Half bridge DC-DC converter topology

On Sat, 7 Jul 2012 21:04:29 -0400, "P E Schoen" <paul@peschoen.com>
wrote:

>"John Larkin"  wrote in message 
>news:s62hv7purs4b8qi6ekob2snu621fijeea1@4ax.com...
>
>> This IR driver chip is pretty nice:
>
>> https://dl.dropbox.com/u/53724080/Circuits/ESM/ESM_power_B.pdf
>
>> at least for fixed duty cycle.
>
>I have some similar products, IR2104, IR2136, IRS2001, IRS24531. The 
>IRS2153D http://www.irf.com/product-info/datasheets/data/irs2153d.pdf seems 
>similar to the IR21531 and in fact the added 1 on the part number signifies 
>a smaller deadband. I like the idea of the built-in timer for 
>self-oscillation although I also like to drive with a PIC for more accurate 
>frequency.
>
>The topology of your application shows the transformer primary driven 
>through capacitors, and that might be even better than what I have with a 
>center tap between two capacitors.

If you need a lot of filter cap, center-tapping the filter caps makes
sense. My 24 volt input is already DC, so I didn't need a lot of
filtering.


 It seems to me that an output voltage 
>control of sorts could be done by adjusting the frequency of the square 
>wave. And the start-up surge would be easily controlled by starting with a 
>small duty cycle and ramping up to 50%. That would require a bridge driver 
>like the IRS2001 which has separate high and low drivers.

The center-tapped filter cap is interesting in that it reduces DC
inrush surge. The low side of the primary powers up at V+/2. Of
course, you still have to charge the caps on the load side.


>
>For my low voltage high power application the capacitors would need to be 
>able to handle high ripple current with low ESR. Those are somewhat rare and 
>expensive. Here is a 22,000 uF 63V cap with 20A ripple for $38.
>http://www.digikey.com/product-detail/en/PEH200MJ5220MB2/399-5653-ND/2193731
>
>For a 24V system with half-bridge I figure that's about 12V RMS so for 1000 
>watts I need about 80 amps. So four of those would be $150.
>
>It seems better to use polypropylene film. Here is a 10uF 300V capacitor 
>with 2.9mOhms ESR and 15A ripple, for $3.51ea/10:
>http://www.digikey.com/product-detail/en/B32674D3106K/495-2915-ND/1277679
>
>The high voltage is "wasted" but the cost is much better. 6 in parallel is 
>still reasonable, $21. And I can easily go to a 48V system or even higher 
>with no worries. In the EV world it would be very useful to have a 144V-144V 
>booster to get 288V for a 240 VAC motor drive. Probably about 20 kW. And 
>probably best to build with multiple 2 kW units in parallel. $50/kW would be 
>an acceptable selling price.
>

Full bridge operation mitigates some of those problems. I think.

"John Larkin"  wrote in message=20
news:u7qhv7tbk3msqllivg0lpb5bbc1etcp9ib@4ax.com...

> If you need a lot of filter cap, center-tapping the filter caps
> makes sense. My 24 volt input is already DC, so I didn't need
> a lot of filtering.

My application will be powered by batteries, so very little filtering is =

needed. Mostly for high frequency current surges to compensate for cable =

inductance and battery ESR.

> The center-tapped filter cap is interesting in that it reduces DC
> inrush surge. The low side of the primary powers up at V+/2. Of
> course, you still have to charge the caps on the load side.

They also draw a huge surge on the line side from the battery. The =
series=20
capacitor draws nothing until the output of the half-bridge starts=20
oscillating. I tried using a 5% duty cycle and I got 65 VDC output. =
Should=20
work to ramp up the duty cycle for a soft start, or for output =
regulation.

> Full bridge operation mitigates some of those problems. I think.

Well, yes, I think so. Or I could go back to the direct drive push-pull =
CT=20
topology. So there are trade-offs, and direct coupling may be best. But =
it=20
seems that the series capacitance helps reduce the spikes. Maybe a full=20
bridge with a capacitor in series with the primary? Could there be a =
problem=20
with resonance?

