khlor
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Homebrew adjustable switched PSU
Hello to all,
I hope you are in great health and I hope you have a better new year than this one we all had to survive to.
Greetings aside, I've come here to present my newest creation.
First some pointers to consider, please understand, it will make sense when I show schematics, photos and ratings.
1 - I am not an engineer
2 - Like I've seen on Sulaiman signature, in regards to this at least, I am a hobbyist, not an amateur nor a professional
3 - you could get better performance with better parts and a design around a specialty chip like the tl494 from Texas Instruments
4 - I did the way I did it because I wanted to make it from scratch, in fact my will to do it from scratch was so great I almost used
an astable flip-flop instead a 555.
5 - this is something I meant to do for over three years, spent the last 6 months designing it and spent whole three days building it
6 - it is not optimal, I know, but I do accept criticism and suggestions for future iterations. the main idea was my design but some
parts of the circuit were stitched from bits and pieces I found on stack overflow, homemade-circuits.com and this place
7 - This is a hobby project, if you have to buy all parts I hardly believe it will be cheaper than buying a decent buck converter
module, unless you have all the parts already, most of it could be scrapped from broken ATX power supplies, with exception of the LM7812, LM358 and
the 555 as well as the BC337/BC327(but these are dirt cheap) and the MOSFETS(IRF9540) but if you do the switching on the low side you could get
N-Channel FETS from the ATX power supply, I guess there are two per power supply.
With all of that said, let's go for a bit of background.
I've seen posts here on the forum, even before the creation of the "Electronics" sub-section, most of these posts were under the power-hungry
"Electrolysis" sub-section and these posts regarded the need for a power supply with an adjustable constant-current feature and adjustable voltage for
mainly electrolysis and chlorate cells many solutions were given, the main solution was to buy a buck-converter board with such features and there are
plenty of cheap solutions, I've used one of these, the cheap ones can hardly give 3A, not bad. some other were to use a resistor or even a light bulb
as a resistor, simple but far from being constant current. and other of the suggested solutions was to use the lm317. and as a power source a big
transformer or a power brick(laptop charger)/wall wart.
While the buck-converter module is a sensible solution, resistive control is too wasteful and not too practical for electrolysis operations and the
lm317, not to bash on it, I mean the humble lm317 is precise and for low power applications is just what the doctor ordered, but when you scale up
things like dropping from 19.5v at 3A, things get complicated quite easily first lm317 is rated for a maximum of 1.5A, so you'd have to be using at
least 3 in parallel and a huge heatsink, not optimal.
I've seen people talking about making their own, for those who can i is a nice option, but I'd say it is only for the hobby since buck-converter
modules now days are cheap even with this stratospheric inflation, I did mine because I do like electronics and I do it as a hobby that sometimes
yield some cash on the side so I set out to see if it is possible to do so without specialty integrated circuits(IC for short) and it is possible, but
I have no idea if it is any good. Alright, I rambled for long enough, let's get down to brass tacks!
The Schematics
There isn't much special here aside from the fact that my drawing skills aren't top of the line, and I didn't wanted to learn CAD just yet, I tried to
do all in one sheet of paper, but it got too complicated, so I divided in two parts so it is easier to see and also show.
Picture 1: Part A
Sorry for showing a chopped up schematics, part A are the two MOSFETS, with two sides, side A and side B and they work in turns, while side A is on,
side B is off and vice-versa, I decided to go this way because I think it would not put too much of a strain on the source. Bases of Q1 and Q6 should
be connected to pin 3 of the 555 in Part B of this schematic, so bases of Q2 and Q8 should be connected to the line designated "Enable" of part B.
this part doesn't have much going on, just a simple modified gate driver with push pull circuit using BJT transistors.
Picture 2: Part B
Part B has a whole lot more going on, it generates the signal to oscillate the MOSFETS in part A and also enable signals for regulation, those who
understand a bit of electronics will notice that the circuit takes two forms of feedback that feed on base Q1 and Q2, base of Q1 regulates voltage, as
voltage on output rises enough will cause Q1 to activate according to how R11 is set, and will stop the 555 and also cut the enable signal for the
MOSFETS on part A, this is the voltage control, simple but in practice proved to be very effective.
