to the power stratum include input socket (VIN), Output socket (VOUT), Input filter capacitance (C3), Output filter capacitance (C2), d2,sc4519. Components connected to the control formations include output voltage divider resistors (R1,R2), Feedback compensation circuitry (R3,C4,C3,), enable sockets (EN), and synchronous sockets (sync).

3) Add a hole near the SC4519 to connect the Power Circuit ground layer with the single point of the control signal Circuit ground layer.

Figure 23 is the upper row layout of the power PCB. For the sake of the reader's understanding, the power grounding layer and the control signal ground are represented by different colors respectively. Here the input socket is placed above the PCB, and the output socket is placed below the PCB. The filter inductor (L1) is placed on the left side of the PCB and near the power ground layer, while the noise-sensitive feedback compensation circuit (R3,C4,C5) is placed on the right side of the PCB and adjacent to the control signal ground. D2 very close to SC4519 's foot 3 and foot 4. Figure 24 is the lower row layout of the power PCB. The input filter capacitance (C3) is placed on the lower layer of the PCB and is very close to the SC4519 and power-connected formations.

2.5 Switching power supply PCB Layout Example 3

Finally, a multi-output switching power supply PCB layout points are discussed. This power supply has 3 sets of input voltages (12v,5v and 3.3V) and 4 output voltages (3.3V,2.6V,1.8V,1.2V). The power supply is used with an integrated multi-switch controller (Serotech model SC2453). The SC2453 offers a wide input voltage range of 4.5v~30v, two simultaneous buck converters up to 700kHz switching frequency and up to 15A output current, as well as low to 0.5V output voltages. It also provides a dedicated, configurable, positive-pressure linear regulator and a dedicated, adjustable negative-pressure linear regulator. The TSSOP-28 package reduces the required board area. The input current ripple can be reduced by two heterogeneous buck converters. Figure 25 is a schematic diagram of this multi-switch power supply. 3.3V output is generated by 5V, l.2v output is generated by 12V input, 2.6V and 1.8V output by 3.3V input. Since all components on the power supply must be placed on a smaller PCB, the power formation and the control signal strata must be separated. Referring to the points discussed in the previous sections, first the components connected to the power strata in Figure 25 are distinguished from the components connected to the control signal strata, and then the control signal components are placed on the signal strata and close to the SC2453 control signal formation and the power strata through a single point connection. This connection point is typically selected on the control chip's ground foot (SC2453 in the Foot 21). Figure 26 describes in detail the mode of the power supply layout.

Power Typesetting Essentials 8 switching power supply and control signal circuit components need to be connected to different ground planes, these two formations are generally connected by a single point.

3 Conclusion

8 key points for switching power supply PCB layout:

1) bypass ceramic capacitor capacitor can not be too large, and its parasitic series inductance should be as small as possible, multiple capacitors in parallel can improve the impedance characteristics of the capacitance;

2) Inductance of the parasitic shunt capacitance should be as small as possible, the distance between the inductor pin pads is as far as the better;

3) Avoid placing any power or signal line on the stratum;

4) The area of the high-frequency loop should be minimized;

5) The path of the high frequency current in the stratum should be destroyed by the small hole placement;

6) A small circuit on the system board requires different ground layer, the ground layer of the small same circuit is connected with the power supply ground layer by a single point;

7) The driving circuit of the control chip to the upper and lower end FET should be as short as possible;

8) switching power supply power circuit and control signal circuit components need to connect to the small same ground layer, these two strata are generally connected by a single point

Transferred from Wolong public number

Switching power supply PCB layout, basic points of analysis