Electronics > Projects, Designs, and Technical Stuff
Preregulation of a linear bench PSU
Mechatrommer:
why dont use one chip solution? like 50¢ 34063? i once try to build my own discrete pre regulator from some comparator and/or schmitt trigger and/or mcu pwm. but with all the hassle, on another circuit, i just use the one chip solution, all i have to take care of is the noises spike and ground loop. in fact i'm building another one right now. ymmv.
Kleinstein:
A switched mode converter is in deed a real option. However not an easy one, as it takes care to keep higher frequency noise out. The slow tap switching has a chance to produce less higher frequency noise.
However the MC34063 is not a good choice for higher power - if at all it is acceptable when the internal switch is sufficient and efficiency is not important. There are more modern chips available and also older standard ones drive an external MOSFET switch. A cheap old one would be UC2842 and similar.
It is in dead a little odd to look at the low noise tap switching at high power (e.g. 8-10 A current). Especially the slow phase angle like switching versions to get a continuous regulation can also produce quite some ripple / noise problems. Even with just a normal rectifier, at 10 A load current the peak currents at the filter cap can be significant.
Mechatrommer:
--- Quote from: Kleinstein on January 01, 2019, 10:40:15 am ---However the MC34063 is not a good choice for higher power - if at all it is acceptable when the internal switch is sufficient and efficiency is not important.
--- End quote ---
it can drive external mosfet, pnp, npn etc. there are circuits examples in the datasheet for buck, boost, invert etc with or without external switcher (power element). the nice thing is comparator, hysterisis, PWM, logic are all taken cared of in single chip, there is also Timing Capacitor (TC) pin i believe to control switching frequency (but i dont care i just use recommended value) and current limit (Ipk sense) pin as well (which i also disabled in my current circuit). the output is open collector-open emiter bjt (much like optocoupler) so the limit is your imagination.
currently i'm upgrading my earlier version long serving but now dead dual tracking psu to add few features, one of it is smps pre regulation. in the attached picture (bottom part) you can see i have to arrange 34063 on -ve rail to mirror the 34063 on +ve rail (top part), its Vcc is PSU's GND, and its GND is PSU's -ve input voltage (VCC-, -20V in my case), feedback is uA741 full swing +ve to -ve rail supply, i know what fanboys are going to say, but nevermind i made them to work for my need, and dont mind the Vrfs (voltage reference for smps, slightly higher but adjustable than Vref for linear regulation output) its not good enough i should've add buffer to it since i use it in voltage division, i overlooked that until i populate them on pcb.
i dont know what specificity of the OP to use diy version, sorry if its not suitable, just thought its related. if say we want to limit current spike maybe we can slow down the mosfet driver to managable level, in my circuit attached, the driver is maxed to about an amp in and out of the gate, maybe i overstress the 3906/3904 pair but o well. for power element you can put anything you like (AO3400/3401 rated 5A in my case although my psu only rated to 0.5 - 1A) fwiw...
Atom:
it seems we have new people that have joined the discussion :-+
--- Quote from: not1xor1 on December 31, 2018, 08:37:07 am ---
I agree that Blackdog's circuit is much simpler (I made something even simpler as it was for regulation on the negative rail).
When converting from AC to DC you have a fixed voltage loss (max peak-to-peak ripple voltage + max dropout of regulator) plus diode bridge and transformer loss.
In the case of center tap transformer the diode bridge and transformer losses are halved, but the other aren't so saving that fraction of volt wouldn't be so bad. :D
In any case such a complicated circuit is obviously an overkill for a single tap. I designed that just for the puzzle solving fun. :)
But in future I would start from there to replace the triacs in the HP 3 taps switching design.
