# How to wind a push-pull transformer?

Started by August 4, 2018
```I need to make a transformer for a push-pull converter.
The core is a 25.3/14.8/10 toroid made of N87, f=200kHz
(total, 100kHz per switch), Bmax=100mT, PRIMARY_Irms=10A,
PRIMARY_V_MAX=20V. This gives 2x5 turns and this low
value opens many options.

1. 5 turns, each turn close to one another, then the other 5 turns the
same way. Pros: thin winding, short supply wires.

2. 5 turns across the entire length of the core, then the other 5 turns
in between the first winding's gaps. Should be as thin as the first one,
better coupling. Cons: the center tap endings are far apart.

3. 5 turns of a twisted pair of Litz wires. Cons: much thick,
the center taps are far apart.

4. 5 turns of a twisted bundle of, say, 20 two-color wires,*
then the appropriate unbraiding.

5. It doesn't matter.

The second issue: does any standard current density matter at all
in the case of such short primary wires? Even 1mm^2 (total area
of the Litz wire/twisted bundle) results in negligible RDC and
the I^2R approach seems more scientific than any industrial I/area
rule of thumb. I should focus on minimizing RAC. Correct?

Best regards, Piotr
```
```On Saturday, August 4, 2018 at 5:46:37 PM UTC+10, Piotr Wyderski wrote:
> I need to make a transformer for a push-pull converter.
> The core is a 25.3/14.8/10 toroid made of N87, f=200kHz
> (total, 100kHz per switch), Bmax=100mT, PRIMARY_Irms=10A,
> PRIMARY_V_MAX=20V. This gives 2x5 turns and this low
> value opens many options.
>
> 1. 5 turns, each turn close to one another, then the other 5 turns the
> same way. Pros: thin winding, short supply wires.
>
> 2. 5 turns across the entire length of the core, then the other 5 turns
> in between the first winding's gaps. Should be as thin as the first one,
> better coupling. Cons: the center tap endings are far apart.
>
> 3. 5 turns of a twisted pair of Litz wires. Cons: much thick,
> the center taps are far apart.

200kHz is a high enough frequency that skin-effect matters.

https://en.wikipedia.org/wiki/Skin_effect

recoomends 38AWG (0.1mm diameter) between 100 and 200kHz

Making the two windings as five turns of twisted pain minimises leakage inductance, but maximinses inter-winding capacitance.

> 4. 5 turns of a twisted bundle of, say, 20 two-color wires,*
> then the appropriate unbraiding.

The filaments in Litz wire are shuffled to make each current path equal. A twisted bundle might not work out as well.

> 5. It doesn't matter.

It is going to matter, but you have to know a fair bit about where your problem areas are going to be before you can work out which choice works best for you.

> The second issue: does any standard current density matter at all
> in the case of such short primary wires?

Current density matters if you have enough current in the wire to get it warm.

> Even 1mm^2 (total area
> of the Litz wire/twisted bundle) results in negligible RDC and
> the I^2R approach seems more scientific than any industrial I/area
> rule of thumb. I should focus on minimizing RAC. Correct?

The problem is that high frequency current doesn't use the whole area of a solid
wire. With Litz wire you've got less copper cross-section in a given window area, but the resistive loss at a given high frequency is less if you fill the winding window with Litz wire.

https://en.wikipedia.org/wiki/Skin_effect

If you aren't dissipating enough heat in the winding to matter, you might start thinking about a more compact (and cheaper) transformer, but you will get heat dissipation in the ferrite as well, and permeability drops quite fast at the Curie temperature (above 210C for N87)

Transformer design can involves quite a few trade-offs.

--
Bill Sloman, Sydney
```
```On 08/04/2018 09:54 AM, bill.sloman@ieee.org wrote:
> On Saturday, August 4, 2018 at 5:46:37 PM UTC+10, Piotr Wyderski wrote:
>> I need to make a transformer for a push-pull converter.
>> The core is a 25.3/14.8/10 toroid made of N87, f=200kHz
>> (total, 100kHz per switch), Bmax=100mT, PRIMARY_Irms=10A,
>> PRIMARY_V_MAX=20V. This gives 2x5 turns and this low
>> value opens many options.
