What are the risk of overload a inductor in buck converter? - Page 1
Forum section: electronics / projects

author: luis.ramirez
author_rank: Newbie
timestamp: May 10, 2016, 07:14:28 pm
content: I have a buck converter working at 100kHzSo far I have tested the circuit for a 4A output current, but I may need to work with 6A. Diode, transistor, capacitance and inductance values are over-dimensioned for the needs of the ripple and power consumption and output ripple, but I don't have the characteristics of the inductor besides the inductance so I don't know the max current neither the working frequency.I have tested the circuit in the range of 80khz till 100khz with and output current of 4A and the transfer function has a linear behavior in those ranges, Do I risk to blow the circuit before notice a change in the linearity of the circuit or and overheating of components by increasing the load?Since I can control the duty cycle I can made a soft-start to have low overshoot.Note 1: The inductor has been taken from a generic pc power supply 400wNote 2: Get a comercial core/inductor is not an option.

author: Fungus
author_rank: Super Contributor
timestamp: May 10, 2016, 07:19:51 pm
content: Quote from: luis.ramirez on May 10, 2016, 07:14:28 pmYeah, yeah... but did you put your finger on it to see how hot it gets?

author: ez24
author_rank: Super Contributor
timestamp: May 10, 2016, 07:23:14 pm
content: Quote from: Fungus on May 10, 2016, 07:19:51 pmI think AofE says to wet your finger so you can hear the sizzle before the skin burns

author: luis.ramirez
author_rank: Newbie
timestamp: May 10, 2016, 07:40:46 pm
content: Quote from: ez24 on May 10, 2016, 07:23:14 pmNot sure if that's a joke, but yeah I did it, it does get very hot, is around 50 degrees with the 4A load.

author: Fungus
author_rank: Super Contributor
timestamp: May 10, 2016, 08:00:57 pm
content: Quote from: luis.ramirez on May 10, 2016, 07:40:46 pmNot at all. Temperature is a good indicator.Quote from: luis.ramirez on May 10, 2016, 07:40:46 pmSo...maybe 25 degrees above ambient at 4A?Now try it at 3A, 5A and plot a graph of the temperatures. If it looks good then go for 6A.My guess is that 6A will catch fire but you never know.You might need heatsinks/fans to make it work but you need to think very hard before putting it in a closed box.

author: luis.ramirez
author_rank: Newbie
timestamp: May 10, 2016, 08:34:37 pm
content: So there is some kind of linear relation between core saturation and temperature? or it will be just about power dissipation in the ESR of the inductor?

author: ez24
author_rank: Super Contributor
timestamp: May 11, 2016, 12:23:43 am
content: Quote from: luis.ramirez on May 10, 2016, 07:40:46 pmNo joke, it will save you from getting a burn and the component is hot enough if you hear the sizzle.I forgot what temp it sizzles. I'd rather use a IR meter. I am getting too old to get burned

author: Fungus
author_rank: Super Contributor
timestamp: May 11, 2016, 06:37:15 am
content: Quote from: luis.ramirez on May 10, 2016, 08:34:37 pmIt's more that when you have no data, you have to create your own (aka. "Suck it and see").You want 6 amps? Try 6 amps (gradually) and see what happens.The only danger is from it catching fire. If it doesn't catch fire then you can measure all the other stuff.PS: This is how the graphs in datasheets are created - by applying a condition and measuring.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 11, 2016, 06:39:07 am
content: What type of inductor is it? Probably, it's either near saturation already, or will saturate soon after.An additional 50% range is unreasonable to expect from just any inductor.Saturation increases the ripple fraction and quickens the loop response, leading to increased heat in the transistors (for average current mode control), reduced output current (peak current mode), and probable loop oscillation (average inductance drops, making the modulator much faster than the control loop is expecting).Tim

author: luis.ramirez
author_rank: Newbie
timestamp: May 11, 2016, 04:12:08 pm
content: Quote from: T3sl4co1l on May 11, 2016, 06:39:07 amI don't have the datasheet I just tested it's linearity in the range mentioned above 0-4A at 100 kHz. the inductor was taken from an pc power supply I know that's far from been the best option by I really don't have access to a better option.

author: station240
author_rank: Unknown
timestamp: May 11, 2016, 04:54:14 pm
content: Quote from: luis.ramirez on May 11, 2016, 04:12:08 pmSo what was the rating of the circuit you removed the inductor from ?Eg 12V rail @ 20AIf it's from the mains side, eg PFC inductor then I think it's harder to figure out.

author: luis.ramirez
author_rank: Newbie
timestamp: May 11, 2016, 09:27:46 pm
content: Quote from: station240 on May 11, 2016, 04:54:14 pm12V rail @ 15A

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 12, 2016, 06:05:30 am
content: Oh well then who cares......But what about losses? Is it ferrite core (rod or drum, usually), or toroid (powdered iron, usually yellow-white; sometimes green-blue)?Powdered iron will probably get quite hot regardless, just because it's such poor material.Tim

author: Buriedcode
author_rank: Super Contributor
timestamp: May 12, 2016, 09:00:34 am
content: I haven't seen anyone suggesting 'scope the switch node' yet. Put a small resistance in series with the inductor and look at the voltage across it on your scope, whilst slowly increasing the load current. You should see a straight ramp and then as it enters saturation, the waveform will start to show a 'knee' as the current shoots up non-linearly.Generally, for ferrite and non-powder cores, saturation current is when the inductance drops by 10% of its value, or rather *to* 90% of its value.

author: Fungus
author_rank: Super Contributor
timestamp: May 12, 2016, 09:11:24 am
content: Quote from: Buriedcode on May 12, 2016, 09:00:34 amI did say "when you have no data, you have to create your own (aka. "Suck it and see")"That sort of covers it.

author: luis.ramirez
author_rank: Newbie
timestamp: May 13, 2016, 11:46:42 pm
content: Quote from: T3sl4co1l on May 12, 2016, 06:05:30 amI think It's powdered iron it's a yellow inductor taken from a cheap power supply.