Thanks,

Paul

"legg"  wrote in message =
news:20mgv7l56cicii89gkkvi9jpj01b1de9qb@4ax.com...

> There is nothing shameful about working with circuits below 1KW.
> It's the most practical method of developing preliminary physical
> models and prototypes.

> The principles involved apply at all power levels. What changes
> is the physical limitations of materials and methods, when
> scaled. Your circuit is a good example. Try running the simulation
> with reduced transformer Lp, while maintaining turns ratio. Even
> with a coupling coefficient of 0.99, your current values are
> dominating permissible Di/Dt in the power transfer, due
> to leakage terms.

> In real life, Lp will be selected only to be high enough so that
> magnetizing energy doesn't dominate function unintentionally and
> copper losses are minimized, so long as core losses remain manageable.

> Another example - see what happens when 50nH is present in battery
> lead wiring....

There's an interesting thread in the DIYelectricCar forum that shows how =

much battery lead and motor lead wiring inductance and resistance can =
affect=20
the waveforms and operation of a rather simple but high-power PWM DC=20
traction motor controller. I tried to give some advice, but I think much =
of=20
the difficulty is the OP's lack of understanding as well as poor wiring =
and=20
measurement techniques:=20
http://www.diyelectriccar.com/forums/showthread.php/open-revolt-igbt-driv=
er-blew-igbts-74629p8.html

> Neither of these factors would be so noticeable at 100w. One of
> the major purposes of modeling in software is that these effects
> can be predicted before hardware is constructed.

The transformer design seems to be the major problem, especially at =
higher=20
frequencies. I need to rethink my original method of using thicker wire, =
and=20
go to a bifilar winding with multiple smaller strands in parallel. But I =

don't know just how to determine leakage inductance, and I'm not sure if =
my=20
LCR meters are good enough to measure it after building one. I just =
guessed=20
at the 0.99 for the coupling. I know everything works better if I set it =
to=20
1.00, but that's unrealistic. I have sometimes added external inductance =
to=20
the model, and that seems to work a little better. And I also guessed =
the=20
magnetizing inductances shown in this simulation, but I used measured =
values=20
for my previous simulations for the toroid transformer at 2 kHz.

> There's usually not much calculus required, it's usually just basic
> algebra, with a little trig thrown in.

> http://cp.literature.agilent.com/litweb/pdf/5952-4020.pdf
> http://www.onsemi.com/pub_link/Collateral/SMPSRM-D.PDF

Those are some excellent references. You're right, it's not that hard to =

understand. But it will take some time to grasp all the pros and cons of =

various topologies.

> DIYers may try to use readily available subassemblies and materials
> from similar, if not directly related applications. It's just as
> important to understand the potential and limitations of these
> materials as it is when developing from scratch.

Yes, but it's probably a good idea to see what is commonly available, =
such=20
as the cores and bobbins I have seen on eBay. I have some ferrite cores =
and=20
bobbins that I picked up from who knows where, and even where the parts =
that=20
have markings I've had a hard time finding data. For a much smaller (2W) =

DC-DC transformer I was able to get samples from Lodestone Pacific and=20
Cosmo, but they have rather high minimum order levels.

I think there are two approaches to engineering and design, with one =
extreme=20
(perhaps the Tesla method) being a very careful and specific design =
process=20
which should produce a product that works pretty much as specified, to =
the=20
other extreme (perhaps the Edison method), where you might make 99 =
mistakes=20
running simulations and building actual units, and with good measurement =

techniques and some experience and "instinct", coming up with a winner =
on=20
try #100. I admire Tesla, and others who can use a very solid =
theoretical=20
approach, but I find the Edison method more to my liking.

