Circuit problem confusion - Page 1
Forum section: electronics / beginners

author: Benjam
author_rank: Regular Contributor
timestamp: May 01, 2022, 08:02:25 pm
content: I encountered some confusion while studying this circuit diagram below, and I have marked a few points of confusion with red circles.1: This inductor is a coupled inductor, PN is: MSD1583-183MED (https://www.mouser.com/datasheet/2/597/msd1583-220521.pdf), my confusion is why is a coupled inductor used here instead of a fixed inductor? Can I replace it with a fixed inductor?2: PN is: AFK477M35H32T-F, this 470uF aluminum electrolytic capacitor, can I replace it with MLCC capacitor? For example, can I replace this 470uF with a 10uF/35V MLCC capacitor (20 pcs)? Of course, MLCC is not cheap, I understand that, I just want to know what will be the difference in the actual circuit effect?3: Why is this defined as DGND instead of AGND and what is its basis?One additional question, how do I calculate its amperage, i.e., 15V?A, how many amps?I'm sorry my question may be stupid, but I'm a self-taught newbie, and I hope I can get some help, thanks guys!

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 01, 2022, 08:51:33 pm
content: The main problem with a simple inductor is that it does not have enough wire connections.Q16 is switching one leg of the transformer to GND, so the corresponding winding in the transformer either has Vin over it, or it's open.This way the amount of current through that winding (and thus the energy input) can be regulated precisely.The output voltage can be either higher or lower then the input voltage.If it was only a single inductor, then it would be a step-up converter, and the (15V) output voltage can never be lower then the Input voltage Vin, because then there would just be a DC current through the inductor and D11.An additional advantage of coupled inductors in a transformer is that the coils can have different number of turns, and this helps if there is a big difference between input and output voltage.The datasheet also is clear about this. The chip is sold as a step-up converter.However, on page 27 of the datasheet, the SEPIC (Single Ended Primari Inductance Converter) is also shown, and it is used there to get a 3.3V output from an input that varies between 3V and 5V. (Thus can either be higher or lower than the output voltage)Analyzing such circuits is quite easy if you remember the basic properties of inductors.* If you put a voltage over them, the current through them starts increasing (and it builds up a magnetic field in the transformer core)* The current can not be interrupted abruptly. It has to go somewhere when the switch opens.* After the switch opens, the rate at which the current reduces depends on the voltage it has to push again. It's all: V = L * dI/dt.The energy is not stored in the "inductor" coil, but in the magnetic field. and the transformer really does not care from which of the inductors the energy is coming from, nor to which side the energy goes when the magnetic field collapses, but it has to go somewhere.And just as with normal transformers, the output voltages of the windings represent the (relative) number of turns for each winding.

author: Benjam
author_rank: Regular Contributor
timestamp: May 01, 2022, 09:16:34 pm
content: Quote from: Doctorandus_P on May 01, 2022, 08:51:33 pmThank you for your patient answer to help me, I seem to understand some of it, but then I don't seem to understand it. If you could give me direct answers based on which 3 questions I asked, I might be able to get some inspiration from them.

author: Benjam
author_rank: Regular Contributor
timestamp: May 01, 2022, 09:20:37 pm
content: Quote from: Doctorandus_P on May 01, 2022, 08:51:33 pmSo it's a Flyback converters circuit?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 02, 2022, 12:52:31 am
content: 1). You really have to understand what the circuit does before you can go change parts for other parts. I think you want to use a step-up configuration instead of sepic.2). Probably not. Too low ESR can cause problems with SMPS circuits. With an SMPS you also can't just throw some parts together and hope it will work.3). All the GND's are connected to each other, but where and how they are connected is an important part of SMPS design. Page 20 of the datasheet has some hints about PCB layout. An SMPS has both parts that generate lots of electrical noise, and parts that need to be kept free of noise. The sensitive parts are the voltage reference inside the IC, and the resistors of the feedback loop, because those are used to measure the output voltage.I had another look at the datasheet, and in one of the example circuits they use SANYO POSCAP 6TPE150MI for the output caps, and three of them (so 450uF total).I find it a bit unusual that an SMPS switching at a relatively high frequency needs such big output capacitors.In another example in the schematic, they use MURATA GRM21BR60J106K and this is a ceramic cap.I'm no expert in SMPS design and am a bit out of my league here.A pretty good read about SMPS design is: Switching Regulators for Poets A Gentle Guide for the Trepidatious Jim Williams September 1987https://html.duckduckgo.com/html?q=switching+regulators+for+poets

author: Benjam
author_rank: Regular Contributor
timestamp: May 02, 2022, 06:40:14 am
content: Quote from: Doctorandus_P on May 02, 2022, 12:52:31 amYou are a good man, a good teacher, thank you for helping me, I will try to continue doing research again, sooner or later I will understand the understanding.

author: Benjam
author_rank: Regular Contributor
timestamp: May 02, 2022, 09:06:28 am
content: Another doubt, the output of this circuit 15V is isolated?

author: Benjam
author_rank: Regular Contributor
timestamp: May 02, 2022, 09:36:02 am
content: I probably understand that the 15V is not isolated because they use a common PGND

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 02, 2022, 10:03:57 am
content: The output is definitely not isolated. The feedback from R105 and R103 is also taken directly from the output.For an isolated output this loop would have to be cut. Usually with a TL431 on the secondary side and an opto coupler.Also a magic word: "Hot Loop".Research this a bit. It There are some good explanations out there to get you into the right mindset of why and how to optimize PCB layout.

author: Benjam
author_rank: Regular Contributor
timestamp: May 02, 2022, 10:56:25 am
content: Quote from: Doctorandus_P on May 02, 2022, 10:03:57 amThanks!!!