As for the current control it took a bit more of work, as current is flow through the circuit it will generate a voltage in R13 as this voltage is too
small to be detected by the transistor I used the first half of an LM358 and feed the output though R4 and R5 (variable resistors) forming a voltage
divider network and then feed it to the base of Q2, so, I regulate a limit based on the 0.65-0.7V needed to activate the transistor to use it as
reference voltage, thus as current increase the amplified voltage generated in R13 will turn on Q2 causing the same effect as Q1.
In my research I learned recently that this is what they call "skip cycle" and it is far different and far less reliable than the proper way of doing
this which would be PWM control. I tried sveral different tricks to increase stability on this feedback loop and as things are I guess it is "good
enough" current control is a little finicky but once you set what you want it is stable regardless on how the load varies and voltage control is
actually pretty good, I got pleased on how it works right out the gate.
I neglected to put on the schematics, but between Vcc and Ground you should put a large capacitor rated for at least 25v at a minimum of 2000uF, but I
guess that 4-5mF would be good to keep things stable down the line,
Second thing, Vcc can be anythere from 16V to 22V 25 would be pushing it, but I believe that if you swap all capacitors to be rated to 50v you could
go as high as the maximum voltage of the LM7812 without much worries, just remember adapt the gate driver circuit as well. If you want to use lower
voltage( <16V >12V ), I it is possible, but you should replace the LM7812 for a LDO of 12V or perhaps a LM7809 would work without the need to
modify anything, but since I am using IRF9540N going bellow 11V will be detrimental and will cause them to heat up a lot! these are my considerations.
Schematics Notes:
Put a large capacitor(electrolytic) between Vcc and Ground;
OUT+ in part B should be connected to OUT+ from part A;
put 0.1uF ceramic capacitors(code 103) before and after the lm7812 and a couple on the output to help dampen the noise and voltage ripple;
R13 is supposed to be a 10A shunt resistor, as I did not had that on hand, I've decided to use 6 1Ohm +/- 5% 5W resistors in parallel, these aren't
best quality nor best suited for the job of shunt, because as they heat up their resistance increase, I calculated and it is around 0.16 Ohm, but when
I do U=RI it returns 0.18-0.22Ohm(I assume it is those 5% plus thermal variations) , so it is unreliable, also, when the circuit is drawing more or
less 4.9A the voltage drop goes over 1V(1050mV) which represent nearly 5W on losses so, a proper shunt resistor would increase efficiency figures
quite significantly.
here are a few pictures of the finished board, I did on a perf board i never did PCB etching and for now I have no interest in doing so since most of
what I've do is early prototypes that may never go into "production" as per say.
Something else I neglected to mention, for the digital/voltage ammeter module I put the positive wire( thin red) connected to the 12v output of the
7812, the negative(thin black) I didn't connected, the rest is just like the image bellow(Picture 3), just replace the battery for (OUT+ and OUT) - as
shown in part B:
Picture 3: Wiring diagram for digital ammeter/voltmeter module
Picture 4: Finished 1
Picture 5: Finished 2
Picture 6: Finished 3
The digital ammeter/voltmeter do have a slight difference from the values I see in my multimeter, the voltage it shows is around 0.4V higher than when
I measure, the current show 0.1A than the measured. I trust my multimeter more, that thing was expensive.
Now for the ratings
The circuit has the 555 oscillating at 33.3kHz and duty cycle was roughly 52% however during tests the gate of the MOSFETS got pretty hectic, due to
cycle skipping, I assume.
My tests were made with a resistive load a 24V 250W halogen lamp HLX 64655 Xenophot made by Osram(not sure what is used for, but was expensive for a
light bulb), not the best load since resistance increases when heated, best test would be with something that has a negative thermal coefficient of
resistance but it was all I had on hand that wouldn't fry with the voltage and power ratings I'd be testing
with this load I made some tests, the power source for the circuit was a19.5V, 65W power brick to recharge dell laptops. and it is on the final
assemble as show in pictures 4 and 5.
