Author Topic: How to add soft-start feature to a PSU?  (Read 6722 times)

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Offline Arjunan M RTopic starter

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How to add soft-start feature to a PSU?
« on: June 22, 2019, 05:41:15 am »
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.
 

Offline TheHolyHorse

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Re: How to add soft-start feature to a PSU?
« Reply #1 on: June 22, 2019, 02:16:46 pm »
I think you'll need to upload new images with higher resolution, you can't really make out any of it.  :)
 

Offline Arjunan M RTopic starter

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Re: How to add soft-start feature to a PSU?
« Reply #2 on: June 22, 2019, 02:18:35 pm »
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.
 

Offline Arjunan M RTopic starter

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Re: How to add soft-start feature to a PSU?
« Reply #3 on: June 22, 2019, 02:20:05 pm »
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 ;)
 

Offline Arjunan M RTopic starter

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Re: How to add soft-start feature to a PSU?
« Reply #4 on: June 22, 2019, 02:21:18 pm »
Click on that prescicion bench power supply.pdf
 

Offline TheHolyHorse

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Re: How to add soft-start feature to a PSU?
« Reply #5 on: June 22, 2019, 02:45:48 pm »
Oh dang I'm stupid, sorry |O
 

Offline Arjunan M RTopic starter

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Re: How to add soft-start feature to a PSU?
« Reply #6 on: June 22, 2019, 02:48:14 pm »
Oh dang I'm stupid, sorry |O
I was also stupid when i came to the forum :)
 

Offline TheHolyHorse

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Re: How to add soft-start feature to a PSU?
« Reply #7 on: June 22, 2019, 02:54:24 pm »
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.
 

Offline TheHolyHorse

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Re: How to add soft-start feature to a PSU?
« Reply #8 on: June 22, 2019, 03:02:58 pm »
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.
 
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Offline Arjunan M RTopic starter

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Re: How to add soft-start feature to a PSU?
« Reply #9 on: June 22, 2019, 03:29:08 pm »
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 :phew:.
I am trying to do most of the things in the hardware side as possible.
 

Offline Arjunan M RTopic starter

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Re: How to add soft-start feature to a PSU?
« Reply #10 on: June 22, 2019, 03:32:52 pm »
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" ;D.
I am designing the PCB right now.
All the components are laid out.I have to now rout 300+ connections :phew:(most of em are GND and VCC and I already connected about 75)
 

Offline Arjunan M RTopic starter

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Re: How to add soft-start feature to a PSU?
« Reply #11 on: June 22, 2019, 03:34:52 pm »
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 :phew:.
I am trying to do most of the things in the hardware side as possible.
And an 16X2 LCD  :-\
 

Online David Hess

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Re: How to add soft-start feature to a PSU?
« Reply #12 on: June 22, 2019, 03:36:54 pm »
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.
 
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Offline Arjunan M RTopic starter

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Re: How to add soft-start feature to a PSU?
« Reply #13 on: June 22, 2019, 03:37:36 pm »
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.
 

Offline Arjunan M RTopic starter

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Re: How to add soft-start feature to a PSU?
« Reply #14 on: June 22, 2019, 03:41:23 pm »
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:
 

Offline Arjunan M RTopic starter

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Re: How to add soft-start feature to a PSU?
« Reply #15 on: June 22, 2019, 03:43:16 pm »
I don't know why i get such low res image when i take a schreenshot.
 

Offline not1xor1

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Re: How to add soft-start feature to a PSU?
« Reply #16 on: June 22, 2019, 04:26:36 pm »
please correct that awful prescicion

it is precision from Latin praecisio (-onis)
 
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Offline Arjunan M RTopic starter

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Re: How to add soft-start feature to a PSU?
« Reply #17 on: June 22, 2019, 04:31:21 pm »
please correct that awful prescicion

it is precision from Latin praecisio (-onis)
;D
This topic is getting off topic.
 

Offline TheHolyHorse

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Re: How to add soft-start feature to a PSU?
« Reply #18 on: June 22, 2019, 06:23:14 pm »
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" ;D.
I am designing the PCB right now.
All the components are laid out.I have to now rout 300+ connections :phew:(most of em are GND and VCC and I already connected about 75)

Whopsie gg english :)
 

Offline TheHolyHorse

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Re: How to add soft-start feature to a PSU?
« Reply #19 on: June 22, 2019, 06:29:46 pm »
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 :phew:.
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 ;D

BTW out of curiosity what micro are you using?

« Last Edit: June 22, 2019, 06:31:35 pm by TheHolyHorse »
 

Offline Arjunan M RTopic starter

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Re: How to add soft-start feature to a PSU?
« Reply #20 on: June 23, 2019, 02:33:45 am »
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 :phew:.
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 ;D

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.
 

Offline Arjunan M RTopic starter

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Re: How to add soft-start feature to a PSU?
« Reply #21 on: June 23, 2019, 02:35:20 am »
Just using Arduino uno because I have three of them sitting here without any use.
 

Offline exe

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Re: How to add soft-start feature to a PSU?
« Reply #22 on: June 23, 2019, 07:29:10 am »
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.
 

Offline Arjunan M RTopic starter

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Re: How to add soft-start feature to a PSU?
« Reply #23 on: June 23, 2019, 08:01:01 am »
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.
 

Offline Arjunan M RTopic starter

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Re: How to add soft-start feature to a PSU?
« Reply #24 on: June 23, 2019, 08:02:16 am »
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.
 


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