Bpkpk of +/-0.137T for LLC converter core at 100kHz - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: March 30, 2022, 08:40:30 pm
content: Hi,We wish to do an LLC converter. (100kHz frequency)We wish to choose a PQ5050 core. (N87 material)The Bpkpk will be +/-0.137T.From the below, that means 6W of loss in the core. Do you think that would be manageable with a fan right next to it?N87 materialhttps://www.tdk-electronics.tdk.com/download/528882/71e02c7b9384de1331b3f625ce4b2123/pdf-n87.pdfPQ5050 corehttps://www.tdk-electronics.tdk.com/inf/80/db/fer/pq_50_50.pdf

author: TimNJ
author_rank: Super Contributor
timestamp: March 30, 2022, 10:01:34 pm
content: Any amount of loss* is manageable with a big enough fan.And surely there is copper loss? If anyone is to give a reasonable guess, you'd really need to know the total loss of the transformer.TDK et. al. provide temperature rise vs. power dissipation graphs in many of their datasheets. These graphs should be taken with a grain of salt since, often times, power supplies are crammed into hot enclosures with little circulating air. These graphs assume large open air space.https://product.tdk.com/system/files/dam/doc/product/ferrite/ferrite/ferrite-core/catalog/ferrite_mz_sw_pq_en.pdf*terms and conditions apply

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 30, 2022, 10:28:56 pm
content: I would expect that's fine. Even more so with N49 at higher frequencies.Tim

author: Faringdon
author_rank: Super Contributor
timestamp: March 31, 2022, 08:06:23 pm
content: Thanks, my worry woudl be the ferrite going over t(curie).As you know, ferrite has a thermal conductivity of about 0.35...compared to aluminium which is 200.....as you know, ferrites cant even be heatsink as it would mean field heating of the heatsink....though it simply woudlnt be possible to draw the heat out of the ferrite, due to 0.35= therm cond.

author: TimNJ
author_rank: Super Contributor
timestamp: March 31, 2022, 10:26:30 pm
content: You can heatsink ferrite with some success.Sure, it'sthermal effusivity(new term to me) is not amazing, but it might still be better than nothing.Assuming equal core and copper loss, maybe ~12W total. PQ5050 is pretty huge. With a fan, it doesn't sounds too bad.If you extrapolate the temperature rise curve to 12W, looks like temperature might be ~100C, with only natural convection cooling...which assuming normal commercial/industry temperature range, puts you at 140 - 150C max...whichistoo much. But, even a little bit of forced airflow goes a long way.Maybe just consider hot-spots deep within this giant PQ5050 transformer. Might explain why planar tends to become more popular at higher power; there is less thermal gradient between inner layer and outer layers/core.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 31, 2022, 10:52:40 pm
content: I see figures more like 3-5 W/(m K), which isn't terrible, that's better than some thermal (heatsink) pads.Wire is probably worst off, being surrounded by less-conductive resins (enamel, varnish, potting)?Note that saturation flux falls off with temp, decreasing hyperbolically approaching Tc. So running much over say 100C might not be a good idea when Tc is only 150-200C, it's not much farther to go before you're saturating the thing and core/wire loss goes way, way up. MnZn tend to have higher Tc, making this a relatively moot point at commercial temperatures.Quote from: TimNJ on March 31, 2022, 10:26:30 pmDid... they build that thing with corrugated cardboard?And yeah, that's an advantage of planar, the wide aspect ratio gives a short path and wide area to heat conduction from the winding. Downside is, you trade that for not having much conductor in the first place (PCB foils). Still, efficiency tends to be quite good.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: March 31, 2022, 11:33:05 pm
content: Quote from: T3sl4co1l on March 31, 2022, 10:52:40 pmWhat, you've never heard of UL Class B corrugated cardboard before? Get with the times!