>>
>> 1. 5 turns, each turn close to one another, then the other 5 turns the
>> same way. Pros: thin winding, short supply wires.
>>
>> 2. 5 turns across the entire length of the core, then the other 5 turns
>> in between the first winding's gaps. Should be as thin as the first one,
>> better coupling. Cons: the center tap endings are far apart.
>>
>> 3. 5 turns of a twisted pair of Litz wires. Cons: much thick,
>> the center taps are far apart.
>
> 200kHz is a high enough frequency that skin-effect matters.
>
> https://en.wikipedia.org/wiki/Skin_effect
>
> recoomends 38AWG (0.1mm diameter) between 100 and 200kHz
>
> Making the two windings as five turns of twisted pain minimises leakage inductance, but maximinses inter-winding capacitance.
>
>> 4. 5 turns of a twisted bundle of, say, 20 two-color wires,*
>> then the appropriate unbraiding.
>
> The filaments in Litz wire are shuffled to make each current path equal. A twisted bundle might not work out as well.
>
>> 5. It doesn't matter.
>
> It is going to matter, but you have to know a fair bit about where your problem areas are going to be before you can work out which choice works best for you.
>
>> The second issue: does any standard current density matter at all
>> in the case of such short primary wires?
>
> Current density matters if you have enough current in the wire to get it warm.
>
>> Even 1mm^2 (total area
>> of the Litz wire/twisted bundle) results in negligible RDC and
>> the I^2R approach seems more scientific than any industrial I/area
>> rule of thumb. I should focus on minimizing RAC. Correct?
>
> The problem is that high frequency current doesn't use the whole area of a solid
> wire. With Litz wire you've got less copper cross-section in a given window area, but the resistive loss at a given high frequency is less if you fill the winding window with Litz wire.
>
> https://en.wikipedia.org/wiki/Skin_effect
>
> If you aren't dissipating enough heat in the winding to matter, you might start thinking about a more compact (and cheaper) transformer, but you will get heat dissipation in the ferrite as well, and permeability drops quite fast at the Curie temperature (above 210C for N87)
>
>
> Transformer design can involves quite a few trade-offs.
>

I like this series of slides from the University of Colorado course, it
uses the method of Lagrange multipliers to optimize (minimize) total
copper loss for given transformer requirements

<http://www.endos.com.tr/dosya/Ch14slide.pdf>
```
```bill.sloman@ieee.org wrote:

> 200kHz is a high enough frequency that skin-effect matters.

Yes, I know that. The question is not what wire I should use,
because I am able to design it properly, but how to make the
dual primaries correctly using a proper wire.

> Making the two windings as five turns of twisted pain minimises leakage inductance, but maximinses inter-winding capacitance.

I am thinking of using 20+ twisted wires, then unbraiding and
shuffling the endings to make the tap. Leakage inductance is
the source of spikes on the MOSFET drains, but what harm could
the capacitance do (without going to the extremes, i.e. causing
resonance exactly where you don't want it to be)?

> It is going to matter, but you have to know a fair bit about where your problem areas are going to be before you can work out which choice works best for you.

There will be a prototype, of course, but I want to avoid visiting

> Current density matters if you have enough current in the wire to get it warm.

I'd say if you have enough resistance, that is length.
And enough layers to stop heat spreading.

> The problem is that high frequency current doesn't use the whole area of a solid
> wire.

Yes, proximity + skin effects combined. But this part I can manage.

> Transformer design can involves quite a few trade-offs.

Sure, for example I am not going to use the optimal number of turns,
because it is a multi-output PSU. Bumping the number of secondary turns
requires a similar primary adjustment, and that increases B and R.
But the additional losses are less than making the voltages too big
and then dropping them to the desired level. Since the initial B=90mT,
127mT is still fine. The core is a tad too big, but once again I am
geometry-limitated.

Best regards, Piotr

```
```On 08/04/2018 05:31 PM, Piotr Wyderski wrote:
> bill.sloman@ieee.org wrote:
>
>> 200kHz is a high enough frequency that skin-effect matters.
