Low Power: TL494 or SG3525? - diyAudio
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Old 1st May 2007, 06:59 AM   #1
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Default Low Power: TL494 or SG3525?

I received a boss ava-650 from a coworker for $10. One channel was bad, opened it up and one power supply had cooked! Aside from that, on the same channel, the output terminals were melted. Someone had messed it up good.

Rather than trying to fix the dirty beat up amp ( way too much wattage for me anyway), I parted it out. I got a KIA494 from it ( TL494). I also got four ultrafast diode pairs, two 1 inch torioids, and 4 good IRFz24's. While the z24's aren't exactly what I would use for this use due to low current rating and high z_on, I can use them for prototyping. I have two stp55n06's that I can use in a more serious amp.

I am really only needing 60 watts or so.

I have some schematics for some amps with the TL494 in it. It does not seem too complex, but the lack of ability to direct drive the mosfets means adding two small transistors.

For prototyping purposes and getting my first smps up and running, should I use the 494 or should I just order a 3525?

TSD88~
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Old 1st May 2007, 02:26 PM   #2
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Either will work well but here are some things you should consider.

If the supply will be really simple (no regulation, no thermal protection, no more than a total of 4 FETs...), the 3525 will make the layout and construction a bit easier. Since there will be no buffer for the 3525, it may be damaged if the FETs fail. Mount it in a socket. It will be easier to replace.

The 494 has 2 error amps. If you 're going to use regulation AND thermal protection (using a thermistor), the 494 will be better. One error amp can be used for each function.

The 3525 has a soft start function. For small supplies, it's not really needed but it's easy to use if you want soft start. It can be done with the 494 but not as easily.

When you're building your first supply, you should use new FETs. If the FETs are not 100% OK, it could cause you to spend a lot of time troubleshooting the circuit.
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Old 1st May 2007, 04:57 PM   #3
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I am not going to need any regulation as I don't see the need for it with a small amp like will be connected. Thermal protection might be something I would consider, but again I design my amps with enough heatsink. The STP55ne06 FETs should'nt but merly get warm to the touch with a supply this small. I know that the 3525 will definatly make the layout simpler, and I am going to order a few regardless sometime. Its just that I have the 494 to experiment with right now. Also, the STP55ne06 came from a working UPS backup unit. I figure if I fry something with my bench supply oh well, I'll buy new parts for the 'real' project. Better to fry freebies than ones you paid for!


About the transformer: How do I know what frequency to operate it? Should I just shoot for between 50 and 100khz or does it really matter? The transformer used 22 AWG magnet wire for both the primary and secondaries.
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Old 1st May 2007, 05:55 PM   #4
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I'd suggest running it at ~30kHz. For the TL494, a 1000pf cap and a 20k ohm resistor will give you approximately 30kHz.

For testing, connect pins 1 and 16 to ground.

Connect pins 2 and 15 to pin 14 (5v).

Connect pin 13 to pin 14.

Connect pin 4 to ground.

Pin 7 is ground.

Connect pins 8 and 11 to B+ (through a 1 ohm resistor for protection of the IC).

Pin 12 can be connected to B+ for testing. It would need to be connected to a switched source later.

Pins 9 and 10 will drive the FETs or driver transistors. If you want to use the IC without driver transistors, you can use a 2 watt resistor between 51 and 68 ohms as a pulldown resistor on the outputs of the IC. The resistors will run a bit warm. This will be OK to drive only 2 FETs (one per output). If you use more FETs, you'd need to use driver transistors.

You should use 3-4 strands of 22g in parallel for the primary windings. I don't know what core you have but 4+4 on the primary would be a good starting point.

The 1" toroids were likely filters and won't work well for a transformer core.
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Old 1st May 2007, 07:20 PM   #5
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no, they were the transformers. They were the only transformers on the board, and the primaries went to the fets and the secondaries went to the UF diodes. The actual size is 1 1/4 inch OD, 3/4" window, and 5/8" heighth. I know they are small, but they should be good for about 100 watts each and will work for testing purposes.

For wire, all I have is 22agw magnetic wire from the shack, so I am going to strand it ( 2 or 3 strands) and then wind it.

Should I interwind the primary and secondary or do I not have to? It seems that on Rod's site , in the picture he has he just had the primary and secondary on seperate sides.
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Old 1st May 2007, 07:41 PM   #6
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For low inductance leakage, you need to have the windings on top of each other. If you don't, you're likely to have huge spikes where the FETs turn on/off.
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Old 1st May 2007, 07:48 PM   #7
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I wound 4-0-4 turns on the primary of dual stranded 22awg. When I power up , I can't even keep the B+ line connected to the bench supply without it melting and a FET burning. while underpowered, The fets I was using were good ( IRFz24n). I did not have a load on the supply so the FETs should not have burned...

By the way I am using KTA1023 transistors as fet drivers.
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Old 1st May 2007, 07:51 PM   #8
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Can you post a schematic of the circuit between the 494 and the gate of the FET?
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Old 1st May 2007, 08:03 PM   #9
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Here it is. I found this on the internet, I cant remember where, but it seems right. I tried connecting it as you said , but still got the same problem with the fets exploding. As I only have one 2sa1266, I used KTA1023's as drivers.
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File Type: jpg tl494pwr.jpg (33.5 KB, 804 views)
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Old 1st May 2007, 08:13 PM   #10
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That circuit looks OK.

Remove the transformer and connect a 1k ohm resistor between B+ and the center leg of each FET. With the power supply powered up, measure the DC voltage on the gates and the drains (the center legs). They should all read ~1/2 B+. The black meter lead should be connected to ground for these measurements.
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