Electronics > Beginners
Inductor Discharge Rate and Other Questions
T3sl4co1l:
--- Quote from: TheDood on January 31, 2020, 10:36:21 am ---Ok, I think I'm comprehending that charging up and then fully discharging is going to result in greater core losses (lower Q will be more effected than higher Q factor), and that you can achieve greater power output to your load given a % efficiency target utilizing CCM. This is due in part because the current doesn't have to go from 0 to X and then back to 0 in CCM, and the difference in operation results in the opportunity to flow greater amounts of current given a set time period, Hz, (and target efficiency)?
If I were using a powdered iron common mode choke as an AC line filter, but then a ferrite or HF core for the switching inductor; due to the average nature of the PF corrected waveform, I shouldn't need to worry (too much) about core losses in the powdered iron common mode choke when HF inductor is switched at high frequencies, correct?
What about ain air core, no saturation, but probably be a large component for a 300W PFC application?
--- End quote ---
CMC doesn't matter -- it sees a few volts and even less (AC) current, at least if you've done everything else correctly (like, not shorting the switching node to the heatsink to ground, forcing the full switching waveform across the CMC).
As it happens, powdered iron doesn't achieve as high mu as ferrite, so it's almost never used for CMCs. Sometimes used for small CM or DM chokes, where the loss helps dampen resonances with other components in the 10-100MHz range. But not for the dominant 0.1-10MHz range, just needs more inductance down there.
Also, you're drawing switching currents from the mains, differentially; there's a relatively large film cap there (usually ~1uF) to keep the switching ripple voltage low, but it's still going to develop a few volts say. CMC doesn't help with this, it's DM.
What ends up happening in most filters, the leakage of the CMC serves as the L of a pi filter -- two film caps either side of the CMC combine to give a 3rd order filter, hopefully with a cutoff below Fsw so the attenuation is good. Handy, saves adding another component.
Have seen one design (based on a CCM PFC controller by STMicro) that switches at very low frequencies, when the input voltage is low -- that is, near zero-crossing. The emissions are chirping over several octaves, over the course of a mains cycle. Nominal was supposed to be like 200kHz but it dips down to maybe 1/10th that near zero crossing. You can actually see the ripple voltage at the input, with an ordinary scope probe.
This is okay in the US, where the FCC regulates emissions only down to 150kHz, but some CE standards (and I don't know offhand if it's applicable for consumer or ITE or what equipment) start at 9kHz, so they may need an improved filter for use over there.
I did actually end up adding a single choke to that design, to improve the DM filtering below 300kHz. Handily, there was a footprint already on the board, so it was an assembly change only. (Actually a 2nd CMC footprint, the one leg of which got jumpered out, and the other leg got the DM choke.)
As for air core, they're larger for the same Q, and usually have a lot of external field, e.g. solenoids and loops. Toroids can be made, but with some difficulty, in which case you are better off using a powder core anyway, saving on turns and copper losses.
Hmm, think it's been a long time since I've specified an air core inductor, in a commercial design, where the inductor handled significant reactive power.
And that was an induction heater, where the reactive power (up to 100kVA in that case, I think) is hard to handle any other way. Ferrites need to be pretty big to handle that much, and there's no good way to get the core loss out of them (add a water cooling plate? yeah..). And the winding likely still needs to be water-cooled copper tubing, so you aren't saving any fab cost.
Also had problems with that design, where the coil inside the enclosure was nuking the enclosure itself. (Imagine that, right, an induction heater that heats with induction?) Hah, we had one proto enclosure made from steel, we called it the pizza oven because the bottom side literally got that hot. The final aluminum version still got awfully hot, but a few ferrite plates in key areas got that down to a touchable temperature, if still a ways off from "cool".
--- Quote ---Haha you the man, I was talking more thermodynamics as opposed to electrodynamics, heat and enegy transfer.
--- End quote ---
Oh; that actually doesn't help very much, unfortunately. Thermodynamics is energy statics (the "dyn" is referring to energy, not its flow :) ). Rarely, quasi-static flows. Doesn't really say anything about time-dependent processes, or non-equilibrium flows like what we have here -- after all, we're switching inductors back and forth to demand some power flow.
