Basic transformer question - number of turns depends on freq

A

Al Borowski

Guest
Hi,

For a university assignment, I have to design and construct a switchmode
regulator (flyback type). It has to drop 10V to 5V, using a certian
chip. I can choose the frequency to be between 20-100kHz.

Before I dive into the equations, could someone please tell me if the
number of turns required on the transformer depends on the switching
frequency? For instance, if a higher frequency means less turns, then
I'll go for it. If the number of turns is independant of frequency then
I'll use 100kHz for minimum ripple.

thanks alot,

Al
 
Al Borowski <aj.borowski@erasethis.student.qut.edu.au> wrote:
Hi,

For a university assignment, I have to design and construct a switchmode
regulator (flyback type). It has to drop 10V to 5V, using a certian
chip. I can choose the frequency to be between 20-100kHz.

Before I dive into the equations, could someone please tell me if the
number of turns required on the transformer depends on the switching
frequency? For instance, if a higher frequency means less turns, then
I'll go for it. If the number of turns is independant of frequency then
I'll use 100kHz for minimum ripple.
Click to expand...
Oh dear.
You really need to read your books/notes a bit more.

Saturation in transformers is a bad thing.
If the magnetic flux in the transformer goes over a limit, it
stops behaving as a transformer.
Look up saturation, and "leakage inductance"

All else being equal, higher frequencies mean lower peak fluxes, as
the energy transfer happens more often, so there is lowere energy to
be stored in each transfer.

A lower inductance is usually used at higher frequencies, so there are
fewer turns.

Look up how inductance varies as number of turns.

If all else fails, read the datasheet.
 
Thanks for the reply.

Ian Stirling wrote:
Al Borowski <aj.borowski@erasethis.student.qut.edu.au> wrote:

Hi,

For a university assignment, I have to design and construct a switchmode
regulator (flyback type). It has to drop 10V to 5V, using a certian
chip. I can choose the frequency to be between 20-100kHz.

Before I dive into the equations, could someone please tell me if the
number of turns required on the transformer depends on the switching
frequency? For instance, if a higher frequency means less turns, then
I'll go for it. If the number of turns is independant of frequency then
I'll use 100kHz for minimum ripple.


Oh dear.
You really need to read your books/notes a bit more.
Click to expand...
As I said, I haven't done *any* reading on this yet :) I just wanted a
quick answer before I 'hit the books'. Luckily I have plenty of time
before this is due.

Saturation in transformers is a bad thing.
If the magnetic flux in the transformer goes over a limit, it
stops behaving as a transformer.
Look up saturation, and "leakage inductance"
Click to expand...
I'm OK with these - I think I understand the theory. Starting tomorrow
I'll get stuck into the assignment, but I first thought I'd ask here -
because I knew someone could tell me the answer right away :)

All else being equal, higher frequencies mean lower peak fluxes, as
the energy transfer happens more often, so there is lowere energy to
be stored in each transfer.
Click to expand...
I thought so, which explains why the core size can be smaller, right?

A lower inductance is usually used at higher frequencies, so there are
fewer turns.
Click to expand...
Thanks! That makes sense.


Look up how inductance varies as number of turns.

If all else fails, read the datasheet.
Click to expand...
No worries, I plan to do so.

cheers,

Al
 
Al Borowski <aj.borowski@erasethis.student.qut.edu.au> wrote:
Thanks for the reply.

snip
No worries, I plan to do so.
Click to expand...
Post in case of any problems...

There seem to be a lot of people who post saying "help me, I've
got 12 hours to complete the assignment and haven't read any books,
or done any work, can you please do all of the work for me".

Good luck.
 
I read in sci.electronics.design that Ian Stirling
<root@mauve.demon.co.uk> wrote (in <zrzdc.31353$Y%6.3891316@wards.force9
..net>) about 'Basic transformer question - number of turns depends on
frequency? (Not turns ration, number of turns)', on Fri, 9 Apr 2004:
Al Borowski <aj.borowski@erasethis.student.qut.edu.au> wrote:

Thanks for the reply.

snip
No worries, I plan to do so.

Post in case of any problems...

There seem to be a lot of people who post saying "help me, I've
got 12 hours to complete the assignment and haven't read any books,
or done any work, can you please do all of the work for me".

Good luck.
Click to expand...
Indeed. Al is a student, and an Australian student to boot. Yet he gave
a polite reply to a rather stern post, and he's got plenty of time for
his project. This man will go far!
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
"Al Borowski" <aj.borowski@erasethis.student.qut.edu.au> wrote in
message news:4076a41c$0$16601$5a62ac22@freenews.iinet.net.au...
| Hi,
|
| For a university assignment, I have to design and construct a
switchmode
| regulator (flyback type). It has to drop 10V to 5V, using a certian
| chip. I can choose the frequency to be between 20-100kHz.
|
| Before I dive into the equations, could someone please tell me if the
| number of turns required on the transformer depends on the switching
| frequency? For instance, if a higher frequency means less turns, then
| I'll go for it. If the number of turns is independant of frequency
then
| I'll use 100kHz for minimum ripple.
|
| thanks alot,
|
| Al
|

There is no 'right' answer to your question.

