Fullbridge SSTC Design Questions
Chris_Knight, Sun Jan 24 2016, 05:59AMI 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.
Re: Fullbridge SSTC Design Questions
Inducktion, Sun Jan 24 2016, 06:03AM
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.
4. Don't cross the lines.
Inducktion, Sun Jan 24 2016, 06:03AM
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.
4. Don't cross the lines.
Re: Fullbridge SSTC Design Questions
Sigurthr, Sun Jan 24 2016, 06:08AM
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.
Sigurthr, Sun Jan 24 2016, 06:08AM
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.
Re: Fullbridge SSTC Design Questions
Justin, Sun Jan 24 2016, 08:11AM
It seems the IRFP460 is so popular because they're seen in all of these old designs.
FCA47N60 will work great.
Justin, Sun Jan 24 2016, 08:11AM
It seems the IRFP460 is so popular because they're seen in all of these old designs.
FCA47N60 will work great.
Re: Fullbridge SSTC Design Questions
loneoceans, Sun Jan 24 2016, 06:44PM
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.
loneoceans, Sun Jan 24 2016, 06:44PM
Chris_Knight wrote ...
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.
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.Re: Fullbridge SSTC Design Questions
Chris_Knight, Sun Jan 24 2016, 07:13PM
loneoceans - I sent you a PM. I like the commerical GDTs, I've wound maybe half a dozen over the years and it's never been any fun.
Chris_Knight, Sun Jan 24 2016, 07:13PM
loneoceans - I sent you a PM. I like the commerical GDTs, I've wound maybe half a dozen over the years and it's never been any fun.
Re: Fullbridge SSTC Design Questions
Inducktion, Sun Jan 24 2016, 08:18PM
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.
Inducktion, Sun Jan 24 2016, 08:18PM
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.
Re: Fullbridge SSTC Design Questions
loneoceans, Sun Jan 24 2016, 09:12PM
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+.
loneoceans, Sun Jan 24 2016, 09:12PM
Inducktion wrote ...
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.
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+.Re: Fullbridge SSTC Design Questions
Chris_Knight, Sun Jan 24 2016, 09:30PM
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.
Chris_Knight, Sun Jan 24 2016, 09:30PM
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.
Re: Fullbridge SSTC Design Questions
dexter, Mon Jan 25 2016, 06:55AM
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
dexter, Mon Jan 25 2016, 06:55AM
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.
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
Re: Fullbridge SSTC Design Questions
Benjamin, Mon Jan 25 2016, 05:01PM
I have had very good results using FGA60N65SMD in a half bridge at 400 kHz. Gate drive chip is a single UCC27425P which is inverting and non inverting so you only need one. The GDT is 15 turns trifilar.
Benjamin, Mon Jan 25 2016, 05:01PM
I have had very good results using FGA60N65SMD in a half bridge at 400 kHz. Gate drive chip is a single UCC27425P which is inverting and non inverting so you only need one. The GDT is 15 turns trifilar.
Re: Fullbridge SSTC Design Questions
Chris_Knight, Tue Feb 09 2016, 10:13AM
It's funny I have the whole thing planned and ready to built, yet the one thing that I can't seem to find closure with is the secondary aspect ratio.I am using 32 AWG on a 3.5" diameter form. The two options I can't really seem to decide between are:
Q is 212, toroid is 2.25"*8"
I'm at odds because the 10" has a pretty high H/D ratio, while the 8" is simply shorter; will the 10" universally give better performance. Plus, though this reason is pretty vain, I think the lower aspect ratio looks more aesthetically pleasing.
JavaTC Screenshots
Chris_Knight, Tue Feb 09 2016, 10:13AM
It's funny I have the whole thing planned and ready to built, yet the one thing that I can't seem to find closure with is the secondary aspect ratio.I am using 32 AWG on a 3.5" diameter form. The two options I can't really seem to decide between are:
- 10" winding for 1040 turns and H/D ratio of 2.86:1
- 8" winding for 833 turns and H/D ratio of 2.3:1
Q is 212, toroid is 2.25"*8"
I'm at odds because the 10" has a pretty high H/D ratio, while the 8" is simply shorter; will the 10" universally give better performance. Plus, though this reason is pretty vain, I think the lower aspect ratio looks more aesthetically pleasing.
JavaTC Screenshots
Re: Fullbridge SSTC Design Questions
Sigurthr, Wed Feb 10 2016, 02:13AM
FWIW I've found very little impact of the H:W ratio in my SSTCs. I think TwirlyWhirly555 can confirm this has he has an array of very low ratio coils which perform equally as well.
My biggest coil is 4" (ID) pvc with 18" of 30ga winding.
