Getting started with Switcher design

[C Egernet]

I've seen switching power supplies mentioned occasionally here.

I'd like to learn about designing such power supplies: choice of
controller chips, MOSFETs and inductors. I know about the principle of
buck and boost but that is about it.

Where should I start? There must be a good app note to get going. (I
realize that not all app notes are created equal, thus the question.)

A first project could be a 12V (a couple of amps) power supply for a
communications receiver I've put together. Is a flyback topology
feasible?


Chris

 

[Michael Black]

On Tue, 3 Aug 2010, C Egernet wrote:

I've seen switching power supplies mentioned occasionally here.

I'd like to learn about designing such power supplies: choice of
controller chips, MOSFETs and inductors. I know about the principle of
buck and boost but that is about it.

Where should I start? There must be a good app note to get going. (I
realize that not all app notes are created equal, thus the question.)

A first project could be a 12V (a couple of amps) power supply for a
communications receiver I've put together. Is a flyback topology
feasible?

Why would you want to do that? A receiver isn't going to draw much

current, yet to use a switching supply means you have to deal with
all the noise that such supplies generate, not a good thing when
building a receiver.

The effort in making the power supply as noise free as possible might
be a useful learning process, if there was an actual need for higher
current, but it's not.

As for the rest, define what you want to do. Do away with transformers?
Provide regulation without a lot of loss?

The best place to start would seem to be a hybrid, start with a 60Hz
stepdown transformer, rectifiers and a filter capacitor, and then build
a switching regulator. Learn from that, don't get lost in extra stages,
and then later you can go to a full switching supply.

Michael

 

[Tim Wescott]

On 08/03/2010 10:23 AM, Michael Black wrote:

On Tue, 3 Aug 2010, C Egernet wrote:

I've seen switching power supplies mentioned occasionally here.

I'd like to learn about designing such power supplies: choice of
controller chips, MOSFETs and inductors. I know about the principle of
buck and boost but that is about it.

Where should I start? There must be a good app note to get going. (I
realize that not all app notes are created equal, thus the question.)

A first project could be a 12V (a couple of amps) power supply for a
communications receiver I've put together. Is a flyback topology
feasible?

Why would you want to do that? A receiver isn't going to draw much
current, yet to use a switching supply means you have to deal with
all the noise that such supplies generate, not a good thing when
building a receiver.

Perhaps for his stated purpose -- to learn? Building quiet switchers
used to be real voodoo, but there's a lot of interest in them and hence
a lot of knowledge floating around out there. The biggest "trick" that
I'm aware of is controlled slew rates on the gate drives, to slow down
the edges on the FET outputs. You trade switching loss for EMI, which
is a nice way to guarantee employment.

The effort in making the power supply as noise free as possible might
be a useful learning process, if there was an actual need for higher
current, but it's not.

Your "low current" may be my "high current", though.

As for the rest, define what you want to do. Do away with transformers?
Provide regulation without a lot of loss?

The best place to start would seem to be a hybrid, start with a 60Hz
stepdown transformer, rectifiers and a filter capacitor, and then build
a switching regulator. Learn from that, don't get lost in extra stages,
and then later you can go to a full switching supply.

Amen.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[Tim Wescott]

On 08/03/2010 08:23 AM, C Egernet wrote:

I've seen switching power supplies mentioned occasionally here.

I'd like to learn about designing such power supplies: choice of
controller chips, MOSFETs and inductors. I know about the principle of
buck and boost but that is about it.

Where should I start? There must be a good app note to get going. (I
realize that not all app notes are created equal, thus the question.)

A first project could be a 12V (a couple of amps) power supply for a
communications receiver I've put together. Is a flyback topology
feasible?

From what power source? The wall? A battery? What voltage battery?
What constraints do you want to place on yourself? What don't you care
about?

If you're going to go off the wall, search on "off-line switchers" for
information.

AFAIK flyback topologies are generally most useful when you're going up
in voltage.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

 

[Hammy]

On Tue, 03 Aug 2010 10:42:24 -0700, Tim Wescott <tim@seemywebsite.com>
wrote:

On 08/03/2010 08:23 AM, C Egernet wrote:
I've seen switching power supplies mentioned occasionally here.

I'd like to learn about designing such power supplies: choice of
controller chips, MOSFETs and inductors. I know about the principle of
buck and boost but that is about it.

Where should I start? There must be a good app note to get going. (I
realize that not all app notes are created equal, thus the question.)

A first project could be a 12V (a couple of amps) power supply for a
communications receiver I've put together. Is a flyback topology
feasible?

From what power source? The wall? A battery? What voltage battery?
What constraints do you want to place on yourself? What don't you care
about?

If you're going to go off the wall, search on "off-line switchers" for
information.

AFAIK flyback topologies are generally most useful when you're going up
in voltage.

A flyback is useful for more then just boosting voltages. Its simple
to derive multiple output voltages either negative or positive
polarity. It provides galvanic isolation from the rectified line
voltage.Its one of the least complex lowest part counts converters for
offline power conversion.

