The phase lead is a coil in series with the feedback burden resistor. Which coil is a question with a surprisingly thin paper trail, three worries attached to it that turn out to be unfounded, and one failure that keeps happening.
The algorithm
1. measure T_d (short the inductor out and look)
2. compute T_ZVS (from C_o(tr) and your dead time)
3. L = R · tan(360° · f · (T_d + T_ZVS)) / (2·pi·f)
4. check: 45 degrees preferred, 60 maximum
5. if it will not fit, make the driver faster rather than the coil largerStep five is the one people skip. Past 60 degrees the amplitude across the network has collapsed enough that you are trading signal for time, and the answer is a shorter delay chain, not more inductance.
What people actually fit
The Coilcraft 7M3 slug-tuned series, by device type:
- 7M3-123, 9 to 15 µH: works well with TO-247 IGBTs.
- 7M3-153, 11 to 19 µH.
- 7M3-223, 17 to 28.
- 7M3-333, 25 to 41.
- 7M3-393, 29 to 49: works well with CM200 and CM300 bricks.
- 7M3-563, 42 to 70.
The generation before it, from Ward's own bill of materials for the UD2.1 revB, was the Coilcraft SLOT TEN-5-09 at 16, 29.4 and 42 µH, and the TEN-5-10 at 25, 42.3 and 60. Style 5 is a ferrite core in a ferrite sleeve.
Ward's original specification, since it settles an argument
From the UD2.1 revB schematic, dated March 2011:
- L1 = "10 to 60 µH", with a three pin header to short it out.
- R1 = 51 Ω, 2 W, the feedback burden.
- C32 = 150 pF across that burden.
- R17 = 5.1 Ω, 0.5 W, the overcurrent burden, on a separate transformer.
Ward never published his reasoning. There are no schematics or BOMs from him for the UD2.5, 2.7 or 2.9; his files stop at the 2.1 revB. The arctan(XL/R) relation appears in published form only on Kaizer's calculator, attributed to ArcAttack.
Mount it on the board
faraday: because it was dangling there it seemed to be picking up enough EMF from the Tesla coil that the signal going to the comparator got corrupted.
If you wind your own, use a toroid, which closes its field, rather than an air coil. profdc9's recipe is about 70 turns for 50 to 60 µH. The alternative is a bank of fixed inductors in series with a jumper across each, which has the advantage that fixed parts publish an Irms and an Isat.
The one failure that repeats
It is mechanical, and it is the tuning slug.
faraday, 2018: if I try to turn the slug down very far, not even close to bottoming, into the inductor, either the slug or the ceramic tuning tool will fracture.
Coilcraft's own explanation, via their product quality supervisor: they could not get the old coil form material a few years ago and had to switch to a glass filled one, which is not as smooth, so the ferrite sticks to it.
profdc9 cracked a bit off one. Mads Barnkob has destroyed his Slot-7 and Slot-10 inductors too. Turn them gently and do not run them to the stop.
Three worries that are not real
"One amp of feedback current is an overload." It is the design target of the whole UD2.x family. loneoceans aims for about 1 A of feedback current and picks the CT ratio to suit, so a bridge expected to run at 500 A peak gets a 500:1 transformer. WaskaLabs specify around 1 A at peak for the UD2.9-X. The UD3 is specified to handle 1 A peak regardless of topology. And Ward's 51 Ω at 2 W only makes sense at hundreds of milliamps RMS.
"The trim inductor will saturate, overheat or drift." Twenty years of forum archives contain no report of it. An eleven-post thread devoted specifically to choosing these parts does not mention current once. The one time it was seriously proposed as a cause, it turned out to be gate resistors at 4.7 Ω instead of 12.
"These parts are only rated 100 mA." That figure traces to the Piconics ST series, where an ST303-3F is 15 to 30 µH at 40 mA and 200 mW. But that part is 2.5 by 5 mm, a microwave component on a 0-80 screw. A 7 to 10 mm slug can is an order of magnitude more volume. For comparison, Coilcraft's 142, 143 and 144, the same mechanical class as the Slot Ten, publish 5.5 to 13.4 A RMS.
What the lead is for and what it is made of is the phase lead itself, and the floor half of the calculation above is the ZVS piece.