Introduction to Power Supply PCB Design Guidelines
Recently, the direction of development and production of switching power supplies based on powerful integrated circuits has been actively developing in the world.
The power of these power supplies ranges from hundreds of milliwatts to hundreds of watts. Power supply PCB design guidelines specialists have developed a number of families of such microcircuits.
This makes it possible to build switching power supplies with an input supply voltage in the range from 16 to 400 V.
The advantages of PI chips are as follows:
- Eliminate up to 50 external discrete components, significantly reducing system cost.
- They contain on one chip a high-voltage field-effect transistor, a controller, a soft-start starting circuit, remote control, programmable current limiting, protection against overload, against low and high input supply voltage, and against overheating of the crystal.
- In standby mode, they provide EcoSmart mode, which significantly reduces consumption from the mains.
- Reduce development and production time.
- Increase the manufacturability of manufacturing.
Why Power Supply PCB Design Guidelines are Critical to Know?
Power supply pcb design guidelines is critical to the design of a compact, high-performance power supply.
Each case should be considered based on the requirements of a specific real-world problem.
The use of complete “proven” boards as part of the device significantly accelerates the development and implementation of one or another device node.
National Semiconductor’s demo and test boards are a good choice and proven solution for evaluating the capabilities of LMZ modules.
All boards are optimized for maximum performance, have the smallest footprint and are easy to use, have a minimum of external passive elements located on both sides of the PCB, and have low output voltage ripple that can be absorbed by installing additional ceramic capacitors.
When self-developed, the manufacturer’s declared electromagnetic and thermal performance characteristics of the mod-
LMZ power supply lines can be obtained using 4-layer PCBs with 1 oz pads, although 2-layer PCBs are sufficient, but thicker layers. Four-layer boards with thicker layers are nevertheless preferred because the heat is more evenly distributed between the layers and the board will be smaller.
To help developers, professional Power supply pcb design guidelines provide the use of ready-made Reference Design computer models, made in the Altium CAD system, or use the WEBENCH Power Designer design systems on the National Semiconductor website.
With these additional development tools and professional Power supply pcb design guidelines, the creation of a high quality power supply can be accomplished in an extremely short time.
Using PI Expert Suite 5.0 for Power Supply PCB Design Guidelines
In fig. 1 shows a typical schematic diagram of a switching power supply based on the TOPSwitch-GX microcircuit.
Figure: 1. Power supply PCB design
According to the Power supply pcb design guidelines, switching power supply circuitry with an output power of up to 70 W and a 50 Hz AC supply with an input voltage range of 85 to 265 V has overvoltage and undervoltage protection and an external maximum output power limitation.
All of the above protections are set by means of external resistors: the overvoltage protection operation threshold is set by the R9 resistor, the lower input voltage threshold is set by the R11 resistor, and the maximum output power level is set by the R10 resistor.
If desired, all of them can be turned off by directly connecting the L and X pins to the minus of the microcircuit (pin S).
This circuit is an example of designing a switching power supply, where the highest efficiency was the optimization criterion.
With such an output power, it was possible to choose a less powerful TOP246Y microcircuit, but this would lead to an increase in the power losses in the key MOS transistor of the microcircuit.
That is why, in this case, one of the most powerful microcircuits of this family, TOP249Y, was chosen.
In addition, to reduce the losses in the output rectifier, two 20-amp diodes were chosen, operating in parallel for a total load of only 3.6 A.
To Build a Similar Wwitching Power Supply, You Need to Make the Following Calculations:
- Select U1 chip according to the maximum output power and input supply voltage.
- Calculate the values of resistors R9, R10 and R11, as well as R4, R5, R6.
- Calculate the value of the input capacitance of the low-pass filter C1.
- Calculate the output high-pass filter C2, C3, C14 and L1.
- Select the type of core and calculate the size of the air gap and the number of turns in all windings of the power transformer.
- Determine the parameters of the output rectifier diode.
- Determine and calculate the ratings of the elements, the high-voltage surge voltage limiting circuit at terminal D of the U1 microcircuit.
Nearly all of these calculations can be performed using PI Expert Suite 5.0, developed by PI.
In fig. Figure 2 shows the appearance of the PI Expert Suite 5.0 working window.
Figure: 2.the appearance of the PI Expert Suite 5.0 working window
In order to open a new project, you must press the NEW button, which is located in the upper left corner.
A window will appear on the screen, as shown in Fig. 3. It offers to enter the parameters of the input voltage, from which the projected power supply should operate.
Figure: 3. parameters of the input voltage
There are several typical supply voltages to choose from. You can choose standard ranges of input supply voltages, or set your own range.
To do this, you must first select User Defined in the desired voltage range (AC Defaults – alternating current, HV DC – high voltage, direct current; LV DC – low voltage, direct current), and then set the minimum and maximum input voltage (Voltage, V) … You can also set the frequency of the power supply (Line Frequency, Hz), which is usually 50 Hz (standard household), 400 Hz or 1 kHz.
After that, click the “Next” button to go to the next window ( Fig. 4 ).
Figure: 4.the next window
Here you are prompted to enter the parameters of the output voltage and current of your power supply. To do this, press the “Add” button and fill in the “Voltage, V” columns – the required output voltage and “Current, A” – the maximum output current. Then click “OK”.
Voltages and currents can be entered for multiple output channels as required. Below in the “Total Power, W” column you will see the total output power.
If you have entered the voltage or current incorrectly, or decided to delete one or several channels altogether, you can use the “Remove” buttons to delete the selected channel and “Edit” to change the parameters.
After that, click the “Next” button to go to the next window (Fig. 5).
