How to add soft-start feature to a PSU? - Page 1
Forum section: electronics / beginners

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 22, 2019, 05:41:15 am
content: I am designing a 18V 1A linear PSU.How can I add soft-start feature to it?Which needs a slew rate of 18V/100ms or higher upto 500ms?Here's the schematic of the psu.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 22, 2019, 02:16:46 pm
content: I think you'll need to upload new images with higher resolution, you can't really make out any of it.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 22, 2019, 02:18:35 pm
content: Quote from: TheHolyHorse on June 22, 2019, 02:16:46 pmi kow that but I also uploaded the pdf version.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 22, 2019, 02:20:05 pm
content: Quote from: TheHolyHorse on June 22, 2019, 02:16:46 pmWelcome to the EEVblog Forum

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 22, 2019, 02:21:18 pm
content: Click on that prescicion bench power supply.pdf

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 22, 2019, 02:45:48 pm
content: Oh dang I'm stupid, sorry

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 22, 2019, 02:48:14 pm
content: Quote from: TheHolyHorse on June 22, 2019, 02:45:48 pmI was also stupid when i came to the forum

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 22, 2019, 02:54:24 pm
content: Nice to hear I'm not the only oneI'm actually working on a PSU as well, it's all layout all I have to do is order the PCB but I'm a bit hesitant. It feels like I've done something wrong, but there's only one way to know isn't it.Anyway I'm not gonna ramble about my stuff, lets stay on topic.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 22, 2019, 03:02:58 pm
content: As for the topic, can't you just set the voltage slowly. I assume you're gonna control everything with some micro so doing that in software will be ezpz.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 22, 2019, 03:29:08 pm
content: Quote from: TheHolyHorse on June 22, 2019, 03:02:58 pmYeah, but the code is already complicated enough for me.With 3 devices on I²C and 1 on SPI.I am trying to do most of the things in the hardware side as possible.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 22, 2019, 03:32:52 pm
content: Quote from: TheHolyHorse on June 22, 2019, 02:54:24 pmit's "it's all laid out" not "it's all layout".I am designing the PCB right now.All the components are laid out.I have to now rout 300+ connections(most of em are GND and VCC and I already connected about 75)

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 22, 2019, 03:34:52 pm
content: Quote from: Arjunan M R on June 22, 2019, 03:29:08 pmAnd an 16X2 LCD

author: David Hess
author_rank: Super Contributor
timestamp: June 22, 2019, 03:36:54 pm
content: The common way is to delay the startup of the reference, maybe with an under-voltage lockout, or limit the rise time of the reference voltage applied to the error amplifier.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 22, 2019, 03:37:36 pm
content: Quote from: TheHolyHorse on June 22, 2019, 02:54:24 pmI am also having that feeling of I have done something wrong or stupid.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 22, 2019, 03:41:23 pm
content: Quote from: David Hess on June 22, 2019, 03:36:54 pmThat error amp is for current not voltage:Or are you talking about this:

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 22, 2019, 03:43:16 pm
content: I don't know why i get such low res image when i take a schreenshot.

author: not1xor1
author_rank: Frequent Contributor
timestamp: June 22, 2019, 04:26:36 pm
content: please correct that awfulprescicionit isprecisionfrom Latinpraecisio (-onis)

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 22, 2019, 04:31:21 pm
content: Quote from: not1xor1 on June 22, 2019, 04:26:36 pmThis topic is getting off topic.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 22, 2019, 06:23:14 pm
content: Quote from: Arjunan M R on June 22, 2019, 03:32:52 pmWhopsie gg english

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 22, 2019, 06:29:46 pm
content: Quote from: Arjunan M R on June 22, 2019, 03:29:08 pmI see, I'm more experienced with software dev than electronics so that just seemed obvious to me.You could do a simple RC thing with a large time constant, but that's gonna affect it all the time not only during start up. I think I'm to much of a noob to come with any meaningful advise when it comes to doing it in hardwareBTW out of curiosity what micro are you using?

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 23, 2019, 02:33:45 am
content: Quote from: TheHolyHorse on June 22, 2019, 06:29:46 pmYes I am going to RC things to make softstart.Just to simplify things a bit I am using an off the shelf arduino uno(ATmega328P).I first decided to use just a MCU IC but it consumes a lot of space with the oscillators and all the things it needs to work.When using a board like arduino uno I can just put it near the pcb and connect.That simplifies the board design andMakes sure that I don't blow up something because i am programming it when everything is connected.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 23, 2019, 02:35:20 am
content: Just using Arduino uno because I have three of them sitting here without any use.

