Electronics > Projects, Designs, and Technical Stuff
Mosfet paralleling reliability in real life - more smaller ones vs fewer bigger
T3sl4co1l:
--- Quote from: MagicSmoker on September 21, 2019, 11:44:05 am ---the penalty in power throughput at >100kHz is not nearly as bad as you might think because max flux swing has to be greatly curtailed to keep core losses under control, anyway. Practically speaking, the advantage in transformer utilization of bipolar vs. unipolar operation at fsw >100kHz is more like 20%, rather than a doubling. Still, one shouldn't be hard-switching 650V at 200kHz, anyway, so rather a moot point.
--- End quote ---
Ah yeah -- that's one of those crusty old things that I've manged to internalize without consideration! Thanks for bringing that up.
Back in the day down at like 20-50kHz, you'd be able to push Bsat, but it's still hard today to find materials that will handle that much at 200kHz (even 100kHz in modest sizes). Modern materials like N97 and 3F46 are quite good, but not quite that good. And that explains the relative prevalence of otherwise-kinda-shitty topologies -- like two-switch forward -- in modern application. :)
So put that on the bonfire of obsolete design information as well. 8)
Tim
Miyuki:
I would like to use ETD shape core, it have relative big area to volume ratio so I can push flux swing high (about 200mT at 200kHz) with sustainable temperature rise
But it is far far from saturation even at single ended topology
Also switching is just half hard, at turn off since turn on is covered by leakage inductance at almost zero current and have just Coss losses
//edit:
wonder what will do to switching losses driving mosfets off very hard with like -10V with minimal gate resistance and rely on diodes to clamp that spike
mzzj:
--- Quote from: Miyuki on September 21, 2019, 08:36:18 am ---Transmited power is "just" about 1500W, but it is constant current / power and need high possible output voltage at low current
Why would body diode conduct any bigger portion of time, it is unwanted and just some nasty osculations do it.
With small parallel transistors current in each shall be enough small to allow reasonable switching speed and losses.
With one device I will need to go with SiC device or accept high losses and problem with cooling
--- End quote ---
This sounds pretty much like welding machine..
15 years ago Kemppi was using matched 3xparallel IGBT's and 2-switch Forward converter topology on smaller welding machines that I'm familiar with. 160kHz switching frequency.
BUT suitable IGBT's for 650v supply voltage operation (ie 800 to 1200v rating ) are lot less common than 600v rated ones.
Miyuki:
--- Quote from: mzzj on September 21, 2019, 06:30:42 pm ---15 years ago Kemppi was using matched 3xparallel IGBT's and 2-switch Forward converter topology on smaller welding machines that I'm familiar with. 160kHz switching frequency.
BUT suitable IGBT's for 650v supply voltage operation (ie 800 to 1200v rating ) are lot less common than 600v rated ones.
--- End quote ---
Yes at 400V It will be easy to use Infineons H5 and they can switch even at this frequency
But 800-900V ones are very limited choice
And 1200V ones are slow
MagicSmoker:
--- Quote from: Miyuki on September 21, 2019, 12:54:22 pm ---I would like to use ETD shape core, it have relative big area to volume ratio so I can push flux swing high (about 200mT at 200kHz) with sustainable temperature rise
--- End quote ---
You sure about that? I'm working on a transformer design right now that uses a state-of-the-art ferrite similar to TDK/EPCOS N97 and at 200mT/200kHz it wpuld have a loss ratio of 1.06kW/m3, or somewhere between 2x and 5x what is tolerable. In fact, the ideal flux swing is more like 100mT, at which point the loss ratio drops to 165W/m3 (the power of the B exponent at work), but that still is enough to cause a temperature rise of 67C over ambient (which happens to be ideal, as there is a loss minimum at 100C, similar to many other power ferrites).
--- Quote from: Miyuki on September 21, 2019, 12:54:22 pm ---Also switching is just half hard, at turn off since turn on is covered by leakage inductance at almost zero current and have just Coss losses
--- End quote ---
Eh? Turn-on is very lossy in a MOSFET-based hard-switched converter due to the energy stored in Coss. You really will need to either use SiC MOSFETs or else go with a ZVS or above-resonance topology.
--- Quote from: Miyuki on September 21, 2019, 12:54:22 pm ---what will do to switching losses driving mosfets off very hard with like -10V with minimal gate resistance and rely on diodes to clamp that spike
--- End quote ---
Makes no difference with MOSFETs. Or IGBTs, for that matter. The only reason to bring the gate negative during turn-off is to prevent shoot-through from Miller capacitance of the opposite leg turning on, but that is not a consideration in the two-switch forward.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version