Synchronous buck converter with NE555 and LM311 - Page 1
Forum section: electronics / beginners

author: pdoriam
author_rank: Newbie
timestamp: January 24, 2014, 01:06:28 pm
content: Since this is my first post... Hi all!The topic itself is the question... is it possible to design a synchronous buck converter with NE555 for PWM and LM311 for feedback?Meaning I want to drive the NFET with NE555's pin 3 and the PFET with pin 7.I'm still learning about buck converters and I'm following this:http://www.nlvocables.com/images/buck_converter/circuit_sync.jpgTrying to design a CCM buck converter with the following parts:IRF9540IRF540100uH inductorNE555LM3117805 (to drive the NE555 and LM311 and prevent fluctuations on the input)If I'm not totally off my rocker...I'd love to hear your ideas (or links) for the circuit with proposed partsTIA,Pedro

author: Rigby
author_rank: Super Contributor
timestamp: January 24, 2014, 03:12:17 pm
content: What up, Pedro?Please make a country selection in your profile so we can see what country you're in. It'll help if & when you ask about locale-specific stuff.A) I'm not sure a 555 can oscillate fast enough to make a good switch mode converter.B) There are chips that have all (most) of the needed circuitry in them. You just pop on an inductor and some caps and you're done. I realize this isn't as fun as building your own, and I don't know if you're doing this for fun or if you're doing it because you need a switching buck converter.C) Usually folks start with linear voltage regulators. Have you made one of these?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 24, 2014, 03:15:56 pm
content: The N and P transistors take the same drive voltage, give or take shoot-through delay. It may be easier to live with shoot through (using a snubber to manage the switching losses) than to add the delay and extra drivers.What kind of efficiency are you expecting? If it's more a proof of concept, you can get away with stupid things, like driving the PMOS directly from pin 7 with a massive 100 ohm pullup resistor.You actually don't want the 7805, because the 555's pin 3 will only pull up to 3 or 4V, hardly enough for an IRF540; likewise pin 7 may not tolerate more than 5.5V (I'd have to check, I don't remember if it's a true open collector or if it has supply diodes attached).You will still want some sort of voltage reference (assuming you want a regulated output), for which anything will do. TL431 is standard, and has the nice feature that it can play double duty as the error amplifier.LM311 however has an awful lot of gain, and no compensation. Were you intending it be hysteretic regulation?Tim

author: pdoriam
author_rank: Newbie
timestamp: January 24, 2014, 04:37:09 pm
content: Tim and Rigby thank you for your kind answersFirst off, to clear any doubts (if there were any lol) I'm a programmer who recently picked up electronics so... yes... I'm a noob@Rigby: country updated, thx for bringing that upA) the intended use is a buck led driver so perhaps I don't need that high a frequency?B) It's as much for learning as is for fun as one can buy one of those for a cpl bucks on EbayC) yes I didinefficient little buggers though@Tim: overwhelming lolafaik pin 7 is an open collector (kindly correct me here if I'm wrong).as for the 7805: good point there!Vdrop is typically ~2.5V@200mA, right? I can use another, higher voltage, LR, say a 7810, as I have an assortment of those in different values.as for the LM311 not quite sure what you mean... I intended to feedback its output to the non inverting input via a 10K resistor as to give it some hysteresis... is that it?First time I heard about TL431 .. will have to look at its datasheet again to let it "sink in" 1st lol. But thx for the reference!For the record... this is what got me started:http://www.instructables.com/id/Poormans-Buck/?ALLSTEPSFirst I decided that I didn't need all that mess... and got away with simply slamming a 10ohm resistor at the gate of a NFET and connecting that to one of the arduinos pwm outputs... pure 0-100% duty cycle.. niceThen I thought I should learn more about buck converters and looking at the link's circuit it struck me how wildly frequencies would vary depending on load. So I ditched it and came up with this half cooked ideaI got to the point where I assembled the circuit and then looking at the ne555... I thought ... WTH am I doing? How in the heck am I going to connect this to both FETS *without* turning them on at the same time!Thanks for your thoughts guys, appreciated