Thanks,

Paul=20

On Sun, 8 Jul 2012 03:07:48 -0400, "P E Schoen" <paul@peschoen.com>
wrote:

>"legg"  wrote in message news:20mgv7l56cicii89gkkvi9jpj01b1de9qb@4ax.com...
>
>> There is nothing shameful about working with circuits below 1KW.
>> It's the most practical method of developing preliminary physical
>> models and prototypes.
>
>> The principles involved apply at all power levels. What changes
>> is the physical limitations of materials and methods, when
>> scaled. Your circuit is a good example. Try running the simulation
>> with reduced transformer Lp, while maintaining turns ratio. Even
>> with a coupling coefficient of 0.99, your current values are
>> dominating permissible Di/Dt in the power transfer, due
>> to leakage terms.
>
>> In real life, Lp will be selected only to be high enough so that
>> magnetizing energy doesn't dominate function unintentionally and
>> copper losses are minimized, so long as core losses remain manageable.
>
>> Another example - see what happens when 50nH is present in battery
>> lead wiring....
>
>There's an interesting thread in the DIYelectricCar forum that shows how 
>much battery lead and motor lead wiring inductance and resistance can affect 
>the waveforms and operation of a rather simple but high-power PWM DC 
>traction motor controller. I tried to give some advice, but I think much of 
>the difficulty is the OP's lack of understanding as well as poor wiring and 
>measurement techniques: 
>http://www.diyelectriccar.com/forums/showthread.php/open-revolt-igbt-driver-blew-igbts-74629p8.html

Did you actually try either of the suggestions in your model?

>> Neither of these factors would be so noticeable at 100w. One of
>> the major purposes of modeling in software is that these effects
>> can be predicted before hardware is constructed.
>
>The transformer design seems to be the major problem, especially at higher 
>frequencies. I need to rethink my original method of using thicker wire, and 
>go to a bifilar winding with multiple smaller strands in parallel. But I 
>don't know just how to determine leakage inductance, and I'm not sure if my 
>LCR meters are good enough to measure it after building one. I just guessed 
>at the 0.99 for the coupling. I know everything works better if I set it to 
>1.00, but that's unrealistic. I have sometimes added external inductance to 
>the model, and that seems to work a little better. And I also guessed the 
>magnetizing inductances shown in this simulation, but I used measured values 
>for my previous simulations for the toroid transformer at 2 kHz.
>
Rest assured, both magnetizing and leakage inductance are directly
measurable, using basic inductance meters, or indirectly, using simple
calculations based on recorded transient current waveforms, at easily
developed power levels. Their ratio produces the coefficient of
coupling required for more accurate simulation at higher power levels.

An unaltered winding pair will have the same leakage inductance, with
respect to each other, regardless of the core material or other
external environmental factors.

The effects of leakage on power transfer and Di/Dt is most easily
demonstrated in low power flyback circuits. 

http://www.ti.com/lit/ml/slup078/slup078.pdf

The same Di/Dt limitation occurs in power transfer where the winding
polarity and direction of current flow is expected to reverse.

The point is - if your leakage is mistakenly made artificially high or
low in the simulation, you'll get effects that aren't reproduceable in
real life. Choosing a high coupling coefficient should produce low
leakage in the simulation - but by picking an unrealistically high
magnetizing inductance, you also ballooned the associated leakage term
out of the ballpark for a low turns count, high frequency
construction.

<snip>
>> DIYers may try to use readily available subassemblies and materials
>> from similar, if not directly related applications. It's just as
>> important to understand the potential and limitations of these
>> materials as it is when developing from scratch.
>
>Yes, but it's probably a good idea to see what is commonly available, such 
>as the cores and bobbins I have seen on eBay. I have some ferrite cores and 
>bobbins that I picked up from who knows where, and even where the parts that 
>have markings I've had a hard time finding data. For a much smaller (2W) 
>DC-DC transformer I was able to get samples from Lodestone Pacific and 
>Cosmo, but they have rather high minimum order levels.
>
Look for dead equipment that previously performed at roughly the same
high frequency power level. This stuff is marketed by weight as
non-ferrous scrap. You're looking for re-usable core assemblies.
Preformed bobbins aren't often used at this power level - ground
insulation and formers being constructed as required.

>I think there are two approaches to engineering and design, with one extreme 
>(perhaps the Tesla method) being a very careful and specific design process 
>which should produce a product that works pretty much as specified, to the 
>other extreme (perhaps the Edison method), where you might make 99 mistakes 
>running simulations and building actual units, and with good measurement 
>techniques and some experience and "instinct", coming up with a winner on 
>try #100. I admire Tesla, and others who can use a very solid theoretical 
>approach, but I find the Edison method more to my liking.
>
>Thanks,
>
>Paul 

The issue here is the 

On Sun, 08 Jul 2012 03:07:48 -0400, P E Schoen wrote:

> The transformer design seems to be the major problem, especially at higher
> frequencies. I need to rethink my original method of using thicker wire,
> and go to a bifilar winding with multiple smaller strands in parallel.

