Two series pass devices?

He has a forum. You could just go there and ask him. I'm sure he would love to discuss it with you.

A pertinent previous post:

contains a link to this schematic:

Does your SED post pertain to pass transistors Q5 and Q6 shown in the schematic?

Danke,

Nevermind. The Youtube video finally downloaded. The pass transistors you ask about are Q1 and Q2.

Danke,

That's the craziest design I've ever seen. It must be a deliberate parody.

Disclaimer: I find EEVblog practically unwatchable, so I haven’t looked at the schematic.

One reason to separate the current limit and voltage regulator functions is to sharpen up the current limiter by putting it inside the voltage regulator loop.

I commonly do that with LND150 depletion FETs in HV bias supplies for APDs and SiPMs. They look like ~1k resistors below about 1.5 mA, which is enough to hurt the gain linearity, but the feedback loop effectively knocks that down to way under an ohm.

Cheers

Phil Hobbs

It's a small lab power supply powered by USB-C. It has several microprocessors, layers of isolators and switching regs and linear regs, a custom enclosure with custom-machined connectors, a custom planar transformer, a custom heat sink, a zillon layer PCB, and what must be tens of KLOCs. It's apparently been in design for about 5 years.

Squeaky talks about it in an almost bearable tone.

Really, it's funny enough to have a look at. Skip the blather and look at the schematic pages.

(I had hernia surgery today and am still goofy from the drugs, so this sort of nonsense fit right in.)

Thanks, good call, I'd completely forgotten!

piglet

The opamp is OPA2180 zero drift/auto zero type with inverse parallel input protection diodes. D4 looks mega kludgy - if it were mine I'd investigate using a different opamp or taking the error signal from the voltage follower used for metering. Still I guess in real life the voltage drop error across D4 at nanoamp opamp input bias current is near negligible?

piglet

Glad to have supplied some light relief!

piglet

Yes. I am used to seeing the outputs of voltage and current error amps orr-ed together to control just one series pass device.

piglet

Yes, Ricky's suggestion is indeed excellent. And what do you know? Someone at the forum already asked and Dave doesn't remember:

Regardless, it's my intention to find the time to digest the video first, then post questions to Dave at the forum.

Danke,

Using a chopamp in that kluge is yet another absurdity.

I must agree; there's THREE power pass transistors, one for the switch mode coarse regulator, one for current, one for voltage, and you could easily limit the current at the switch mode node (there's a switch-current-sense resistor there anyhow). The design has all the hallmarks of being an exercise in using medium-scale-integration blocks, each with a single function. That's how you end up needing two microprocessors to obey two instructions: supply-this-voltage except limit-to-this-current.

One nice item, though, is worth mentioning: it displays BOTH the target-or-actual output voltage and output current at the same time. One of my bench supplies has one-analog-meter-does-both; simpler, but a pain to use.

no it is not, one mcu is for an isolated USB interface, the other one for running the display/keypad/menu/supply

Good quote, appropriate to that power supply:

It was the embodiment of the idea that perfection is not when there is nothing left to add but when there is nothing left to take away.

Keith Houston in Empire of the Sum

As the wise man said,

Cheers

Phil Hobbs

Phil Hobbs

PDP-11 had the RESET instruction. As in full hardware powerup-type reset. Saved wear and tear on the power switch.

It also has the Land Mine instruction,

MOV -(PC), -(PC)

which copied itself one word below, and executed from there.

Before the vacuum-column tape transport was invented, early IBM tape drives allegedly had an instruction SST, for “stop and stretch tape”.

Cheers

Phil Hobbs

Maybe it was prototyped to help prototype other USB-C powered power supplies: