> requiring transistors that can efficiently switch on and off at high speed under high power.
Right. Switching power supplies need to go from off to on fast. In the full on state, the resistance is near zero (millohms with modern MOSFETs), and there's little heat dissipation. In the full off state, the resistance is near infinite (megohms with modern MOSFETs), with little heat dissipation because the current is near zero. During the transition, the switching element dissipates power as a resistor. The less time spend in transition, the less heat generated and the higher the efficiency.
Today the components are so good this is easy and efficiencies have passed 90%. That wasn't true in the Apple 2 era. Power transistors had higher OFF resistances, lower ON resistances, and slower slew rates. The better power transistors cost more. A cost-effective power supply that wouldn't overheat was tough to engineer.
(I've designed and built a switching power supply. Worked fine, could handle no-load and a dead short,
and didn't blither all over the RF spectrum. Probably had twice the parts cost a good designer could achieve.)
Not sure what is the puzzle here: the most likely explanation is that Holt himself had very narrow historic knowledge of the provenance of the design he had learned off when doing oscilloscope power supply design, told his boss Jobs, and he then just repeated like a parrot what Holt told him.
Contemporary example would be a ceo who will repeat the hallucination of an LLM as fact and then some guy on the internet will spend days refuting it.
The Apple II power supply was the first switching PS I had ever seen. And I still saw a lot of linear ones post-Apple II… From the article, perhaps the IBM switching PS, four years after the Apple II, then more or less cemented the switching PS for consumer electronics.
Are switching PSUs more expensive to produce than linear ones? Because, if you think about it, they took several decades to move from higher-end to lower-end products: my Atari 800XE (the third generation of the Atari 800, introduced 1985, so 8 years after the Apple II) still had a linear PSU. So did most cheaper mobile phones (most prominently the good old Nokia feature phones) up until around 2005.
Yes for a few reasons: more components, more noise filtering needed for sensitive devices, and more design theory and analysis is needed. A linear supply is nothing more than a high current op-amp. A switching supply requires understanding currents in the time domain to choose or design the right inductor. It's just more.
However, they don't need the same heatsinking as linear supplies, so they're less expensive in that regard. If designing an optimal heatsink (which has the second order effect of determining what else around the part will heat up because of the regulator) is a lot of work, then maybe the NRE will be the same. But overall linear regulators are fewer and simpler components so the BoM cost is a lot lower.
The materials in a 60hz transformer I think are the main BOM cost for linears? Relatively, transformers are say 2x cheaper than in 1970 (steel and copper are relatively MORE expensive than they used to be but the manufacturing improved) - but chips are more like 100x or 1000x cheaper. The high frequencies of switchers let you shrink the transformer (less materials) so its a big win.
Also the waste heat of linears constrains your design in terms of weight, power density, how big you need to make the enclosure, etc.
For reference I looked at what I'd need to buy on AliExpress to power my laptop off the cheapest linear PSU I could find (say 20v 5A), something like: https://www.aliexpress.com/item/1005003397909174.html - it weighs 4kg.
Jobs mischaracterized the innovation, and the author is technically correct (the best kind), but it's a shame that the piece appears to want to bury Holt's actual accomplishment. Holt's work was innovative in the same way that Woz's Disk II controller was. He didn’t invent the underlying technology, and he did create an elegant, product-defining implementation of a known (but difficult) technique.
Author here. Can you clarify what you consider Holt's innovative power supply accomplishment to be? I spent an excessive amount of time studying Holt's power supply to find out what made it special, but couldn't find anything particularly important.
Yeah it's nothing special. I find the whole Apple innovation thing quite tedious if I'm honest. I've seen some milspec stuff from around that era that purposely misused an LM723 as an SMPS. This was done without any fanfare at all even though it was quite interesting.
I second your analysis, there's nothing special at all about it. By the standards of the time it was a unusual to see a switch-mode supply in a computer, but the supply itself isn't unique when compared to contemporary designs. Mix "unusual application of a known technology" with "Apple fanboyism" and you get "Holt revolutionized power supplies" or whatever the claim is.
> To start his own company, Boschert knew he’d have to leave his 50-hours-per-week job at Microwave Associates in Sunnyvale, Calif. The ultimate motivator came when he became the custodial parent of his children. Boschert Inc. was formed in 1970 as he entered the consulting field. “I was designing power supplies to make money so we could eat,” he says.
Apple didn't do X they just shipped the new X components.
Apple takes the time to evaluate new components, bothers to adapt to the new components, re-optimizes other components around the new components, ships the new components.
Eventually,... eventually someone else does too.
Not always the pattern.
But the number of ways people try to punch holes in an earned reputation is remarkable. They got the reputation for doing some things right. Maybe not everything right, maybe not the things someone thought were right, or someone thought should be right, or what they would have done, or what someone thinks they claimed they did, that they actually didn't claim...
What an excellent example of Brandolini's law: “ The amount of energy needed to refute bullshit is an order of magnitude bigger than that needed to produce it.”
No. There was no "unit". This was before the days of modular PC PSUs and switching wall warts (which started proliferating only later). So it was just a custom circuit that used commodity components. For these components, the volume of orders from Apple would have been tiny compared to overall market demand.
Right. Switching power supplies need to go from off to on fast. In the full on state, the resistance is near zero (millohms with modern MOSFETs), and there's little heat dissipation. In the full off state, the resistance is near infinite (megohms with modern MOSFETs), with little heat dissipation because the current is near zero. During the transition, the switching element dissipates power as a resistor. The less time spend in transition, the less heat generated and the higher the efficiency.
Today the components are so good this is easy and efficiencies have passed 90%. That wasn't true in the Apple 2 era. Power transistors had higher OFF resistances, lower ON resistances, and slower slew rates. The better power transistors cost more. A cost-effective power supply that wouldn't overheat was tough to engineer.
(I've designed and built a switching power supply. Worked fine, could handle no-load and a dead short, and didn't blither all over the RF spectrum. Probably had twice the parts cost a good designer could achieve.)
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