Forums

Small transformers and LTSpice simulation

Started by Grant April 22, 2011
I have a design that could use three or four winding toroid transformer.  

In the LTSpice notes they suggest keeping the transformer coupling at 
1.0 or -1.0 otherwise there's lots of high frequency noise generated, 
I'm not sure if that's real world, or the simulation going silly?

What I'm wondering is just how much coupling should I expect to see in 
a real world transformer of around five to ten turns on a half or 
three-quarter inch diameter toroid?  Like the ones you see on PC mother-
boards.  Or the larger mag-amp ones from PC power supplies that come 
with two or three windings.

I have lots of toroids recovered from power supplies, no idea of what 
inductance they have until I power up some circuit and try to match 
LTSpice inductance with observed waveforms ;)  I'm guessing 33uH at 
the moment for 40 or 50kHz operation.

I do notice that over-voltages start at close to ideal coupling, for 
example 0.97 can give a nasty over-voltage spike on the leading edge 
in LTSpice.  I don't know how much of that to expect in a real circuit, 
any guidance here?

What I plan to do is drive a transformer with a current limited latch 
circuit, +ve edge turns on a 'hc74 flip flop, current sense through an 
npn will pull 'hc74 reset line down.  Should be safe enough to watch 
the waveforms.  

Frequency of interest is 20 to 100kHz, current up to 2A through N-chan
MOSFET driving the transformer, small snubber on primary as suggested 
by LTSpice, secondaries are standard flyback, pair of schottky diodes 
and caps.  

I might even use a 555, as it has a -ve reset line?  :)  Save me building 
a separate oscillator, add the voltage cutoff and it's done, cheap'n'nasty.

Can one make a 50KHz oscillator from half an 'HC74? and an R + RC?  Is there 
a odd numbered ring of inverters hiding in there?

Thanks,
Grant.
On a sunny day (Sat, 23 Apr 2011 00:12:32 +1000) it happened Grant
<omg@grrr.id.au> wrote in <u223r6dt2g62ep2pdfabjmhkga6isgde1s@4ax.com>:

>I have a design that could use three or four winding toroid transformer. > >In the LTSpice notes they suggest keeping the transformer coupling at >1.0 or -1.0 otherwise there's lots of high frequency noise generated, >I'm not sure if that's real world, or the simulation going silly? > >What I'm wondering is just how much coupling should I expect to see in >a real world transformer of around five to ten turns on a half or >three-quarter inch diameter toroid? Like the ones you see on PC mother- >boards. Or the larger mag-amp ones from PC power supplies that come >with two or three windings. > >I have lots of toroids recovered from power supplies, no idea of what >inductance they have until I power up some circuit and try to match >LTSpice inductance with observed waveforms ;) I'm guessing 33uH at >the moment for 40 or 50kHz operation. > >I do notice that over-voltages start at close to ideal coupling, for >example 0.97 can give a nasty over-voltage spike on the leading edge >in LTSpice. I don't know how much of that to expect in a real circuit, >any guidance here? > >What I plan to do is drive a transformer with a current limited latch >circuit, +ve edge turns on a 'hc74 flip flop, current sense through an >npn will pull 'hc74 reset line down. Should be safe enough to watch >the waveforms. > >Frequency of interest is 20 to 100kHz, current up to 2A through N-chan >MOSFET driving the transformer, small snubber on primary as suggested >by LTSpice, secondaries are standard flyback, pair of schottky diodes >and caps. > >I might even use a 555, as it has a -ve reset line? :) Save me building >a separate oscillator, add the voltage cutoff and it's done, cheap'n'nasty. > >Can one make a 50KHz oscillator from half an 'HC74? and an R + RC? Is there >a odd numbered ring of inverters hiding in there? > >Thanks, >Grant.
Why use a separate oscillator, just use feedback :-)
On Apr 22, 4:12=A0pm, Grant <o...@grrr.id.au> wrote:
> I have a design that could use three or four winding toroid transformer. =
=A0
> > In the LTSpice notes they suggest keeping the transformer coupling at > 1.0 or -1.0 otherwise there's lots of high frequency noise generated, > I'm not sure if that's real world, or the simulation going silly? > > What I'm wondering is just how much coupling should I expect to see in > a real world transformer of around five to ten turns on a half or > three-quarter inch diameter toroid? =A0Like the ones you see on PC mother=
-
> boards. =A0Or the larger mag-amp ones from PC power supplies that come > with two or three windings.
Wind one and find out. Measuring the inductance of one winding on it own, and the same winding with the other winding short-circuited, should give you a pretty clear indication The transformer equations V1=3DL1.dI1/dt + M.DI2/dt and V2=3D M.dI1/dt + L2/DI2/dt tell you what's going on, though for tight coupling you may have to figure in the resistance of the windings and maybe even their self-capacitance M =3D the coupling times the square root of L1 times L2. Ferrite is about a thousand times more permeable than free space, so couplings can be around 0.999, if you've been careful about keeping the leads close together away from the core. -- Bill Sloman, Nijmegen
"Bill Sloman" <bill.sloman@ieee.org> wrote in message 
news:1e591fc5-53b7-4bf1-865f-25d4b3f4ac8e@q30g2000vbs.googlegroups.com...
On Apr 22, 4:12 pm, Grant <o...@grrr.id.au> wrote:
>Wind one and find out. Measuring the inductance of one winding on it >own, and the same winding with the other winding short-circuited, >should give you a pretty clear indication
I've read somewhere that with toroids you inevitably end up with a leakage inductance of at best the self-inductance of a half-turn around the toroid...
>The transformer equations V1=L1.dI1/dt + M.DI2/dt and V2= M.dI1/dt + >L2/DI2/dt tell you what's going on...
Yes, although those basic equations are a rather surprising distance away from the "ideal transformer" equations, though (...where impedances are scaled by the turns squared ratio and that's it...). Heck, it's still a handful of lines of algebra away from the not-so-obvious result that transformers act as impedance inverters!
> M = the coupling times the square root of L1 times L2.
Yeah, although if one is trying to determine it experimentally, I believe the results are often a bit more accurate going with the approach of measuring the inductance of the two coils connected in series, once such that one of the coils adds to the total flux ("Ladd") and then such that that same coil reduces the total flux ("Lops"), and then use M=(Ladd-Lops)/4 . ---Joel
On 04/22/2011 09:34 AM, Bill Sloman wrote:
> On Apr 22, 4:12 pm, Grant<o...@grrr.id.au> wrote: >> I have a design that could use three or four winding toroid transformer. >> >> In the LTSpice notes they suggest keeping the transformer coupling at >> 1.0 or -1.0 otherwise there's lots of high frequency noise generated, >> I'm not sure if that's real world, or the simulation going silly? >> >> What I'm wondering is just how much coupling should I expect to see in >> a real world transformer of around five to ten turns on a half or >> three-quarter inch diameter toroid? Like the ones you see on PC mother- >> boards. Or the larger mag-amp ones from PC power supplies that come >> with two or three windings. > > Wind one and find out. Measuring the inductance of one winding on it > own, and the same winding with the other winding short-circuited, > should give you a pretty clear indication > > The transformer equations V1=L1.dI1/dt + M.DI2/dt and V2= M.dI1/dt + > L2/DI2/dt tell you what's going on, though for tight coupling you may > have to figure in the resistance of the windings and maybe even their > self-capacitance > > M = the coupling times the square root of L1 times L2. > > Ferrite is about a thousand times more permeable than free space, so > couplings can be around 0.999, if you've been careful about keeping > the leads close together away from the core.
But not unity! Have you seen 0.999? I've seen numbers more like 0.95 to 0.99, but I don't get out much, transformer-wise. -- 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
On Fri, 22 Apr 2011 10:07:59 -0700, Tim Wescott <tim@seemywebsite.com> wrote:

>On 04/22/2011 09:34 AM, Bill Sloman wrote: >> On Apr 22, 4:12 pm, Grant<o...@grrr.id.au> wrote: >>> I have a design that could use three or four winding toroid transformer. >>> >>> In the LTSpice notes they suggest keeping the transformer coupling at >>> 1.0 or -1.0 otherwise there's lots of high frequency noise generated, >>> I'm not sure if that's real world, or the simulation going silly? >>> >>> What I'm wondering is just how much coupling should I expect to see in >>> a real world transformer of around five to ten turns on a half or >>> three-quarter inch diameter toroid? Like the ones you see on PC mother- >>> boards. Or the larger mag-amp ones from PC power supplies that come >>> with two or three windings. >> >> Wind one and find out. Measuring the inductance of one winding on it >> own, and the same winding with the other winding short-circuited, >> should give you a pretty clear indication >> >> The transformer equations V1=L1.dI1/dt + M.DI2/dt and V2= M.dI1/dt + >> L2/DI2/dt tell you what's going on, though for tight coupling you may >> have to figure in the resistance of the windings and maybe even their >> self-capacitance >> >> M = the coupling times the square root of L1 times L2. >> >> Ferrite is about a thousand times more permeable than free space, so >> couplings can be around 0.999, if you've been careful about keeping >> the leads close together away from the core. > >But not unity!
LTSpice shows such nice clean results with unity! I wanted to know how far away to make the value.
> >Have you seen 0.999? I've seen numbers more like 0.95 to 0.99, but I >don't get out much, transformer-wise.
0.99 or 0.98 looked more like real life for the thing I was investigating. If was after a range to expect, I'll try it soon to see how close LTSpice is to reality. I'm bouncing between two different topologies, perhaps I should simply flip a coin to decide which one first ;) Grant.
On Fri, 22 Apr 2011 15:04:33 GMT, Jan Panteltje <pNaonStpealmtje@yahoo.com> wrote:

