30v 10A bench supply schematic. Is it any good???

[FotatoPotato]

Hello everyone,

I have been interested in building a bench power supply for a while now and just recently I got a 24v 10A transformer, and I want to put it to use. Because I am new to building power supplies I decided to just look up schematics for power supplies that would meet my requirements. I found this schematic and it looks pretty good. Even though it's only rated at 4A I’m sure that if I just beef up the transistors and power traces/ components then it would be fine for delivering 10A. Let me know if this is a good schematic and if I should build it.

Thanks

EDIT: I'm looking for 12v @10A for the High current and 30v @ 5A

[schmitt trigger]

Question yourself: 300 watts output from a 240 VA transformer?
Because this is a linear power supply, expect no more than about 170 watts output.

[FotatoPotato]

Sorry, I should have been more specific. Im looking for 12v @ 10A and 30v @ about 5A

[Audioguru]

The schematic is a partially modified Greek kit from at least 13 years ago and Chinese copies are available today. I fixed some of its problems by beefing up many parts and using two output transistors (with emitter resistors) and a 28V/4.3A (120VA) power transformer.
When it had a 24V transformer its output voltage could not reach 30V when loaded to 3A, maybe 25VDC.

MC34071 opamps in a DIL package are not available anymore but a TLE2141 is available in a DIL package,has the same pin numbers and has the same spec's.

For an output of 10A then the BD139 driver transistor might have a heart attack and the opamp driving it might not be powerful enough.

[FotatoPotato]

Would replacing the Drive and Power transistors with a high power Mosfet and adding a mosfet gate driver Ic help with achieving a 10A output?

[Audioguru]

A Mosfet will produce exactly the same heat as a power transistor. But its gate-source voltage is high which will reduce the output voltage of the supply.

[FotatoPotato]

Ah, Ok so pretty much no difference to efficiency and power output between a Mosfet and a power trannie. Would It make more sense to build a Buck-boost converter and feed it 15v @20A so that I can get a higher efficiency? I get the 15v @ 20A from the transformer because it can be configured to output 12v @ 20A or 24v @10A, the 15v is what I get after it goes through a full bridge rectifier and filter capacitor.

[Cliff Matthews]

...MC34071 opamps in a DIL package are not available anymore..

There's always these cheap helpers (SOIC-8 to DIP-8) https://www.sparkfun.com/products/13655


Perhaps the OP hasn't seen this one (using 2 vintage DIL's to control TIP142's)
http://www.ve2ums.ca/chasse/Serge/Atelier/Projets/Membres/VE2EMM/alimentation_ang.htm

[FotatoPotato]

The PSU you linked is a great help and I really appreciate it, I might actually make it because it meets my needs and the components aren't too expensive. Do you think it would be possible to replace the TIP142 NPN Darlington BJT with a N-channel Mosfet for better efficiency?

Thanks!   

[Cliff Matthews]

The PSU you linked is a great help and I really appreciate it, I might actually make it because it meets my needs and the components aren't too expensive. Do you think it would be possible to replace the TIP142 NPN Darlington BJT with a N-channel Mosfet for better efficiency?

Thanks!   

http://home.earthlink.net/~schultdw/psupply/

[FotatoPotato]

Again, another SUPER useful resource, Thanks so much 

[dmills]

Your 12V @ 10A is actually by far the nastier requirement for a simple linear supply capable of 30V at any current at all.

The reason is that for ANY simple linear supply the power dissipated in the pass transistor is (input voltage - output voltage) * current, essentially independent of the pass device technology.
For a simple minded linear like your are trying for the input voltage must be greater then 30V (So that you can get 30V at the output), which means that at 12V @ 10A you are dissipating at least 30 - 12 = 18V * 10A = 180W, and it only gets worse as the output voltage drops.
The '3055 lacks the safe operating area to do this at temperature, quite apart from being the '741 of the power transistor world (Old and crap, and beloved of a certain generation of hobbiest), I would be looking at a dozen of the things to handle this, and would almost certainly use something else in practise.
If you wanted such a supply to supply say 1V @ 10A, you would have at least 290W being dissipated in the pass bank.

Yea there is a reason everyone gets clever to a greater or lesser extent when building high current variable output power supplies.