Test #1
Input: 0.08A - 19.45V - 1.556W
Output: 0.51A - 0.2V - 0.102W
Efficiency: 6.5%
Test #2
Input: 1.71A - 19.45V - 33.26W
Output: 4.06A - 4.61V - 18.71W
Efficiency: 56.25%
Test #3
Input: 1.80A - 18.83V - 33.89W
Output: 4.14A - 4.75V - 19.66W
Efficiency: 58.0%
Test #4
Input: 2.15A - 18.7V - 40.20W
Output: 4.81A - 5.5V - 26.45W
Efficiency: 65.8%
The sampling is too little to be scientific, I know, the testing wasn't made using automated tools, it was made by hand, step by step I had to use 3
multimeters and a lot of imagination. However from a glance it is possible to see that it shows some characteristics of a buck converter, low
efficiency at low power ratings and efficiency goes up with the power ratings, I had no other load to test with but I'm sure this little project of
mine can give little excess of 5A over long periods of time, the MOSFETS don't even get too hot, you can touch them and hold touch without getting
burns.
And aside the low power tests I am mildly satisfied with the results, or I was until I got my cheap osciloscope thing to see what was on the output,
and the voltage ripple is... regrettable, I've measured as high as 1500mV of pure ripple. I know these cheap chinese osciloscopes aren't precision
tools, however it does help to have an idea of what is going on. noise was low though... but it happened only when I was clamping down heavy on the
current, however it cannot be ignored that even 100mV ripple is still high(so I've read), so I'd say this doesn't qualify for lab power supply and I
don't think it will be good to use with electronics testing or precision work, but if all you need is something with an adjustable constant current
feature for electrolysis/chlorate, I think it'll do just fine. I tried some adjustments(already on schematics) to improve the ripple situation and it
got bellow 300mV for the most part.
Picture 7: Tests
Final Considerations
It was a pretty fun project to build, at times I almost gave up, and I did one using the tl494 based on a video from AKA Kasyan, but for me it didn't
worked quite well, so I referred to the datasheet from Texas Instrument for help and got it working. it is way easier, but I wanted to do something
unnecessary and for fun, so I decided to do it iwth the 555, but as stated before, I almost did it with just transistors, but I opted for a better
stability on the signal generation. most of the parts I had on hand, and use of a "high side switching" is simpler, but not obligatory, instead of the
P-Channe MOSFET with little modification you could use any power N-Channel MOSFET.
With regards to applications, I guess this circuit can e used as a charger, for lead-acid batteries, no problem, and electrolysis and chlorates
production, it is adjustable, both voltage and current, and it can keep a constant current, so long your shunt is a good one, otherwise you may notice
a slight current drop as the resistance of the shunt resistor rises. Also, a good shunt can increase efficiency, since almost 5W have been calculated
to be lost just for this, almost 12.5% so that is something to think about.
I am sorry for the long text, I wanted to keep it all short and sweet, but this in on itself is a lot to unpack.
if you have questions, suggestions or considerations, please I'd like to know.
EDIT: corrections on the text above picture 3
[Edited on 31-12-2024 by khlor]
"NOOOOOO!!! The mixture is all WROOOOOOONG!"
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Rainwater
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Hot dam the perf board. I love it.
Nothing wrong with a cheap scope ether..
Just a note, something i wish I new when I first started.
A capacitor's capacitance will decreases under load as the voltage rating is reached.
Sometimes up to 90% at rated voltage. Its a marketing ploy manufacturers use
And the devil is in the datasheets.
What frequency is your ripple at? 33k? 1500mv of ripple. You mean one point five volts? Thats a lot.
Are you measuring that at the output of your psu, or the input of your load? Wire leads(feeder circuits) make a big difference.
For example that 5 amps you list, going through a 16 awg Al lead 40mv per foot, then another 5mv per through hole device with the standard 0.4mm lead
Trace thickness plays a role to.
Consider a cheap hotplate from the dollar store, as a load. The stove cooking heating elements are a nicrome wire with nice linear properties. And is
usually rated for about 900 watts 120v version 15 ohms. 220version 33ohms.
