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Registered Member #58280
Joined: Sat Jan 09 2016, 06:48AM
Location:
Posts: 43
I have a roughly 500W, H-bridge SSTC that I'm planning. Resonance Frequency is about 250kHz, with a 4" diameter, 10" tall secondary with a 2*8" toroid. I'm on 120V mains.
1. I originally planned on using IRFP260's, but I'm concerned about their 200v rating, 170v DC from mains I believe is cutting it quite close. Will I be ok, or will I need to turn to alternatives? I was thinking about the IRFP460s, but I'm wondering if their 20A rating is too low. If both those MOSFETs are unsuitable, will something like the FCA47N60 suffice? (600v, 47A)
2. What driving currents do I need for the fullbridge MOSFETs? I'm not confident that a pair of UCC37321/2s will be able to drive the GDT for the fullbridge. If I'm correct, will these options work?
Micrel MIC4452ZT (12A, 25ns)
IXYS IXDD630MYI (30A, 11ns)
If the 9A UCC's are sufficient, will a cheaper option like the Micrel MIC4421ZN (9A, 25ns) also work?
3. What is a good GDT core material type?
4. Any PCB design tips? I'm shooting for a 2 layer board.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
1. I wouldn't use IRFP260's. You generally want at least a 2x margin when it comes to voltage ratings (at least, with mains anyway!) and 200 volts is cutting it far too close.
The IRFP460's would be ok but their on state resistance is a bit high. The FCA47N60's would be my personal choice for this.
2. You can actually parallel UCC drivers by just stacking them on top of eachother.
3. GDT core material, you generally want a ferrite core that has a high saturation limit. You don't want to saturate your core when driving your transistors.
I would go with a >400V fet, 200v is cutting it too close. Remember to be mindful of the gate capacitance in addition to the timings, on resistance, and drain currents.
*a little aside; why is it everyone loves the IRFPs so much? They have HUGE on resistances compared to modern alternatives!
Your gate current is determined by several factors, most notably the gate capacitance and your resonant frequency. That being said the UCCs should do fine. The reason I say this is that I've driven large capacitance gates like the FDL100N50F at up to 1MHz on the UCC chips. If you keep gate capacitance per gate the same, keep GDT parameters the same, but double frequency it doubles your gate current. Likewise you can double your gate capacitance and halve your frequency and keep the same gate current. If you want to use something else besides the UCCs go with the IXYS gate drive chips, they're fantastic.
A SSTC is only as good as its gate drive, don't skimp out by using cheap parts.
Here's the cores I use for my GDTs. Same core I use in the 1MHz coil pumping those hefty gates. Really great stuff.
I've never PCB mounted an inverter bridge itself. I always do point to point construction to keep parasitic and stray inductances to a minimum. The same principles should apply there though; watch your trace impedances, keep them low as possible. Be mindful of transient voltages and space the traces accordingly.
I have a roughly 500W, H-bridge SSTC that I'm planning. Resonance Frequency is about 250kHz, with a 4" diameter, 10" tall secondary with a 2*8" toroid. I'm on 120V mains.
1. I originally planned on using IRFP260's, but I'm concerned about their 200v rating, 170v DC from mains I believe is cutting it quite close. Will I be ok, or will I need to turn to alternatives? I was thinking about the IRFP460s, but I'm wondering if their 20A rating is too low. If both those MOSFETs are unsuitable, will something like the FCA47N60 suffice? (600v, 47A)
2. What driving currents do I need for the fullbridge MOSFETs? I'm not confident that a pair of UCC37321/2s will be able to drive the GDT for the fullbridge. If I'm correct, will these options work?
Micrel MIC4452ZT (12A, 25ns)
IXYS IXDD630MYI (30A, 11ns)
If the 9A UCC's are sufficient, will a cheaper option like the Micrel MIC4421ZN (9A, 25ns) also work?
