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Old 12th February 2005, 02:09 AM   #11
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Default Re: 2SC2335 Transistors

Quote:
Originally posted by N-Channel
Kins-

Those are the main switching transistors, and since they will see over 320V across them, they are most likely 400V, 8-15A rated NPNs.

Not a real good choice if you want alot of current on the primary. You could change them to some good N-Channel MOSFETs rated at 60V. Any N-Channel MOSFET over a 30A drain current ratinf will do- there are many choices.

As for your AT transformer cores, if they came out of a 200-250W AT box, then they are probably good for about 60-80W continuous power, with extended peaks of 200-250W. That is a dirty little secret behind all those impressive ATX power supply ratings. 500W (continuous) from an ATX box? I don't think so. for a 500W rated unit, more like 120W continuous, even with forced-air cooling.

Anyway, I'm babbling again, I always do that. In all honesty, 80W CONTINUOUS power PER channel is more than plenty for your needs. And remember about using two PWM supplies in synchronicity. It will look cool, work great, impress your friends and get you all the girls!
go to www.onsemi.com and find some NPN's that will work in this application. if you pick up a textbook on smps design you'll see that the stress on the switching transistors is pretty severe. if you tool around the National Semi and OnSemi websites you'll find some interactive software which aids the design process and demonstrates the peak voltages and currents at criitcal nodes.

but an ATX supply will chug along and provide one heck of a lot of current -- what it won't do is remain stable if you demand relatively little current -- that's why the output inductors have to be oversized for ham radio operation (SSB and Class C) -- less so when you move down the food chain to audio and class AB operation.

a lot of ATX supplies are poorly designed -- the magnetics is one area where you have to be able to calculate with more than a four-function calculator. i think that Carly Fiorina personally designed the one I had with an HP computer -- another example of incompetence.

i just ripped apart a power supply for a Dell network server -- while the solder was really tough even with my big Weller gun I found two cold solder joints which probably caused premature device failure (or at best wonky behavior).
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Old 12th February 2005, 10:22 AM   #12
kinser is offline kinser  Israel
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So,
I have a few Q's.
1. If I where to rewind the AT xformers for 12v center tap on primary and an output of about 40-0-40=56Vdc+/- on secondrys, could I use each one to power a moded P3A amp? because N-Channel said that they are 60-80 watts continues,200-250 watts extreme peak?
2. I downloaded some Power supply software from National Semi. its called "Switchers Made Simple", have any of you used it?

Thanks,
Kinser
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Old 12th February 2005, 01:15 PM   #13
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if I recall, the "Simple Switcher" series relates to their (Nat Semi) line of buck and boost converters, but not to a push-pull or forward converter.

Before we go any further, tell me the component values on pins 4, 5 and 6 of the TL494 so we can get an idea of the switching frequency and dead time.
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Old 12th February 2005, 02:25 PM   #14
kinser is offline kinser  Israel
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ok, out of pin 4 is a 10k resistor, out of pin 5 is a 1nF(102) cap and out of pin 6 is an 18 k resistor. all the other leades of the resistors and cap meat together.!

----------------------
4 | 5| 6 |
\ | \
/ - /
\ - \
/ | /
|---- |----- |


Kinser
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Old 12th February 2005, 02:55 PM   #15
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they all meet together and go to ground.

I have a web page with a car SMPS using TL494.

www.djquan.angelcities.com/smps.html
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Old 12th February 2005, 03:04 PM   #16
kinser is offline kinser  Israel
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djQUAN,
Thans,
I found you site a few dayes ago... only problem is what transistors to use, and the transformer calculation. I would love to get 500 watts out of it, but i dont know how to calculte it....