Have you simulated the design? Results vs real-life?
Great job too.
"You can't do that" - challenge accepted
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Twospoons
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Well, looks like you got it basically working, so congrats on that.
A couple of things: you are pushing the gate voltage on your mosfets to damn near breaking point - they are only rated for 20V. You only need -10V on
the gate to get those things fully turned on - anything more is just extra gate charge that will add to the heating.
Also your reference voltage appears to be just the Vbe of a transistor, which is wildly unstable with temperature - so you can expect your output
voltage to vary wildly too, as things heat up. Same goes for your current regulation.
The inductors are unshielded so likely radiating a lot of noise. Bear that in mind if you are using other instrumentation near this PSU.
Realistically, while I applaud the use of BJTs and 555's, the better route would be to use TI's Webench tool - give it your input and output
requirements and it spits out a list of suitable chips, complete with schematics and simulations. The PSU chips they have will give you an accurate
voltage reference, cycle by cycle current limiting (protect your MOSFETs), appropriate loop compensation, very low ripple, under voltage protection,
overload protection, soft-start, better EMI, smaller parts, and better that 90% efficiency.
EE's like me don't do this stuff from scratch without a really compelling reason. TI's chips are just too nice.
Helicopter: "helico" -> spiral, "pter" -> with wings
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khlor
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Me too, I mean, it is better than a breadboard and you can freeform or easily adjust as you go, it is perfect for prototyping!
Quote: Originally posted by Rainwater  |
Just a note, something i wish I new when I first started.
A capacitor's capacitance will decreases under load as the voltage rating is reached.
Sometimes up to 90% at rated voltage. Its a marketing ploy manufacturers use
And the devil is in the datasheets.
|
Okay, this is new for me and I find it disturbing, I didn't bothered with datasheets since most of the caps I am using were scrapped from electronic
thrash. on this note, it means that i.e. if I use 50v 1000uF capacitors I may have better performance and less ripple than the 25v 1000uF, right?
| Quote: Originally posted by Rainwater |
What frequency is your ripple at? 33k? 1500mv of ripple. You mean one point five volts? Thats a lot.
Are you measuring that at the output of your psu, or the input of your load? Wire leads(feeder circuits) make a big difference.
For example that 5 amps you list, going through a 16 awg Al lead 40mv per foot, then another 5mv per through hole device with the standard 0.4mm lead
Trace thickness plays a role to. |
So... this is the not good part, it was, one point five volts of ripple, but this was before, now it gets to a maximum of 0.4V but usually 0.25-0.30V
still high, but not as bad as before. About the frequency, it is not at the 33kHz, since my circuit uses cycle skipping instead of PWM, the frequency
was around 90kHz-125kHz(at least I assume this to be the cause of the frequency discrepancy or some sort of hysteresis loop).
The measurements were made right at the contacts sticking out of the box, and I did measurements right from the board as well.
and I went crazy with the "traces" using not just tin/lead but also copper wires to reinforce the whole thing.
I think that some filters may help with that, perhaps a choker, will have to test, but efficiency will decrease, for sure.
| Quote: Originally posted by Rainwater |
Consider a cheap hotplate from the dollar store, as a load. The stove cooking heating elements are a nicrome wire with nice linear properties. And is
usually rated for about 900 watts 120v version 15 ohms. 220version 33ohms.
Great job too. |
Yes, I'll, in fact, thanks to you, I remembered I do have some nichrome wire, so, I guess I can test the maximum rating of it after all!
just in my head, alright, I'm done with the lame jokes... but seriously, no I never played with simulation software it is on my to-do list, the most
thought I gave it was when I decided to build it, the dream was at least 7-8 amps at 10V, I am barely scratching 4.8 amps at 3.5V, and the old regular
pen and paper.
and thanks for the compliments.
"NOOOOOO!!! The mixture is all WROOOOOOONG!"
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khlor
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| Quote: Originally posted by Twospoons | Well, looks like you got it basically working, so congrats on that.