AKA litz wire. Suggested by many already.

> But
> I don't know just how to determine leakage inductance,

I've told you once...

> and I'm not sure if
> my LCR meters are good enough to measure it after building one.

What are they? What specification? You might be pleasantly surprised. Most
commercial LCR meters should be up to the job.

> I just
> guessed at the 0.99 for the coupling. I know everything works better if I
> set it to 1.00, but that's unrealistic. I have sometimes added external
> inductance to the model, and that seems to work a little better. And I
> also guessed the magnetizing inductances shown in this simulation, but I
> used measured values for my previous simulations for the toroid
> transformer at 2 kHz.

Don't guess - measure.

<snip> 

> I think there are two approaches to engineering and design, with one
> extreme (perhaps the Tesla method) being a very careful and specific
> design process which should produce a product that works pretty much as
> specified, to the other extreme (perhaps the Edison method), where you
> might make 99 mistakes running simulations and building actual units,
> and with good measurement techniques and some experience and "instinct",
> coming up with a winner on try #100.

How do you know that try #100 is the optimal solution, without calculation?

> I admire Tesla, and others who can
> use a very solid theoretical approach, but I find the Edison method more
> to my liking.

Whose electrical distribution system is almost universally used today?

Edison's, or Tesla/Westinghouse ?

-- 
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
                                       (Richard Feynman)

On Sunday, July 8, 2012 9:07:48 AM UTC+2, P E Schoen wrote:
> "legg"  wrote in message news:20mgv7l56cicii89gkkvi9jpj01b1de9qb@4ax.com.=
..
>=20
> > There is nothing shameful about working with circuits below 1KW.
> > It's the most practical method of developing preliminary physical
> > models and prototypes.
>=20
> > The principles involved apply at all power levels. What changes
> > is the physical limitations of materials and methods, when
> > scaled. Your circuit is a good example. Try running the simulation
> > with reduced transformer Lp, while maintaining turns ratio. Even
> > with a coupling coefficient of 0.99, your current values are
> > dominating permissible Di/Dt in the power transfer, due
> > to leakage terms.
>=20
> > In real life, Lp will be selected only to be high enough so that
> > magnetizing energy doesn't dominate function unintentionally and
> > copper losses are minimized, so long as core losses remain manageable.
>=20
> > Another example - see what happens when 50nH is present in battery
> > lead wiring....
>=20
> There's an interesting thread in the DIYelectricCar forum that shows how=
=20
> much battery lead and motor lead wiring inductance and resistance can aff=
ect=20
> the waveforms and operation of a rather simple but high-power PWM DC=20
> traction motor controller. I tried to give some advice, but I think much =
of=20
> the difficulty is the OP's lack of understanding as well as poor wiring a=
nd=20
> measurement techniques:=20
> http://www.diyelectriccar.com/forums/showthread.php/open-revolt-igbt-driv=
er-blew-igbts-74629p8.html
>=20
> > Neither of these factors would be so noticeable at 100w. One of
> > the major purposes of modeling in software is that these effects
> > can be predicted before hardware is constructed.
>=20
> The transformer design seems to be the major problem, especially at highe=
r=20
> frequencies. I need to rethink my original method of using thicker wire, =
and=20
> go to a bifilar winding with multiple smaller strands in parallel.

Litz wire is multiple small strands in parallel, but it's supposed to be br=
aided rather than twisted, so that each wire sees exactly the same field - =
averaged over the whole length of the winding. If the various wires see dif=
ferent fields, hooking them together makes a sort of shorted turn.

http://www.litz-wire.com/pdf%20files/Round_Litz_Recommended_Operating_Frequ=
ency_28-48_AWG_R3.07.09.2010.pdf

could be interesting reading

> But I  don't know just how to determine leakage inductance,=20

The standard technique is to measure the inductance of one winding with ano=
ther winding shorted. You can do better with an impedance bridge, by allowi=
ng for the resistance of the shorted winding, but an inductance meter is of=
ten good enough.