Maybe that's where you're getting tripped up? Because, LR circuits have an equilibrium, and store energy, that's a starting point right? But you're getting lost because you're looking at one tree (energy storage) instead of seeing the forest (continuous energy consumption -- power dissipation!). Or forest fire, perhaps I should say.
Tim
TheDood:
--- Quote from: T3sl4co1l on January 31, 2020, 11:16:30 am ---
--- Quote from: TheDood on January 31, 2020, 10:36:21 am ---Ok, I think I'm comprehending that charging up and then fully discharging is going to result in greater core losses (lower Q will be more effected than higher Q factor), and that you can achieve greater power output to your load given a % efficiency target utilizing CCM. This is due in part because the current doesn't have to go from 0 to X and then back to 0 in CCM, and the difference in operation results in the opportunity to flow greater amounts of current given a set time period, Hz, (and target efficiency)?
If I were using a powdered iron common mode choke as an AC line filter, but then a ferrite or HF core for the switching inductor; due to the average nature of the PF corrected waveform, I shouldn't need to worry (too much) about core losses in the powdered iron common mode choke when HF inductor is switched at high frequencies, correct?
What about ain air core, no saturation, but probably be a large component for a 300W PFC application?
--- End quote ---
CMC doesn't matter -- it sees a few volts and even less (AC) current, at least if you've done everything else correctly (like, not shorting the switching node to the heatsink to ground, forcing the full switching waveform across the CMC).
As it happens, powdered iron doesn't achieve as high mu as ferrite, so it's almost never used for CMCs. Sometimes used for small CM or DM chokes, where the loss helps dampen resonances with other components in the 10-100MHz range. But not for the dominant 0.1-10MHz range, just needs more inductance down there.
Also, you're drawing switching currents from the mains, differentially; there's a relatively large film cap there (usually ~1uF) to keep the switching ripple voltage low, but it's still going to develop a few volts say. CMC doesn't help with this, it's DM.
What ends up happening in most filters, the leakage of the CMC serves as the L of a pi filter -- two film caps either side of the CMC combine to give a 3rd order filter, hopefully with a cutoff below Fsw so the attenuation is good. Handy, saves adding another component.
Have seen one design (based on a CCM PFC controller by STMicro) that switches at very low frequencies, when the input voltage is low -- that is, near zero-crossing. The emissions are chirping over several octaves, over the course of a mains cycle. Nominal was supposed to be like 200kHz but it dips down to maybe 1/10th that near zero crossing. You can actually see the ripple voltage at the input, with an ordinary scope probe.
This is okay in the US, where the FCC regulates emissions only down to 150kHz, but some CE standards (and I don't know offhand if it's applicable for consumer or ITE or what equipment) start at 9kHz, so they may need an improved filter for use over there.
I did actually end up adding a single choke to that design, to improve the DM filtering below 300kHz. Handily, there was a footprint already on the board, so it was an assembly change only. (Actually a 2nd CMC footprint, the one leg of which got jumpered out, and the other leg got the DM choke.)
As for air core, they're larger for the same Q, and usually have a lot of external field, e.g. solenoids and loops. Toroids can be made, but with some difficulty, in which case you are better off using a powder core anyway, saving on turns and copper losses.
Hmm, think it's been a long time since I've specified an air core inductor, in a commercial design, where the inductor handled significant reactive power.
And that was an induction heater, where the reactive power (up to 100kVA in that case, I think) is hard to handle any other way. Ferrites need to be pretty big to handle that much, and there's no good way to get the core loss out of them (add a water cooling plate? yeah..). And the winding likely still needs to be water-cooled copper tubing, so you aren't saving any fab cost.
Also had problems with that design, where the coil inside the enclosure was nuking the enclosure itself. (Imagine that, right, an induction heater that heats with induction?) Hah, we had one proto enclosure made from steel, we called it the pizza oven because the bottom side literally got that hot. The final aluminum version still got awfully hot, but a few ferrite plates in key areas got that down to a touchable temperature, if still a ways off from "cool".
--- Quote ---Haha you the man, I was talking more thermodynamics as opposed to electrodynamics, heat and enegy transfer.