In the window that opens, you are asked to specify the following items:
Topology – voltage converter architecture:
a) Flyback – flyback architecture. It is the cheapest solution for output currents <6 A. Advantages: Simple circuit (no choke required to store energy in the output circuit). Disadvantages: higher output ripple current (higher cost of output capacitors).
b) Forward – forward thrust architecture. It is the cheapest solution for output currents> 6 A. Advantages: lower output ripple current (lower cost of output capacitors). Disadvantages: more complex circuit (requires a choke to store energy in the output circuit).
Family – A family of microcircuits.
a) DPA-Switch – DC-DC voltage converter 24/48 V with power up to 100 W;
b) LinkSwitch-TN – AC-DC voltage converter of very low power (IOUT <360 mA);
c) LinkSwitch – AC-DC voltage converter of very low power (up to 4 W);
d) TinySwitch and TinySwitch-II – low power AC-DC voltage converter (up to 23 W);
f) TOPSwitch-GX – high power AC-DC voltage converter (up to 290 W).
Package – type of microcircuit case:
P – Plastic DIP.
G – DIP for surface mounting.
Y – TO-220.
R – TO-263.
F – TO-262.
Frequency – fixed switching frequency (in kHz).
Opti.Type – select the direction in which the circuit will be optimized (only for the TOP family of microcircuits). This means that the optimization will be based on the minimum cost of the power supply or the maximum efficiency.
Alternatively, a synchronous rectifier (Synchronous Rectification) can be used. However, this is only possible when operating with a low input supply voltage and when using DPA microcircuits).
Then click “Next” to go to the next window (Fig. 6).
Here it is proposed to give a name to the project (New Design File Name); if necessary, enter the width of the margin of the power transformer frame window (Safety Margin); enter the region where you are (Region); system of parameters (SI Units) and manual start of the optimizer (Manual Start Point). Make all the settings and click the “Finish” button: the program will automatically calculate the power supply and show a window with the calculation result (Fig. 7).
Figure: 7. calculation resul
To view the file with the calculatiotn result (the version of the file and report is given below), you need to click the “Design Result” tab.
The main parameters required for designing a switching power supply are highlighted in yellow.
In addition, you can use the block diagram (“Block Diagram” tab), which shows the structural electrical diagram of the power supply.
You can make an automatic selection of the most suitable standard size of the power transformer (by pressing the “Auto Pick” button), or manual (by placing the cursor on any power transformer from the offered range).
With the power transformer selected, click OK to automatically recalculate. When choosing a power transformer, you need to pay attention to the five upper cells in the window:
- StartUp Pk Flux Density, milli Tesla – maximum operating induction of magnetization of the magnetic core. Should not exceed the maximum permissible saturation induction for a given type of core material (usually no more than 300 mT).
- Layers – the number of layers of the primary winding. It is desirable that the number of layers be an integer (1, 2, 3 … N) – this will simplify the manufacture of the transformer.
- Main Turns – the number of turns in the main secondary winding (stabilized channel).
- Gap Length, mm – the width of the non-magnetic gap in the core of the magnetic circuit. It should not be too small (less than 100 microns) or too large (more than 3 mm). (Problems with gap milling may occur.)
In this program, when choosing a power transformer, a hint is provided: the program immediately shows its main parameters in the five upper cells. Depending on the value of one or another parameter, the color of the inscription inside the cells changes (it is very convenient for a quick assessment and selection of the most suitable size of the power transformer in a given situation).
The inscription can take four colors: blue – the most suitable, brown – with a large margin (redundant), red – little acceptable or unacceptable for this case.
Power supply PCB
Optimization of Parameters for the Selected Power Transformer
Often, the developer, due to various reasons, is limited in the choice of a power transformer, therefore the program provides for the function of entering an additional standard size.
In order to enter the parameters of the required power transformer, you need to open the “Custom Transformers” tab, check the “Use Custom Transformers” checkbox and click “Add”
To obtain detailed information about the elements of the circuit, you must select the element you are interested in and click on it once with the left mouse button. After that, a plate with circuit parameters will appear.
You can also change the previously set parameter in it, and after you click “OK”, the program will automatically calculate with the new parameters.
And, before starting to design your own devices, it is always very useful to study the instructions for use of microcircuits provided by the manufacturer, as well as the reference project.
Since the development of the hardware is the most “conservative” in the project and it is impossible to quickly alter or change it in the same way as the firmware of a microcontroller or FPGA changes, then, first of all, it is here that you want to avoid design errors.
The article summarizes the recommendations of several leading manufacturers of microcircuits for the design of printed circuit boards and the placement of components on boards.
These recommendations should be taken into account when designing devices with a physical Ethernet layer, since each of them is a complex analog-digital project operating at a sufficiently high frequency.
The design must take into account numerous requirements, ranging from galvanic isolation to ESD protection. The Printed Circuit Board (PCB) is the most important component in determining electromagnetic noise, electrostatic discharge (ESD) and overall performance.
The quality of the entire device as a whole depends on the fulfillment of all these requirements in the PCB design.
The main goal of Power supply pcb design guidelines is to reduce the noise introduced by digital circuits and the general background noise, as well as provide shielding between the internal circuitry on the PCB and external equipment.
These PCB design requirements should apply to the entire PCB design, not just to products with an Ethernet physical layer.
Additional Useful Information About Power Supply PCB Design Guidelines
To speed up and simplify calculations, you may need:
- DAK (Design Accelerator Kit). All such kits contain an already functioning switching power supply board or a parts list and a printed circuit board. With their help, you can assemble a functioning power supply, as well as complete documentation for it.
- Designer handbook, including:
- product selection guide;
- Datasheet for products;
- list of Application notes;
- design ideas;
- See descriptions on Design Accelerator Kits.