How to build an adjustable switching power supply using LM2576 [Buck Converter, CC-CV]

By Hesam Moshiri, Anson Bao

Abstract

Switching power supplies are known for high efficiency. An adjustable voltage/current supply is an interesting tool, which can be used in many applications such as a Lithium-ion/Lead-acid/NiCD-NiMH battery charger or a standalone power supply. In this article, we will learn to build a variable step-down buck converter using the popular LM2576-Adj chip.

Features

Cheap and easy to build and use

Constant current and constant voltage adjustment [CC, CV] capability

1.2V to 25V and 25mA to 3A controlling range

Easy to adjust the parameters (optimum use of variable resistors to control the voltage and current) 

The design follows the EMC rules

It is easy to mount a heatsink on the LM2576

It uses a real shunt resistor (not a PCB track) to sense the current

[1]: Circuit Analysis

Figure-1 shows the schematic diagram of the power supply. The heart of the circuit is the LM2576-Adj chip. It is a popular, cheap, and handy buck converter IC. According to the LM2576 datasheet: “TS2576 Series are step-down switching regulators with all required active functions. It is capable of driving 3A load with excellent line and load regulations. These devices are available in fixed output voltages of 3.3V, 5V and adjustable output version. TS2576 series operates at a switching frequency of 52kHz thus allowing smaller sized filter components than what would be needed with lower frequency switching regulators. It substantially not only reduces the area of board size but also the size of a heat sink, and in some cases, no heat sink is required. The ±4% tolerance on output voltage within specified input voltages and output load conditions is guaranteed. Also, the oscillator frequency accuracy is within ±10%. External shutdown is included. Featuring 70μA (typical) standby current. The output switch includes cycle-by-cycle current limiting, as well as thermal shutdown for full protection under fault conditions” [1].

Figure-1

Schematic diagram of the switching buck converter

Capacitors C1 and C2 are used to reduce input noise. D1, L1, C3, C4, and PS1 are the typical buck converter circuit ingredients. C3 and C4 are used parallelly instead of a single capacitor because using parallel capacitors reduces the final capacitor’s ESR value. Simply it means using two 470uF capacitors in parallel is better than using a big 1000uF capacitor.

R1 to R4 construct a shunt resistor. I have used four 0.5R-1%-1W resistors that make an accurate 0.125R-4W resistor. The current flow over this resistor generates a voltage drop, which we used it to sense the current. 

REG1 makes a fixed 9V supply for the IC1 [2]. IC1 is used to amplify the voltage drop on the shunt resistor because small current flows do not make a big voltage drop over a 0.125R resistor. So we have to use an amplifier here. IC1 is configured as a non-inverting amplifier that can deliver 820x gain maximum. Potentiometer R7 defines the gain, so the minimum gain is around 4x. Therefore this potentiometer defines the maximum output current.

The potentiometer R6 adjusts the output voltage. The diode D2 blocks the feedback voltage path to IC1. otherwise, we are not able to adjust the voltage and current simultaneously. I have included the D2’s voltage drop into consideration and have compensated it using the IC1 gain.

C5, C6, and C7 are used to reduce the noise. C6 defines the cut-off frequency of the amplifier that will not amplify high-frequency noises. R6 and R7 values are selected wisely. So by turning the potentiometers, you will experience smooth voltage/current changes. 

According to the EMC guidelines, I/O lines that transmit/receive signals through cables/wires (especially high frequency), should be placed near each other (for example on one edge of the board). Otherwise, the potential difference between the ground return paths will cause noise or interference. More importantly, where the main circuit itself runs at high frequencies. Although our circuit does not deal with high frequencies, it is always a good practice to follow the guideline.

[2] PCB Board

Figure-2 shows the designed 2 layers PCB board. I used the SamacSys provided schematic symbols and PCB footprints for the LM2576 [3] and LM358N [4] because I did not have the libraries and designing component libraries from scratch is a time-consuming process. The service is free and industry-designed (IPC standard). I use Altium Designer to design schematics and PCBs, so I use the introduced CAD plugin [5] (figure-3).

Figure-2

PCB layout of the switching power supply

Figure-3

LM2576 and LM358 components selection from SamacSys Altium CAD plugin

[3] Assembly

Figure-4 shows a 3D view of the assembled PCB board and figure-5 shows a real photo of the assembled board. I used a semi-homemade PCB board just to test the circuit and prove the concept, but you should use a professional PCB fabrication company such as PCBWay because now you are sure about the true operation of the circuit. Besides, the quality of the PCB is important for many designs. If you deal with currents higher than 1.5A, then simply mount a U or L shape heatsink on the PS1.

Figure-4

A 3D view of the assembled PCB board

Figure-5

A view of the semi-homemade assembled PCB board 

[4] Test and measurement

You can apply the maximum voltage of 30V to the input. The LM2576-Adj (PS1) can accept input voltages up to 40V, but REG1 (78L09) can tolerate the maximum of 35V (absolute maximum rating) at the input. REG1 plays an important role in the stability of the amplifier (IC1), therefore reducing 10V from the input voltage threshold is a wise decision.

To set your desired voltage, simply connect a voltmeter (or a multimeter in the voltage setting) to the output and turn the R6 multiturn potentiometer. To set your desired current limit, simply connect an ammeter (or a multimeter in the current setting) to the output and turn the R7 multiturn potentiometer to set your desired current limit. Do not prolong this process because holding the output on the short circuit condition is not recommended.

[5] Bill of Materials

Table-1 shows the bill of materials. Just follow the script, build the circuit, and have fun ?

Table-1

Bill of materials

You can download the Gerbers or order the PCB here

References

[1]: https://www.onsemi.com/pub/Collateral/LM2576-D.PDF

[2]: https://www.onsemi.com/pub/Collateral/LM358-D.PDF

[3]: https://componentsearchengine.com/part.php?partID=531545

[4]: https://componentsearchengine.com/part.php?partID=671517

[5]: https://www.samacsys.com/altium-designer-library-instructions