> and I'm not sure > if my=20
> LCR meters are good enough to measure it after building one. I just guess=
ed=20
> at the 0.99 for the coupling.

We got 0.98 for a heavily gapped RM14 core (Al 160nH/T^2), and expect close=
r to 0.999 for an ungapped RM14 core.=20

> I know everything works better if I set it to=20
> 1.00, but that's unrealistic. I have sometimes added external inductance =
to=20
> the model, and that seems to work a little better. And I also guessed the=
=20
> magnetizing inductances shown in this simulation, but I used measured val=
ues=20
> for my previous simulations for the toroid transformer at 2 kHz.
>=20
> > There's usually not much calculus required, it's usually just basic
> > algebra, with a little trig thrown in.
>=20
> > http://cp.literature.agilent.com/litweb/pdf/5952-4020.pdf
> > http://www.onsemi.com/pub_link/Collateral/SMPSRM-D.PDF
>=20
> Those are some excellent references. You're right, it's not that hard to=
=20
> understand. But it will take some time to grasp all the pros and cons of=
=20
> various topologies.
>=20
> > DIYers may try to use readily available subassemblies and materials
> > from similar, if not directly related applications. It's just as
> > important to understand the potential and limitations of these
> > materials as it is when developing from scratch.
>=20
> Yes, but it's probably a good idea to see what is commonly available, suc=
h=20
> as the cores and bobbins I have seen on eBay. I have some ferrite cores a=
nd=20
> bobbins that I picked up from who knows where, and even where the parts t=
hat=20
> have markings I've had a hard time finding data. For a much smaller (2W)=
=20
> DC-DC transformer I was able to get samples from Lodestone Pacific and=20
> Cosmo, but they have rather high minimum order levels.

EPCOS and Ferroxcube parts are widely available in Europe - I've got EPCOS =
parts from Farnell without much trouble - they did once ship me some 10-pin=
 parts as if they were the 12-pin parts that I'd actually ordered, but they=
 sorted it out within a couple of days. The Farnell website connects you di=
rectly to EPCOS data sheets, which are pretty detailed.
=20
> I think there are two approaches to engineering and design, with one extr=
eme=20
> (perhaps the Tesla method) being a very careful and specific design proce=
ss=20
> which should produce a product that works pretty much as specified, to th=
e=20
> other extreme (perhaps the Edison method), where you might make 99 mistak=
es=20
> running simulations and building actual units, and with good measurement=
=20
> techniques and some experience and "instinct", coming up with a winner on=
=20
> try #100. I admire Tesla, and others who can use a very solid theoretical=
=20
> approach, but I find the Edison method more to my liking.

The Edison appraoch turns into the Tesla approach when you've done it often=
 enough to have the kind of feel for what's going on that you can turn into=
 mathematical models. LTSpice now allows you to use the John Chan model to =
simulate real inductors with hysteresis. The one part that we modelled and =
tested suggests that the model works pretty well.

--=20
Bill Sloman, Nijmegen

"legg"  wrote in message =
news:i43jv7177gekoem8hs1hmbnbi06rcntbrp@4ax.com...

> Did you actually try either of the suggestions in your model?

I tried the 50 nH in series with the battery with minimal observed =
change. I=20
also rewound the ferrite core transformer I plan to use as a first=20
approximation to see what to expect. The core is about 2" square and =
1/2"=20
thick. I have two layers of 36 turns #16 AWG each followed by two =
bifilar=20
windings each consisting of 3 parallel windings of #16 AWG. So the total =
72=20
turn winding is 19.95 mH and the six turns reads 0.10 mH. It should be a =

12:1 ratio but the ratio of the square roots of the inductance comes to=20
14.14. Probably an error in the measurement.

But previously I read 8.38 mH for the 72 turns, so either there was more =
of=20
a gap previously or there had been some shorted turns. I had used 10 =
turns=20
of #12 AWG and I had to squash it to fit the core around it.