>On a sunny day (Sat, 23 Apr 2011 00:12:32 +1000) it happened Grant ><omg@grrr.id.au> wrote in <u223r6dt2g62ep2pdfabjmhkga6isgde1s@4ax.com>: > >>I have a design that could use three or four winding toroid transformer. >> >>In the LTSpice notes they suggest keeping the transformer coupling at >>1.0 or -1.0 otherwise there's lots of high frequency noise generated, >>I'm not sure if that's real world, or the simulation going silly? >> >>What I'm wondering is just how much coupling should I expect to see in >>a real world transformer of around five to ten turns on a half or >>three-quarter inch diameter toroid? Like the ones you see on PC mother- >>boards. Or the larger mag-amp ones from PC power supplies that come >>with two or three windings. >> >>I have lots of toroids recovered from power supplies, no idea of what >>inductance they have until I power up some circuit and try to match >>LTSpice inductance with observed waveforms ;) I'm guessing 33uH at >>the moment for 40 or 50kHz operation. >> >>I do notice that over-voltages start at close to ideal coupling, for >>example 0.97 can give a nasty over-voltage spike on the leading edge >>in LTSpice. I don't know how much of that to expect in a real circuit, >>any guidance here? >> >>What I plan to do is drive a transformer with a current limited latch >>circuit, +ve edge turns on a 'hc74 flip flop, current sense through an >>npn will pull 'hc74 reset line down. Should be safe enough to watch >>the waveforms. >> >>Frequency of interest is 20 to 100kHz, current up to 2A through N-chan >>MOSFET driving the transformer, small snubber on primary as suggested >>by LTSpice, secondaries are standard flyback, pair of schottky diodes >>and caps. >> >>I might even use a 555, as it has a -ve reset line? :) Save me building >>a separate oscillator, add the voltage cutoff and it's done, cheap'n'nasty. >> >>Can one make a 50KHz oscillator from half an 'HC74? and an R + RC? Is there >>a odd numbered ring of inverters hiding in there? >> >>Thanks, >>Grant. > >Why use a separate oscillator, just use feedback :-)
The interesting thing about putting up ideas here is the brain bending _simple_ questions some raise :) Because I didn't think of it? Plus I'm trying to get an idea what to expect before wiring something up to 24V 100AH battery, things go bang, or hiss and weld themselves together while the fuses are slowly deciding to open... I welded an Anderson connector the other day, shorted out the battery, silly thing to do. At least I didn't have dancing cables hissing and jumping about, inside. Grant.
On Apr 22, 6:55=A0pm, "Joel Koltner" <zapwireDASHgro...@yahoo.com>
wrote:
> "Bill Sloman" <bill.slo...@ieee.org> wrote in message > > news:1e591fc5-53b7-4bf1-865f-25d4b3f4ac8e@q30g2000vbs.googlegroups.com... > On Apr 22, 4:12 pm, Grant <o...@grrr.id.au> wrote: > > >Wind one and find out. Measuring the inductance of one winding on it > >own, and the same winding with the other winding short-circuited, > >should give you a pretty clear indication > > I've read somewhere that with toroids you inevitably end up with a leakag=
e
> inductance of at best the self-inductance of a half-turn around the toroi=
d... Only if you wind it wrong. We've been through this here in times past.
> >The transformer equations V1=3DL1.dI1/dt + M.DI2/dt and V2=3D M.dI1/dt + > >L2/DI2/dt tell you what's going on... > > Yes, although those basic equations are a rather surprising distance away=
from
> the "ideal transformer" equations, though (...where impedances are scaled=
by
> the turns squared ratio and that's it...). =A0Heck, it's still a handful =
of
> lines of algebra away from the not-so-obvious result that transformers ac=
t as
> impedance inverters! > > > M =3D the coupling times the square root of L1 times L2. > > Yeah, although if one is trying to determine it experimentally, I believe=
the
> results are often a bit more accurate going with the approach of measurin=
g the
> inductance of the two coils connected in series, once such that one of th=
e
> coils adds to the total flux ("Ladd") and then such that that same coil > reduces the total flux ("Lops"), and then use M=3D(Ladd-Lops)/4 .
Thanks. I hadn't thought of - or read about - that approach. Winding resistance is still going to complicate the interpretation of the result - you've got much the same current circulating in the second coild with both approaches. -- Bill Sloman, Nijmegen
On a sunny day (Sat, 23 Apr 2011 07:07:44 +1000) it happened Grant
<omg@grrr.id.au> wrote in <2rq3r61jvejaf31mlkp3uaf6d3utk4prq7@4ax.com>:

>>>Can one make a 50KHz oscillator from half an 'HC74? and an R + RC? Is there >>>a odd numbered ring of inverters hiding in there? >>> >>>Thanks, >>>Grant. >> >>Why use a separate oscillator, just use feedback :-) > >The interesting thing about putting up ideas here is the brain bending >_simple_ questions some raise :) Because I didn't think of it? Plus >I'm trying to get an idea what to expect before wiring something up to >24V 100AH battery, things go bang, or hiss and weld themselves together >while the fuses are slowly deciding to open... I welded an Anderson >connector the other day, shorted out the battery, silly thing to do. > >At least I didn't have dancing cables hissing and jumping about, inside. > >Grant.
Yes, true, at that power level I would use a dedicated chip, measure real current in the switches, and stop the PWM on a cycle by cycle basis, like here: http://panteltje.com/panteltje/pic/pwr_pic/ Notice the small current transformer working into a PIC comparator, using the PIC's hardware feature to stop the PWM (decently). Then you can control the current reference so you get cycle by cycle voltage control, something I need to add to this one day (it is now hysteretic), I do a simple HV stabiliser here with a PIC too, up to about 1300 V software controlled: That one has no current trip, runs of a wallwart. Here is an other stabilised (500 V IIRC) PIC PWM supply, in this and the first one the PIC does the LCD drive, calculations and communication too. http://panteltje.com/panteltje/pic/gm_pic/
On Sat, 23 Apr 2011 09:38:07 GMT, Jan Panteltje <pNaonStpealmtje@yahoo.com> wrote:

>On a sunny day (Sat, 23 Apr 2011 07:07:44 +1000) it happened Grant ><omg@grrr.id.au> wrote in <2rq3r61jvejaf31mlkp3uaf6d3utk4prq7@4ax.com>: > >>>>Can one make a 50KHz oscillator from half an 'HC74? and an R + RC? Is there >>>>a odd numbered ring of inverters hiding in there? >>>> >>>>Thanks, >>>>Grant. >>> >>>Why use a separate oscillator, just use feedback :-) >> >>The interesting thing about putting up ideas here is the brain bending >>_simple_ questions some raise :) Because I didn't think of it? Plus >>I'm trying to get an idea what to expect before wiring something up to >>24V 100AH battery, things go bang, or hiss and weld themselves together >>while the fuses are slowly deciding to open... I welded an Anderson >>connector the other day, shorted out the battery, silly thing to do. >> >>At least I didn't have dancing cables hissing and jumping about, inside. >> >>Grant. > >Yes, true, at that power level I would use a dedicated chip, measure real current in the switches, and stop the PWM >on a cycle by cycle basis, like here: > http://panteltje.com/panteltje/pic/pwr_pic/ >Notice the small current transformer working into a PIC comparator, using the PIC's hardware feature to stop the PWM (decently).
Actually I modeled the output latch to be like the comparator reset latch that some of the power PIC chips have, with external level shifting to 10V gate drive. Not used a current transformer for feedback yet, haven't looked to see how big the signal is. I remember when you put that circuit up many months ago. I recall rightly the PIC comparator reference can be varied so the switching basically is happening in hardware, supervised by the software. A timer setting the latch and output, the comparator terminating power cycle? What happens if your feedback coil wired backwards? One reason I modeled the power circuit was to make sure it settled safe if the controller stalled, that meant AC coupled gate drive so that open loop step would not cause over-current. Very nice to simulate that little bit, the rest of the circuit left to real world design, I wouldn't try to do the lot in a sim. So now I have a failsafe hardware driver waiting for my to catch up with PIC chip and an input sensor opamp.
> >Then you can control the current reference so you get cycle by cycle voltage control, >something I need to add to this one day (it is now hysteretic),
Possibly we agree on this :)
> >I do a simple HV stabiliser here with a PIC too, up to about 1300 V software controlled: >That one has no current trip, runs of a wallwart. > >Here is an other stabilised (500 V IIRC) PIC PWM supply, >in this and the first one the PIC does the LCD drive, calculations and communication too. > http://panteltje.com/panteltje/pic/gm_pic/
I've yet to do PIC serial comms, bought some serial to USB bridge modules to try for that. Keep getting distracted with other stuff though. No programming PICs for months now. Grant.