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Old 11th November 2003, 06:41 PM   #1
MaXiZ is offline MaXiZ  Italy
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Default Can someone explain to me?

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What's the sense of blue part? And becouse if i don't connect it the wire on transformer blow up..?
Just to understand...
THanx!
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Old 11th November 2003, 08:04 PM   #2
Eva is offline Eva  Spain
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MOSFET output capacitance toghether with transformer distributed inductance, capacitance and resistance [between adjacent turns] forms a parasitistic resonator with somewhat high Q

Resonance frequency is usually in the range 1..40Mhz depending on number of turns, wire resistance, turn area, proximity of adjacent turns, external MOSFET and diode capacitances, external parasitistic resistance, etc...

Those RC networks in the schematic are used to damp these RF oscillations, at least to some extent

I encourage you to get an oscilloscope [20Mhz double trace or better] and use it to look at waveforms, this way most things will turn into self-explanatory [in the past I learnt a lot this way after buying a Hameg HM-407-2 and spending lots of time in experimentation, sometimes more time than I had ]

Oscillations can also be dampened by carefully winding of the transformer, look at the picture attached

This transformer has two 6-turn primaries wound on a toroidal ferrite core of 33mm outer diameter, each winding is made with 4 wires of 0,75mm diameter to get a total power rating better than 250VA at 50Khz

Note the separation between turns to minimize capacitance and reduce resonant Q

Also note the interleaving of the primary and secondary windings [alternated turns] to maximize equivalent inductor lenght and minimize leakage inductance of the windings

Secondary windings are not shown for clarity but same rules apply to get an optimum transformer, primaries should be located internally and secondaries externally to minimize stray magnetic fields [that are already minimun in a toroidal transformer]

I recommend adding a thin insulation layer between primaries layer and secondaries layer to minimize inter-winding capacitance [this also dampens resonances] and get better galvanic isolation

To get output voltages right you must add some light load in order to dissipate extra energy transferred to the secondary due to ringing and overshoot allways present, 10..100mA of load should do in your case

If you get an oscilloscope please tell and then I will try to explain the method used to figure out R and C values for the oscillation damping networks

PD: Sorry for bad english

PD2: Picture is poor resolution but well focused so you can zoom it 2:1 or 3:1 to watch it with more detail
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Old 12th November 2003, 05:39 PM   #3
MaXiZ is offline MaXiZ  Italy
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Ok thanx for explain to me but i've another problem to solve at the moment..
I connect all the circuit (the IRF are connected at an SG3524 @130Khz, 65 per rail) and if i'm not wrong if i don't start the oscillator should not happeing nothing! ABSOLUTELY NOTHING right? Just becouse should not be there current in the transformer.. Right? But if the IRF blow up.. explode!!??! What's maybe? A wrong soldering or what? Since i've lost 6 IRF i would like to found a solution to this.. I've alredy build anothe SMPS but all gone right but on this SMPS i've this problem and all the connection appear to me to be the same.. Is there some strange phenomenon?
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Old 12th November 2003, 09:36 PM   #4
Eva is offline Eva  Spain
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Obviously MOSFETs are turning on when they shouldn't

For example, if you leave gate open, floating or driven by a very high impedance then the transistor will turn-on on itself starting to pass an undefined amount of current as soon as you put voltage between D and S

Check driver circuit, to get some reliability you must make it capable of sinking current from the gate preventing Vgs to exceed 1V even when oscillator and driver circuit are unpowered

PD: A trick to blow less components and waste less money when experimenting is to connect a light bulb of the right voltage and power in series with your power supply, for low currents it will act as a low-value resistor but if current gets higher it will act as a passive current limiter [In your case I suggest 12V 21W or so to test the converter, with little or no load of course]
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