So I'm hoping to get 5 to 10 volts per turn at 50 kHz, and with 12 V P-P =

square wave on the 3 turn primary I should get about 288 V P-P on the=20
secondary. I figure the transformer has 3.8 uH/sqrt(turns) so 3 turns =
should=20
be about 35 uH. That should be about 11 ohms at 50 kHz so I'll have =
about 1=20
amp magnetizing current. Since I expect up to 50 amps at 12V for 600 =
watts=20
that's only 2% so it seems OK. This is where I usually just put it on =
the=20
test bench and run the voltage up and see where the current starts =
rising=20
faster than voltage and shows saturation. And I can also look at the =
current=20
waveform. Otherwise I'd need to go through the calculations, and since I =

don't really know what material this is I still have to wing it.

> Rest assured, both magnetizing and leakage inductance are directly
> measurable, using basic inductance meters, or indirectly, using simple
> calculations based on recorded transient current waveforms, at easily
> developed power levels. Their ratio produces the coefficient of
> coupling required for more accurate simulation at higher power levels.

I do have a better LCR meter with a 10kHz signal, but I haven't used it =
for=20
awhile. I realize I can measure the magnetizing and leakage inductance =
but I=20
wasn't sure how to calculate it by the physical construction of the=20
transformer.

Does a coupling factor of 0.99 mean that the leakage inductance is 1% of =
the=20
magnetizing inductance? And is it analogous to the regulation of the=20
transformer? Our high current 60 Hz transformers typically have a =
regulation=20
of 5-10% which means that they will put out 20x to 10x of their normal=20
rating into a short, which is essentially how we use them. However, the =
open=20
circuit current draw is probably 1/10 that at the normal output current, =
so=20
that would indicate the leakage inductance would be about 1% of the=20
magnetizing inductance.

For instance, we may have a transformer rated at 480V input and 4.8 =
volts=20
output at 1000 amps. The open circuit current draw might be 1 amp, while =
it=20
would draw 10 amps at its rated 4.8 kVA output, and 100 amps into a =
short=20
circuit. Thus the magnetizing inductance would be 785 mH. The leakage=20
inductance would be 7.85 mH. Does this sound about right?

> An unaltered winding pair will have the same leakage inductance, with
> respect to each other, regardless of the core material or other
> external environmental factors.

> The effects of leakage on power transfer and Di/Dt is most easily
> demonstrated in low power flyback circuits.

> http://www.ti.com/lit/ml/slup078/slup078.pdf

> The same Di/Dt limitation occurs in power transfer where the winding
> polarity and direction of current flow is expected to reverse.

> The point is - if your leakage is mistakenly made artificially high
> or low in the simulation, you'll get effects that aren't reproducible
> in real life. Choosing a high coupling coefficient should produce low
> leakage in the simulation - but by picking an unrealistically high
> magnetizing inductance, you also ballooned the associated leakage
> term out of the ballpark for a low turns count, high frequency
> construction.

> Look for dead equipment that previously performed at roughly
> the same high frequency power level. This stuff is marketed by
> weight as non-ferrous scrap. You're looking for re-usable core
> assemblies. Preformed bobbins aren't often used at this power
> level - ground insulation and formers being constructed as required.

For one-off designs that's a good idea, but I want to be able to order =
the=20
right materials so that once it works, it will be reproducible and=20
manufacturable. The cores don't seem to be terribly expensive. But maybe =

it's useful to see how such transformers are made.

I wish I had the time and energy to devote to learning all the details =
of=20
such designs, but I have many more irons in the fire and I need to=20
concentrate on those, where I know a bit more what I'm doing and mostly =
I=20
just need to attend to some details.

Thanks for everyone's help.

Paul=20

On Monday, July 9, 2012 9:53:21 AM UTC+2, P E Schoen wrote:
> "legg"  wrote in message news:i43jv7177gekoem8hs1hmbnbi06rcntbrp@4ax.com...