--- End quote ---
Oh; that actually doesn't help very much, unfortunately. Thermodynamics is energy statics (the "dyn" is referring to energy, not its flow :) ). Rarely, quasi-static flows. Doesn't really say anything about time-dependent processes, or non-equilibrium flows like what we have here -- after all, we're switching inductors back and forth to demand some power flow.
Maybe that's where you're getting tripped up? Because, LR circuits have an equilibrium, and store energy, that's a starting point right? But you're getting lost because you're looking at one tree (energy storage) instead of seeing the forest (continuous energy consumption -- power dissipation!). Or forest fire, perhaps I should say.
Tim
--- End quote ---
Chuckling out loud as I read these comments. Forest fire lol, Id love to see that beast, I bet it was fun building/expirementing/cooking pizza lol
Heat flux is rate of thermal transfer, and an example of energy transfer modeled by thermo, its been years though and I wish I remebered it all. Ill get the hang of electrical and all your guys' jargon, it'll just take me a min or 2 or 200000000000 lol
It's early morning here and brain is spent, Ill review the important parts tmrw, thanks again Tim.
TheDood:
Why and how am I flowing so much current?
The idea was to check bulk cap at ZC and adjust the boost switch's PWM to create a steady-ish voltage at bulk cap with V boosted above line, then run multiple LED loads from the bulk cap, each with its own inverter and capacitive dropper with the inverter Hz controlling current through LED load.
The circuit simulates flowing a ton of current through the diode after the boost inductor. Are the inverter switches upside down? I feel like I'm missing something. I thought the capacitor in the capacitive dropper would limit current and I can't understand why im flowing so much current through the diode but hardly any at the load. The pic shows 100kHz inverter, but even at lower Hz, there's a ton of conducted current D1 and barely any through the load cct?
T3sl4co1l:
Surely C2, C6 are typos? Hhahahaha, 400V gate to source on M2, M6.
C1 is also clearly too large to show anything of interest in a transient sim; it might as well be a DC voltage source of whatever. Which can be a good strategy in a simulation, to show that the stuff connected to it is behaving without having to work with the dynamics of the voltage loop.
And also, because the VDC fixes the voltage regardless of what current is drawn from it -- it can be divided in two, i.e., two equal VDC sources, one for the left half and one for the right half. That is, the boost stage, and the inverter stage, can be simulated completely independently of each other, speeding up the simulation of each.
The same is true of V2, which can be set to a DC value for say a few milliseconds at a time, during which the operation of the boost stage can be evaluated. A millisecond is 24 cycles as shown. Really only need one or two cycles to see the switching itself. Repeat for other voltages, then finish for changing voltage, and there you go.
Tim
TheDood:
--- Quote from: T3sl4co1l on February 11, 2020, 04:29:11 am ---Surely C2, C6 are typos? Hhahahaha, 400V gate to source on M2, M6.
C1 is also clearly too large to show anything of interest in a transient sim; it might as well be a DC voltage source of whatever. Which can be a good strategy in a simulation, to show that the stuff connected to it is behaving without having to work with the dynamics of the voltage loop.
And also, because the VDC fixes the voltage regardless of what current is drawn from it -- it can be divided in two, i.e., two equal VDC sources, one for the left half and one for the right half. That is, the boost stage, and the inverter stage, can be simulated completely independently of each other, speeding up the simulation of each.
The same is true of V2, which can be set to a DC value for say a few milliseconds at a time, during which the operation of the boost stage can be evaluated. A millisecond is 24 cycles as shown. Really only need one or two cycles to see the switching itself. Repeat for other voltages, then finish for changing voltage, and there you go.
Tim
--- End quote ---
Thanks Tim,
Do triacs offer greater VDS options than pFETs? How about Rdson? When I probe C1 I only see a max of 80V, are you saying that the pETS are shorting through their VDS rating in the sim? Or why is it drawing so much current? Ya the snubber is a bit overkill lol I was half ass calculating but based on boost L instead of parasitic, I barely skimmed this link and saw a couple equations but didn't read anything, should have lol...
https://www.digikey.com/en/articles/techzone/2014/aug/resistor-capacitor-rc-snubber-design-for-power-switches
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