How to add soft-start feature to a PSU?

[Arjunan M R]

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.

[TheHolyHorse]

I think you'll need to upload new images with higher resolution, you can't really make out any of it. 

[Arjunan M R]

I think you'll need to upload new images with higher resolution, you can't really make out any of it. 

i kow that but  I also uploaded the pdf version.

[Arjunan M R]

I think you'll need to upload new images with higher resolution, you can't really make out any of it. 

Welcome to the EEVblog Forum

[Arjunan M R]

Click on that prescicion bench power supply.pdf

[TheHolyHorse]

Oh dang I'm stupid, sorry

[Arjunan M R]

Oh dang I'm stupid, sorry

I was also stupid when i came to the forum

[TheHolyHorse]

Nice to hear I'm not the only one I'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.

[TheHolyHorse]

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.

[Arjunan M R]

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.

Yeah, 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.

[Arjunan M R]

Nice to hear I'm not the only one I'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.

it'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)

[Arjunan M R]

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.

Yeah, 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.

And an 16X2 LCD 

[David Hess]

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.

[Arjunan M R]

Nice to hear I'm not the only one I'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.

I am also having that feeling of I have done something wrong or stupid.

[Arjunan M R]

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.

That error amp is for current not voltage:

Or are you talking about this:

[Arjunan M R]

I don't know why i get such low res image when i take a schreenshot.

[not1xor1]

please correct that awful prescicion

it is precision from Latin praecisio (-onis)

[Arjunan M R]

please correct that awful prescicion

it is precision from Latin praecisio (-onis)

This topic is getting off topic.

[TheHolyHorse]

Nice to hear I'm not the only one I'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.

it'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)

Whopsie gg english

[TheHolyHorse]

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.

Yeah, 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.

I 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 hardware

BTW out of curiosity what micro are you using?

[Arjunan M R]

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.

Yeah, 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.

I 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 hardware

BTW out of curiosity what micro are you using?

Yes 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 and
Makes sure that I don't blow  up something because i am programming it when everything is connected.

[Arjunan M R]

Just using Arduino uno because I have three of them sitting here without any use.

[exe]

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.

[Arjunan M R]

Btw, LT3080xST, afaik, has a minimal load specification. You should pull 1mA from it or so, or it will not regulate well small currents.

I have a constant current source in the schematic.
It's a current mirror.

[Arjunan M R]

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.

Just using IRFZ44N for the same reason as arduino uno.

[Arjunan M R]

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.

Stm32 is expensive.
I will stick with what i have to keep the cost down.

[Arjunan M R]

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.

Yeah 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.

[Arjunan M R]

I think paralelling D2 and D8 is redundant, imho. A uni-directional TVS would work the same, imho. What was the reason behind it?

Yes a unidirectional TVS would work fine .

[TheHolyHorse]

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.

Stm32 is expensive.
I will stick with what i have to keep the cost down.

Well 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.

[Arjunan M R]

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.

Stm32 is expensive.
I will stick with what i have to keep the cost down.

Well 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.

Yeah , but then I have to buy the programmer and learn how to programm it.

[Arjunan M R]

Yes, it's cheaper than arduino.

[Siwastaja]

Yeah , but then I have to buy the programmer

$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.)

learn how to programm it.

This 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 in one 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.

[TheHolyHorse]

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.

Stm32 is expensive.
I will stick with what i have to keep the cost down.

Well 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.

Yeah , but then I have to buy the programmer and learn how to programm it.

You 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.

[TheHolyHorse]

Yeah , but then I have to buy the programmer

$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

You don't have to buy anything but a usb connector for like 0.4€

[Siwastaja]

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.

[TheHolyHorse]

I jumped in the STM32 world (from 8-bit AVR) by directly designing it in in an actual customer project and promising delivery in one 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.

Yeah I've only played with some AVR stuff before as well so this will be my first project with the STM32

[exe]

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.

[Kleinstein]

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.

[TheHolyHorse]

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.

You can get boards from JLC for almost nothing and solder LQFP with 0.5mm pitch.

[Arjunan M R]

 Then what is the best way to add current limiting?

[Arjunan M R]

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.

You can get boards from JLC for almost nothing and solder LQFP with 0.5mm pitch.

But in my country i can't order from JLC.(that's not a problem)

[TheHolyHorse]

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.

I'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

[TheHolyHorse]

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.

You can get boards from JLC for almost nothing and solder LQFP with 0.5mm pitch.

But in my country i can't order from JLC.(that's not a problem)

There are other cheap chines pcb manufacturer that will do the job.

[TheHolyHorse]

Then what is the best way to add current limiting?

Mine is just using some mosfet as pass transistor. It's basically doing what the voltage regulator is doing internally.

[Arjunan M R]

Then what is the best way to add current limiting?

Mine is just using some mosfet as pass transistor. It's basically doing what the voltage regulator is doing internally.

Yeah, thats much simpler .
Can you post the schematic.

[TheHolyHorse]

Then what is the best way to add current limiting?

Mine is just using some mosfet as pass transistor. It's basically doing what the voltage regulator is doing internally.

Yeah, thats much simpler .
Can you post the schematic.

you can download the entire project from https://github.com/JeppeSRC/PowerSupply

[Arjunan M R]

Here is mine.

[TheHolyHorse]

Here is mine.

BTW 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.

[TheHolyHorse]

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.

[Arjunan M R]

Here is mine.

BTW 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.

Sorry.I didn't save it into project specific library.What can I do.
Are you having problems with footprints.

[TheHolyHorse]

Here is mine.

BTW 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.

Sorry.I didn't save it into project specific library.What can I do.
Are you having problems with footprints.

Not the footprints only library symbols not showing up.

[Arjunan M R]

 After looking at your  schematic it turns out that you are doing I limiting as i am doing

[TheHolyHorse]

Yes

[Arjunan M R]

Then what is the best way to add current limiting?

Mine is just using some mosfet as pass transistor. It's basically doing what the voltage regulator is doing internally.

[TheHolyHorse]

Then what is the best way to add current limiting?

Mine is just using some mosfet as pass transistor. It's basically doing what the voltage regulator is doing internally.

Oh well I just meant that I used mosfets instead of volt regs.

[exe]

Oh well I just meant that I used mosfets instead of volt regs.

You 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.

[TheHolyHorse]

Oh well I just meant that I used mosfets instead of volt regs.

You 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.

.
Yeah, I'm using FQP17P10 I think that's the name. 20 ish volts and 4A

[exe]

Yeah, I'm using FQP17P10 I think that's the name. 20 ish volts and 4A

Cool, this one has SOA down to DC. That's rare nowadays

[TheHolyHorse]

Yeah, I'm using FQP17P10 I think that's the name. 20 ish volts and 4A

Cool, this one has SOA down to DC. That's rare nowadays

Yeah I hope I get it working 

[Arjunan M R]

Here is mine.

BTW 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.

Sorry.I didn't save it into project specific library.What can I do.
Are you having problems with footprints.

Not the footprints only library symbols not showing up.

Fixed it, try now .

[BravoV]

Cool, this one has SOA down to DC. That's rare nowadays

Becareful, the SOA shown that its has DC chart, it is not realistic temperature as 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.

[TheHolyHorse]

Cool, this one has SOA down to DC. That's rare nowadays

Becareful, the SOA shown that its has DC chart, it is not realistic temperature as 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.

I'll have to see if it explodes If it doesn't great, if it does well now I've learned something new.