What happens to CCM SMPS when load current drops? - Page 1
Forum section: electronics / beginners

author: igor78
author_rank: Contributor
timestamp: November 17, 2022, 12:30:26 am
content: I'm looking at a typical buck converter that operates in CCM mode. 350kHz.FR9888https://datasheet.lcsc.com/lcsc/1811151534_Fitipower-Integrated-Tech-FR9888SPGTR_C80507.pdfI want to convert 12v to 7V, with 350mA current. It will be a led driver, so I'm pretty sure the load will always be 300-350mA.The minimum inductor value formula for such current (page 11)In my case, it's ~100uHWhich is 10 times more than they recommend at the beginning of the datasheet - 10uH. I understand why. To operate in CCM mode, I need the current always flowing through, so with less current, I need either a larger inductor or faster switching.What happens if undersize the inductor? The coil current will run out to zero, and then what? Is this the output capacitor that will be powering the load? It does not have much energy, so until the next cycle, the load voltage drops to 0V?Is this what is called DCM mode? Output ripple becomes 7V?This makes little sense, as SMPS can supply any amount of current, right? But it looks like they can only supply the amount of current they were designed for.Are there ICs that can do CCM and then fallback to DCM and FR9888 is not one of then? Or all buck converters can handle the light loads?I think there is a hole in my understanding.

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 17, 2022, 01:06:59 am
content: If you undersize the inductor it might saturate, acting like a short, current won't drop to 0 but to orbit levels, blowing everything apart.About the load dropping, the voltage will self adjust, trying to maintain the current constant.

author: MathWizard
author_rank: Super Contributor
timestamp: November 17, 2022, 03:47:28 pm
content: I don't remember reading that about inductors. So once they are saturated and the magnetic field can't grow any bigger, they loose their reactance ? And if you brought an inductor to saturation at 1 frequency, could you still be unsaturated at lower frequencies ? I have to review some vector calculus, before I go back to the physics of magnetic fields and all that.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 17, 2022, 04:34:52 pm
content: Quote from: DavidAlfa on November 17, 2022, 01:06:59 amthat statement only valid for boost converter right? not buck converter.

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 17, 2022, 04:55:40 pm
content: An inductor is an inductor, once saturated will lose its ability to generate the inductance which limits and filters the input pulses.Normally the switch output makes ex. 24V pulses, the inductor smoothens them, outputting a lower constant voltage, but once it becomes saturated the output will get that "raw" 24V pulse because there's nothing filtering it anymore.That's in buck converter. In other topologies using a primary/secondary, the converter will just blow apart, the current peak won't be transferred to the secondary fue the said saturatin state.I'm not an expert at magnetics, but I think that's pretty much it.