Buck converter - Page 1
Forum section: electronics / beginners

author: Xenon_K
author_rank: Contributor
timestamp: April 10, 2025, 07:00:00 pm
content: Hello everyone, this is my first project and I would really appreciate any help.I’m trying to build a buck converter. I started putting something together in LTspice, which I later want to build in real life.I'm still not very familiar with how to properly choose component values or which parts are generally good for different sections of the circuit.Just now I realized that my PWM IC only allows a 50% duty cycle per output.Can someone recommend a different IC that ideally is also available in LTspice and can handle a duty cycle from 0 to 100%?I chose 12V as my Vcc and wanted to adjust the output voltage using a potentiometer.Can anyone help or give me general advice?Thank you very much!

author: MathWizard
author_rank: Super Contributor
timestamp: April 10, 2025, 08:27:40 pm
content: Well does it have to be an IC in the sim ? Or do you also want one you can buy and use on a breadboard ?There's a NE555 timer model, in the MISC parts, it's not hard to find adjustable duty cycle configurations for it. And it's a very very common and wide-use IC, for all kinds of circuits. So any hobbyist should have a few.I think the typical max current from a 555 timer is 200mA. At some point in big powerful circuits, that won't be enough, and then you could build some higher current gate drive, and just use the 555 as the clock.IDK much about the brandname IC's in LTSpice, or common buck IC's either.If you don't know, can also make squarewave voltage or current sources, using the PULSE or PieceWiseLinear options, under advanced settings in the source-box.For a bridge type setup, with the 2 gate's to drive out of sync, you still might beable to use a single 555, but with an added inverter, like a common emitter BJT circuit. But some circuits like these require a dead time, with both gates fully off, so yeah, maybe that's why a real controller with 2 phase gate drive is needed.

author: Benta
author_rank: Super Contributor
timestamp: April 10, 2025, 09:53:06 pm
content: Why 0...100%?That would either turn it completely off or completely on (short from input to output).Define your specifications in regard to output voltage range. A 0...12 V output is not realistic.

author: Andy Chee
author_rank: Super Contributor
timestamp: April 11, 2025, 01:41:23 am
content: Quote from: Xenon_K on April 10, 2025, 07:00:00 pmGeneral advice is to provide more information about your application.Is this going to be bench power supply?Is this going to be a motor speed controller?Is this going to be a LED lamp dimmer?Is this going to be an electro-shock therapy device?Is this a learning platform for buck topology experimentation?

author: Konkedout
author_rank: Frequent Contributor
timestamp: April 11, 2025, 04:17:19 am
content: One approach which may be instructional is a hysteretic controller. We used to build some of these in the 1980s. We called them "ripple switchers." Nowadays it is relatively obsolete.1) You need a voltage reference. In LTSpice it can just be a voltage source, but if you want something more realistic, I recommend a TL431.2) You need a couple of comparators, such as LM339 or LM311. One comparator controls output voltage and the other one limits inductor current. Both comparators need to have hysteresis such as maybe 30 mV hysteresis for 5V output ripple and 500 mA hysteresis for 3A current limit. When either comparator says "turn off" the power switch you need to turn it off.3) You need a power switch. This could be a fast PNP bipolar power transistor or a Pch MOSFET or an Nch MOSFET. MOSFETs will need adequate gate drive for fast switching edges. An Nch MOSFET will need a higher bias voltage for gate drive.4) I recommend a schottky freewheel diode. Synch rectification sounds good but I would not attempt that on a first attempt.

author: Smokey
author_rank: Super Contributor
timestamp: April 11, 2025, 04:30:05 am
content: Check out TI webench. Full schematics with component values for most of their partshttps://www.ti.com/tool/WEBENCH-CIRCUIT-DESIGNER

author: Picuino
author_rank: Super Contributor
timestamp: April 11, 2025, 07:33:16 am
content: Maybe you could start with a regulator with everything integrated. It is inexpensive and simple to use:LM2575https://www.ti.com/product/LM2575Note that there are versions for fixed voltage output and versions with adjustable output voltage (this is the one you are interested in).EDIT: Version of Microchip with 3A output currentLM2576WThttps://ww1.microchip.com/downloads/aemDocuments/documents/APID/ProductDocuments/DataSheets/LM2576-52kHz-Simple-3A-Buck-Regulator-DS20006238A.pdf

author: Picuino
author_rank: Super Contributor
timestamp: April 11, 2025, 11:29:32 am
content: During the simulation you will have no problems with the track routing, but when you are going to mount the regulator in practice, you must take into account the basic rules of track routing so that the regulator works correctly and does not emit too much EMI noise.AN-1149 Layout Guidelines for Switching Power Supplieshttps://www.ti.com/lit/an/snva021c/snva021c.pdfAN-2155 Layout Tips for EMI Reduction in DC / DC Convertershttps://www.ti.com/lit/an/snva638a/snva638a.pdfAN-139: Power Supply Layout and EMIhttps://www.analog.com/media/en/technical-documentation/app-notes/an-139.pdf