>
> Yes, I know that. The question is not what wire I should use,
> because I am able to design it properly, but how to make the
> dual primaries correctly using a proper wire.
>
>> Making the two windings as five turns of twisted pain minimises leakage inductance, but maximinses inter-winding capacitance.
>
> I am thinking of using 20+ twisted wires, then unbraiding and
> shuffling the endings to make the tap. Leakage inductance is
> the source of spikes on the MOSFET drains, but what harm could
> the capacitance do (without going to the extremes, i.e. causing
> resonance exactly where you don't want it to be)?
>
>> It is going to matter, but you have to know a fair bit about where your problem areas are going to be before you can work out which choice works best for you.
>
> There will be a prototype, of course, but I want to avoid visiting dead-ends.
>
>> Current density matters if you have enough current in the wire to get it warm.
>
> I'd say if you have enough resistance, that is length.
> And enough layers to stop heat spreading.
>
>> The problem is that high frequency current doesn't use the whole area of a solid
>> wire.
>
> Yes, proximity + skin effects combined. But this part I can manage.
>
>> Transformer design can involves quite a few trade-offs.
>
> Sure, for example I am not going to use the optimal number of turns,
> because it is a multi-output PSU. Bumping the number of secondary turns
> requires a similar primary adjustment, and that increases B and R.
> But the additional losses are less than making the voltages too big
> and then dropping them to the desired level. Since the initial B=90mT,
> 127mT is still fine. The core is a tad too big, but once again I am
> geometry-limitated.
>
>     Best regards, Piotr
>
>
>
>
Here's how far I got:
maxima file and a .csv based on mostly epcos datasheets. no idea if any of this is correct

-------------------------------xfrmr.mac---------------------------------------
/*
try to guess the power capability of the cores.
using the AP estimate in slup126.pdf

makelist([
(ui[e+1][2]+ui[e][2])/2,
(ui[e+1][1]-ui[e][1])/(ui[e+1][2]-ui[e][2])
],e,2,length(ui)-1,1);
write_data([["Id","Ud'"],%],"dif1.csv",comma);
*/
kill(all);

Po(AP,K,B,f_t):=(AP*100000000)^(3/4)*K*B*f_t;

cores:makelist(cores[e],e,2,length(cores),1);
pwr:makelist(Po(cores[e][6]*cores[e][9],0.014,200e-3,100000),e,1,length(cores),1);

/*
the pm is a tad too big (3000W vs 520W)
They(EPCOS) don't include the ring cores
in their simulation tool
want something more detailed..
The plan:
1)guess rtherm from v/surface
2)calc deltaB and currents for equal dissipation Pcore=Pwinding
3)check the temp rise is ok

1)R_th
(EPCOS Ferrites and accessoires)
R_th ~=1/sqrt(V_e)
(a point from their R_th graph)
R_th(300e-9 m^3)=100K/W
100=K/sqrt(300e-9)
K = 0.0547722557505166
therefore:
*/
R_th(V_e) :=0.0548/sqrt(V_e);
delta_T(P_v,R_th) :=P_v*R_th;

/*
transferrable power:
Ptrans=C*delta_B*f*A_e*A_n*J
A_N is copper cross section m^2
A_e is core cross section mm^2
J current density A/m^2
C=1 for push pull
delta_B flux dens variation Vs/m^2
f freq 1/s

Power loss:
P_v=P_vc+P_vj
P_vj=I^2*N*R_cu
P_vc(B,f)=K*f^(1+x)*B^(2+y)
x,y 0..1
delta_B=V*t/(N*A_e)

find the koeff K and exp y for core loss
logexpand:super
B,P
30e-3,7e+3
90e-3,100e+3
dy/dx=
(log(100e+3)-log(7e+3))/(log(90e-3)-log(30e-3));
2.420562799417347

y=k*x+d
y=log(P)
k=dy/dx
x=log(B)
d=?
d = 8.781048544054926
log(P)=2.42*log(B)+8.7810485
P=6509.699302573376*B^(2+0.420562799417347)
y=0.4...