author: exe
author_rank: Unknown
timestamp: June 23, 2019, 07:29:10 am
content: Q6 doesn't look right to me, too big for the job, imho. Why not using something smaller? Gate capacitance can be a stability problem if PSU supports CC mode.I think paralelling D2 and D8 is redundant, imho. A uni-directional TVS would work the same, imho. What was the reason behind it?As for MCU, for my PSU I chose stm32f373 as it has three DACs and and two or so 16bit ADC (datasheet claims 14bits noise-free). This way I reduced BOM, and avoided i2c/spi devices (and writing drivers for them). But I'm yet to finish the project, so not sure how the MCU is good for the job.Btw, LT3080xST, afaik, has a minimal load specification. You should pull 1mA from it or so, or it will not regulate well small currents.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 23, 2019, 08:01:01 am
content: Quote from: exe on June 23, 2019, 07:29:10 amI have a constant current source in the schematic.It's a current mirror.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 23, 2019, 08:02:16 am
content: Quote from: exe on June 23, 2019, 07:29:10 amJust using IRFZ44N for the same reason as arduino uno.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 23, 2019, 08:03:50 am
content: Quote from: exe on June 23, 2019, 07:29:10 amStm32 is expensive.I will stick with what i have to keep the cost down.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 23, 2019, 08:08:10 am
content: Quote from: exe on June 23, 2019, 07:29:10 amYeah I might want to change the MOSFET.Its input capacitance is 1470pF.I have a pull down resistor of 4K7 at the input.So, it will take 4 µs to turn on or off the MOSFET. It's a lot. I will change the MOSFET.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 23, 2019, 08:10:34 am
content: Quote from: exe on June 23, 2019, 07:29:10 amYes a unidirectional TVS would work fine.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 23, 2019, 10:09:23 am
content: Quote from: Arjunan M R on June 23, 2019, 08:03:50 amWell not really, the STM32F373 cost less than MCP3421A0T-ECH and MCP4922 combined and you'll get everything in one package. And as exe stated higher precision SDADC and IIRC the internal DACs are also better.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 23, 2019, 10:13:59 am
content: Quote from: TheHolyHorse on June 23, 2019, 10:09:23 amYeah , but then I have to buy the programmer and learn how to programm it.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 23, 2019, 10:14:59 am
content: Yes, it's cheaper than arduino.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 23, 2019, 10:18:26 am
content: Quote from: Arjunan M R on June 23, 2019, 10:13:59 am$1.42:https://www.ebay.com/itm/USB-To-RS232-TTL-UART-PL2303HX-Auto-Converter-USB-to-COM-Cable-Adapter-Module/201539573206?hash=item2eecb1d5d6:g:sUoAAOSwyXVbaQjC(using the bootloader. I have never used anything else than the built-in UART bootloader, or my own bootloaders written later. This is just fine, ignore the naysayers.)QuoteThis is true, there's quite a bit of learning curve involved. The bright side is, it's going to be a very universal skill, over wide range of projects.I jumped in the STM32 world (from 8-bit AVR) by directly designing it in in an actual customer project and promising delivery inone month. Funny times. In the end, the customer was satisfied despite delays (took two months to have anything work at all), and I learned a lot, but it happened quickly, and with the customer, we designed a proper V2 (using STM32 as well) a year later, with more realistic schedules.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 23, 2019, 10:19:09 am
content: Quote from: Arjunan M R on June 23, 2019, 10:13:59 amYou can just plug in the usb download the DFU tool from st and flash it real easy. As for the programming it will probably be a bit more complicated, but that all depends on programming experience. The arduino will by far be easier, so if you're more comfortable with that go for it.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 23, 2019, 10:21:50 am
content: Quote from: Siwastaja on June 23, 2019, 10:18:26 amYou don't have to buy anything but a usb connector for like 0.4€

author: Siwastaja
author_rank: Super Contributor
timestamp: June 23, 2019, 10:24:19 am
content: Yeah, some indeed have the USB bootloader. UART works for the smaller STM32's not equipped with the USB peripheral or factory bootloader functionality for it.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 23, 2019, 10:44:02 am
content: Quote from: Siwastaja on June 23, 2019, 10:18:26 amYeah I've only played with some AVR stuff before as well so this will be my first project with the STM32

author: exe
author_rank: Unknown
timestamp: June 23, 2019, 11:06:24 am
content: To me stm32 has a huge learning curve (may be because I'm on Linux). But I don't know an alternative that would be that universal. I use Chibios for firmware. I don't use CubeMX because the code and API it generates is absolutely not to my liking. It's fine for led blinking, but when I tried more sophisticated programs I found Chibios much easier to use. Still difficult nonetheless.BTW, I'm giving esp32+8Mb psram + micropython a chance. But it doesn't have many IO pins, so I'm stuck with SPI displays.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 23, 2019, 11:15:10 am
content: The choice of µC is probably the least problem. One difficulty with the STM32 is that they come in really small cases only. Not everybody can solder this, especially on a board with limited quality. One can do something like AVR or PIC18 on a prototype board - the STM32 essentially needs a professional made board with solder mask.A big problem of the circuit shown is that it starts with a voltage regulator and kind of adds a poor current limit as a an afterthought. This sytems tends to oscillate. For the same reason Dave's 1 st Version of the µ-supply never got finished. Designing a lab supply (combined voltage and current regulation) is not that easy as it sounds. Using a voltage regulator chips usually does not really help - it is more like making things more difficult.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 23, 2019, 11:21:48 am
content: Quote from: Kleinstein on June 23, 2019, 11:15:10 amYou can get boards from JLC for almost nothing and solder LQFP with 0.5mm pitch.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 23, 2019, 11:22:37 am
content: Then what is the best way to add current limiting?

