Chaining Voltage Regulators? - Page 1
Forum section: electronics / beginners

author: MuhScopeBroke
author_rank: Contributor
timestamp: May 13, 2023, 11:46:24 am
content: Hello all,I'm working on a project where we have a 24V battery that supplies 3 different rails, 12V, 5V and 3V3.I am curious to see what you all think is more efficient power-wise, having the 24V batteries run straight to voltage regulators that output 5V/3V3 or (rather than using the 24V battery) use the output of the 12V voltage regulator as the input.The voltage regulators I'm using are some cheap Chinese boards from amazon so no data sheets unfortunately.Might be hard to answer without that data but what would you do?Thanks

author: Siwastaja
author_rank: Super Contributor
timestamp: May 13, 2023, 11:58:56 am
content: If all linear regulators, it doesn't matter - efficiency is the same.Calculate power dissipated per regulator in each possible configuration, though - use the configuration which heats the regulators as evenly as possible.

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 13, 2023, 12:00:12 pm
content: Better to run parallel regulators from the 24V battery. If you run the 5V regulator after the 12V then the 3V after the 5V then the 12V regulator has to supply the current enough for all 3 circuit. For example if you have 3 5A regulators and you run one after another you have only 5A total. Running them in parallel you have 15A total.

author: AndyBeez
author_rank: Frequent Contributor
timestamp: May 13, 2023, 12:31:15 pm
content: Agreed above - with buck converter boards. So we can assume your Vregs are 'buck' converters and not linear regulators? Regardless of supplier, the devices will have a datasheet for the parts somewhere. Unless they are so cheap they could only be fakes?btw If you drive 24V into a 5V or 3V3 linear regulator, you will produce a lot of heat. Possibly going out of spec too. If you daisy chain linear regulators you will still waste a lot of energy. It's a battery, so think efficiency.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 13, 2023, 04:40:53 pm
content: Quote from: BeBuLamar on May 13, 2023, 12:00:12 pmYou can't make that kind of generalization. For example, for a linear 24 to 12V and 5V system, case 1:Vin = 24VI12 = 0.01AI5 = 1ABoth regulators inputs paralleled from 24V:Preg_12V = (24-12)V * 0.01A = 0.12WPreg_5V = (24-5)V * 1A = 19WTotal: 19.12WRegulators in cascade:Preg_12V = (24-12)V * 1.01A = 12.12WPreg_5V = (12-5)V * 1A = 7WTotal: 19.12WAssuming similar regulator packages, you can clearly see cascaded solution is superior for heatsinking.Different case:I12 = 1AI5 = 1AInputs in parallel:Preg_12V = (24-12)V*1A = 12WPreg_5V = (24-5)V*1A = 19WTotal 31WRegulaters cascaded:Preg_12V = (24-12)V*2A = 24WPreg_5V = (12-5)V*1A = 7WTotal 31WIn this case, parallel input solution is easier to heatsink. So it depends on load currents. This is basic circuit analysis and Ohm's law, ignore stupid rules of thumbs and calculate and choose wisely.In the world of switch mode converters, it's quite common to have hybrid solution of switchers plus linear regulators. These are often chained, like doing the +12V from +24V using a switcher, then going down to 5V and 3.3V using linear regs. Although if the current from 5V is high, then it would be appealing to use a switcher for that too (either from +24V, or +12V, there is no generalization which is better), and only use a linear reg to go from 5V to 3.3V.One reason to chain regulators is, the lower you place your max Vin requirement, the wider the choice for parts. You can find some super low cost, miniatyrized regulator ICs with Vin_max = 6V or 7V.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 13, 2023, 04:50:42 pm
content: Quote from: AndyBeez on May 13, 2023, 12:31:15 pmOnly if you use a lot of current. Sometimes linear regulators are suitable even for large voltage differentials simply because the circuits are so low-power. Think about a 3.3V microcontroller running at 0.5mA and controlling a relay, coil of which is driven from that 24V source directly. Efficiency% is crap, but if absolute power consumption is still low enough, that does not matter.

author: MrAl
author_rank: Super Contributor
timestamp: May 14, 2023, 06:02:15 am
content: Quote from: MuhScopeBroke on May 13, 2023, 11:46:24 amHi,Let's look first at the two regulator case.We have a 20v constant input source and a 10v regulator and a 5v regulator.The 10v regulator has to handle 1 amp for the 10v output. The 5v regulator has to handle 1 amp.If the 10 and 5v regulators are connected to the 20v source directly, the power wasted because of the 10v regulator is 10 watts and the power wasted by the 5v regulator is 15 watts. Total 25 watts.If the 10v regulator is connected to the 20v source directly and the 5v regulator is connected to the 10v regulator, then the 5v regulator wastes 5 watts and the 10v regulator wastes 20 watts (2 amps at 10 volts drop). Total 25 watts.However, in the second case the 10v regulator has to handle 2 times the power as in the first case while the 5v regulator now only has to handle 1/3 of the power as in the first case. This indicates that the actual part numbers will come into play here. If the 5v regulator can handle 15 watts then the first case may be ok, but if it can only handle 10 watts then the first case will not work at all. Similarly, if the 10v regulator can not handle 20 watts then the second case will never work either.All of this depends highly on the size of the heat sink used for each regulator also.

author: mariush
author_rank: Super Contributor
timestamp: May 14, 2023, 06:18:53 am
content: Quote from: MuhScopeBroke on May 13, 2023, 11:46:24 amIt depends on the amount of current on each rail.The higher the input - output difference, the less efficient a step-down regulator would be. So it may make sense to have a 24v-12v converter but not so much a 24v - 3.3v converter.Some step-down converters need minimum loads, let's say 5-10mA in order to regulate properly or to not go into PFM or some other skip modes where the voltage ripple could be higher and efficiency much worse.So it may be preferable to load the 24v - 5v step-down regulator with a step-down 5v-3.3v or a linear regulator (if the current amount on 3.3v is low enough), as the quiescent current of the regulator plus maybe 1-2mA into a stand-by/power-on led could be enough to keep the 24v-5v regulator happy, with a minimum load of a few mA.In some scenarios it could also be a plus to have a linear regulator dissipate some energy as heat, for example to have 5v to 3.3v LDO dissipate a watt or so of power heating up the area. For example maybe it's a device that has to live on a truck or car in a compartment that could get very cold... in that case that watt of heat could be beneficial.