Introduction: Switching power supply is a power topology widely used in electronic circuits. Whether it is a complex CNC machine tool or a compact electronic device, as long as the device is connected to some kind of power supply, the switching power supply circuit is indispensable.
Regardless of the design and function of the circuit, an incorrect or faulty power supply unit may cause serious failure of the product.
This article will talk about PCB switching power supply design.
4 components of switching power supply circuit
First of all, to design a switching power supply circuit, you need to have clear circuit requirements and specifications. The power supply has 4 important parts:
1. Input and output filters
2. Driver circuits and related components for drives, especially control circuits.
3. Switching inductor or transformer
4. Output bridge and associated filters
The input and filter section is where the noisy or unregulated power line connects to the circuit.
Therefore, the input filter capacitor needs to be evenly spaced from the input connector and the driver circuit, and a short connection length must always be used to connect the input section to the driver circuit.
The highlighted area in the above figure shows the close placement of the filter capacitors
Driver circuits and related components for drives, especially control circuits.
The driver mainly consists of internal MOSFETs and sometimes external switching MOSFETs. The switching line always switches on and off at a very high frequency and creates a very noisy power line. This part always needs to be separated from all other connections.
For example, the high-voltage DC line directly connected to the transformer (for a flyback SMPS) or the DC line directly connected to the power inductor (for a switching regulator based on a buck or boost topology) should be separated.
In the figure below, the highlighted signal is the high voltage DC line and the signal is routed in a separate manner from other signals.
The highlighted signal is the high voltage DC line, which is routed separately from other signals.
One of the noisiest lines in a switch mode power supply design is the drain pin of the driver, whether it is an AC to DC flyback design, or a low power switch mode power supply design based on a buck, boost or buck-boost topology. It always needs to be separated from all other connections and needs to be very short, as this type of routing often carries very high frequency signals. The best way to isolate this signal line from the others is to use a PCB cutout, either through milling or dimensional layers.
In the image below, the isolated drain pin connection is shown at a safe distance from the optocoupler, and the PCB cutout will eliminate any interference from other routing or signals.
Isolated drain pin connection at a safe distance from the optocoupler
Another important point is that the driver circuit almost always has very sensitive feedback or sense lines (sometimes more than one, such as input voltage sense line, output sense line) and the driver operation is completely dependent on sensing feedback. The length of any type of feedback or sense line should be shorter to avoid noise coupling. These types of lines always need to be separated from power, switching or any other noisy lines.
The following diagram shows the individual feedback lines from the optocoupler to the driver.
Separate feedback line from optocoupler to driver
Not only that, the driver circuit may also have multiple types of components such as capacitors, RC filters required to control the operation of the driver circuit. These components need to be placed close to the driver.
Switching Inductor or Transformer
The switching inductor is the largest available component in any power board, second only to the bulk capacitor. A poor design is to route any type of connection between the inductor leads. It is important not to route any signals between the power or filter inductor pads.
Electronic components pictures
Also, whenever a transformer is used in a power supply, especially in an AC-DC SMPS, the main purpose of the transformer is to isolate the input from the output. Sufficient distance is required between the primary and secondary pads. One of the best ways to increase creepage distance is to apply PCB cutoffs using a milling layer. Never use any type of wiring between the transformer leads.
PCB cut between switch transformers
Output bridge and associated filters
The output bridge is a high current Schottky diode that dissipates heat according to the load current. In a few cases, it is necessary to create a PCB heat sink using copper planes in the PCB itself. The heat sink efficiency is proportional to the PCB copper area and thickness.
There are two commonly used copper thicknesses in PCBs, 35 microns and 70 microns. The higher the thickness, the better the thermal connectivity and the smaller the PCB heat sink area. If the PCB is double-layered and some heating space in the PCB is not available, both sides of the copper plane can be used and a common via can be used to connect the two sides.
The following image is an example of a PCB heat sink for a Schottky diode created on the bottom layer.
Example of a PCB heat sink for a Schottky diode created at the bottom layer
The filter capacitor immediately after the Schottky diode needs to be placed very close to the transformer or switch inductor to make the power loop through the inductor, bridge diode and capacitor very short. In this way, the output ripple can be reduced.
