5 yrs in and I never did make a buck convertor - Page 1
Forum section: electronics / beginners

author: paulca
author_rank: Super Contributor
timestamp: February 07, 2023, 07:07:08 pm
content: I've used buck convertors and stuff, but I always avoided trying to make one figuring it involved too many equations.Took a notion tonight and stuck a 100mH inductor into a breadboard put a scope on it and just started playing. With a just a manual push switch it was rather boring. I couldn't even get the inductor to ring.I added a capacitor so I would have something to measure, but of course with no load it just charged up and sat there staring at me. So I put a 330R across the capacitor and finally I started to see some inductor like effects on the spamming the switch. So I swapped the switch for a PWM signal from the sig gen at 5V SQR +2.5V so it's non-negative. Though an behold I could now adjust the output voltage relative to the input voltage with the duty cycle.A buck convertor. Not a very good one and you learn a lot by doing. There was a -15V spike on the drop of the PWM. One of those, "Ah ha!" moments and I went to find a diode. No schotty's so any ole signal diode and the negative spike disappeared.The final few improvements had to wait until I added an actual power source and a mosfet, rather than using the PWM input signal to the inductor directly. So with the bench PSU suppling 5V and the Siggen giving 5V PWM, a tiny jelly bean enhancement mosfet and... a pain. The N channel just couldn't produce solid PWM input because of the VGS requirements. I could drive the thing really really hard on teh gate, up to 20V with the sig-gen, but nope. Rubbish, 50% output at best.Quick google and swapped the mosfet to the low side and ... while it makes the output floating wildly all over the place, it was producing a stable 3.3V output with 5V input and about a 20% duty cycle and 8mA.So I put a few LEDs on it and not much changed. Decided to power a ~100mA 3.3V breadboard with it. Which worked. Although with a 5V supply the buck convertor topped out at 3.4V. Probably all the losses in the mosfet/diode etc. Still it was supplying 500mW and the noise was a 250mVpp 24Mhz ringing off the inductor at the 50kHz PWM signal.I thought I'd share as a reminder, as long as you are careful enough, it's often far easier to just start playing with things rather than reading the theory and/or forum posts.Maybe tomorrow I'll try and boost convertor. Although having seen the 20V floating offset on the floating buck, I might want to investigate a P channel highside or other ground referenced topology.

author: Kim Christensen
author_rank: Super Contributor
timestamp: February 08, 2023, 01:02:31 am
content: QuoteWell, you can easily make aboost with an N channel FET, coil, diode, and capacitor.

author: redkitedesign
author_rank: Regular Contributor
timestamp: February 08, 2023, 06:24:10 am
content: Quote from: paulca on February 07, 2023, 07:07:08 pmI've made a few buck converters from scratch, figuring out all the equations etc, but that was 20 yrs ago. Nowadays, I pretty much always use a manufacturers design tool to figure one out. Finetune it a little, adjust for parts allready in use on other designs and let the tool recalculate all parameters.I immediately admit that my work is usually for small-series, so there is more pressure on the design cost than the part cost. Thus a TI convertor instead of a cheap chinese with google-translated documentation.It's a bit like long division. I learned it in school, and can still do it if needed, but in practice I always grab a tool that does the work for me.

author: james_s
author_rank: Super Contributor
timestamp: February 08, 2023, 07:04:40 am
content: Yeah the design tool or even just the datasheets now show you nearly everything you need. If you're doing something outside the box you might need to run all the calculations the old fashioned way but I've rarely found that to be necessary.