Almost every consumer and or industrial electronic device under a
100W is getting its power from a flyback converter.

The OP should go and browse ONsemis and Fairchilds site for offline
controllers for tons of info includeing examples and sources for small
quantity magnetic's.

I don't know why you would use anything other then a flyback for 100W
or less for a line powered converter when primary to secondary
isolation is a must. They are even the prefered topology for TELCO
power conversions all kinds of examples for 48Vdc conversion useing a
flyback are avaliable on google.

 

[Jon Kirwan]

This,

http://www.onsemi.com/pub_link/Collateral/SMPSRM-D.PDF

this,

http://www.jaycar.com.au/images_uploaded/dcdcconv.pdf

this,

http://focus.ti.com/docs/training/catalog/events/event.jhtml?sku=SEM401014

these,

http://focus.ti.com/docs/training/catalog/events/event.jhtml?sku=SEM401001
http://focus.ti.com/docs/training/catalog/events/event.jhtml?sku=SEM405006
http://focus.ti.com/docs/training/catalog/events/event.jhtml?sku=SEM406008
http://focus.ti.com/docs/training/catalog/events/event.jhtml?sku=SEM408003

and this?

http://www.smps.us/Unitrode.html

Jon

 

[C Egernet]

Perhaps for his stated purpose -- to learn?

True.

Right now, the main purpose is to learn about these beasts.

Building quiet switchers used to be real voodoo, but there's a lot of interest in them and hence
a lot of knowledge floating around out there.

It's the "floating around" bit, I'd like to deal with. That is, where
do I find some of this knowledge.

I suggested application notes myself but unless one has some prior
knowledge each and every one of them seems to be the latest and the
greatest. Some pointers would be helpful.


Chris

 

[C Egernet]

Why would you want to do that?  A receiver isn't going to draw much
current, yet to use a switching supply means you have to deal with
all the noise that such supplies generate, not a good thing when
building a receiver.

Fair comment.

I suggested this because, (a) I need one, (b) I thought it a not too
extreme goal. A good point about the noise, though.

The effort in making the power supply as noise free as possible might
be a useful learning process, if there was an actual need for higher
current, but it's not.

:-(

Again, even if I, following your advice, should do this particular
project with a linear power supply, learning about switchers is still
interesting.

As for the rest, define what you want to do.  Do away with transformers?
Provide regulation without a lot of loss?

Getting rid of heat and heatsinks is definitely a priority. My
question about flyback was motivated by the notion that they include
transformers and provide galvanic isolation.

The best place to start would seem to be a hybrid, start with a 60Hz
stepdown transformer, rectifiers and a filter capacitor, and then build
a switching regulator.  Learn from that, don't get lost in extra stages,
and then later you can go to a full switching supply.

Excellent idea. I have a small (too small for the above project)
potted mains transformer in my junk box that could get me going.


Chris

 

[C Egernet]

The OP should go and browse ONsemis and Fairchilds site for offline
controllers for tons of info includeing examples and sources for small
quantity magnetic's.

Good point about small quantity magnetics.

ONsemi's tutorial page has 24 documents alone! I'll have a look.


Thanks

Chris

 

[C Egernet]

On Aug 4, 10:11 am, Jon Kirwan <j...@infinitefactors.org> wrote:

This,

http://www.onsemi.com/pub_link/Collateral/SMPSRM-D.PDF

Wow, even the first one will keep me occupied for a while.

Thanks, Jon.


Chris

 

[Paul E. Schoen]

"C Egernet" <egernet@hushmail.com> wrote in message
news:fde6aaee-a338-4148-913f-163f9d010517@y11g2000yqm.googlegroups.com...
On Aug 4, 10:11 am, Jon Kirwan <j...@infinitefactors.org> wrote:

This,

http://www.onsemi.com/pub_link/Collateral/SMPSRM-D.PDF

Wow, even the first one will keep me occupied for a while.

Thanks, Jon.

============================================================

Here is an oldie (1987) but goodie from Linear Technology (Switching
Regulators For Poets):
http://cds.linear.com/docs/Application%20Note/an25fa.pdf

Also, you can easily simulate these and other circuits using LTSpice or
SwitcherCAD which is available free on the website: http://www.linear.com/

Paul

 

[roma]

C Egernet <egernet@hushmail.com> wrote in news:d17ba18f-
870e-4082-b862-78d1f1900df5@i28g2000yqa.googlegroups.com:

I've seen switching power supplies mentioned
occasionally here.

I'd like to learn about designing such power supplies:
choice of
controller chips, MOSFETs and inductors. I know about
the principle of
buck and boost but that is about it.

Where should I start? There must be a good app note to
get going. (I
realize that not all app notes are created equal, thus
the question.)

A first project could be a 12V (a couple of amps) power
supply for a
communications receiver I've put together. Is a flyback
topology
feasible?


Chris

Have there to start

http://members.shaw.ca/roma/eleven.html
roma