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Old 20th August 2006, 03:54 PM   #221
chas1 is offline chas1  Dominican Republic
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Default answer's to question's

Luka


In question number four is that 65 Amps or 6.5 Amps, if 65 Amps you surely want to use a Bridge for 74 Volt @65 Amps is aprox 5kW however if the value 6.5 Amps then 74 Volts @ 7 amps is aprox 500 watts but the turn's ratio is not the limiting factor for output current, topology,core size along with frequency and wire size are the determining factors for power :

Example an ETD-49 core used in a bridge has the following Power capabilties.

Freq: 48 kHz @ 1200 watts , 72 Khz @ 1800 watts, 96 kHz@ 2400 watts.

However you must be able to wind the number of turns with the necessary wire size on the bobbin.

For the current value of 65 Amps requires copper foil of the proper dimension.

Refering to the datasheet for MC33025 or MC34025

The 1k resistor feeds the current ramp input (pin 7) and connected to pin 9 which is current limit input.

For the other info refer to page 127 & 128 of Marty Brown's book
all is detailed there

One question, what are the specifications of your design?

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Old 21st August 2006, 11:00 AM   #222
luka is offline luka  Slovenia
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chas

You did not understand me.For the moment I am talking about BUCK converter,where I need to limit secondary current to 65 Amps @ only 13v output, and input being 45v running at ~50kHz.By turn ratio I mean ratio for current trafo(1 pri, sec=?), to limit output to 65 Amps.THIS IS NOT FOR HALF,FULL BRIDGE.
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Old 21st August 2006, 12:24 PM   #223
chas1 is offline chas1  Dominican Republic
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Default Current transfomer design

Luka

It matters not the topology, a current transformer design is the same. In your previous post you made no mention that this was for a current transformer.

I would think that you might want to visit Magnetics website and download this software CTD-1.3 , it will help you with the design part.

The link below will explain and help you calculate the proper values for the burden resistor.


http://www.coilwinder.com/current_tr...and_theory.htm

Most current transformers use one turn for the primary and up to 200 for the secondary. The burden resistor calculation is the meat of current transformer design along with core selection. If you are
winding your own and you don't know the core properties use a ATX yellow core wind about 200 turns for secondary put a 1k 1% resistor across secondary follow this with a diode with a 1k 1% resistor for a load,filter load with about .2ufd, set the supply for a 50% duty cycle and measure the voltage across the 1k 1 % load. Note: Don't forget the 1 turn thru center for primary, proper wire size for the current you expect.

Note: the secodary can be wound with #36 AWG or smaller

The voltage you measure should represent your normal output current. If this is 65 amps then the voltage you measured represents 65 amps. Divide that by the 1k and that will give you the volts per amp.

The trick is on the secondary side you want a voltage that represents a magnitude of current thru your transformer ie 1 volt per amp or in your case maybe 1 volt per 6.5 amps.

Since you have the book "High Frequency Switching Power Supplies theory and design " take a look at pages 161 & 162 all the info you need is also there.

If this does not help let me know and I will post a circuit along with calculations.



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Old 22nd August 2006, 06:09 AM   #224
luka is offline luka  Slovenia
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Buck...
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Old 22nd August 2006, 12:21 PM   #225
luka is offline luka  Slovenia
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Buck 2...
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Old 22nd August 2006, 01:16 PM   #226
luka is offline luka  Slovenia
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I don't know next thing. I have connected my SG so that both outputs drive the fet.So if freq of SG is set to 50kHz, is then my output freq. 100kHz?

If output is 50 then my inductor will have value 17.8uH, I'll use 19 or 20uH.How does that sound?
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Old 23rd August 2006, 01:00 PM   #227
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luka,

Your output frequency will be twice the switching frequency. This is true for all double-ended converters: push-pull center-tap, half-bridge, full-bridge, and single-ended Buck PWMs where both outputs drive the main transistor. Simply put, the output frequency is the same as the oscillator frequency, 100kHz, which is double the switching freq. Substituting 20uH for 17.8uH should be fine.


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Old 23rd August 2006, 01:22 PM   #228
luka is offline luka  Slovenia
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Since didn't think ahead to much I realised that my SG conf. as it was couldn't function properly.When the output D.C. should be biggest, was oposite of that therefor I would need to use SG1527.So I have decided to go with Uc3842.And it is current mode .Freq. will be 50kHz so I will put 20uH for inductor.
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Old 25th August 2006, 11:34 AM   #229
luka is offline luka  Slovenia
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Since this UC was made for flyback, I have a problem fully open mosfet, because Uc drives it to gnd, but here it would have to drive it to mosfets drain.

Would it be OK if I would make floating gnd for uc, so that it would drive mosfet fully
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Old 26th August 2006, 10:00 AM   #230
luka is offline luka  Slovenia
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Default 1. or the 2.

On this site you calculate the value of inductor. For Vin=42v,Vout=14v Iout=30amps, Freq.=50kHz you get value of ~16uH, but if you calculate it from A. Pressman and you insert he same values you get twice as big as before (~32uH).Which value is the right one?
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