Inductor current rating and temperature - Page 1
Forum section: electronics / projects

author: smbaker
author_rank: Regular Contributor
timestamp: March 30, 2019, 01:34:38 am
content: I have some switching power supply projects, and I chose this inductor, PA4309.104NLT, rated 3.5A. The issue I have is that I'm measuring it's temperature at around 130C with 1.4A going through it, and 190C with 2.2A going through it. This greatly exceeds the 125C rating of the inductor. Plus, it smells bad.Datasheet says "The rated current as listed is the heating current that the part can withstand with acceptable temperature rise in typical applications".So what am I missing here? It's not like one typically heatsinks an inductor, does one?Scott

author: Circlotron
author_rank: Super Contributor
timestamp: March 30, 2019, 01:55:02 am
content: Is the rating with forced air blowing on it? Also, some inductors can change their magnetic properties when they are too hot, but come good again when they cool down.

author: drussell
author_rank: Super Contributor
timestamp: March 30, 2019, 02:17:45 am
content: How is the inductor being used in the circuit?

author: nukie
author_rank: Frequent Contributor
timestamp: March 30, 2019, 02:44:55 am
content: Not some but all magnetics will lose performance so it's best to go up a notch with the saturation spec. Keep heat away from magnetics.I am pretty sure inductors are not suppose to smell bad. What is the peak current at the inductor?

author: smbaker
author_rank: Regular Contributor
timestamp: March 30, 2019, 03:42:50 am
content: Quote from: drussell on March 30, 2019, 02:17:45 amIt's the main inductor in an LM2576 buck converter. LM2576 datasheet says to oversize by 20-30%, hence my choice of a 3.5A inductor. Perhaps I should have choose a greater value of inductance. I'm usually building 5V boost converters where the 100uH is appropriate, this time I built a 12V where 150uH or 220uH may have been more appropriate.I can derate the circuit down to an amp or so and be within a more reasonable temperate. I'm just curious more than anything.Scott

author: AndyC_772
author_rank: Super Contributor
timestamp: March 30, 2019, 06:42:43 am
content: What's the efficiency of the power supply? How does it vary with load?It sounds as though the inductor might be saturating, which means it'll behave more like a resistor, and losses will go up dramatically.What do you see if you scope the switching waveform? Does it look clean?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: March 30, 2019, 07:28:01 am
content: Hi,I think that the Pulse PA4309.104NLT iswildly optimisticallyrated for 3.5A.Consider this:The DC Resistance is given as 405mat 25C.adjusting for a typical operating temp of 100C and calculating power dissipation from DCR alone (Ignoring core losses)And then dissipating that much power in this size deviceFurther If I search Digikey asking for 100uH 3 to 4A and in stock I get (partial result shown):The pulse parts stand out, other vendors are 40 to 80mDraw your own conclusionHave you tried using one of the, much bigger, inductors recommended on the TI datasheet?Other consideration, the LM2576 is first generation simple switcher. You would benefit from a later device with a higher switching frequency.Regards,Jay_Diddy_B

author: smbaker
author_rank: Regular Contributor
timestamp: March 30, 2019, 03:30:07 pm
content: Thanks Jay,I do have some LM2596 on order, which is a 150 KHz regulator, and I have some XL4005 on order, which is a 300 KHz regulator. The LM2596 should be a drop-in replacement. The XL4005 has an annoyingly different pinout and is used in some cheap eBay switching, so I figured I'd give it a shot too.I can't go with a physically larger inductor as board space does not allow.Scott

author: emu_herder
author_rank: Newbie
timestamp: March 30, 2019, 04:25:04 pm
content: I'd stay away from those unshielded drum cores in general, they are all kinds of trouble.In that footprint I'd guess that you'd need to be around 22uH to get up to 3.5A. Look at Bourns SRR1208-220ML for instance, that's a shielded 0.5" 22uH inductor with a 3.5A rating. At 150kHz you may need more like 33-47uH though. If you can handle being limited to 3A I'd try the 33uH from that series at 150kHz.

author: smbaker
author_rank: Regular Contributor
timestamp: April 05, 2019, 12:59:53 am
content: I received my LM2596150KHzswitching regulators today! Pictures of the regulator and the switching waveform are attached!I saved so much money buying these on eBay rather than digikey.

author: jbb
author_rank: Super Contributor
timestamp: April 05, 2019, 01:18:26 am
content: Note inductors often have 2 current ratings: the saturation current (above which available inductance drops off rapidly) and thermal current. Saturation currents are often higher and thus may be used for marketing...

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: April 05, 2019, 02:22:25 am
content: Quote from: jbb on April 05, 2019, 01:18:26 amI agree.In the case of the Pulse datasheet, it is defined in note 3:Jay_Diddy_B

author: Siwastaja
author_rank: Super Contributor
timestamp: April 05, 2019, 05:49:10 pm
content: The datasheet only lists ratings based on DC copper loss, which is only relevant when passing a DC current through the inductor.In a switching converter, there is an additional AC loss, caused by copper AC loss (skin effect) plus core loss. Sometimes the AC loss is negligible; sometimes it's dominant! It's highly dependent on the material properties of the inductor core. Some core materials are completely unsuitable for switch mode converters. Some are so freaking good that youcanignore the AC loss completely and simply work with the datasheet DC current rating.But, with no data about the AC loss in the datasheet, your only way is to measure, which is basically what you have done (based on temperature measurement), and found the inductor unsuitable.Nowadays, many inductor manufacturers do provide AC loss analysis software (often web based), based on their measurements and modeling. If available, use such software to find the total loss.

author: smbaker
author_rank: Regular Contributor
timestamp: April 06, 2019, 06:54:56 pm
content: Quote from: smbaker on April 05, 2019, 12:59:53 amI'm surprised nobody commented on (what I believe to be) fake LM2596. Maybe it flew under the radar.I've received fake vintage ICs before, most notably the SP0256A-AL2, but this is the first time I've received a fake on something as mundane as a voltage regulator. The IC is clearly marked LM2596, but the waveform appears to be 50KHz, not 150KHz. Can someone confirm my logic that this has to be a fake?