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Old 23rd May 2019, 04:38 PM   #121
Salas is offline Salas  Greece
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"For different brand LEDs the temperature coefficient of the forward voltage can vary between –1 and –20 mV/◦C":
http://web.vu.lt/tmi/a.zabiliute/wp-...acterstics.pdf

With the particular LEDS bar and having the LM334 intentionally located at a closer distance to the main radiator there is an acceptable Vout negative drift overall for my purposes. Few tens of mV usually.

BTW the 51R should not connect to the Vref node but to the input DC rail in your simulation.
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Old 23rd May 2019, 11:49 PM   #122
jackinnj is offline jackinnj  United States
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Quote:
Originally Posted by Salas View Post
With the particular LEDS bar and having the LM334 intentionally located at a closer distance to the main radiator there is an acceptable Vout negative drift overall for my purposes. Few tens of mV usually.
Yupp, this is just a bit of an academic exercise -- design with two LED's, with two cathode to cathode zeners, and a simple resistor. (Correction you pointed out made!)
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Old 24th May 2019, 02:37 AM   #123
Salas is offline Salas  Greece
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Here's a practical exercise:

Stick a thermocouple next to the LEDs bar, load the thing, circle above with a hot air gun across the main sink over to the bar. Output semis & Vref stuff reside there. Take it from 28C to 65C, stop. Went down to 40C because it took time to react. Snap pics as in before and after second-rate marketing adverts. Oops... better than expected
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Old 24th May 2019, 08:58 AM   #124
Dimdim is offline Dimdim  Greece
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I finally managed to sort-of complete my build of the prototype board Salas was kind enough to provide. Compared to Nikos' other (shunt) power supplies, this power supply is by far the easiest one to build. Pretty much anyone with a soldering iron should be able to build it. No matching or measuring of parts is required and it is very hard to put the wrong part in the wrong place. Just make sure that the LED bar gets soldered on with the correct orientation. For the moment I'm powering my unit with my lab power supply, while I'm deciding where to test it first. Having set it up for 5.1V output with no load (3 LEDs lit), the output will drop only by 30mV when having to output ~280mA. Very nice. The reg itself with no load will burn about 11mA. For the moment I have it powering an RPi 3 with no peripherals, which draws about 265mA at idle.

IMG_20190523_234919 (Medium).jpgIMG_20190524_001844 (Medium).jpg

The rectifier diodes that I put in (MUR810G, no heat sinks) don't even get warm to the touch. Same goes for the output transistor's heat sink (input voltage is about 12VDC). Next up is soldering-on a couple of filtering capacitors and connecting an actual transformer. I plan on testing it on my RPi3, my SBT and probably on something more demanding like a NAS box.
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Old 24th May 2019, 06:19 PM   #125
Salas is offline Salas  Greece
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Very nice so far. The moment you load it, the pass transistor conducts current and increases Vbe a little. That little gets deducted from Vout. Thus no over-voltage reaction. Such behavior is benign for feeding digital gear. There's no control loop to run compensate the loss and overshoot.

When you want to know its dynamic output resistance, make a move from say 2A to 3A load and compare Vout before output cabling. You should find about 0.02Ω dynamic resistance.

Also, in open plan placement like it is operating now, you will probably find 30-40mV upward thermal drift after 5 minutes and not a 10mV drop as in my example with hot air blasting. That one was for imitating a closed box rising ambient.

When in open plan, the LEDS are far enough from the board level sink to be much affected, and they will not contribute as much negative mV settling drift. So the closer located CCS chip's positive current drift dominates yet a little and feeds them bit more current pushing up their mV VF more than they compensate by their natural thermal mV VF loss.

In any case you should not find wide enough Vout drift differences from cold boot to hot loaded and back again that may worry you. But test to confirm.
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Old 25th May 2019, 06:52 AM   #126
jimk04 is offline jimk04  United Kingdom
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With respect to all relevant parties, how does the L adapter compare to xrk971s Simple cap multiplier psu board.?
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Old 25th May 2019, 11:49 AM   #127
jackinnj is offline jackinnj  United States
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The output voltage of the L-Adapter would appear to be stable with respect to line voltage. The cap multiplier output voltage is directly related to line voltage.

Maybe a picture or two help. Making cap values equal, the L-Adapter has superior PSRR and pretty decent regulation. The cap multiplier (a generic here using NMOS) has decent PSRR but the output tracks the input once the loss across the FET is taken into account:
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Old 25th May 2019, 01:27 PM   #128
skunark is offline skunark  United States
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What is the rise-time requirement for USB power? I assume as we put this together we will want to test ourselves to avoid issues with a raspberry pi.

Update:
Still looking for the USB spec but a TI document hints at 50ms.
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Old 25th May 2019, 02:27 PM   #129
Salas is offline Salas  Greece
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I don't know the USB RT spec, but I have tested the RPi3 and a 12V Win 10 mini-PC on this PSU without any issues. They both booted fine and held fine even when pressed for CPU usage % on multimedia streaming without freezes or resets. I many times pushed them to stuttering and they would always recover.
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Old 25th May 2019, 04:25 PM   #130
jimk04 is offline jimk04  United Kingdom
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@jackinnj thankyou for a comprehensive answer.

If only I understood it
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