The main observed effects of ratio in SSTCs is just the EM field effects of the topload and the coupling of the primary. If the secondary is shorter the topload's e-field has more effect on the secondary coil's field (and can potentially be seen as a shorted turn to the primary's field if close enough). Also, the primary would have a higher coupling as it now has a greater degree of intersection with the secondary.
The main limiting factors are then eddy currents and the related coil heating, and unwanted arcing at undesirable places.
Sigurthr, Wed Feb 10 2016, 02:13AM
FWIW I've found very little impact of the H:W ratio in my SSTCs. I think TwirlyWhirly555 can confirm this has he has an array of very low ratio coils which perform equally as well.
My biggest coil is 4" (ID) pvc with 18" of 30ga winding.
The main observed effects of ratio in SSTCs is just the EM field effects of the topload and the coupling of the primary. If the secondary is shorter the topload's e-field has more effect on the secondary coil's field (and can potentially be seen as a shorted turn to the primary's field if close enough). Also, the primary would have a higher coupling as it now has a greater degree of intersection with the secondary.
The main limiting factors are then eddy currents and the related coil heating, and unwanted arcing at undesirable places.
Re: Fullbridge SSTC Design Questions
ZakWolf, Thu Feb 11 2016, 12:29AM
ZakWolf, Thu Feb 11 2016, 12:29AM
Justin wrote ...
It seems the IRFP460 is so popular because they're seen in all of these old designs.
FCA47N60 will work great.
WUT? Thats a terrible choiceIt seems the IRFP460 is so popular because they're seen in all of these old designs.
FCA47N60 will work great.
Re: Fullbridge SSTC Design Questions
Sigurthr, Thu Feb 11 2016, 01:30AM
IRFP260 != IRFP460.
See the 460's datasheet:
270mOhm RdsON.
20A Id continuous.
Sigurthr, Thu Feb 11 2016, 01:30AM
ZakWolf wrote ...
Justin wrote ...
It seems the IRFP460 is so popular because they're seen in all of these old designs.
FCA47N60 will work great.
WUT? Thats a terrible choiceIt seems the IRFP460 is so popular because they're seen in all of these old designs.
FCA47N60 will work great.
IRFP260 != IRFP460.
See the 460's datasheet:
270mOhm RdsON.
20A Id continuous.
Re: Fullbridge SSTC Design Questions
Justin, Thu Feb 11 2016, 03:24AM
We were talking about the IRFP460, not the IRFP260N which is only rated for 200V which wouldn't be suitable for this project. Both of them are really old and there are much better options available.
The FCA47N60 is a great choice, albeit an expensive one. He could just as easily use some FGH40N60 or FGH60N60 IGBTs if he wanted to.
Justin, Thu Feb 11 2016, 03:24AM
ZakWolf wrote ...
Justin wrote ...
It seems the IRFP460 is so popular because they're seen in all of these old designs.
FCA47N60 will work great.
WUT? Thats a terrible choiceIt seems the IRFP460 is so popular because they're seen in all of these old designs.
FCA47N60 will work great.
We were talking about the IRFP460, not the IRFP260N which is only rated for 200V which wouldn't be suitable for this project. Both of them are really old and there are much better options available.
The FCA47N60 is a great choice, albeit an expensive one. He could just as easily use some FGH40N60 or FGH60N60 IGBTs if he wanted to.
Re: Fullbridge SSTC Design Questions
Chris_Knight, Thu Feb 11 2016, 03:45AM
Ended up using the FGH40N60 IGBTs @ 400kHz. Loneoceans reports good results at these frequencies with them. It's pretty amazing how far IGBTs have come along, I remember 6 or so years ago when I first got interested IGBTs were pretty much sub 250kHz.
Chris_Knight, Thu Feb 11 2016, 03:45AM
Ended up using the FGH40N60 IGBTs @ 400kHz. Loneoceans reports good results at these frequencies with them. It's pretty amazing how far IGBTs have come along, I remember 6 or so years ago when I first got interested IGBTs were pretty much sub 250kHz.
Re: Fullbridge SSTC Design Questions
ZakWolf, Thu Feb 11 2016, 03:48AM
I just happened to be looking at at 250 it was not ment to be the 460 mosfet. With that being said The FCA47..POS has 10x on and off time 3x the rise and 2x the fall. The irf460 is just a bit slower the the 250. not sure what you guys are talking about
ZakWolf, Thu Feb 11 2016, 03:48AM
Justin wrote ...
We were talking about the IRFP460, not the IRFP260N which is only rated for 200V which wouldn't be suitable for this project. Both of them are really old and there are much better options available.
The FCA47N60 is a great choice, albeit an expensive one. He could just as easily use some FGH40N60 or FGH60N60 IGBTs if he wanted to.
ZakWolf wrote ...
