Ampere doubts - Page 1
Forum section: electronics / beginners

author: Benjam
author_rank: Regular Contributor
timestamp: April 28, 2022, 08:11:48 am
content: I see this block diagram below, how many amps should a 12V power supply provide to support stable operation? Is 3A + 1.2A + 3A + 2A + 3A + (2x0.3A) = 12V12.8A required, or is 12V3A sufficient?

author: tszaboo
author_rank: Super Contributor
timestamp: April 28, 2022, 08:20:14 am
content: For any LDO the output current is equal to the input current. For any Buck converter, or DC-DC converter the output power is equal to the input power. And then you include some inefficiency into the system, say you multiply the result by 1.2, or the expected value from the datasheetAlso, see where the LDOs are powered from, because some LDO will be powered by a DC-DC. You work your way from the right to the left, and fill out for each node the current, the voltage and the power.

author: Fredderic
author_rank: Regular Contributor
timestamp: April 28, 2022, 09:17:24 am
content: I'd assume the answer to the question, is simply that you need to provide for the maximum of required voltages, and the sum of required currents. (Including consideration of overheads, and if there are groups of current drawing components where only one group can ever be active at a time, then it is likewise the maximum of the sums for each group.)

author: Nominal Animal
author_rank: Super Contributor
timestamp: April 28, 2022, 11:57:01 am
content: You have both DC-DC converters and LDOs.First, add the LDOs input current (which is the same as their output current) to the output current requirements of the DC-DC converters they hang off of. This is because linear regulator input current is the same as their output current.That gives you the voltages and currents the DC-DC converters need to provide. I think they are 1.0V @ 2.0A, 1.8V @ 0.5A, 1.5V @ 4A, and 3.3V @ 2.9A; but do check. We multiply and sum these to get the overall DC-DC converter output power: 2.0 W + 0.9 W + 6.0 W + 9.6 W = 18.5 W.We divide that by the efficiency of the DC-DC converters to get the expected input power. Something like 75% efficiency should be a realistic estimate; note that one should look at the efficiency curves of the DC-DC converters at that specific output current and voltage. (18.5 W / 0.75 ≃ 24.7 W)If we look at theMPM3630datasheet, we can find the expected efficiency curves for 12V input:- 1.0V @ 2.0A output: efficiency ≃ 77%- 1.8V @ 0.5A output: efficiency ≃ 75%- 1.5V @ 4.0A output: efficiency ≃ 85%- 3.3V @ 2.9A output: efficiency ≃ 90%Thus, the input power would be 1.0 V · 2.0 A / 0.77 + 1.8 V · 0.5 A / 0.75 + 1.5 V · 4.0 A / 0.85 + 3.3 V · 2.9 A / 0.90 ≃ 21.5W.Of course, nothing is as good in practice as it is on paper, so if we decrease the efficiencies to say 70%, 70%, 75%, and 80%, the input power would be 2.0 W / 0.70 + 0.9 W / 0.70 + 6.0 W / 0.75 + 9.6 W / 0.80 ≃ 24.1 W.At 12V input voltage, the input current to the DC-DC converters would be 21.5 W / 12 V ≃ 1.8 A, or 24.1 W / 12 V ≃ 2.0 A, or 24.7 W / 12 V ≃ 2.1 A, depending on what efficiency figures you use.Note that this is steady-state, DC voltages and currents.Momentarydraw can be somewhat more, so you do need some headroom in the input current as well, so that if the current draw is higher for short durations (say, fractions of a second), the voltages stay stable.Based on this – and I'm hoping you do your own research, using same or similar steps, to find your own conclusions! –, I think a power supply that can provide 3A at 12V to the DC-DC converters should work just fine, and have a couple of watts to spare for peripherals and LEDs and such, if needed.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 28, 2022, 09:05:45 pm
content: Is this an homework assignment?As said, before, for an LDO, the input current is the same as the output current (Except maybe for a few uA)A small SMPS generally has an efficiency between 70% and 95%

author: Benjam
author_rank: Regular Contributor
timestamp: April 29, 2022, 06:54:52 am
content: Quote from: Nominal Animal on April 28, 2022, 11:57:01 amThank you very much for your explanation, very clear, you are a master!

author: Benjam
author_rank: Regular Contributor
timestamp: April 29, 2022, 06:55:56 am
content: Quote from: Doctorandus_P on April 28, 2022, 09:05:45 pmPlease respect a middle-aged man who taught himself circuits, I was just a music teacher before. THANKS!

author: Nominal Animal
author_rank: Super Contributor
timestamp: April 29, 2022, 07:17:47 am
content: Quote from: Benjam on April 29, 2022, 06:54:52 amNo, I am just a verbose uncle bumblefug hobbyist... but happy to help!

author: Benjam
author_rank: Regular Contributor
timestamp: April 29, 2022, 07:41:04 am
content: Quote from: Nominal Animal on April 29, 2022, 07:17:47 amLove you...