As how I understand it, there are two ways of converting DC. One is by using a voltage regulator (zener diode, feedback combination etc), and the other is by using a buck boost converter. Firstly, if there are other methods, please tell me!
How do lab bench power supplies work? I've heard that buck-boost supplies are noisy (for precise lab requirements) and I know that voltage regulators basically drop the remaining voltage as wasted energy.
Now to my question: I have only seen a single transformer in any lab supply, so obviously the output voltage of it is fixed. If say the bench supply's max voltage output is 24V, then I'm assuming the output of the transformer would be ~24+V.
If this is going (after AC-DC conversion) to a voltage regulator, and say I pick 1V fixed o/p voltage and connect the bench supply to a load that draws 1A current. If I understand it correctly) the lab supply will waste at least (P=VI=[24-1]*1) 23W and have an efficiency of just 4%.
Is this true? If so, the heat output would increase dramatically as selected output voltage is lowered (and current increased). So much energy would be wasted? Not to mention heat sinking etc.
What am I missing? Is there a better way? Or is this a necessary sacrifice for getting a precise power supply?
Best Answer
Many supplies nowadays use both a switching regulator and a linear regulator together. The two are set up to track each other, so the voltage dropped by the linear regulator is never excessive.
So for example, say you set the supply to 5V, you might have a 24V input which the switching regulator drops down to 6-7V, then the linear regulator drops that down to the desired 5V. This way you get the benefit of the switchers efficiency with the quiet output of the linear regulator.
However, as mentioned by Passerby, efficiency and weight is not always so much of a concern compared to reliability and quietness, so many simply use a purely linear supply and deal with the losses involved.
To keep from being ridiculously inefficient however, generally many linear supplies will have more than one tap on the transformer which are switched between at appropriate points in the range (e.g. instead of dropping from 24V to 1V, it would switch to, say, a 5V tap so a lot less power is dissipated) You can usually hear the clicks of the relays at certain points in the voltage range.