One nice trick for example is to use a transformer having a couple of secondary windings and have a 'range' switch which changes the connections from parallel to series as the voltage is turned up.
If for example you had such a thing putting out say 18V in parallel mode and 36V in series mode (quite reasonable) then @ 12V, 10A your pass device is now only dropping (18 - 12) * 10A = 60W, a third of the case for the simple minded design.

The ultimate expression of this idea is a switched mode preregulator setup to maintain the input to the linear regulator at say a volt above its output, which is what most designers these days seek to do, it trades complexity for a MASSIVE reduction in heat, and components are cheaper then heatsinks. 

Regards, Dan.
 

[FotatoPotato]

I like the points you bring up and I never thought about the fact that I would be dissipating almost 300W of heat stepping down 30V to 12v @10A. I do however think the design will be fine because I can think of only an EXTREELMY small amount of projects where I would actually need 12v @ 10A. I just want that power as a possibility so that if I need to supply something for maybe 30 seconds to a minute with such a large amount of current then I Could. I have played around with building a SMPS but I never got anywhere and it eventually proved to be too complicated for my beginner skills (or lack there of). For now I’ll probably just stick to the more simple linear design.

[dmills]

There is not actually all that much in hobby electronics that needs anything like that much current, personally I would build a supply capable of say 20- 25V @ 1 amp and see if you ever need more (20 or so V chosen to allow a 12-0-12 transformer secondary).
This will suffice for most of the little micro sort of projects, and if you were to build a dual output one would also cover most audio projects (+-15V is common there).

This is a decidedly easier project in all sorts of ways, and in all probability by the time you really need more then that you will have the skills to cope with building something clever.

I have **MUCH** bigger power supplies in the lab, I nearly never use them.

regards, Dan.

[FotatoPotato]

That is ture, I have only had a handfull of times when I ever needed more than 5A, mostly with 100W LED's and peltier modules, I'll still build this supply so that it is "capable" of delivering more than 5A but I'll probably never use more than 3A. And like you said, by the time I need more than 10A, I'll have the know-how to build a proper switchmode supply that can handle that kind of power. 
 

[ogden]

I have only had a handfull of times when I ever needed more than 5A, mostly with 100W LED's and peltier modules

Exactly. When it's about higher power than my small lab supply can do, I just bring out my old 400W desktop PC power supply (and some fuses too). There's quite a lot of information around about using such for loads that are not PC. You can even slightly adjust voltage up or down with simple tweak: http://electronics.pl7.de/power-supplies/converting-computer-power-supplies-psu-to-stabilized-13-8-v-dc-20-a/

When you need higher voltage @ high power - just buy it. Today for ~30$ you can get quite decent 24V 20A "brick". If you need variable voltage and/or current regulation - add DPS5015 (Dave recently blew such up) and you are quite covered. Rather than having 300W linear lab supply "beast" on the table, I put my projects there.

[FotatoPotato]

I have wanted to learn how to build a completer SMPS, not just a large step down transformer combined with a Buck-boost converter, but a true SMPS that converts the 110v AC directly to my desired output. The only thing keeping me away is the complexity and fact that I would be dealing with 110V AC and somewhere around 230V DC, so if I make one mistake then there goes my face, my room and my circuit breakers. And I'm not too keen on having something like that happening. Maybe one day I'll make a schematic and post it here and have people tell me if I made a bomb or a functional power supply.

[ogden]

I have wanted to learn how to build a completer SMPS, not just a large step down transformer combined with a Buck-boost converter, but a true SMPS that converts the 110v AC directly to my desired output. The only thing keeping me away is the complexity and fact that I would be dealing with 110V AC and somewhere around 230V DC

You don't build soldering iron and multimeter, right? Or did you? - Then why do you need to build supply? SMPS design is very complex stuff, only knowledge you can learn while designing SPMS is that you shall not design SMPS, especially while you are beginner. Better build something that is really worth your time and resources.

[FotatoPotato]

Definitely, trying to design one now would be a waste of time, there are many other more useful things that I could work on that would teach me some really useful stuff.

[Audioguru]

A TIP142 darlington has a maximum base-emitter voltage loss of 3.5V at 10A. Many Mosfets have a 10V gate-source voltage loss at 10A.
So if you use Mosfets then the maximum output voltage is 6.5V less, unless you are lucky enough to find very sensitive Mosfets that have a loss of only 4.5V.

