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Old 16th March 2005, 06:45 AM   #11
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I'm right there with you man, only 16, got me some good sounds subs, but just blew my amp and am out of money for a new one that can push 500x1 at 1 ohm, so looking to build one myself...

I have been all over, reading everything I can get my hands on. I have really been interested in the smps project at esp and have all of the major parts to build one from my blown amp...

just a few questions, toroid or e-core, i have a very large toriod, at least 2 in. in diamter, but i also have an EDT49 core.
also looking at the mosfets, going with 4 irf540 per rail
got good capacitors out the wazoo cause my blown amp was filled with low esr 105 degree rated capacitor for the supply

ready to build, but i should i make the pwm circuit on a seperate vero board and transfer the signal to the mosfets by wire, or have the mosfets on the board too, keep in mind that this supply will be pulling about 30A at 12V per rail...
Thanks
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Old 16th March 2005, 06:53 AM   #12
ssanmor is offline ssanmor  Spain
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For the core, make sure it is ferrite (not iron powder or similar).

About making the modulator in a separate board, keep in mind that the gate connections should be as short as possible. If that's completely impossible, use a pair of twisted wires to gate and source of the mosfets to minimize inductance of the loop, that could cause ringing and undesired turn-on of any mosfet just when it can do more harm.

Best regards and have luck!

Sergio
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Old 16th March 2005, 07:12 AM   #13
ssanmor is offline ssanmor  Spain
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By the way, IRF540 is a very good choice. Easy to get and very robust. If you can, use IRF540N variation, it is even better and have lower Rds(on), that is important:

IRF540: Rds(on)=77mohm. For 30A, if your use 4 per primary side, you have 77/4=19.25mohm. At 30A, P dissipated due to resistive losses = I^2*R=17W aprox.
IRF540N: Rds(on)=44mohm. P dissipated is now 9.9W in the same conditions.

BTW: Try to use International Rectifier mosfets, they are rated for avalanche, etc. Are much more reliable than other brands (and I don't have relationship with them )

Best regards,
Sergio
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Old 16th March 2005, 09:52 AM   #14
Alme is offline Alme  Ukraine
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If it's real 500W RMS amp then current consumed from battery can be much higher, over 50A. IRF540 is 100V device and not quite suitable to operate from 12V battery, the usual and cheap FET choice for 12V car amp SMPS is IRFZ44N ->55V, Rds(on)=16mohm - about 3 times lower losses. Slightly better and more expensive are IRFZ46/48N. Even better are IRF1010 or IRF3205 (the last you will need only 2 pairs).
Toroid core is preferrable to ETD at input voltages as low as car battery. It's been mentioned a lot about it here.
Your 2-inch toroid core is sufficient for such SMPS. Use 25-30kHz switching and 4 turns each half primary winding for good efficiency.
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Old 16th March 2005, 10:07 AM   #15
ssanmor is offline ssanmor  Spain
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Alme is right, although IRF540(N) has the advantage of being readily available and cheap.
However, if you need the ultimate in efficiency, use for example the ones indicated in my article in ESP: Onsemi's NTP75N06, they feature Rds(on)=9 mohms, so with 1 or 2 of them per phase you can be ok. Besides, they worked very reliably for me.

About the toroid, if it is ferrite (that's very important), the size is ok in principle.
ETD39 is also suitable and easy to wind.

About frequency, the lower you go, the less switching losses you will have in the mosfets (they can be nearly as important as Rds(on) losses). Although the size of the transformer required will be bigger too. But I agree in that 30-60KHz is a good start.

Best regards,
sergio
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Old 16th March 2005, 01:55 PM   #16
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SoundNERD-

Have you tried the Onsemi SG3525 or the MC33025? Both are available as samples from ONsemi.

The SG3525 PWM IC has totem-pole outputs (makes driving the '75N06s much easier), and the MC33025 is the current-mode version of the '3525.

The MC33025 has a very high speed PWM and error amplifier, and with current-mode operation, it all but eliminates core saturation in c.t. push-pull topologies mentioned a few posts ago. Its totem-pole outputs will drive up to +/-2A, good for MOSFETS with really big gate capacitances.

Best of luck,

Steve
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Old 16th March 2005, 02:05 PM   #17
ssanmor is offline ssanmor  Spain
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For my prototype I used 2xNTP75N06 per phase, driven with a SG3525 and a totem-pole transistor stage, but I am sure that they can be driven very well directly with the SG3525 chip, as Rod remarked.

Best regards
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Old 16th March 2005, 03:51 PM   #18
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Default SG3525

Yes, they can. I think the source/sink of the outputs is something like +/-400mA, which should be enuff for the 75N06s.
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Old 16th March 2005, 05:16 PM   #19
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For my 250W SMPS I had to speed up the FET turn-off to keep them cool enough to still use SMT devices. The attached PDF shows a schematic fragment of the unregulated version of the power supply.

-Casey Walsh
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File Type: pdf psfragment.pdf (62.8 KB, 309 views)
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Old 17th March 2005, 12:51 PM   #20
Alme is offline Alme  Ukraine
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SmarmyDog, appears that you use SG3525 at about highest possible frequency of 200kHz (400kHz oscillator). I can imagine that maybe PNP pull-down helps to discharge gates at such frequency, with only 5 ohm in series. In my practice with this chip ut to 100kHz operation, no significant improvement was noticed with changing gate resistors 10 to 68 ohm, with up to 4 pcs. FETs driven from each output (usually IRFZ44N type). But, I noticed improvement at once after decreasing dead-time, certainly, not too critically to keep preventing shoot-thru. If used more than 4 pairs to drive (yes there were such cases), then I had added npn-pnp follower to each output and shorted 'discharge' resistor to have minimum dead-time.
Anyway from my experience, transformer design influences more on total efficiency than those gate things - especially in PWM topology with DC choke between diode bridge and output capacitors.
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