Apple II Power Supply Efficiency Question - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: May 03, 2023, 12:54:28 pm
content: I have a question regarding the Apple II power supply.According to the explanation and schematics (https://mirrors.apple2.org.za/Apple%20II%20Documentation%20Project/Books/W.%20Gayler%20-%20The%20Apple%20II%20Circuit%20Description.pdf) the power supply has higher efficiency, and one reason was due to not using a heavy transformer.I'm a bit confused because it still uses a transformer and not sure where the efficiency is improved by placing the transformer after the rectifier versus the traditional prior.Does anyone know?

author: Siwastaja
author_rank: Super Contributor
timestamp: May 03, 2023, 01:30:22 pm
content: Switch mode power supplies offer better efficiency than mains transformer, linear regulated power supplies because switch mode supplies internally provide feedback, regulating the output voltage by accurately controlling the amount of power spent from the grid. Compare this to classic, simple mains-powered transformer followed by rectification and regulation: while the transformer itself can be highly efficient, it will produce a varying voltage, which needs to be made too high, so that it's always guaranteed to be above the minimum required, for the linear regulation step afterwards; which is what wastes a lot of power.For something that does not care about the exact voltage, a classic transformer + rectifier has good efficiency as you can skip the linear regulator stage completely. But computers need regulated voltages so that is not an option.Size and weight are reasons to use a switch-mode converter, as well.

author: TimNJ
author_rank: Super Contributor
timestamp: May 03, 2023, 04:36:31 pm
content: It's not really a matter of "the order in which the transformer and rectifiers are arranged". Besides, for a switch-mode power supply, there are rectifiers before and after the transformer. The differences is that in the "traditional design", the transformer is subject to 50-60Hz and in a switch-mode design, the transformer is subject to 10's - 100's of KHz.At 50-60Hz, each cycle is relatively long, so when some voltage is applied across the magnetizing inductance of the transformer (the primary), there is a long period of time for the flux density to build up. In order to keep the core from saturating at low frequency, the core needs to be relatively large.At 100KHz, each cycle is relatively short. If the same transformer above was operated at 100KHz, during each cycle, the flux density would only build up to a tiny fraction of the 50/60Hz example.By this logic, we can reduce the size of the 100KHz transformer's core until the flux density rises to somewhere below the saturation point of the core.(e.g. to the same flux density of the 50-60Hz example). The key is: Higher operating frequency leads to core size reduction.In practice, the core materials of the low frequency and high frequency transformers are different. So, in the above example, you cannot really take a low frequency transformer and operate it at high frequency due to core material frequency/loss limitations. Low frequency mains transformers typically are constructed from silicon steel and high frequency switch-mode transformers use ferrite.I'm not sure how efficient the Apple II power supply really was -- It may not have been that great, but I suppose better than the traditional approach. I feel the main advantage (at that time) was in size/weight reduction, even though it is still not that small. For comparison, a 50VA 50/60Hz transformer is maybe 4" x 2" x 2" @ 1.5lb. Add in some filter caps and rectifiers and yes it will get pretty big, I suppose.http://www.righto.com/2012/02/apple-didnt-revolutionize-power.htmlBut, I will assume they did the analysis on this and that it made sense!