Ken Smith said:
Fred Bartoli
[...]
At 2MHz, 2x6 turns it's 36uH and 100mA magnetizing current. I don't want
much more and I can't have more turns (leakage inductance will be
unmanageably high).
If it wasn't for your interwinding capacitance issue, I'd suggest twisting
the windings together.
First a couple of things I'm sure you've already thought of:
The leakage inductance makes this:
L1 T1 L3
------))))----+----- --------------)))))----
! ! !
) ) (
L2 ) ) (
) ) (
! ! !
--------------+----- --------------------
T1 is a ideal transformer.
I assume that the primary of this is effectively across the capacitor of a
tuned circuit. (At least at higher power) This would be a normal way to
do things.
If the leakage inductance L3 can be predicted, you could place a capacitor
in series to cancel it. This makes it a double tuned system but the Q of
the secondary side will be quite low so it shouldn't be a major problem.
You could put a capacitor across the secondary and make it a highish Q
tuned circuit. Unfortunately for your application, I don't think this is
in the cards.
Now for my real suggestion:
I've made lower interwinding capacitance at the cost of leakage inductance
by linking cores with single grounded turn. I'm thinking that you may be
able to do the same in a more extreme form like this:
Rod soldered into hole
/
*************
* * * <- Plumbing pipe cap
*! * !*
! * ! <- copper tube
! TTTTT ! <- T= core
!---- * ----! <- copper disk (washer)
! TTTTT !
! * !
*! * !*
* * *
*************
By turning the single grounded turn into totally enclosing shorted copper,
the leakage inductance may be within reason even with the larger number of
turns that the lower Al cores need.
Ken, you've almost described what I'm going to do (well have already done on
the proto), but with some minor modifications.
Effectively 2 cores, with a "one" turn in between.
The first core is pushpull hard driven at 100% duty ratio.
Then the series of both cores leakage inductances (about 500nH each xformer,
seen from the 'single turn' side) is resonated out with a cap.
The secondary side of the second xformer has 2 small parallel caps to limit
dV/dt and thus parasitic currents closed on the shield.
Then the differences:
primary side of the second core has a center tap to ground in order to
balance parasitics currents to the primary side shield. Just a single
grounded turn doesn't do. The secondary side is already balanced.
The inter-cores loop has more than one turn to reduce loop current (10W have
to pass there) and ease these parasitic currents balancing. Plus we need an
even turn number to make a center tapped winding
Then we need shielding between the "1 turn" primary and the isolated side
secondary. 0.1pF capacitance is already way too much and if you carefully
think about the voltage distribution along the turns, CM voltage can't be
perfectly balanced, even in a perfect world.
So we have complete shielding of both sides, with both shields crossing the
second core between primary and secondary.
Funnily enough, the shields, which have to extend right in the middle of the
core (but be careful, don't close the loop
, see high induced voltage in
the center leg (a few volts), and some adjustment is necessary to compensate
for the minute dissymetry that will inevitably appear (flux leakages are
pretty high) and create some unwanted common mode current.
The rather crude proto (dead bug on a plain GND plane) show that the induced
CM voltage into both shields has to be carefully equaled.
Once that's done, the CM current is well below 1uA, down to the 200nA level
on each harmonics. I probably can expect better with a clean PCB and tightly
coupled tracks (this isn't well controlled now).
Now, I've also found some available cores (43 material) so I should have a
more definitive view of all this next week, before making a clean PCB.