try again for K
K=3.398769238238504E+7

P_B(B):=3.398769238238504E+7*B^(2+0.420562799417347);

now K and x for P(f)
P(f)=K*f^(1+x)
f,Pv
30e3,30e3
300e3,600e3
dy/dx=
(log(600e+3)-log(30e+3))/(log(300e+3)-log(30e+3));
1.301029995663981
d = - 3.103303974733933
P(f):=K*f^(1+0.301029995663981)
K = 0.04490060658064694
P_f(f):=0.04490060658064694*f^(1+0.301029995663981)

now how2 combine?
P_vc(B,f)=K*f^(1+x)*B^(2+y)
K=guessed 11.4
*/
P_vc1(B,f):=11.4*f^(1+0.301029995663981)*B^(2+0.420562799417347);
/*
winding losses:
want those in terms of window area. filling factor 0.4
*/
P_vj(I,l,rho,A_n):=I^2*l*rho/(A_n*0.4);
/*
more accurate l based on inner and outer winding radius
circular bobbins:
fullratsimp(integrate(2*%pi*r,r,r1,r2)/(r2-r1));
l=%pi*r2+%pi*r1;

ring cores:
l1(h,da,di):=2*(h+da-di);
l2(h,da,di):=2*(da/2+di/2+h+di);
l(h,da,di):=(l1(h,da,di)+l2(h,da,di))/2;

vind=dphi/dt
vind=db/dt*a
B=integral(v/a)dt t=0 t=T/2
B=V*(T/2-0)/A
B=V*T/(2*A)
V=Vin/N
*/

delta_B(V,T,N,A_e):=V*T/(2*N*A_e);
B(delta_B):=delta_B/2;
P_vc(f,V,N,A_e,V_e):=P_vc1(B(delta_B(V,1/f,N,A_e)),f)*V_e;
/*
assume p and s winding same voltage + current
if I keep P=u*i const what's the opt u/i, N
with min loss?
P_vc increases times 2^2.5 for half N, double v, half core area
increases times (2^(1.3))/(2^(2.5))=0.43 for twice f
P_vj incr times 4 for twice I, 2 for 2 l, 0.5 for twice A_n

*/
I1:24;
V1:31.4;
N1:4;
RN:800/16;
I2:500e-3;
V2:800;
N2:N1*RN;

pvj_l:makelist(P_vj(I1*N1,cores[e][10],2.3e-8,cores[e][9]*(1-0.417))+P_vj(I2*N2,cores[e][10],2.3e-8,cores[e][9]*(0.417)),e,1,length(cores),1);
pvc_l:makelist(P_vc(100e3,V2,N2,cores[e][6],cores[e][7]),e,1,length(cores),1);
rth_l:makelist(R_th(cores[e][7]),e,1,length(cores),1);
delta_t_l:makelist(delta_T(pvj_l[e]+pvc_l[e],rth_l[e]),e,1,length(cores),1);
/*
I1*N1/A1=I2*N2/A2
A1/A2=(I1*N1)/(I2*N2)
A1/A=0.7172413793103448/1.7172413793103448;
0.417...
(area ratio prim/sec winding based on rms currents
from the simulation I1,I2)

find the minima?
best_N:makelist(
solve(
diff(
P_vj(I*BN,cores[e][10],2.3e-6,cores[e][9])+P_vc(100e3,V,BN,cores[e][6],cores[e][7]),BN
)=0,BN),e,1,length(cores),1);

it's too dumb for that..
have to log and then..

logsolve:true...
??