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 23, 2019, 11:24:09 am
content: Quote from: TheHolyHorse on June 23, 2019, 11:21:48 amBut in my country i can't order from JLC.(that's not a problem)

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 23, 2019, 11:26:47 am
content: Quote from: exe on June 23, 2019, 11:06:24 amI'm gonna do everything from scratch, that's all part of the fun(for me at least). If you just wanna get things running you'll probably be better of using something like Chibios like you're doing. But I've never been a fan of that, not that there's anything wrong with em I just like programming

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 23, 2019, 11:28:05 am
content: Quote from: Arjunan M R on June 23, 2019, 11:24:09 amThere are other cheap chines pcb manufacturer that will do the job.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 23, 2019, 11:31:42 am
content: Quote from: Arjunan M R on June 23, 2019, 11:22:37 amMine is just using some mosfet as pass transistor. It's basically doing what the voltage regulator is doing internally.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 23, 2019, 11:34:51 am
content: Quote from: TheHolyHorse on June 23, 2019, 11:31:42 amYeah, thats much simpler.Can you post the schematic.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 23, 2019, 11:38:14 am
content: Quote from: Arjunan M R on June 23, 2019, 11:34:51 amyou can download the entire project fromhttps://github.com/JeppeSRC/PowerSupply

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 23, 2019, 12:00:02 pm
content: Here is mine.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 23, 2019, 12:06:38 pm
content: Quote from: Arjunan M R on June 23, 2019, 12:00:02 pmBTW if you're making your own symbols and footprints save them in a project specific library and include that library with the project files. RN some symbols are missing.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 23, 2019, 12:20:51 pm
content: Also "a trap for you players" like dave would've said. Check the minimum soldermask width. If you look at the attached images you'll see that on one image it's just two big blocks instead of each individual pad. It's gonna be real difficult to solder if you don't get that soldermask between the pads.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 23, 2019, 12:30:22 pm
content: Quote from: TheHolyHorse on June 23, 2019, 12:06:38 pmSorry.I didn't save it into project specific library.What can I do.Are you having problems with footprints.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 23, 2019, 12:33:02 pm
content: Quote from: Arjunan M R on June 23, 2019, 12:30:22 pmNot the footprints only library symbols not showing up.

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 25, 2019, 12:55:28 pm
content: After looking at your schematic it turns out that you are doing I limiting as i am doing

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 27, 2019, 05:32:42 am
content: Yes

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 27, 2019, 09:49:44 am
content: Quote from: TheHolyHorse on June 23, 2019, 11:31:42 am

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 27, 2019, 09:55:42 am
content: Quote from: Arjunan M R on June 27, 2019, 09:49:44 amOh well I just meant that I used mosfets instead of volt regs.

author: exe
author_rank: Unknown
timestamp: June 27, 2019, 09:58:58 am
content: Quote from: TheHolyHorse on June 27, 2019, 09:55:42 amYou mean discrete regulator?What fet did you use and what is the voltage/current accross it? Most fets are not rated for linear operation, but if it is beefy and battle-tested then you probably can get away with it.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 27, 2019, 10:14:52 am
content: Quote from: exe on June 27, 2019, 09:58:58 am.Yeah, I'm using FQP17P10 I think that's the name. 20 ish volts and 4A

author: exe
author_rank: Unknown
timestamp: June 27, 2019, 01:59:18 pm
content: Quote from: TheHolyHorse on June 27, 2019, 10:14:52 amCool, this one has SOA down to DC. That's rare nowadays

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 27, 2019, 04:53:53 pm
content: Quote from: exe on June 27, 2019, 01:59:18 pmYeah I hope I get it working

author: Arjunan M R
author_rank: Regular Contributor
timestamp: June 29, 2019, 06:31:08 am
content: Quote from: TheHolyHorse on June 23, 2019, 12:33:02 pmFixed it, try now.

author: BravoV
author_rank: Super Contributor
timestamp: June 29, 2019, 10:05:53 am
content: Quote from: exe on June 27, 2019, 01:59:18 pmBecareful, the SOA shown that its has DC chart, it isnot realistic temperatureas the TC at 25C, especially its not a linear mosfet.Once crossed the 25C, you just can not interpolated that DC chart, as sometimes the max current will be reduced significantly, especially for non linear mosfets.An example of a proper and true linear mosfet FBSOA, watch the yellow highlighted areas and compare both charts.

author: TheHolyHorse
author_rank: Regular Contributor
timestamp: June 29, 2019, 09:51:18 pm
content: Quote from: BravoV on June 29, 2019, 10:05:53 amI'll have to see if it explodesIf it doesn't great, if it does well now I've learned something new.