In an 'ideal' world higher frequency would equate to an overall smaller
inductor with, perhaps, fewer turns.

Unfortunately

At low frequencies.Core losses are less so you are saturation limited,
0.3T. Skin and proximity effects are less so you can use larger wire
sizes. At high frequencies. Core losses are greater so you are loss
limited, <0.2T Skin and proximity effects are greater so you have to use
wire ropes.

Discontinuous operation gives you high peak and RMS currents with higher
copper losses implying thicker wire and more winding area. Continuous
operation has lower RMS currents and losses but needs more inductance,
more turns, and suffers from the right half plane zero.

These are just a few of the design decisions you will have to make and
justify. There are probably more and at the end of the day you have to
get out your pen and paper and run through some iterations.

I hope you have some good sums.

DNA
 
On a sunny day (Fri, 09 Apr 2004 23:29:00 +1000) it happened Al Borowski
<aj.borowski@erasethis.student.qut.edu.au> wrote in
<4076a41c$0$16601$5a62ac22@freenews.iinet.net.au>:

Hi,

For a university assignment, I have to design and construct a switchmode
regulator (flyback type). It has to drop 10V to 5V, using a certian
chip. I can choose the frequency to be between 20-100kHz.

Before I dive into the equations, could someone please tell me if the
number of turns required on the transformer depends on the switching
frequency? For instance, if a higher frequency means less turns, then
I'll go for it. If the number of turns is independant of frequency then
I'll use 100kHz for minimum ripple.
Yes, the higher the freq, the less turns you need.
Click to expand...
This because there is less inductance needed for a low (zero load)
current in the transformer.
You can calculate that from the switch time and L with i = t / L
Beware of saturation.
 
Al Borowski wrote:
Hi,

For a university assignment, I have to design and construct a switchmode
regulator (flyback type). It has to drop 10V to 5V, using a certian
chip. I can choose the frequency to be between 20-100kHz.

Before I dive into the equations, could someone please tell me if the
number of turns required on the transformer depends on the switching
frequency? For instance, if a higher frequency means less turns, then
I'll go for it. If the number of turns is independant of frequency then
I'll use 100kHz for minimum ripple.

thanks alot,

Al
Click to expand...
Turns per volt depends on the frequency, the core material, and the
power level (ie: the size of the core).
 
Thanks to everyone who replied... clearly I need to bone up on
electromag theory :)

cheers,

Al
 
I read in sci.electronics.design that Al Borowski <aj.borowski@erasethis
..student.qut.edu.au> wrote (in <4077cc91$0$16606$5a62ac22@freenews.iinet
..net.au>) about 'Basic transformer question - number of turns depends on
frequency? (Not turns ration, number of turns)', on Sat, 10 Apr 2004:
Thanks to everyone who replied... clearly I need to bone up on
electromag theory :)

You may find this easily-memorable equation useful:
Click to expand...

E = BAWN

E = voltage (volt)
B = induction ('flux density') (tesla)
A = area of cross-section of core (m^2)
W [omega] = angular frequency (rad) = 2 [pi]f
f = frequency (hertz)
N = number of turns.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
You may find this easily-memorable equation useful:

E = BAWN

E = voltage (volt)
B = induction ('flux density') (tesla)
A = area of cross-section of core (m^2)
W [omega] = angular frequency (rad) = 2 [pi]f
f = frequency (hertz)
N = number of turns.
Click to expand...
......if you can figure out whether it's peak to peak, peak, average,
or RMS for the flux quantity.

For voltage, rms or instantaneous values are assumed, unless stated
otherwise. This is not so for flux density.

RL
 
I read in sci.electronics.design that R.Legg <legg@magma.ca> wrote (in
<e715b5cc.0404101333.3d6335c5@posting.google.com>) about 'Basic
transformer question - number of turns depends on frequency? (Not turns
ration, number of turns)', on Sat, 10 Apr 2004:
You may find this easily-memorable equation useful:

E = BAWN

E = voltage (volt)
B = induction ('flux density') (tesla)
A = area of cross-section of core (m^2)
W [omega] = angular frequency (rad) = 2 [pi]f
f = frequency (hertz)
N = number of turns.

.....if you can figure out whether it's peak to peak, peak, average,
or RMS for the flux quantity.

For voltage, rms or instantaneous values are assumed, unless stated
otherwise. This is not so for flux density.
Click to expand...
It is so for ALL quantities (that can have p-p, peak, average and r.m.s
values - you don't normally get peak-to-peak d.c. resistance!). Why
should there be an exception for B?
If you understand SI units, it is obvious that E in that equation is
Click to expand...
r.m.s. if B is r.m.s.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
 
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