*/

------------------------------------------cores.csv--------------------------------------------------
Name,al,mu_i,sum_L_over_A,L_e,A_e,V_e,mass,A_n,l_w
"R10",900e-9,1500,3.07e+3,24.07e-3,7.83e-6,188e-9,0.9e-3,28.27e-6,28.325e-3
"R12",1330e-9,2200,2.08e+3,31.17e-3,14.96e-6,466e-9,2.4e-3,49.02e-6,38.25e-3
"R16",1420e-9,2200,1.95e+3,38.52e-3,19.73e-6,760e-9,3.7e-3,72.38e-6,44.65e-3
"R29.5",2880e-9,2200,0.96e+3,73.78e-3,76.98e-6,5680e-9,27e-3,283.5e-6,87.55e-3
"R102",2880e-9,2200,0.96e+3,255.3e-3,267.2e-6,68220e-9,330e-3,3400.49e-6,222.05e-3
"PM62/49",9200e-9,1400,0.191e+3,109e-3,570e-6,62000e-9,280e-3,292.5e-6,198.86e-3
"PM50/39",7400e-9,1340,0.227e+3,84e-3,370e-6,31000e-9,140e-3,196.3e-6,97.232e-3
"ETD29/16/10",2860e-9,2160,0.947e+3,72e-3,76e-6,5470e-9,28e-3,95e-6,53e-3
"ETD39/20/13",2700e-9,1600,0.74e+3,92.2e-3,125e-6,11500e-9,60e-3,178e-6,69e-3
"ETD49/25/16",3800e-9,1630,0.54e+3,114e-3,211e-6,24100e-9,124e-3,269.4e-6,86.7e-3
"ETD59/31/22",5300e-9,1590,0.38e+3,139e-3,368e-6,51200e-9,260e-3,365.6e-6,106.61e-3
"RM14",6000e-9,1670,0.35e+3,70e-3,200e-6,14000e-9,74e-3,140e-6,71.63e-3
"E80/38/20",4590e-9,1710,0.470e+3,184e-3,392e-6,72300e-9,360e-3,1628e-6,157.8e-3
"E71/33/32",10000e-9,1740,0.218e+3,149e-3,683e-6,102000e-9,520e-3,1628e-6,160e-3
"E56/24/19",6900e-9,1730,0.31e+3,107e-3,340e-6,36400e-9,184e-3,281.78e-6,113.8e-3
"E55/28/25",9860e-9,2300,0.239e+3,123e-3,420e-6,52000e-9,260e-3,360e-6,164e-3
"E55/28/21",6300e-9,1760,0.350e+3,124e-3,353e-6,44000e-9,216e-3,375.55e-6,117.0e-3
"E42/21/20",5200e-9,1690,0.41e+3,97e-3,234e-6,22700e-9,116e-3,172e-6,100e-3
"E42/21/15",3950e-9,1710,0.548e+3,97e-3,178e-6,17300e-9,88e-3,177e-6,87e-3
"E36/18/11",3100e-9,1500,0.68e+3,81e-3,120e-6,9670e-9,50e-3,122.55e-6,76.4e-3

```
```On 08/04/2018 05:31 PM, Piotr Wyderski wrote:
> bill.sloman@ieee.org wrote:
>
>> 200kHz is a high enough frequency that skin-effect matters.
>
> Yes, I know that. The question is not what wire I should use,
> because I am able to design it properly, but how to make the
> dual primaries correctly using a proper wire.
>
>> Making the two windings as five turns of twisted pain minimises leakage inductance, but maximinses inter-winding capacitance.
>
> I am thinking of using 20+ twisted wires, then unbraiding and
> shuffling the endings to make the tap. Leakage inductance is
> the source of spikes on the MOSFET drains, but what harm could
> the capacitance do (without going to the extremes, i.e. causing
> resonance exactly where you don't want it to be)?
>
>> It is going to matter, but you have to know a fair bit about where your problem areas are going to be before you can work out which choice works best for you.
>
> There will be a prototype, of course, but I want to avoid visiting dead-ends.
>
>> Current density matters if you have enough current in the wire to get it warm.
>
> I'd say if you have enough resistance, that is length.
> And enough layers to stop heat spreading.
>
>> The problem is that high frequency current doesn't use the whole area of a solid
>> wire.
>
> Yes, proximity + skin effects combined. But this part I can manage.
>
>> Transformer design can involves quite a few trade-offs.
>
> Sure, for example I am not going to use the optimal number of turns,
> because it is a multi-output PSU. Bumping the number of secondary turns
> requires a similar primary adjustment, and that increases B and R.