A couple of things: you are pushing the gate voltage on your mosfets to damn near breaking point - they are only rated for 20V. You only need -10V on
the gate to get those things fully turned on - anything more is just extra gate charge that will add to the heating.
Also your reference voltage appears to be just the Vbe of a transistor, which is wildly unstable with temperature - so you can expect your output
voltage to vary wildly too, as things heat up. Same goes for your current regulation.
The inductors are unshielded so likely radiating a lot of noise. Bear that in mind if you are using other instrumentation near this PSU.
Realistically, while I applaud the use of BJTs and 555's, the better route would be to use TI's Webench tool - give it your input and output
requirements and it spits out a list of suitable chips, complete with schematics and simulations. The PSU chips they have will give you an accurate
voltage reference, cycle by cycle current limiting (protect your MOSFETs), appropriate loop compensation, very low ripple, under voltage protection,
overload protection, soft-start, better EMI, smaller parts, and better that 90% efficiency.
EE's like me don't do this stuff from scratch without a really compelling reason. TI's chips are just too nice.
|
yeah, the gate voltage was a huge oversight, and yes, my reference voltage is Vbe, after things warm up it gets "stable"
and I agree, TI chips are good, though, first time I hear about Webench, I know TI like to spoil designers, I mean their datasheets are so good, but a
tool like that, I'll have a look. with that in mind, I did this for the absurdity, for fun, and I don't realistically think there's any other reason
anyone should be doing something like it. but that is my opinion.
EDIT: and I like BJTs, and discrete components, as for the inductors, even the cheap scope was catching the EM radiation when the box was open
[Edited on 31-12-2024 by khlor]
"NOOOOOO!!! The mixture is all WROOOOOOONG!"
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Keras
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Really, I’m not sure why you bother with all those discrete transistors while you could order ICs that would do all the regulation for you and boost
your efficiency way up to the 90%s.
I know this is less fun than building the whole shebang from scratch, but if you value energy… :p
For example this IC costs less than € 1 over here (unit pricing), probably you can get it for a handful of nickels.
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Sulaiman
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Nice. messy, but nice, and it works.
I understand the desire to make discrete circuits from scratch.
ICs would make the job much simpler
buying a ready made psu is even easier, and probably cheaper,
(enclosure,connectors,knobs,switches,initial failures etc.)
but where is the fun in that?
good effort.
......................
to me, chemistry is similar,
almost every chemical that I've synthesised is cheaper and probably purer if I buy it,
but I keep going.......
CAUTION : Hobby Chemist, not Professional or even Amateur
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bnull
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Did you include a fuse on the mains side?
You need short-circuit protection on the output.
You're using caps rated to 25 V in a place with 19.5 V plus that huge ripple. Change them for 50 V caps. Better yet, measure the voltage between pins
for each electrolytic capacitor in the circuit and substitute them by others rated to at least twice the measured value. Trust me, you don't want to
push them.
I know they are quite old, but you may want to take a look at the electronics books at WorldRadioHistory. Allan Lytel, John Potter Shields, and Rufus P. Turner have some books on power supplies from which you can adapt something.
Search for the service manual of a commercial power supply. These manuals have schematics of the device (sometimes with labels on the
blocks, like "switching", "regulation", "thermal protection" etc.).
Simulate. You can get results pretty close to real life without risk of burning or blowing up stuff. The time I spend on simulations more than
compensates the time I would spend trying to fix a major screw up (chiseling away melted insulation, for example). They're not perfect but they beat
pen and paper for me, especially for complex circuitry.
Always read the datasheets and test the components. I have some capacitors whose capacitance is roughly half of what it should be.
That's an interesting project.
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khlor
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| Quote: Originally posted by Keras | Really, I’m not sure why you bother with all those discrete transistors while you could order ICs that would do all the regulation for you and boost
your efficiency way up to the 90%s.
I know this is less fun than building the whole shebang from scratch, but if you value energy… :p
For example this IC costs less than € 1 over here (unit pricing), probably you can get it for a handful of nickels.
|
I know, in fact I did already one using the tl494 and cost was next to nothing I got 4 ics for about US$ 2, but this is the hobby, the fun is in doing
something you aren't exactly supposed to.
| Quote: Originally posted by Sulaiman | I understand the desire to make discrete circuits from scratch.