[dmills]

Yea but producing an isolated gate drive voltage 12V or so above the source terminal is not exactly rocket science....

Point still stands that a linear passbank will be horrible efficiency wise a lot of the time, and not just when it is up against the input rail (Which is actually best case).

Regards, Dan.

[rob77]

A TIP142 darlington has a maximum base-emitter voltage loss of 3.5V at 10A. Many Mosfets have a 10V gate-source voltage loss at 10A.
So if you use Mosfets then the maximum output voltage is 6.5V less, unless you are lucky enough to find very sensitive Mosfets that have a loss of only 4.5V.

there is no such a thing as  "gate-source voltage loss" , so please don't spread BS.

you need a voltage at least Vgs above source's potential to controll the N-chan mosfet. but that has nothing to do with the output voltge drop on the mosfet you can still have a voltage drop Vds of few millivolts while Vgs at 12V! the gate is isolated and you can think of a N mosfet as a variable resistor which needs to be controlled by a voltage higher than the voltage on the source pin. the source pin is usually the output of a PSU so you need higher gate voltage than the output. you can easily solve this by a small dc-dc converter (e.g. MC34063) providing the needed higher voltage for the control circuit of the mosfet and clamping the Vgs with a zener.

[ogden]

there is no such a thing as  "gate-source voltage loss" , so please don't spread BS.

mosfet you can still have a voltage drop Vds of few millivolts while Vgs at 12V!

Now explain please - supply voltage coming into N-FET linear regulator is 30V and it is only voltage rail of the supply. To open gate, 12V voltage above source needed which would be at 29.99V considering few mV drop as you say. But hey - we have single rail which is 30V. How do you open N-mos? IMHO in this case gate shall receive 29.99+12V above ground Or am I missing something here?

[edit] Ok. Didnt come to hang yourself. In short: introducing switching crap into regulator chain like MC34063 and clamping(!) Vgs of N-MOS linear regulator with zener is worst crime which can be done to linear lab supply.

[rob77]

there is no such a thing as  "gate-source voltage loss" , so please don't spread BS.

mosfet you can still have a voltage drop Vds of few millivolts while Vgs at 12V!

Now explain please - supply voltage coming into N-FET linear regulator is 30V and it is only voltage rail of the supply. To open gate, 12V voltage above source needed which would be at 29.99V considering few mV drop as you say. But hey - we have single rail which is 30V. How do you open N-mos? IMHO in this case gate shall receive 29.99+12V above ground Or am I missing something here?

[edit] Ok. Didnt come to hang yourself. In short: introducing switching crap into regulator chain like MC34063 and clamping(!) Vgs of N-MOS linear regulator with zener is worst crime which can be done to linear lab supply.

i don't see how a zener used to protect the gate of as mosfet  is a crime    and a low current switcher can be easily filtered down to virtually no noise , so i can't see a crime there either.

but after your last post i'm pretty sure you have a major gap in understanding of a mosfet as a series pass element... the voltage at the gate is NOT DIRECTLY RELATED to the output voltage , it's not like a NPN emiter follower. with a mosfet you control the resistance of the drain-source path by changing the gate voltage . if the threshold voltage of the mosfet is for example 5V and saturation is at 10V Vgs then you control the whole output range (for example 0 to 30V) of the PSU with that 5V swing (5 to 10V)  at the gate.

[dmills]

The low noise, simple way to get that extra rail is usually to just do a voltage doubler off the main transformer secondary, current is pretty negligible, so the caps can be small.
Small isolated switchers can work, but why bother when you can get something usable with a couple of diodes and caps?

It has to be said, that I have personally never really seen the virtues of mosfets in this application, the limiting factor if usually more Safe Operating Area then saturated switch efficiency, and jellybean BJTs usually have much better SOA then modern jellybean power mosfets for the simple reason that power mosfets are usually designed as switching devices (There are exceptions mainly aimed at audio, but they are less common then they were).

BJTs also usually parallel more easily then mosfets in practise because they do not have that annoying variation in threshold voltage to the same extent so simple emitter resistors are sufficient to get reasonable current sharing out of the things.

Regards, Dan.