3. What is a good GDT core material type?
4. Any PCB design tips? I'm shooting for a 2 layer board.
Sounds like a nice project! As mentioned, the IRFP260/460s are pretty old and don't exactly have the best characteristics. I'd recommend any of the newer 500V/600V MOSFETs, including the FCA/H lines from Fairchild. Alternatively, many of the modern 600V IGBTs will do nicely as well.
A pair of UCCs should be sufficient to drive the FETs, depending on your desired gate drive voltages and drive frequency. At the usual 12 / 15V drive, it will do fine. Alternatively you can use a single fet driver IC to drive a fet pair buffer output for great drive capability. Any of the options you listed looks good as well, though I'd focus more on designing a good gate drive transformer and reducing your leakage inductance. The N30 or T85 cores work great at your desired frequency.
Designing a good 2 layer PCB is ideal since you can really design it to keep the inductances low and make a proper laminated bus which is very difficult to otherwise solder up. In fact I recently designed a board for exactly what you seem to be doing:
It features a full-bridge design, capable of running on both 120V and 240VAC input, and uses commercial GDTs and a discrete FET driver. Feedback can be done via the usual antenna, direct secondary feedback or current transformer feedback. The driver has a flip flop as well for easy conversion into a DRSSTC, and allows plug-in interrupter cards.
The SSTC was designed primarily to be operated in a mains-synced fashion allowing for very long straight sparks (and does not require a bus cap) but there are footprints on the board allowing for easy customaization and a laminated low-inductance bus; similarly for gate drive traces. Boards were also designed to fit inside a ATX power supply box - probably isn't ideal from a magnetics standpoint but works fine and has a cute form factor. It also has a pretty overkill heatsink :P
I have extra boards if you're interested to play with them as well. ; drop me a PM if you're interested.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
Actually, instead of going with any of the ones you listed a SiC Fet would probably be the best in terms of ease of driving and overall characteristics. Only 660 pF of gate capacitance vs 5.9 nF! It's also fastly faster, supports higher voltage, and has lower on state resistance. Only downside is lower current rating.
Actually, instead of going with any of the ones you listed a SiC Fet would probably be the best in terms of ease of driving and overall characteristics. Only 660 pF of gate capacitance vs 5.9 nF! It's also fastly faster, supports higher voltage, and has lower on state resistance. Only downside is lower current rating.
Plus they're cheaper than the Fairchild ones too.
SiC fets are really nice, but they're pretty expensive (for $10, prices are certainly coming down! .. but still about 2-4x more expensive), and they also require an asymmetrical gate drive which requires discrete gate drivers - can't just drop them in as regular MOSFETs or IGBTs using regular GDT drive. Just something to take note of. Here's a pretty good IGBT for $4.60 on Mouser, . Otherwise other good IGBTs include FGH60N60SMD and FGA65N60SMD, which often come up cheap distributors like arrow electronics. I have ran these to about 400kHz+.
Registered Member #58280
Joined: Sat Jan 09 2016, 06:48AM
Location:
Posts: 43
I haven't worked directly with SiC, I work with GaN MOSFETs, which aim towards replacing sillicon in low power, high frequency operations, as opposed to the high power applications of most SiC - but yeah, much too expensive for not much benefit. Unlike the lab, I don't run off government money!
As for IGBTs... I'm a bit hesitant to use them. Most of them have upper rated limits of 150-200kHz - I know many have reported good performance at 300kHz and above - but I design conservatively. My coil will be around 250-300khz, which is cutting it a little too close - unless there's something I don't know about.
Registered Member #42796
Joined: Mon Jan 13 2014, 06:34PM
Location:
Posts: 195
Chris_Knight wrote ...
As for IGBTs... I'm a bit hesitant to use them. Most of them have upper rated limits of 150-200kHz - I know many have reported good performance at 300kHz and above - but I design conservatively. My coil will be around 250-300khz, which is cutting it a little too close - unless there's something I don't know about.
i used chinese fake FGH60N60SMD for 2 SSTC at ~200KHz and 395KHz in half bridge configuration driven through a GDT from a single MCP14E5 chip and had no failure
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Fullbridge SSTC Design Questions