> But the additional losses are less than making the voltages too big
> and then dropping them to the desired level. Since the initial B=90mT,
> 127mT is still fine. The core is a tad too big, but once again I am
> geometry-limitated.
>
>     Best regards, Piotr
>
>
>
>

The layered windings have higher capacitance but lower leak inductance sectioned ones have higher
leak inductance and lower winding capacitance.
There's some formulas in the Transformer and Inductor Design Handbook (google will find a pdf)(but none for toroids)
slup126.pdf is also interesting, but careful it seems to use the cgs System.
for guessing an rtherm, Epcos' PDF_Application.pdf uses 1/sqrt(V_e)

```
```On Sunday, August 5, 2018 at 1:32:13 AM UTC+10, Piotr Wyderski wrote:
> bill.sloman@ieee.org wrote:
>
> > 200kHz is a high enough frequency that skin-effect matters.
>
> Yes, I know that. The question is not what wire I should use,
> because I am able to design it properly, but how to make the
> dual primaries correctly using a proper wire.
>
> > Making the two windings as five turns of twisted pain minimises leakage inductance, but maximinses inter-winding capacitance.
>
> I am thinking of using 20+ twisted wires, then unbraiding and
> shuffling the endings to make the tap.

Simple twisting doesn't give you Litz wire. You want each wire to cut the same amount of magnetic flux, and single twisting doesn't give you that.

> Leakage inductance is
> the source of spikes on the MOSFET drains, but what harm could
> the capacitance do (without going to the extremes, i.e. causing
> resonance exactly where you don't want it to be)?

The inter-winding capacitance has to be dischraged and recharged whenever the polarity flips. If you want to switch fast, that's a lot of current during switch-over (when the voltage across your switching transistors is high) and it pushes up your switching losses and radiated interference.

Baxandall resonant inverters finesse this (which is why Baxandall invented the circuit) but the approach isn't popular if you aren't trying for high turns ratios.

http://sophia-elektronica.com/0344_001_Baxandal.pdf

> > It is going to matter, but you have to know a fair bit about where your problem areas are going to be before you can work out which choice works best for you.
>
> There will be a prototype, of course, but I want to avoid visiting
>
> > Current density matters if you have enough current in the wire to get it warm.
>
> I'd say if you have enough resistance, that is length.
> And enough layers to stop heat spreading.

Heat spreads quite quickly between layers of copper wire. Everything else has a much higher thermal resistance.

<snipped sensible stuff which we don't need to reiterate>

--
Bill Sloman, Sydney

```
```Primary impedance is very low, count on heavy interleaving.  Consider
transmission line transformer design: not because of the bandwidth, but
because of the method to obtain low impedances.

Ideally, primary Zo is around 2-10 ohms, depending on exactly what you meant
(is V_MAX what the switch sees, or what the CT sees?).

Litz is lower losses but higher inductance.  Consider that the whole point
is to allow flux to penetrate the cable.  This gives higher impedances,
which may increase losses elsewhere.

Ideally, you'd have flat ribbon shaped Litz, so you get the paralleling and

Yes, Rac matters, to the extent it may cause the transformer to overheat.
No need to have it stupendously low or anything.

Last time I did something like that, I used 6 strands of #24 in parallel.
Alternating for each side of the primary, P1-P2-... in a flat (multifilar)
winding for the whole primary.  Strands are connected to pins alternately,
giving the interleave and CT winding.  Leakage so low (between ends of the
primary), I didn't even need snubbing for it -- the transistors are
switching slower.  (Slower than I would've liked; they were unfortunately
limited by the inductance of the current sense resistor.)

What's secondary?  If same voltage, throw another set in the bundle,
interleaved, and you're good.  If high voltage, consider that you need to
keep its impedance high, otherwise it'll be swamped by capacitance.

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

"Piotr Wyderski" <peter.pan@neverland.mil> wrote in message
news:pk3lkq\$3te\$1@node2.news.atman.pl...
>I need to make a transformer for a push-pull converter.