Justin wrote ...
It seems the IRFP460 is so popular because they're seen in all of these old designs.
FCA47N60 will work great.
WUT? Thats a terrible choiceIt seems the IRFP460 is so popular because they're seen in all of these old designs.
FCA47N60 will work great.
We were talking about the IRFP460, not the IRFP260N which is only rated for 200V which wouldn't be suitable for this project. Both of them are really old and there are much better options available.
The FCA47N60 is a great choice, albeit an expensive one. He could just as easily use some FGH40N60 or FGH60N60 IGBTs if he wanted to.
I just happened to be looking at at 250 it was not ment to be the 460 mosfet. With that being said The FCA47..POS has 10x on and off time 3x the rise and 2x the fall. The irf460 is just a bit slower the the 250. not sure what you guys are talking about
Re: Fullbridge SSTC Design Questions
ZakWolf, Sun Feb 14 2016, 08:20AM
ZakWolf, Sun Feb 14 2016, 08:20AM
Re: Fullbridge SSTC Design Questions
Chris_Knight, Sun Feb 21 2016, 06:06AM
I'm thinking about using the IXY614 gate driver due to it's RF drive ability (up to 8Mhz) simple package, and high current rating. (14A)
The problem is that only the non-inverting flavor has an enable pin. Am I missing something? Why would the inverting flavor not have an enable pin option?
Also Zak has me confused. Look at the FCA MOSFET, doesn't that have a switching time 10 times longer than the IGBT?
Chris_Knight, Sun Feb 21 2016, 06:06AM
I'm thinking about using the IXY614 gate driver due to it's RF drive ability (up to 8Mhz) simple package, and high current rating. (14A)
The problem is that only the non-inverting flavor has an enable pin. Am I missing something? Why would the inverting flavor not have an enable pin option?
Also Zak has me confused. Look at the FCA MOSFET, doesn't that have a switching time 10 times longer than the IGBT?
Re: Fullbridge SSTC Design Questions
Sigurthr, Mon Feb 22 2016, 02:13AM
Just make a simple TTL inverter (NPN, pull up on collector) and use two non-inverting drivers.
Sigurthr, Mon Feb 22 2016, 02:13AM
Just make a simple TTL inverter (NPN, pull up on collector) and use two non-inverting drivers.
Re: Fullbridge SSTC Design Questions
Wolfram, Tue Feb 23 2016, 08:22AM
Bipolar transistors take a while to come out of saturation, and an inverter made with one can easily take several microseconds to turn off. At typical SSTC frequencies, this can be a large fraction of a cycle, so the circuit will not function correctly, there is some good info here: . The same circuit made with a MOSFET will work much better. An inverter IC like the 74HC14 would be somewhat faster again, and have more symmetrical delay.
Wolfram, Tue Feb 23 2016, 08:22AM
Sigurthr wrote ...
Just make a simple TTL inverter (NPN, pull up on collector) and use two non-inverting drivers.
Just make a simple TTL inverter (NPN, pull up on collector) and use two non-inverting drivers.
Bipolar transistors take a while to come out of saturation, and an inverter made with one can easily take several microseconds to turn off. At typical SSTC frequencies, this can be a large fraction of a cycle, so the circuit will not function correctly, there is some good info here:
. The same circuit made with a MOSFET will work much better. An inverter IC like the 74HC14 would be somewhat faster again, and have more symmetrical delay.Re: Fullbridge SSTC Design Questions
Sigurthr, Tue Feb 23 2016, 07:02PM
Thank you Wolfram, that is actually very valuable information. The nanosecond (and often, single microsecond) scale is hidden from me as I lack the equipment to visualize it.
I've used simple BJT inverters in SSTC drive circuits before, but only as stop-gaps until I got proper gate chips in, guess I just got lucky with the f0.
Sigurthr, Tue Feb 23 2016, 07:02PM
Thank you Wolfram, that is actually very valuable information. The nanosecond (and often, single microsecond) scale is hidden from me as I lack the equipment to visualize it.
I've used simple BJT inverters in SSTC drive circuits before, but only as stop-gaps until I got proper gate chips in, guess I just got lucky with the f0.
Re: Fullbridge SSTC Design Questions
woodchuck, Tue Feb 23 2016, 07:44PM
...which is a good reason not to drive them into saturation. Hence accommodations like Baker clamps or a reverse-biased Schottky diode between base and collector.
woodchuck, Tue Feb 23 2016, 07:44PM
Wolfram wrote ...
Bipolar transistors take a while to come out of saturation...
Bipolar transistors take a while to come out of saturation...
...which is a good reason not to drive them into saturation. Hence accommodations like Baker clamps or a reverse-biased Schottky diode between base and collector.
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