ICs would make the job much simpler
buying a ready made psu is even easier, and probably cheaper,
(enclosure,connectors,knobs,switches,initial failures etc.)
but where is the fun in that?
good effort.
......................
to me, chemistry is similar,
almost every chemical that I've synthesised is cheaper and probably purer if I buy it,
but I keep going....... |
Thanks, I mean, this means a lot when it comes from someone who gets it.
Well... I didn't bothered with fuse on mains side, because the entire circuit is fed from a laptop power adapter(a dell 65W 19.5V) I assume it has its
own fuse, though, I guess a fuse would be important, not just on the input of my circuit but also on the output of it.
Short circuit protection, I though of it, but never implemented even though I should, right there with reverse polarity protection and soft start, it
is a work in progress.
| Quote: Originally posted by bnull | You're using caps rated to 25 V in a place with 19.5 V plus that huge ripple. Change them for 50 V caps. Better yet, measure the voltage between pins
for each electrolytic capacitor in the circuit and substitute them by others rated to at least twice the measured value. Trust me, you don't want to
push them.
|
About the caps, Rainwater mentioned it, and I guess that if I want this ship to sale for real it is time to get some new caps, thanks for bringing
this out, I'll take it to heart.
| Quote: Originally posted by bnull | I know they are quite old, but you may want to take a look at the electronics books at WorldRadioHistory. Allan Lytel, John Potter Shields, and Rufus P. Turner have some books on power supplies from which you can adapt something.
Search for the service manual of a commercial power supply. These manuals have schematics of the device (sometimes with labels on the
blocks, like "switching", "regulation", "thermal protection" etc.).
[...]
Always read the datasheets and test the components. I have some capacitors whose capacitance is roughly half of what it should be.
|
I'll be sure to check those out, in fact, I do favor older books because everything today is integrated or programmed(i.e. pic, avr, arduino), like a
black box and I do enjoy seeing how it used to be done.
And yes, most of what I know today came from service manuals and datasheets, though, I was much more concerned with getting things working rather than
the needed protections of the design, my bad.
And I'll start checking datasheets for capacitors as well, didn't thought it'd be that important.
| Quote: Originally posted by bnull |
Simulate. You can get results pretty close to real life without risk of burning or blowing up stuff. The time I spend on simulations more than
compensates the time I would spend trying to fix a major screw up (chiseling away melted insulation, for example). They're not perfect but they beat
pen and paper for me, especially for complex circuitry. |
Simulations, now days it is getting more widespread. I did spent some time without a proper PC, and I am typing this from a crappy 32gb laptop(I
disassembled it in hopes of upgrading just to discover the motherboard is the same they use on tablets) so, this one is something I've been avoiding,
I understand its importance and how helpful it can be, but truth be told, I never felt enthusiastic about it but whenever possible I'll give it a
shot, hope there are linux tools.
Good to see I'm not the only one who thinks this way.
I'm grateful to see that many replies and many with interesting insights, I'm already rethinking many things and how to rework or make a brand new and
improved iteration, please, keep 'em coming.
"NOOOOOO!!! The mixture is all WROOOOOOONG!"
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Twospoons
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I was thinking about your control loop last night, and theres a very strong possibility your circuit is running in a quasi linear mode, which would
explain why your efficiency is so poor for a buck regulator. Scope the mosfet gate and check the voltages. My guess is that its not switching hard,
but only to around the gate threshold voltage.
