AC voltage rectification for switching IC - Page 1
Forum section: electronics / beginners

author: Godzilla
author_rank: Contributor
timestamp: December 14, 2020, 03:41:27 pm
content: Hello guys, im trying to design a little switching power supply using LM22670, i want to use 230V AC as voltage supply so im gonna lower it with a transformer 230V/12V and rectify it with a full bridge rectifier. After that i was thinking about using a big 4700uF capacitor to decrease the ripple as much as possible and other 2/3 low value capacitors to filter noise. Is this solution gonna be good enough to be the input of the IC? Or am i gonna need more regulation?Ive attached the schematic to be more clear. Thanks.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: December 14, 2020, 06:55:15 pm
content: What is the point of making a bulky linear primary power supply when using a secondary pulse? This, of course, is not bad and will work, but it is more compact and convenient to get the necessary voltage from a single pulsed source of 220V.

author: fourtytwo42
author_rank: Super Contributor
timestamp: December 14, 2020, 07:13:47 pm
content: Can you tell us what is the intended output voltage and maximum load current of the LM22670 is ?There is no point suppressing noise on the input as the switching process generates lots, so you need to pay attention to the output.You only need to suppress the 100hZ ripple on the input (V=IT/C) and provide the LM22670 recommended decoupling

author: Godzilla
author_rank: Contributor
timestamp: December 14, 2020, 09:00:02 pm
content: I want to use it to supply 3.3V at 3A, im learning how to create schematics and PCB so i choose this for a project. Another idea is to run more IC in sync and create more outputs, like 5V and 12V. Honestly i read the datasheet and in the input section it said that i should provide a nice regulated input voltage that shouldnt change on the load current, so i thought that a big cap was what i needed, but yeah 4700u its probably too much, the other caps are what the datasheet advice to use to decouple the input...

author: fourtytwo42
author_rank: Super Contributor
timestamp: December 15, 2020, 09:44:34 am
content: OK so the output power is ~9W according to TI the efficiency is around 80% (on the safe side) so the input power will be ~11W.Your peak DC voltage will be (12*1.4)-1.5 rect drop = 15V, the regulator will work with a minimum input voltage of 5V so the maximum ripple is 10V, in that case the average input voltage would be 5+(10/2) =10V so the input current is ~1A, using C=IT/V gives 1000uF so that is your minimum capacitance, anything bigger will also work

author: Godzilla
author_rank: Contributor
timestamp: December 15, 2020, 01:43:33 pm
content: Oh perfect, is switching regulation less sensitive to ripple than linear regulation? I remember using 4700uF capacitors for my old projects with LM317. Also i have some questions on SMD components choices for the project, can i ask here or should i open a new thread?

author: fourtytwo42
author_rank: Super Contributor
timestamp: December 15, 2020, 04:33:08 pm
content: I was just illustrating how to arrive at the minimum capacitance required. There are two problems with any regulator you should seek to avoid, the worst being the input voltage dipping below the minimum input to output headroom required (I seem to remember the LM317 likes lots of headroom) and the other being the input ripple rejection capability of the regulator control loop.You might also like to allow for your local mains variation in the calculation and any transformer secondary drop on load that I have not included, certainly using 4700uF will give you more margin for such effects.If you want to discuss SMT best to start another thread

author: David Hess
author_rank: Super Contributor
timestamp: December 15, 2020, 09:11:37 pm
content: Quote from: S. Petrukhin on December 14, 2020, 06:55:15 pmThe line transformer provides the necessary galvanic isolation while the switching regulator yields better efficiency than a linear regulator. In the past this configuration was more common but it is still found in some low power applications.Quote from: Godzilla on December 15, 2020, 01:43:33 pmNo, switching regulators do not provide better line or load regulation than linear regulators. These depend on the performance of the error amplifier.

author: Zero999
author_rank: Super Contributor
timestamp: December 16, 2020, 11:09:05 am
content: Switching regulators have a different ripple characteristic, than linear regulators. When given the same load resistance, linear regulator draws an almost constant current from the input, irrespective of the voltage, as long as it's not operating in the drop-out region. A switching regulator will draw roughly the same power, irrespective of the input voltage, so the current will increase, as the voltage drops.

author: Godzilla
author_rank: Contributor
timestamp: December 18, 2020, 10:35:55 pm
content: If i want to build a bench power supply, should i aim for a linear or a switch regulation? The fact that i still need to convert the 230 main voltage to something smaller force me to use bulky transformer, or do i have another way to achieve at least 3A of current in the output with at least 12V?

author: Zero999
author_rank: Super Contributor
timestamp: December 19, 2020, 11:19:58 am
content: Quote from: Godzilla on December 18, 2020, 10:35:55 pmIf you go for linear, you'll have to dissipate the power, using large transistors and a heat sink, but it's generally easier to do and is less noisy. Switched mode power supplies are lighter, but more noisy and not so beginner friendly.If you want to avoid the big transformer. You could always use a ready made switched mode power supply to reduce the mains voltage to something safe and add your own switched mode power supply.