The following figure is an example of a short loop from the transformer output to the bridge diode and filter capacitor.
Example of a short loop from transformer output to bridge diode and filter capacitor
Switching Power Supply Circuit PCB Design Layout Tips
There are some basic switching power supply circuit PCB layout rules that need to be followed to ensure that the PCB design has low noise, low radiated EMI and remains cool.
Specifically, there are the following points:
Try to keep EMI low by properly defining grounds, placing short-circuit paths in the PCB layout, and placing electrical isolation in the PCB to reduce noise coupling.
If noise is present in the layout, features such as envelope tracking are required, or if a particular noise is causing problems in the design, use appropriate input and output EMI filter circuits where necessary.
Use plenty of copper to provide a path for heat to escape from important components, and if needed, you can consider unique case designs, as well as heat sinks or fans over hot components.
Place fast switching, high current circuits, such as MOSFET arrays, to prevent parasitic oscillations during switching power supply design.
Be careful when defining the ground
The first switch mode power supply PCB layout guideline to consider is how the grounds are defined in the layout. When designing a switching power supply circuit, remember that there are five ground points. These can be separated into different conductors to ensure galvanic isolation. These are:
Input large current source ground
Input high current current loop grounding
Output high current rectifier ground
Output high current load ground
Each of these ground connections may exist on physically separate conductors, depending on the need for galvanic isolation in the converter, rectifier, or regulator circuit.
Your power circuits may pick up common-mode noise if ground is capacitively coupled, such as would typically occur through a nearby conductive enclosure. Ground areas in the PCB should be clearly defined on each side of the isolated component, for example:
If for some reason you do need to bridge the grounds to remove some DC offset, a Y-class capacitor is the best choice because it provides high frequency filtering and removes DC offsets between ground planes.
In some switching converter applications, Y-stage capacitors can be used to bridge the grounds.
In some switching converter applications, Y capacitors can be used to bridge the ground
Each high current ground is used as a branch of the current return path, but it should be arranged to provide a low impedance return path for the current. This may require multiple via return ground planes to allow high current with low equivalent inductance.
These points and their potential relative to the system ground become the points for measuring DC and AC signals conducted between different points in the circuit. Because of the need to prevent noise overflow from the high-current AC ground, the negative terminal of an appropriate filter capacitor serves as the connection point for the high-current ground.
The best practice for defining ground areas is to use large planes or polygonal pours. These areas provide a low impedance path to dissipate noise from the DC output, and they can handle high return currents. They also provide a path for heat transfer away from critical components when needed.
Placing ground planes on both sides absorbs radiated EMI, reduces noise, and reduces ground loop errors. While acting as an electrostatic shield and dissipating radiated EMI in eddy currents, the ground plane also separates the power traces and components on the power layer from the signal layer components.
Ground areas in a design can be given multiple names depending on their function. Be careful when defining ground areas in your design and make sure they are connected together correctly.
The ground plane is also important in the system outside of the power PCB layout. Make sure the connection is defined to have low impedance without affecting the assembly.
Common mode noise and conducted ripple are the main noise sources in PCB layout, and when the noise is extreme, they can cause the design to fail EMI testing.
The power and ground planes provide a low impedance connection while providing a path for heat dissipation away from critical parts of the system.
Power and ground planes provide low impedance connections while providing a heat dissipation path away from critical parts of the system
The power and ground planes provide a low impedance connection while providing a path for heat dissipation away from critical parts of the system.
Reducing Ground Bounce in Switching Power Supply PCB Layout
First of all, ground fill is essential, separating different ground planes in the power circuit is another most important thing.
From a circuit perspective, the switching power supply can provide a common ground for all components, but this is not the case during the PCB design stage.
From the perspective of PCB design, the ground is divided into two parts. The first part is the power ground, and the second part is the analog or control ground. These two grounds have the same connection but are very different. Components related to the driving circuit use analog or control ground. These components use a ground plane that creates a low current return path.
On the other hand, the power ground carries a high current return path. Power components are very noisy and can cause undefined ground bounce issues in the control circuits if they are directly connected to the same ground line. The following figure shows how the analog and control circuits can be completely isolated from the other power lines of the PCB in a single-layer PCB.