<snip>
 
> > Rest assured, both magnetizing and leakage inductance are directly
> > measurable, using basic inductance meters, or indirectly, using simple
> > calculations based on recorded transient current waveforms, at easily
> > developed power levels. Their ratio produces the coefficient of
> > coupling required for more accurate simulation at higher power levels.
> 
> I do have a better LCR meter with a 10kHz signal, but I haven't used it for 
> awhile. I realize I can measure the magnetizing and leakage inductance but I 
> wasn't sure how to calculate it by the physical construction of the 
> transformer.

The leakage inductance is independent of the presence of the core, so if you just calculate the mutual inductances of the windings as air-cored coils you can deduce the leakage inductances.
 
> Does a coupling factor of 0.99 mean that the leakage inductance is 1% of the 
> magnetizing inductance?

Sort of. Look at the transformer equation

> And is it analogous to the regulation of the 
> transformer?

No. That's mostly driven by winding resistance and core losses. There's no loss mechanism directly associated with leakage inductance, though if leakage inductance induces huge switching spikes it can indirectly increase your losses.

> Our high current 60 Hz transformers typically have a regulation 
> of 5-10% which means that they will put out 20x to 10x of their normal 
> rating into a short, which is essentially how we use them. However, the open 
> circuit current draw is probably 1/10 that at the normal output current, so 
> that would indicate the leakage inductance would be about 1% of the 
> magnetizing inductance.

Not remotely correct. Thinks about what's happening when the transformer is driving a short - with large currents flowing through the internal resistances.
 
<snip>

-- 
Bill Sloman, Nijmegen

On Mon, 9 Jul 2012 03:53:21 -0400, "P E Schoen" <paul@peschoen.com>
wrote:

>"legg"  wrote in message news:i43jv7177gekoem8hs1hmbnbi06rcntbrp@4ax.com...
>
>> Did you actually try either of the suggestions in your model?
>
>I tried the 50 nH in series with the battery with minimal observed change. I 

You should se a 20Vppk waveform on 'in' and primary current exhibiting
a dual slope. V'in' will show up on the driven end of L1.

>also rewound the ferrite core transformer I plan to use as a first 
>approximation to see what to expect. The core is about 2" square and 1/2" 
>thick. I have two layers of 36 turns #16 AWG each followed by two bifilar 
>windings each consisting of 3 parallel windings of #16 AWG. So the total 72 
>turn winding is 19.95 mH and the six turns reads 0.10 mH. It should be a 
>12:1 ratio but the ratio of the square roots of the inductance comes to 
>14.14. Probably an error in the measurement.
>
>But previously I read 8.38 mH for the 72 turns, so either there was more of 
>a gap previously or there had been some shorted turns. I had used 10 turns 
>of #12 AWG and I had to squash it to fit the core around it.

With a magnetic Xsection of 144mm, estimated for a core of this size
(eg E40/16/12), your 50KHz drive waveform on a three turn winding
produces core flux excursions of +/-140mT, and typical total core
losses of 1.4W. This will vary with input voltage, as the circuit is
unregulated. 

When fully wound, this core's surface temperature will rise 50degC
under a total loss burden of ~4.2W in free air.
>
>So I'm hoping to get 5 to 10 volts per turn at 50 kHz, and with 12 V P-P 
>square wave on the 3 turn primary I should get about 288 V P-P on the 
>secondary. 

With a 24V source, the drive voltage is approximately 24Vppk,
approximately 12Vpk. It's the peak voltage that is produced on the
full wave output rectifier, but only if the total input voltage is
reflected accurately.

Look at the output voltage of your simulation, before the choke.
You'll see that the voltage there only goes positive about a third of
the conversion period. The rest of the time, the voltage is dominated
by the leakage inductance of the simulated transformer during phase
reversal. This loss of conduction period shows up as reduced filtered
output voltage, in spite of expectations due to turns ratio.

This is mostly the effect of leakage inductance occuring in your
model.

While the effect is real, the scale of this effect in your simulation
is masking realistic performance.

RL

"legg"  wrote in message =
news:9dmlv7t8au4i0dp6kfp6vaue44a8dl8lj0@4ax.com...

> On Mon, 9 Jul 2012 03:53:21 -0400, "P E Schoen" <paul@peschoen.com>
> wrote:

>>I tried the 50 nH in series with the battery with minimal observed =
change.