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khlor
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| Quote: Originally posted by Twospoons | | I was thinking about your control loop last night, and theres a very strong possibility your circuit is running in a quasi linear mode, which would
explain why your efficiency is so poor for a buck regulator. Scope the mosfet gate and check the voltages. My guess is that its not switching hard,
but only to around the gate threshold voltage. |
Sorry for the long time, well, I got worried about your earlier reply, since I did not use zener clamps and was already scoping the gates and you're
right, this circuit is working in a quasi-linear mode, the gate voltage starts at around -7V and as I ramp up the voltage pot it raises up to -9.8V
and then it gets switching, little to say I wasn't pleased, since it resulted in the majority of my losses, after plenty on thinking and debugging I
decided to make some changes, I tried to just step the fets and keep the 555 running, but no dice, then I decided to drive the "enable pin" of the
gate driver with a schmitt trigger, it improved the situation considerably, but while scoping the gate I saw that while the majority of pulses were
between -11V and -15V there were some around -5V and -8V and I don't think that is adequate, while messing around withe the circuit and rebuilding
the gate driver from scratch trying to understand what is going on, I am still at a loss, and some parts died, among them, 4x 555, 3x BJTs, 2x FETs,
and 2 resistors. most of the death toll was due to my carelessness while embroiled in testing and frustrated at my little pet project which at this
point devolved into complete madness, for now it'll be shelved since I have no more p-channel FETs, I'm not sure how I'm gonna solve this or perhaps I
might settle for this improvement and put large heat sinks on the FETs and settle for it, I guess my downfall was the and gate baked into the gate
driver, and the -5V pulses are the overlapping rise an fall of the BJTs since they activate independently, but for all my testing I couldn't find a
proper reason so it is just a quickly thrown together theory. however since I have to buy more parts, I'll go for improved caps as well and see if I
can get this to be better or at least as good as it can be with my insane requirements, at any rate I am once again grateful for the tips and help.
"NOOOOOO!!! The mixture is all WROOOOOOONG!"
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Rainwater
National Hazard
Posts: 959
Registered: 22-12-2021
Member Is Offline
Mood: Break'n glass & kick'n a's
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I admire your bravery and confidence. Building this without a
simulation, had it been me, would have resulted in a lot of smoke.
Ltspice is free, easy to use, and many manufacturers have models of their parts freely available.
At least then you can quickly, cheaply and safely try different stuff
"You can't do that" - challenge accepted
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Twospoons
International Hazard
Posts: 1331
Registered: 26-7-2004
Location: Middle Earth
Member Is Offline
Mood: A trace of hope...
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Buck converters usually rely on some form of duty cycle modulation for control - either fixed frequency PWM or variable frequency with fixed on-time
or off time. What you seem to be trying to do is a form of hysteretic control - turning the power on or off to maintain regulation. This is a valid
approach, but is usually only used where the buck converter faces a very large range of loads, and is only engaged for very light loads to improve
light load efficiency (the converter switches between PWM and hysteretic control based on load).
You would need to add a comparator with some hysteresis to your feedback loop to achieve hysteretic control, or you need to modulate the duty cycle of
your 555 oscillator.
Also: stop thinking of BJTs and MOSFETs as switches - they are amplifiers. They can mimic a switch if you drive then hard enough, but they are still
amplifiers.
Helicopter: "helico" -> spiral, "pter" -> with wings
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Sulaiman
International Hazard
Posts: 3742
Registered: 8-2-2015
Location: 3rd rock from the sun
Member Is Offline
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that type of voltage and current meter module has an inconvenient feature:
(I have two of them)
the current sensing is done in a very similar way to your current sensing using R13
so there is a V=I.R voltage across the thick red and black wires
so the voltage across your load is
(your power supply output voltage)-(voltage drop between the current meter thick red and black wires)
ie, the ACTUAL load voltage will reduce as the load current increases
even if your psu gives a perfectly constant output voltage.
.........................
Irrespective of the above,
it is convenient to have the output voltage 'floating' with a third (usually green) terminal that is connected to the chassis and 'earth'
under most circumstances the black and green terminals are connected together,
but you can if required, disconnect the green terminal, for a floating supply.
eg you could connect the -ve terminal to the positive terminal of a 12V battery to get
12V to (12V + psu voltage)
OR
connect the +ve terminal to earth for a negative output voltage.
this feature is most useful when there are multiple PSUs in a system.
including other mains powered instruments
often a high value resistor (eg 1MOhm) is added between the green and black outpuut terminals to shunt any leakage current.
CAUTION : Hobby Chemist, not Professional or even Amateur
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