Analog and control circuits are completely isolated from other power lines
These two parts need to be separated and should be connected in a specific area.
If the PCB is dual layered, this is easy, like the top layer can be used as control ground and all the control circuits should be connected in the common ground plane of the top layer. On the other hand, the bottom layer can be used as power ground and all the noisy components should use this ground plane. But these two grounds are the same connection and connected in the schematic. Now, to connect the top and bottom layers, vias can be used to connect both ground planes at one place. For example, see the image below:
Use vias to connect the two ground planes in one place
The upper portion of the driver has all the power filter related capacitors they use a separate ground plane called power GND, but the lower portion of the driver IC is all the control related components which use a separate control GND. Both grounds are the same connection but created separately. The two GND connections are then connected through the driver IC.
A switching power supply circuit operates by rapidly switching a pass cell between a cutoff operating state and a saturation operating state and providing constant power to an output load.
In cutoff, a high voltage is present across the pass cell, but no current flows. In saturation, a high current flows through the pass cell with very little voltage drop. Because semiconductor switches generate an AC voltage from a DC input voltage, a switching power supply circuit can step up or down the voltage via a transformer and then filter the voltage back to DC at the output.
Pulse Width Modulation (PWM) switching power supplies can operate in forward mode or boost mode. Forward mode power supplies have an LC filter at the output, which produces a DC output voltage based on the voltage time average value of the output obtained from the filter. To control the voltage time average value of the signal, the switching power supply controller changes the duty cycle of the input rectangular voltage.
Buck Conversion vs. Boost Conversion
The boost converter mode power supply connects an inductor directly across the input voltage source when the power switch is turned on. The inductor current increases from zero and reaches its peak value at the same time as the power switch is turned off. The output rectifier clamps the inductor output voltage and prevents the voltage from exceeding the power supply output voltage. When the energy stored in the inductor core is transferred to the output capacitor, the switching end of the inductor drops back to the level of the input voltage.
Meanwhile, a buck converter mode power supply uses the same components but employs a different topology to clamp the inductor's back EMF to a level below the input voltage. The switching action provides the same effect as a boost converter, where the output current oscillates in competition with the charging/discharging capacitor, enabling output power regulation.
Both types of regulator/converter topologies allow switching noise to propagate to the output port of the design, which can be seen as high-frequency ripple on the output.
Buck and boost converter layouts can carry high currents requiring large polygons to contain the heat and prevent power losses.
Buck and Boost Converter Layout
Power supply routing helps ensure low noise operation
Switching power supplies conduct high-frequency noise until the noise frequency reaches about 100 times the switching frequency. The noise frequency then decreases at a rate of -20 to -40 dB per decade. Because switching regulators operate in the "on" and "off" power states, large current pulses with sharp edges flow in switching power supply circuits, generating EMI.
Transitions between ON and OFF power states generate EMI, which can be induced elsewhere in the system if the current loops in the power layout are too large. A switching power supply circuit consists of a power switch loop and an output rectifier loop, which need to be properly routed to prevent excessive noise.
When laying out the power supply, pay special attention to the loop perimeter and the length and width of the traces. Keeping the loop perimeter small eliminates the possibility of the loop acting as an antenna for low-frequency noise. From a circuit efficiency perspective, wider traces also provide additional heat sinking for the power switches and rectifiers.
You can use the active routing routing engine to implement the artificial routing results and arrange the components to allow the switching current loops to proceed in the same direction. Since the current loops conduct in the same direction, the control circuit is coupled to a specific point in the layout. Therefore, the magnetic field cannot reverse along the trace located between the two half-cycles and generate radiated EMI.
When using power supply layout, keep traces that handle high switching currents short, straight, and thick. IPC standards can be used to calculate recommended trace widths, but a rule of thumb is a minimum width of 15 mils per ampere.
EMI filters in switching power supply circuits suppress high-frequency noise caused by high-frequency currents conducted in the DC input and output wiring.
The components in the PCB layout below are closely connected and routed using short and direct traces.
PCB Layout
The components in this PCB layout are closely connected and routed using short and direct traces.
Reference source:
https://circuitdigest.com/article/+https://resources.altium.com/p/
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