Switching Power Supply Circuit Using LM2576 [Adjustable, Buck Converter, CC-CV]
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.
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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 heat-sink on the LM2576
It uses a real shunt resistor (not a PCB track) to sense the current
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” .
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 . 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.
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