--- End quote ---
today i had time to tes blackdog circuit so i used a 12mohm nmos in a to252 package the current of the load was 6/7amps at 25V ....the mosfet died after the first switch, i think the peak current killed it, no big deal it was a chinese mosfet
2nd try .....i don't have low voltage high current mos on hand....i used a 14nk50z to 247 on heatsink .....0.380ohm of rdson 500vds 14A continuos
https://www.st.com/resource/en/datasheet/stb14nk50z.pdf
i know that's not the right mosfet for the job but i wanted to try it, it works, but there is a problem the voltage measured from drain to source is 11V pkpk so 5.5v. @ 6.6A load
power disspiantion is Rdson = 5.5/6.6 = 0.83ohm not good :-DD power disspiation is 0.83*36= 30W (the mos was on a heatsink).
yes that's not the right mosfet but it works, i'm waiting for low voltage high current mos.
for anyone intrested the diode used was byw99w200 : https://www.st.com/resource/en/datasheet/CD00000692.pdf
Happy new year to everyone!!
Atom:
--- Quote from: Kevin.D on December 31, 2018, 09:19:19 pm ---A full wave voltage doubler could be added on to that to give two extra extended ranges. There is one disadvantage I can see
though as it's halving the output capacitance (since 2 caps in series) on the lower voltage (higher current) ranges where its needed most.
Notice on the third range how the top caps gets charged upto double the voltage of the bottom cap to get us the third range (sw1 closed sw2 open).
The available outputs currents and voltages I give on the schematic are what should be theoretically available from the
transformer and don't include rectification losses.
Happy New Year .
--- End quote ---
really intresting i was taught that voltage doublers are used in low current application and for generating supply rails if you have an ac signal nearby withouth using overcoplex circuits...never thought to use it in a power supply, it could be used in a more hv / low current psu.
--- Quote from: Mechatrommer on January 01, 2019, 03:14:12 am ---why dont use one chip solution? like 50¢ 34063? i once try to build my own discrete pre regulator from some comparator and/or schmitt trigger and/or mcu pwm. but with all the hassle, on another circuit, i just use the one chip solution, all i have to take care of is the noises spike and ground loop. in fact i'm building another one right now. ymmv.
--- End quote ---
yes the mc34063 i have some on hand..but still i'm trying to keep the noise as down as possible so a switchmode prereg is excluded...btw that doesn't mean that i'm not intrested in switching prereg...maybe in a smaller and more compact unit than the one that i'm trying right now.
i've now ordered boards from jlcpcb the 100*100 mm pcb incudes a tpa3116 class d amp (but that's irrelevant, props to the author https://github.com/kurozz/kamp_tpa3116d2)
the other 2 boards are one with the LM2679 simple switcher, main candidate to a smaller psu prereg because it's really simple and can deliver quite a bit of power
the second one is based around the LM5116 Synchronous Buck Controller, the board were generated by webench designer and tweaked to make them smaller and better by me.
planning to play around with them and see what i can do, another option would be the xlsemi 4005 or similar but the 32v max input is a bottle neck
LM2679 : http://www.ti.com/lit/ds/symlink/lm2679.pdf
LM5116 : http://www.ti.com/lit/ds/symlink/lm5116.pdf
xl4005 : http://www.xlsemi.com/datasheet/xl4005%20datasheet.pdf
then i discovered that some seller on aliexpress is selling those dcdc industrial modules (3.4mm thick pcb) made from DELTA and MURATA, i'm going to take some of those since they are extremely cheap (4E). maybe the feedback of those one can me modified so we can control the voltage of the output.
MURATA https://it.aliexpress.com/item/Lungo-staffa-con-la-dissipazione-di-calore-piatto-Murata-RBQ-isolamento-ad-alta-potenza-DC-DC/32957694449.html?spm=a2g0s.13010208.99999999.262.42653c00ZZeXhp
DELTA https://it.aliexpress.com/item/Con-dissipazione-di-calore-Delta-Isolato-500-w-ad-alta-potenza-DC-DC-modulo-di-conversione/32955585915.html?spm=a2g0s.13010208.99999999.259.42653c00ZZeXhp
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