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 Cute little FBT
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ScotchTapeLord
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Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
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Posts: 635
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Not enough capacitance would result in hearing 60Hz hum in the audio. It could be clipping, it could be that your driver is too close to the flyback and is getting unwanted feedback (I've known flyback transformers to shut computer mice off from across the room), or it could be that the GDTs don't like the uneven duty cycle and it's showing through at higher powers...
Since it's related to the volume I'm guessing it's one of the first two scenarios I mentioned, but having a GDT really does limit your resolution with PWM. Raising the logic voltage may help a bit with turning the FETs on harder and possible clipping, but be advised that it will also be harder on your chip, which isn't terribly optimized for gate driving...
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Adam Munich
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Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
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Hmm, since I'm running the circuit at only 24v I wonder if the gdt is even necessary... @ 15V the logic is only -9V relative to Vd, so the gate shouldn't break down. Sound like a good idea?
I'll need bigger gate resistors though, 20 ohms allows 750mA to flow and that's too much fo the IC, which can source only 125ma @ 15V.
A small heatsink on the sg3525 may be needed.
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ScotchTapeLord
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Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
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You need some form of isolation between the drivers for the high and low sides. The high side's source sees the full supply voltage and you can't ground it...
Bigger gate resistors cause your FETs to stay in their linear state longer, so there's more heating.
If you don't want to worry about frying things you should consider a completementary UCC37322 and UCC37321 pair to drive your GDT. They're fairly simple to use and you can sample them from TI...
I've blown a fair share of them, though, so you might want to buy a bunch from a dealer just in case (or you could just not run them open-circuit like I did! Not sure why that would make them explode, though...). Also you will definitely find use for them eventually as they are very handy chips.
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radhoo
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Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
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I've got two of this tiny flybacks myself:
Would have preferred bigger ones instead, though 
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Adam Munich
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Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
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Herp derp. Turns out the problem was my batteries were dead. Fully charged the circuit works beautifully, but the fets get pretty dang hot. I need to buy some better fets with a lower rdson.
Anyone want to recommend me a good TO-247 fet? This thing will never see more than 28 volts, and I'm looking for as little heating as possible. Ideally I'd like this to run for maybe thirty minutes before it needs a break.
I'll post some more pix and maybe a vid later.
@radhoo
why all the hate on them? These small ones both work great and look good in a plasma speaker.
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quicksilver
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Registered Member #1408
Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679
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Grenadier wrote ... I made a page: 
That's really quite nice. It's clear, readable and pleasant to see how the design was laid out. I have never really liked the common MOSFET symbol; it can be confusing if crowded, etc.
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Adam Munich
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Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
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Ok so it's completely done now. I came across 19 Irfp250's in a junk box today, so naturally the first thing I did was swap out the 460's. It gets a lot less hot, but the heatsink still wasn't sufficient.
So I dug through my crap and find a nice flat little sink. Some thermal goop and superglue and the heating problem is gone. This thing can run for hours now without any issues.
Some pix;
 
And here's a pic of the back. You can see where the HV found it's way to the +24V rail and into the drain of a mosfet. Oddly enough nothing happened. I solved the problem by using a straw to isolate the HV electrode, and you can see this in the previous two pix.
And a youtube video;
This is a wonderful circuit, and there doesn't seem to be any problems with it. It has great bass response too!
Edit: DAMMIT YOUTUBE. My video contains music copyrighted by blah blah blah, blocked in the following countries.
"Afghanistan, Albania, Andorra, Angola, Anguilla, Antigua and Barbuda, Argentina, Armenia, Austria, Azerbaijan, Bahamas, Bahrain, Bangladesh, Barbados, Belarus, Belgium, Belize, Benin, Bermuda, Bhutan, Bolivia, Bosnia and Herzegovina, British Indian Ocean Territory, British Virgin Islands, Brunei, Bulgaria, Burkina Faso, Burundi, Cambodia, Cameroon, Cape Verde, Cayman Islands, Chad, Chile, Colombia, Comoros, Congo, Congo - Democratic Republic of, Cook Islands, Costa Rica, Cote d'Ivoire, Croatia, Cuba, Denmark, Djibouti, Dominica, Dominican Republic, Ecuador, Egypt, El Salvador, Equatorial Guinea, Eritrea, Estonia, Ethiopia, Falkland Islands (Islas Malvinas), Faroe Islands, Finland, French Guiana, French Polynesia, Gabon, Gambia, Georgia, Germany, Ghana, Gibraltar, Greenland, Grenada, Guadeloupe, Guatemala, Guinea, Guinea-Bissau, Guyana, Haiti, Holy See (Vatican City State), Honduras, Hong Kong, Hungary, Iceland, Iran, Iraq, Jamaica, Jordan, Kenya, Kiribati, Kuwait, Kyrgyzstan, Laos, Latvia, Lesotho, Liberia, Libya, Liechtenstein, Lithuania, Luxembourg, Macao, Macedonia, Madagascar, Malawi, Malaysia, Maldives, Mali, Malta, Marshall Islands, Martinique, Mauritania, Mauritius, Mayotte, Micronesia - Federated States of, Moldova, Monaco, Mongolia, Montserrat, Mozambique, Myanmar, Nauru, Nepal, Netherlands Antilles, New Caledonia, Nicaragua, Niger, Niue, North Korea, Northern Mariana Islands, Norway, Oman, Pakistan, Palau, Panama, Papua New Guinea, Paraguay, Peru, Philippines, Poland, Qatar, Reunion, Romania, Rwanda, Saint Helena, Saint Kitts and Nevis, Saint Lucia, Saint Pierre and Miquelon, Saint Vincent and the Grenadines, Samoa, San Marino, Sao Tome and Principe, Saudi Arabia, Senegal, Seychelles, Sierra Leone, Singapore, Slovakia, Slovenia, Solomon Islands, Somalia, South Korea, Sri Lanka, Sudan, Suriname, Swaziland, Sweden, Switzerland, Syria, Tajikistan, Tanzania, Thailand, Timor-Leste, Tonga, Trinidad and Tobago, Turkey, Turkmenistan, Turks and Caicos Islands, Tuvalu, Uganda, Ukraine, United Arab Emirates, Uruguay, Uzbekistan, Vanuatu, Venezuela, Vietnam, Wallis and Futuna, West Bank, Yemen, Zambia, Zimbabwe "
God damn. I'll make a new vid tomorrow for all you not in the USA
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ZakWolf
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Registered Member #3114
Joined: Sat Aug 14 2010, 08:33AM
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Grenadier wrote ...
This FBT has a permanent home. I used the half bridge design and it definitely cuts don on the heat, A LOT. It still can't run continuously because I used those 460's I pulled from the PSU. (460's have a high rds-on). If I use FETs with a lower resistance it should run forever without a problem.
Could you give me a schematic of the driver please
thanks
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Adam Munich
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Registered Member #2893
Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242
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LittleVenture wrote ...
Could you give me a schematic of the driver please.
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radhoo
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Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
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Could you make another video without copyrighted music? I'd love to see the results!
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