> You should se a 20Vppk waveform on 'in' and primary current
> exhibiting a dual slope. V'in' will show up on the driven end of L1.

Well, V(in) is more like a distorted sine wave with peaks of 27V and =
valleys=20
of 20V. And it does form the upper waveshape of the driven end of L1, so =
I=20
can see where battery lead inductance is a major factor.

I was a bit surprised at how much inductance a length of wire can have. =
I=20
used an inductance calculator and found that a 36" length of battery =
cable=20
0.25" diameter has 1025 nH of inductance.
http://www.consultrsr.com/resources/eis/induct5.htm

But when I used that in the simulation it actually seemed to improve the =

operation. V(in) varies from 25.5 to 20.5 volts (looks like a rectified =
sine=20
wave) and the DC output actually seems to increase. The peaks seem to be =

shifted to the center of the conduction cycle and the current in the =
primary=20
L1 looks almost like a sine wave.

> With a magnetic Xsection of 144mm, estimated for a core of this
> size (eg E40/16/12), your 50KHz drive waveform on a three turn
> winding produces core flux excursions of +/-140mT, and typical
> total core losses of 1.4W. This will vary with input voltage, as
> the circuit is unregulated.

> When fully wound, this core's surface temperature will rise
> 50degC under a total loss burden of ~4.2W in free air.

Sounds reasonable. Do you know of a calculator that can obtain these=20
figures? The following seem to be pretty good:
http://www.bcae1.com/trnsfrmr.htm
http://www.smps.us/magnetics.html
http://www.epcos.com/web/generator/Web/Sections/DesignSupport/Tools/Ferri=
tes/Page,locale=3Den.html

>> So I'm hoping to get 5 to 10 volts per turn at 50 kHz, and with
>> 12 V P-P square wave on the 3 turn primary I should get
>> about 288 V P-P on the secondary.

> With a 24V source, the drive voltage is approximately 24Vppk,
> approximately 12Vpk. It's the peak voltage that is produced
> on the full wave output rectifier, but only if the total input
> voltage is reflected accurately.

> Look at the output voltage of your simulation, before the
> choke. You'll see that the voltage there only goes positive
> about a third of the conversion period. The rest of the time,
> the voltage is dominated by the leakage inductance of the
> simulated transformer during phase reversal. This loss of
> conduction period shows up as reduced filtered output
> voltage, in spite of expectations due to turns ratio.

> This is mostly the effect of leakage inductance occuring
> in your model.

> While the effect is real, the scale of this effect in your
> simulation is masking realistic performance.

With the higher battery lead inductance of 1025 nH, the voltage at the=20
output of the bridge only briefly drops to a minimum of 89V, has a peak =
of=20
413V, an average of 306V, and RMS of 315V. I get input power of 1.03 kW =
and=20
output of 939W for an efficiency of 91%.

With an (unrealistic) lead inductance of 1 nH, the input power is 963 W =
and=20
output is 872 W, for 90.5% efficiency. It is interesting and useful to =
know=20
that realistic battery lead inductance may actually help the operation. =
I'm=20
still using the 0.99 coupling factor.

Thanks for the tips. I can see that, at these power levels, everything =
needs=20
to be modeled as accurately as possible. But in the end I will still =
need to=20
build it and test it.

I still don't know what the problems are for the OP in the thread:
http://www.diyelectriccar.com/forums/showthread.php/open-revolt-igbt-driv=
er-blew-igbts-74629p9.html

It may very well be battery and motor lead inductance, which should be=20
minimized by using twisted or bundled pairs. But I think the main =
problem is=20
measuring the waveforms using a cheap USB scope with long, unshielded =
leads=20
within a giant loop that may be seeing current surges of 100 amps or =
more at=20
8 KHz.

I'm trying to give advice on ways to check what's really happening and =
ways=20
to reduce the problem, but I think there is a lack of understanding and=20
experience. Maybe on my part as well, but I have encountered similar =
bogus=20
waveforms and I have been able to reduce the effects considerably by =
keeping=20
measurement leads tightly twisted and outside of the magnetic loop.

Paul=20