> The core is a 25.3/14.8/10 toroid made of N87, f=200kHz
> (total, 100kHz per switch), Bmax=100mT, PRIMARY_Irms=10A,
> PRIMARY_V_MAX=20V. This gives 2x5 turns and this low
> value opens many options.
>
> 1. 5 turns, each turn close to one another, then the other 5 turns the
> same way. Pros: thin winding, short supply wires.
>
> 2. 5 turns across the entire length of the core, then the other 5 turns
> in between the first winding's gaps. Should be as thin as the first one,
> better coupling. Cons: the center tap endings are far apart.
>
> 3. 5 turns of a twisted pair of Litz wires. Cons: much thick,
> the center taps are far apart.
>
> 4. 5 turns of a twisted bundle of, say, 20 two-color wires,*
> then the appropriate unbraiding.
>
> 5. It doesn't matter.
>
> The second issue: does any standard current density matter at all
> in the case of such short primary wires? Even 1mm^2 (total area
> of the Litz wire/twisted bundle) results in negligible RDC and
> the I^2R approach seems more scientific than any industrial I/area
> rule of thumb. I should focus on minimizing RAC. Correct?
>
> Best regards, Piotr

```
```Tim Williams wrote:

> Last time I did something like that, I used 6 strands of #24 in
> parallel. Alternating for each side of the primary, P1-P2-... in a flat
> (multifilar) winding for the whole primary.

I can use 7x0.3mm TRW Litz for that purpose, 3 strands of it would
give me ~1.5mm^2 cross-section per half-primary and a flat winding.
Sounds very reasonable, thanks!

> What's secondary

There's a whole herd of them. 3.3V, 5V, 9V and 18V are the main
outputs, totalling to ~60W. There will also be a lot of 10V isolated
low-current secondaries for high-side MOSFET drivers. I'd like to
check a push-pull first, because it makes the life of the filters
easy and I could steal some AC to directly feed several magamps in the
critical places. A kind of "one ring to rule them all" approach.

A mundane active-clamp forward is waiting as a relief force.
I would like to try the push-pull first also because of its
neuron de-rusting merits. I haven't made a high-power push-pull
for 15 years. Only the SN6501/IR21531 1W-class gizmos.

Best regards, Piotr
```
```OK, some experimental data.

UCC28084, f_osc=224kHz, V_IN=10V (desired range: ~8..20V),
no feedback to force 50/50 mode, no secondary windings.
R_SENSE=50m, output switches: SQJA62EP-T1_GE3, input cap:
150uF/35V SMD polymer tantalum with 70m of ESR. 3D construction
to minimize the lengths of the wires, ~1cm transformer leads
soldered directly to the cap/drain tabs. BAT83 30mA diodes
between D and S, just in case. No gate resistors. Core
is B64290L0618X087, 25mm OD toroid made of N87.

Trafo #1: 1x7 turns, 20x0.335mm wires twisted together
and then unbraided to make the center-tap primaries.
Spread evenly across the core. Considered the best I can do.

Trafo #2: 2x10 turns of 0.7wire, the first half, the tap
and then the second part, wound tightly. The worst possible
implementation, just for reference.

Results:

Idle current=54mA for Trafo #1, 39mA for T#2. Looks like
the effect of the increased capacitance Bill wrote about.

Gate signals: perfect, sharp, no ringing.

Drain waveforms: strange in both cases. With T#1 and T#2
I have massive ringing, but only on one of the drains.
It's not an output stage issue. If I swap the primary
endings, nothing changes. If I swap the gate signals,
the ringing goes to the other coil. So it originates in
the controller itself.

With T#1 it is always like that:

https://s15.postimg.cc/harvkb1u3/DS1_Z_Quick_Print11.png

With T#2 it started more symmetric:

https://s15.postimg.cc/3tux17el7/DS1_Z_Quick_Print10.png

But by manually squeezing/stretching the primaries I am able
to restore the T#1 situation, i.e. dampen the blue waveform.
I am not able to significantly alter the yellow ringing, though.
50V of ringing in a 10V-powered push-pull looks scary. Moreover,
it continues for the entire off-period.

Best regards, Piotr
```