switched mode power supply - diode

[stevez]

I am doing some learning/experimenting with SMPS, among other things. I am placing an order for parts soon and thought it wise to purchase a few items that I would use. In relatively simply supplies there is a diode, sometimes called a catch diode (maybe other things). I understand it needs to be fast. I could use help in describing the diode characteristics so that I can purchase some common (hopefully) parts to get started. Descriptions or even suggested part numbers would be appreciated.

Right now I plan to stick to 13.8 vdc or less and 2 or 3 amps. I am being encouraged to take 13.8 vdc from car and use buck to get steady 8 volts as a first step. Next step would be to take the 8 volts back up to 13.8 vdc. Whole point is learning but as a practical matter I'd like to have a power supply that will provide a steady 2 amps at 13.8 vdc (or thereabouts) from the nominal 12 volt accessory (used to be cigarette lighter) outlet in a car.

Again, help with describing a diode would be appreciated.

 

[justDIY]

the keyword you want is schottky diodes ... but a newer method of layout is called synchronus recitification, and relies on a combination of highside + lowside switch, eliminating the diode and its associated in-efficiencies.

look to National, Zetex, ST and Maxim for some interesting switch mode controllers and regulators... some from Maxim even have on-board sync rectifiers... all you need is a few caps and an inductor

controller = just the PWM generator and comparator circuits... regulator = pwm, comparator and switching circuits aka integerated switcher aka monolithic switcher

 

[stevez]

Thanks for the reply. For the moment I am just trying to work thru the basics on some relatively simple circuits. While the item I might construct might be less than current technology allows, it's still enough for me to improve my level of understanding. I may even just build an adjustable pulse width generator the FET switch (maybe w/driver IC) and inductor section just to see how things come out then take advantage of more complete technology - so your suggestions are useful and appreciated.

 

[Oznog]

I am being encouraged to take 13.8 vdc from car and use buck to get steady 8 volts as a first step. Next step would be to take the 8 volts back up to 13.8 vdc. Whole point is learning but as a practical matter I'd like to have a power supply that will provide a steady 2 amps at 13.8 vdc (or thereabouts) from the nominal 12 volt accessory (used to be cigarette lighter) outlet in a car.

I'm confused. Why would you make a converter that converts to 13.8v to 8v then back to 13.8v? If you want to just make a supply that produces a regulated 13.8v when the cigarette lighter voltage varies from 14.6v to 10.5v there are more direct ways to do it.

 

[Russlk]

If you are looking for a part, Digi-key is a good place (digikey.com). If you search for "diode", the search engine will help you narrow down the search to a part number.

 

[stevez]

I can work with a relatively constant 8 vdc but would like to get up to 13.8 vdc eventually. Step #1 was to demo a buck regulator - then take the constant 8 vdc and boost to 13.8 vdc. This is a hobby so taking smaller steps works better for me.

FYI - a practical use for the 8 vdc is a handheld transceiver meant to run on it's own internal batteries. It is sensitive to the noise from auto systems. They run quite well and at reduced power at this voltage. At this voltage the batteries aren't likely to be damaged. For more output you can take the supply up to 13.8 vdc with some risk of damage to the batteries.

 

[evandude]

I built a small DC-DC converter that provides a regulated 12v output from the cigarette lighter (which can be as high as 14.5 volts or so, and sometimes drops below 7v when starting the engine).

http://www.eegeek.net/mp3box/DCDC/index.htm

it's a buck-boost converter, uses a pretty standard, cheap controller chip, and is a pretty straightforward design. The design is not my own; When I built it, I was starting from scratch with DC-DC converters, so I followed this guy's design:
**broken link removed**
But, I certainly learned a lot along the way.

 

[stevez]

I took a quick look -very nice. Will look closer tonight. Thanks!!

 

[Hero999]

The link you've provided doesn't work.

Also could you please post the schematic in a free and open file format like .png or .svg.

 

[Russlk]

"Get Opera it's simply a superb browser. "

But it doesn't work with html? That's not superb!

 

[RadioRon]

I just finished work on a small boost converter using MBR0530 diodes. Not sure if that part can handle your current, but its a good part. Higher voltage outputs need MBR0540, same family. Digikey has them.

 

[Mike - K8LH]

I built a small DC-DC converter that provides a regulated 12v output from the cigarette lighter (which can be as high as 14.5 volts or so, and sometimes drops below 7v when starting the engine).

http://www.eegeek.net/mp3box/DCDC/index.htm

it's a buck-boost converter, uses a pretty standard, cheap controller chip, and is a pretty straightforward design. The design is not my own; When I built it, I was starting from scratch with DC-DC converters, so I followed this guy's design:
**broken link removed**
But, I certainly learned a lot along the way.

Pictures look nice. Is there a non-Eagle schematic somewhere? Also, the 'original' link doesn't work.

Regards, Mike

 

[Hero999]

"Get Opera it's simply a superb browser. "

But it doesn't work with html? That's not superb!

Rubbish, I'm viewing this forum in Opera now which is in html and it displays perfectly.