fast switching frequencies in converters - Page 1
Forum section: electronics / general

author: pepelevamp
author_rank: Regular Contributor
timestamp: September 30, 2021, 10:35:03 pm
content: hello delicious humans & offworld imposters.i am interested in smooth, noiseless DC.i'm eyeballing up making my own buck converter on a breadboard for jokes and/or looking at switching / premade power ICsive convinced myself that i want the fastest switching frequency possible so i can filter it off. But here comes my question...the fastest ive seen these chips switch at is like 2Mhz or 3MHz or so. Yet my CPU can switch at like 1000 times as fast.Is it because the mosfet (or ganfet or whatever) has to switch large current? Is that the trade-off? i figure the way they make the chip for power applications means it is bigger & has more capacitance & slower or something, yeah?thanks in advance.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 01, 2021, 03:41:47 am
content: A few things.Power MOSFETs need a certain gate charge to switch. You can only practically push that charge in so fast. This sets a minimum rise / fall time. Switching losses occur when the FET is partly on, so you want the switching time to be a small fraction of the total period.The next problem is parasitic inductance / loop area. These components are physically large for high power so there is some inductance of the switching loop. This limits the rise time no matter how fast your transistors are. GAN switches are fast so they are commonly sold as wire bondable dies to allow a compact switching loop.High speed is not actually great for filtering. Yes it's true that it is slightly easier to filter a 5 MHz fundamental than a 100 kHz fundamental but that isn't the problem. The problem always comes with the high harmonics from the fast rising edges. These tend to exceed the self resonance frequency of your filter components and are difficult to handle. The easist way to remove high harmonics is to reduce the slew rate / rise time. This reduces the high harmonics considerably but costs efficiency. The faster the switching frequency the harder this tradeoff.The main reason to push switch mode frequencies higher is to reduce the size of the primary energy storage inductor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 01, 2021, 07:14:49 am
content: In general, there's a tradeoff between frequency and power level -- independent of device type.Device type is just a scaling factor. GaN does better than SiC, better than Si (IGBT, MOS), better than, uh... SCRs say? And than vacuum tubes, if you want to go really far back...All of them scale with power, so the bigger it is, the slower it is, for the same overall performance (efficiency say).If you don't mind that the efficiency is under 50%, you can crank up any old power amp and go -- doesn't have to be anything very special, Si will go to the low GHz, and GaN, InP, etc. will go into the 10s of GHz (I don't know anything about doing power conversion at 100s GHz, but I'm sure it works out the same; just that, everything has to be so damn small that you're doing it entirely on a single die, at not very much total power -- fractional watts?).For higher efficiency, you need flatter waveforms, so that less time is spent with significant voltage and current at the same time, in the active devices. They can still have rounded edges (gradually changing voltage or current), but the other (current or voltage) must be near zero at the same time. Reactance is hard to avoid, so you do tend to see such waveforms up there -- resonant converters, that is. Which has consequences for timing those flat periods (on and off switching), usually with some combined PWM and frequency control; how exactly, doesn't matter so much (that's left as an exercise for the engineer, eh?), just that it's possible.So, if you want to run at the highest frequencies possible, target a low power level. Choose the higher technology, of course. You might not be able to achieve, say, 5MHz and 1kW. It might have to be 200W, or 100W or even less. Mind,you can always make up the difference by using multiple channels in parallel,and indeed you should for a number of reasons (if they can be phase-interleaved, you can save on input/output ripple current as well; though this is a bit difficult to arrange for a resonant topology).This says nothing about the particular means of reaching such power levels; circuit layout is absolutely critical, and extremely high power densities, even at high efficiency, require equally critical thermal management.Tim

author: David Hess
author_rank: Super Contributor
timestamp: October 02, 2021, 07:44:54 pm
content: Quote from: pepelevamp on September 30, 2021, 10:35:03 pmInstead of looking for the highest possible switching speed, consider parts like the LT1533 and LT3439 which support voltage and current slew rate limiting of the output switches.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 02, 2021, 10:06:28 pm
content: A lower switching frequency mainly needs larger inductors and capacitors. This may cost a little more and can be larger it is still usually the simpler way unless you go very slow (e.g. < 50 Hz).