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Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
mzincali wrote ...
What's the best way to boost the initial 12volts up to 50-100 volts, before feeding to the original primary?
Take a discarded CRT monitor and copy its switching power supply, that has one output specifically to power the flyback circuits. You can probably just cut the whole power supply from the board, after a look to its schematic diagram (find it in specialized web sites) to see what is important. The flyback driver can be copied too, with some simplifications.
Registered Member #4127
Joined: Fri Oct 07 2011, 05:40PM
Location:
Posts: 22
Antonio wrote ...
mzincali wrote ...
What's the best way to boost the initial 12volts up to 50-100 volts, before feeding to the original primary?
Take a discarded CRT monitor and copy its switching power supply, that has one output specifically to power the flyback circuits. You can probably just cut the whole power supply from the board, after a look to its schematic diagram (find it in specialized web sites) to see what is important. The flyback driver can be copied too, with some simplifications.
don't they utilize the 110 coming in for the flyback primary?
Registered Member #4127
Joined: Fri Oct 07 2011, 05:40PM
Location:
Posts: 22
Has anybody done a flyback transformer this way, and would care to comment?
The flyback is being driven by a 555, that through a high voltage transistor, is supplying 110v at high frequency to the primary.
Other questions:
1. Any thoughts on what to replace the "hairdryer" with? 2. What I would really prefer is to have a first stage transformer provide me with a boost up to 100V, which is directly connected to the original primary of a flyback and goes up to 10KV. Better or no? 3. Is 20khz about the sweet spot for flybacks? If not, how do I determine what frequency my primary prefers?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
From this IRFP250 datasheet, with the junction at 150C (fig2) at 10V gate drive (from 555 on 12V supply) at 6A Vds=1V, at 10A Vds=1v5, which is quite a lot compared to the supply of only 12V. but usable.
At 10A peak you need a good power source to maintain 12V, e.g. a charged 12V lead-acid battery with 1,000s uF in parallel, near the mosfet. and even 10A peak is only 45W input at 12V with a 75% duty cycle.
The 200V rating should be adequate provided that you operate well above 10 kHz. If you operate at 10A peak at greater than 10 kHz then 45W is less than 4.5 m Joules per cycle, since the repetitive avalanche rating for this transistor is 19 mJ you could build and operate without a snubber and later experiment with snubbers - which one reduces overall heating the most. Just monitor the heatsink temperature.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
mzincali wrote ...
Has anybody done a flyback transformer this way, and would care to comment?
The flyback is being driven by a 555, that through a high voltage transistor, is supplying 110v at high frequency to the primary.
Other questions:
1. Any thoughts on what to replace the "hairdryer" with? 2. What I would really prefer is to have a first stage transformer provide me with a boost up to 100V, which is directly connected to the original primary of a flyback and goes up to 10KV. Better or no? 3. Is 20khz about the sweet spot for flybacks? If not, how do I determine what frequency my primary prefers?
Yes i built severeral of JK's devices back about 12 years ago as a high school student.
1. I think a hair dryer or incandesent bulb is cheap, there are better components for mucho $$$. 2. well my flyback sees about 65 v and does quite well, i dont know about putting the secondary of one intlo the primary of the other with out more active silicon in between. 3. You have to kinda guess, these things dont seem to be exact, i turn mine up to a DC of 50% and down to 20% for different experiments.
Sulaiman wrote ...
From this IRFP250 datasheet, with the junction at 150C (fig2) at 10V gate drive (from 555 on 12V supply) at 6A Vds=1V, at 10A Vds=1v5, which is quite a lot compared to the supply of only 12V. but usable.
At 10A peak you need a good power source to maintain 12V, e.g. a charged 12V lead-acid battery with 1,000s uF in parallel, near the mosfet. and even 10A peak is only 45W input at 12V with a 75% duty cycle.
The 200V rating should be adequate provided that you operate well above 10 kHz. If you operate at 10A peak at greater than 10 kHz then 45W is less than 4.5 m Joules per cycle, since the repetitive avalanche rating for this transistor is 19 mJ you could build and operate without a snubber and later experiment with snubbers - which one reduces overall heating the most. Just monitor the heatsink temperature.
I worry being new as he is, that the snubber for overvoltage is a good idea since he may botch the adjustment of the pots, over volt the mos, and kill his device. So i think the TVS's are still a good idea.
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
Patrick wrote ...
you say youre putting 12v input, but youre transistor could be seeing 300 plus voltage.
having done quite a few kickback simulations, a 200v+ mosfet shuld be used. depending on cirumstances, kickback is around 170v in a 12v circuit.(again this varies) also, try using a mosfet that is avalanch rated, then you dont even need a tvs diode, as whan this "avalanch rating" is achieved, the mosfet just switches into conducting state to protect itself.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
ben123324 wrote ...
Patrick wrote ...
you say youre putting 12v input, but youre transistor could be seeing 300 plus voltage.
having done quite a few kickback simulations, a 200v+ mosfet shuld be used. depending on cirumstances, kickback is around 170v in a 12v circuit.(again this varies) also, try using a mosfet that is avalanch rated, then you dont even need a tvs diode, as whan this "avalanch rating" is achieved, the mosfet just switches into conducting state to protect itself.
what simulation did you use for the flyback? what factors were assumed?
I used A 1000 volt mosfet which is avalanche rated, and i put 65v into the 30T primary at about 2-4 amps. i have it clamped to 600-800 volts via TVS's. never had a problem due to overvoltage.
Ive used the radio shack crap and their IRF510 (mosfet-100 V) and it always died in flyback and iggy circuits.
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
i agree with the bad specs of the 510, as it has died on me quite a few times as well... i never said that your setubp is bad, i even think its a bit overkill
but with the irf450(or irfp, i can never remember) no matter what the voltage peaks at, it will conduct at the avalanch v. this way, the voltage will shut off, but the inductive current will cause the v to rise untill this rating is achieved, then dumps this built up v into the flyback- read the flyback article on uzzors2k's site.
and as for simulation, i used falstad. i didnt do anything complicated, i just ran a variable frequency through a mosfet with an inductance in series. then you just scope it(play with the inductance too)
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
ben123324 wrote ...
i agree with the bad specs of the 510, as it has died on me quite a few times as well... i never said that your setubp is bad, i even think its a bit overkill
but with the irf450(or irfp, i can never remember) no matter what the voltage peaks at, it will conduct at the avalanch v. this way, the voltage will shut off, but the inductive current will cause the v to rise untill this rating is achieved, then dumps this built up v into the flyback- read the flyback article on uzzors2k's site.
and as for simulation, i used falstad. i didnt do anything complicated, i just ran a variable frequency through a mosfet with an inductance in series. then you just scope it(play with the inductance too)
hth
Im always suspicious of simulations reguarding flybacks, which are not simple inductors, the parasitics matter, along with switching parameters.
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555 flyback transformer MOSFET gets hot.
