SMPS basics

[Thunderchild]

Well I need to make a 12 v to 5 volt converter. Now as of yet I do have the 1.5 amp version of the 7805 but i am not please at chucking out over half the power in heat.

Now the design of a SMPS seems somewhat complicated i suppose because it has to maintain very acurate voltage output and minimal ripple.

so i thought what about a simple switching circuit to come down to like 6 volts and then a linear regulator that only drops 1 volt.

so I started some basic simulation in electronics workbench 5.2 I connected a square wave generator with a duty cycle of 50 % to what is a basic LC low pass filter, now with just those components in the project the obvious happened the voltage very slowly rose to 12 volts, but by connecting just a 30 ohm resistor the voltage was a steady 5 volts and also with a 3 ohm resistor, I found that two LC filters made much less ripple.

Now my question is, is it as simple as that ? or is the simulation program missing out on something ?

 

[Hero999]

i thought what about a simple switching circuit to come down to like 6 volts and then a linear regulator that only drops 1 volt.

That's called a hybred regulator and is really common for high quality switching power supplies.

Now my question is, is it as simple as that ? or is the simulation program missing out on something ?

My advice is get LTSpice and consider using their ICs as a basis for your supply.

What are you powering with your supply?

Do you really need the full 1.5A and is the ripple really that critical?

You can make a pretty good switching regulator from a couple of transistors.
https://www.romanblack.com/smps/smps.htm

 

[Thunderchild]

yes I need 1.5 amps, 1.2 at least as that is what my camera is rated, I want to power it off a 12 volt battery, to eliminate the need for AA batteries and constantly removing the tripod shoe from the camera to get to the batteries, the screw socket is plastic and will end up wearing out, I can commonly use 12 for other stuff as well,

I was also wondering if it was possible to make my own PC power supply, I mean most stuff on mother boards now has it's own regulator anyhow and if the software is correct (speed fan) there are continuous fluctuations of 100 mV or more

 

[gramo]

Whats the 12V from?

 

[Thunderchild]

a led acid battery

 

[Hero999]

I use the MAX649 for this sort of thing, it can easily do what you want and it's current limited too.
https://www.electro-tech-online.com/custompdfs/2007/06/MAX649-MAX652.pdf

I wouldn't worry too much about ripple, you can quite easily get the ripple down to 40mV by using a low ESR electrolytic. Also the camera is a digital appliance and digital units are generally cope with ripple better than analogue appliances.

 

[Thunderchild]

yea I'm sure computer SMPS have a good deal of ripple in there

 

[Thunderchild]

I'm wondering though what exactly is a SMPS controller got in it, I did try once a simulation using a window comparator to switch on and off power so that it stayed within the small window set (ie the acceptable ripple) and charging a condenser that was discharged by the load and recharged by the controoler (window comparator and SR latch) i suppose if i were to throw in an inductor i would have a proper SMPS,

my question really is how do I choose the L ad C values ?

 

[Hero999]

Have a look at the link to the MAX649 I posted.

The calculation is pretty similar for all SMPs.

 

[CheapSlider]

IR mypower

A good starting point is International Rectfier's site for designers.

It contains reference designs and also supports: Chinese 简体中文 ; Korean 한국어 and Japanese 日本語 languages


edit - correction to url