Inductor design

Question

I really need your help on this. I am designing an inductor which is rated for 50 A and 350 Vrms for 85 kHz. I want to get 3 inductance value from one inductor so I decided to tap between the points or to use switch to get the required inductance.

I just want to know if my assumptions are right.

I considered ETD 59 core. Let's say I make 5 turns to get 14 uH and solder both the ends to the bobbin pin (lets consider pin 1 to 2). Again I make 3 turns on the second layer (I start from the side where 1st layer winding ends) I solder both the sides (pin 3 to 4).

Now, if I connect between:

• 1 and 2 I am supposed to get 14 uH (5 turns).
• 3 and 4 I am supposed to get 5 uH (3 turns).

If I short 2 and 3 and measure between 1 and 4 I am suppose to get 36 uH (8 turns) which makes it in series, I just calculated using ferroxcube tool for respective turns and assumed it would give the same.

Here my questions are:

1. Can I use switches and jumpers between the windings as I said and get the mentioned inductance value?
2. If there is a space on the first layer after making 5 turns, can I start with the same layer to make 3 turns? Does mutual inductance come into play in this case?
3. If it's right which way is better to do either making 3 turns in 2nd layer or in the same layer?

PS: Here I am doing tunable network where I am reducing the total size of the circuit by doing 3 inductance value in a single inductor.

I would really appreciate if anyone can tell me what to do with this. I can give you more information on this if you need it. Thank you in advance. Waiting for your valuable answers.

Answer 1

I don't understand the interest in disconnecting and rearranging windings; the worst case is to handle 50A on the whole 36µH, and the others are not specified as any different current so it's not clear that anything else should be done. The "350Vrms" (of waveform undefined) is lurking ominously in the background, but 350V sine on 5µH at 85kHz would be a whopping 131A, so I'm going to assume 350V is the total only for 36µH and the AC current is lower; the 50A figure is then a DC (or LF/mains) bias, which is a common case.

Plugging in 50A DC/bias, 350V (RMS sine) at 85kHz, 36µH, and ETD59 (n97 material) into a calculator, I get an AC current of 18.2A (implying, for a buck converter application for example, a ripple fraction of 50% or so), a suggested minimum-loss design of 17 turns, which must be Litz wire of fairly fine build (0.1mm dia. / 38AWG stranding is quite common), and 800 strands. (Which should be built from multiple rope bundles, e.g. 5 x 160 strands, or 5 x 5 x 32 strand, even.) The core air gap will be 3.65mm equivalent, which will be somewhat larger in practice due to fringing fields; simply adjust the core spacing (or if grinding, take a little off at a time, testing frequently) until the desired value is reached.

My experience has been, this tool consistently overestimates Q by 20-40%, so although it says Q = 434.5, expect more like 350 in practice. Total power dissipation will be 28.2W, on the high side for a core this size, so expect to need air circulation around the part. The next size larger core (maybe E70/33/32, or whatever PQ style you can find above 50mm, or a UI/UU/UR style of similar scale) may be desirable to reduce losses.

The 5 and 14µH taps can be made by simply making 6 and 10 turns and splicing the wire down to connections at each point. Though these taps are a bit in error, and it might be desirable to increase the total turns a little bit to make them more accurate.

Construction itself is nontrivial, as the bobbin pins are not rated for anywhere near 50A, and many in parallel must be used. And you have four connections to make. Instead I would suggest using the core, and a basic (no-pins or "skeleton") bobbin, and using crimp lugs on the litz cable. Use bolted connections and terminal blocks to make connection to your PCB, or bolt directly onto bus bars if this is an industrial module sort of build.

Litz can joined by soldering it into crimp lugs. (Yes, this is the recommended procedure: the crimp strain is relieved when the enamel melts, then is displaced by a film of solder, resulting in an ordinary soldered joint.) Use an open-barrel crimp lug, where the strand ends are visible. Simply crimp as normal, dip in rosin flux, then immerse in a solder pot until the bubbling stops, typically 10-20s. Knock off excess solder and let cool, use a file to clean off the mating surfaces and any gross bumps, and apply heat shrink tubing over the crimp and cable to insulate it.

Answer 2

Can I use switches and jumpers between the windings as I said and get the mentioned inductance value?

Yes, you can use switches and jumpers to select between different tap points on the inductor, and thus achieve different inductance values. This is a common method in power electronics to get variable inductance from a single inductor. When you use switches, ensure they can handle the current and voltage you're working with.

If there is a space on the first layer after making 5 turns, can I start with the same layer to make 3 turns? Does mutual inductance come into play in this case?

Yes, if there's enough space on the first layer after the 5 turns, you can make the additional 3 turns on the same layer. However, note that mutual inductance will play a role. The mutual inductance depends on the geometry and proximity of the windings. If the 3-turn winding is close to the 5-turn winding on the same layer, they'll have a higher mutual inductance as compared to if they were on separate layers. This can affect the effective inductance when the windings are in series or parallel.

If it's right which way is better to do either making 3 turns in the 2nd layer or in the same layer?

The decision can be based on a few factors:

Mutual Inductance: As mentioned, mutual inductance could be higher when the turns are on the same layer. This might result in slightly different inductance values than you've calculated.

Thermal Considerations: Having all turns on the same layer might make the inductor heat up more uniformly. Layers on top might act as insulation for the bottom layers, causing them to heat up more.

Ease of Manufacturing: Sometimes it might be easier for manufacturing processes to have distinct layers rather than continuous windings on the same layer.

AC Resistance: The AC resistance or skin effect can be different if windings are on the same layer versus different layers. This can affect efficiency at high frequencies like 85kHz.

Given your design constraints, if there's space on the first layer, and the slight change in mutual inductance doesn't affect your circuit's operation significantly, it might be more efficient thermally to wind on the same layer. However, if precision in inductance values is a concern, you might prefer separate layers to minimize mutual inductance.

In any design, especially in power electronics, it's crucial to prototype and test your design in the real world to verify theoretical calculations. Remember, core losses, skin effect, proximity effect, and other factors can affect the inductor's performance at high frequencies, and they should be taken into account in your design. Good luck!