Hi,

I am wondering why power supplies are so complex with there voltage regulation when there are IC's like the LM723 about?

I am building a variable power supply that I got from this forum, and it looks great and no doubt will work great. But while I was ordering the list of parts, I was wondering what is the downfall of voltage regulator IC's over complex power supplies?

The big power supply I am building does not work out cheap, I know I will need to use more supplies in future, and the IC's are cheaper to make extra power supplies. Just want to make sure I don't build them, then realise they are unreliable or something.

 

Well, complex power supplies are required for more sensitive devices such as CPU power supply. The amount of current that the load will be using is also another factor of deciding which types of power supply. Some loads require higher current to operate which couldn't be supplied by ICs.

 

Linear regulators such as the LM723, LM317, LM78XX - have been around for many, many years, and are very cheap, and can produce very "quiet" supplies (low in electrical noise, low ripple on the output).

However, they are very inefficient, particularly when used with variable voltage power supplies. This is because the regulator acts as a variable resistor.

Let's say that you have unregulated 25vdc on your filter capacitors after the rectifier bridge, and that you have the regulator biased to output 20v, and your load is drawing a constant 1A. The regulator will have to dissipate (25-20v)*1A = 5 *1 = 5 Watts of power, and the load dissipates 20W. The supply is 80% efficient used in this manner.

However, now you have a load that requires 1A @ 3.3V, so you adjust your regulator output down to 3.3v. The regulator now has to dissipate (25-3.3v)*1A = 21.7 Watts! The load is only dissipating 3.3W. The supply is only 13.2% efficient used in this manner. You will need a very large heat sink.

Switching supplies are much more complex, and have significantly more ripple voltage on the output. However, they retain about the same efficiency percentage across their output range. Some switching supplies are remarkably efficient.