Basic Switch Mode 230-20v PSU

[Luke]

This has probably been asked before but I want to build a psu to
replace a standard linear design to save space. I want to have mains
input and produce 20V @ 5A approx. This is for a model railway booster
circuit and will feed an H bridge running at approx 20KHz.
I'm guessing nowadays that there are switch-mode controllers that with
the addition of a few passive components and perhaps a bypass MOSFET or
2 will do the job in much less space than a std transformer takes but
searching on t' web only produces thousands of pages of information,
all of which are either theoretical or are about various power supply
ICs that are not for this purpose.

I've looked at a few commercially available units and they are nearly
as big as a linear one!!

Can anyone point me to something useful - perhaps there is a specific
name for these ICs if they exist?
Thanks

Luke

 

[John]

This has probably been asked before but I want to build a psu to
replace a standard linear design to save space. I want to have mains
input and produce 20V @ 5A approx. This is for a model railway booster
circuit and will feed an H bridge running at approx 20KHz.
I'm guessing nowadays that there are switch-mode controllers that with
the addition of a few passive components and perhaps a bypass MOSFET or
2 will do the job in much less space than a std transformer takes but
searching on t' web only produces thousands of pages of information,
all of which are either theoretical or are about various power supply
ICs that are not for this purpose.

I've looked at a few commercially available units and they are nearly
as big as a linear one!!

Can anyone point me to something useful - perhaps there is a specific
name for these ICs if they exist?
Thanks

Luke

PC power supplies are switch mode and available with 5 volt outputs in
fairly high amps. Just be aware that many of them need at least a
small load on the 5 volts and/or 12 volt outputs to operate. And they
include the cooling fan ;-)
http://www.allelectronics.com/cgi-bin/item/PS-206/480/250W_SWITCHING_POWER_SUPPLY_.html

Or check the sizes of the supplies offered here:
http://www.mpja.com/listitems.asp?dept=3&main=1
http://www.allelectronics.com/cgi-b...22A,_12_6A,_-12V_.3A,_5V_1.5A,_3.3V_12A_.html
http://www.allelectronics.com/cgi-b...PPLY,_5V_35A,_3.3V_20A,_12V_16A,_12_14A_.html

John

 

[Clint Sharp]

This has probably been asked before but I want to build a psu to
replace a standard linear design to save space. I want to have mains
input and produce 20V @ 5A approx. This is for a model railway booster
circuit and will feed an H bridge running at approx 20KHz.
I'm guessing nowadays that there are switch-mode controllers that with
the addition of a few passive components and perhaps a bypass MOSFET or
2 will do the job in much less space than a std transformer takes but
searching on t' web only produces thousands of pages of information,
all of which are either theoretical or are about various power supply
ICs that are not for this purpose.

I've looked at a few commercially available units and they are nearly
as big as a linear one!!

Can anyone point me to something useful - perhaps there is a specific
name for these ICs if they exist?
Thanks

Luke

Far, far easier to get hold of a 24 volt unit and adjust/modify it to
suit than design something from the bottom up.

 

[Eeyore]

This has probably been asked before but I want to build a psu to
replace a standard linear design to save space. I want to have mains
input and produce 20V @ 5A approx. This is for a model railway booster
circuit and will feed an H bridge running at approx 20KHz.
I'm guessing nowadays that there are switch-mode controllers that with
the addition of a few passive components and perhaps a bypass MOSFET or
2 will do the job in much less space than a std transformer takes but
searching on t' web only produces thousands of pages of information,
all of which are either theoretical or are about various power supply
ICs that are not for this purpose.

I've looked at a few commercially available units and they are nearly
as big as a linear one!!

Can anyone point me to something useful - perhaps there is a specific
name for these ICs if they exist?

If you haven't done it before and think it's easy............ Forget IT !

Graham

 

[Luke]

If you haven't done it before and think it's easy............ Forget IT !

Graham

What about if I get a PSU book? Are worked examples usually easy? I
know the theory is quite involved but I thought there must be some easy
standard circuits for commonly used voltage outputs or at least ICs
available where you just add a few specified components.

Is the problem that there are too many variables involved as in
current/voltage/frequency/noise vs cost etc?

Luke

 

[Chris]

What about if I get a PSU book? Are worked examples usually easy? I
know the theory is quite involved but I thought there must be some easy
standard circuits for commonly used voltage outputs or at least ICs
available where you just add a few specified components.

Is the problem that there are too many variables involved as in
current/voltage/frequency/noise vs cost etc?

Luke

Hi, Luke. You're not getting a lot of responses because you're kind of
way off base on many of your assumptions.

Non-isolated DC-to-DC switchers can be done by a hobbyist (the National
Semi Simple Switcher series comes to mind). But line voltage AC-to-DC
switchers are a lot more complex. Typically, you will rectify the line
voltage to produce 170VDC or 340VDC, to charge up HV caps, then use
high voltage transistors or MOSFETs to push the power through a
specially made transformer. Feedback from the voltage output at the
other side of the transformer is done through an optocoupler. The ICs
which accomplish this are typically on the line side instead of the
isolated side.

All of this means a lot of peril on many fronts for a newbie. As you
mentioned in the OP, there are scads of papers and theoretical
information on how to do this, mostly because it's tough to do well
without a fairly extensive background in electronics engineering.
Nobody writes papers on using a LM7805 regulator, because it's easy.
Not only that, but doing this well requires tools (such as a digital
storage oscilloscope) which just aren't available to hobbyists.

Let's see -- what else. Oh, yes. Switching power supplies are a
commodity item, with vicious competition between manufacturers, on
price but also on size. These guys have dozens of electronics and
manufacturing engineers trying desperately 50 or 60 hours a week to
squeeze every tenth of a cent out of their product, as well as every
cubic millimeter. Assuming as a newbie that you'll get a smaller size
(not to mention a lower cost) than these large manufacturers on a
one-off is, well, kind of earnest, but not realistic.

And the biggest point. Playing with offline switchers is a bit of a
hazardous activity. Any troubleshooting will have to be done with
potentially lethal DC and AC voltages present. In short, it's not
really a job for a newbie. And this is supposed to be an adjunct to
your real project, which I guess is model trains, right?

Do yourself a favor. If you want, shop around and get a 150 or 200
watt switching power supply that will do the job. If you want a 20VDC
5A power supply for your trains (isn't that a little high -- wouldn't
18VDC be a better idea?), just buy one.

Good luck
Chris

 

[Chris]

Chris wrote:

Do yourself a favor. If you want, shop around and get a 150 or 200
watt switching power supply that will do the job. If you want a 20VDC
5A power supply for your trains (isn't that a little high -- wouldn't
18VDC be a better idea?), just buy one.

Good luck
Chris

Sorry:

If you want a 20VDC 5A AC-to-DC switching power supply for your trains,
just buy one.

Good luck with your train set
Chris