Thanks again,
Kinser
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Old 12th February 2005, 03:10 PM   #17
djQUAN is offline djQUAN  Philippines
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for the switching mosfets, you could use anything with >40A and between 50 to 100V rating. I used IRF1302 for the 200W (four in all) and IRF3205 for the 500W (six in all) which I think is overkill but I already had them so it wouldn't hurt.


for the 200W version, I have 6 turns per primary, and 16 turns per secondary. the primary is wound with 3 x 1mm wires, psecondary is 2x 1mm wires. I get +/-35V with that tranny.
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Old 12th February 2005, 03:19 PM   #18
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Default TL494 Frequency

Kinser/Jack,

OK, using the following formula provided by Onsemi, TI and others:

f(clock) = (1.1)/R(t)C(t), where R(t)=18K, and C(t)=1000pF. Therefore, your clock freq should measure approx. 61.1 kHz on the scope, and your switching frequency will be half that, or ~ 30.5kHz.

With 10K on pin 4, I'm not sure of the max duty-cycle (Ton -v- Toff) will be, but if I had to take an educated guess, I would say the 10K resistor limits each output's duty cycle to about 45%. Again, this is just a guess.

As for the transformers, I'm not 100% sure about the max CONTINUOUS power they can handle, but 60-80W sounds right. In George Chryssis' book, the chapter on transformers gives good detail on finding out how much CONTINUOUS powe a core can handle, based on the cross-sectional area of your particular cores.

Kins-

go to this website,

http://www.onsemi.com/pub/Collateral/TL494-D.PDF

and see how simple it is to SYNCHRONIZE two TL494s together to eliminate beat frequencies. These can occur when two (or more) PWMs operate in close proximity to each other on the same (but not exact) frequency. So, one 494 can power channel one of the P3A Amp, and the other 494 can power the other channel.

As for the NatSemi SimpleSwitchers, they are great chips to work with (I've used them myself many times before- with great success), and there are so many different chips for many different applications. However, none of them are suitable for this high of power levels you're working with. -Stick with the '494.

If you're making, say, a +12V to +19V booster (we make/use them for powering our laptops in the field), the LM2587-ADJ is a great chip for this, but for a high power car amp, it's WAY too small. Same story for the rest of the SimpleSwitchers.

I have used the Switchers-Made-Simple software, and it's pretty precise. The values for the components I calculated both using and not using the software were very close, and results from finished circuits using the software were very close to the calculated values.

Steve
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Old 13th February 2005, 12:28 PM   #19
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Quote:
1. If I where to rewind the AT xformers for 12v center tap on primary and an output of about 40-0-40=56Vdc+/- on secondrys, could I use each one to power a moded P3A amp? because N-Channel said that they are 60-80 watts continues,200-250 watts extreme peak?
The 1.41 rms multiplier is for sine waves, (40-0-40) 56v+/-, if you build a switcher out of a tl494 you'll use square waves which when rectified is roughly half pp, or if you have 70v pp square wave on your scope your dc rectified will be rouhgly 35v. Which is why you get +/- 24v for 1:2 ratio push pull smps trnsfor with 12v primary . Each half leg of the primary will see 24v pp (vdc2) and with a 1:2 ratio wind, each half leg of the secondary will have 48v pp for a total of 96v pp making a recitifed 24v+/- or 48v total.
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Old 13th February 2005, 12:35 PM   #20
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Quote:
Originally posted by kinser
djQUAN,
Thans,
I found you site a few dayes ago... only problem is what transistors to use, and the transformer calculation. I would love to get 500 watts out of it, but i dont know how to calculte it....

Thanks again,
Kinser
you'll be hard pressed to get 500W out of the existing transformer -- this is because (among other things, the transformer probably can't handle the physics) --> there just isn't enough space to put the windings -- there is a balancing act between the required inductance of the primary winding, the peak and average current demand, the operating temperature of the transformer and the size of the window available for winding.

i have a torn up ATX supply -- the windings are #22 which will handle 2 amps -- this is fine on the primary of an offline, 200 watt switcher which will see 165 VDC on the primary. figure that your auto battery power supply at 500 watts would see 40 amps --
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