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Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
All of those ferrites would work but I would use the 3F3 material because - you want high permeability to ensure low leakage inductance, so the 3E25 material would seem best. - you don't want large core losses, and from the graph it looks like the 3E25 material peaks in permeability exactly at the frequency that you want to operate at. To reduce core losses I'd go for the next lower permeability material, 3F3. -Also I have good experience with 3C85 at 300 kHz which is very like 3F3.
You should use the SMALLEST core that will work to reduce leakage inductance, cost, losses ..... but use one large enough to make winding and insulation practical. REMEMBER, one or more windings of the GDT will be at live mains voltage, one or more windings will connect to the low-voltage circuitry that it is convenient to adjust whilst running. A good GDT should have low leakage inductance, safe insulation and low loss.
For example - F=300 kHz, +/- 15V primary and secondary voltages. At 300 kHz +/- 50mT should be ok for 3F3 core losses e.g. TN 16/9,6/6.3, Ae = 19.7 mmsq. Phi = total flux = 50mT x 19.7 mm2 = 985 nWb Since Phi also = Integral (e.dt) = e.t for a squarewave, t= 1/2 x 1/F = 1.666 us so e = (985 x 1E-9)/(1.66666 E-6) = 0.591 Volts per turn. So 15V requires 25.38 turns. sounds reasonable. IF N=26 then L=26x26 x 1.16 = 784 uH Magnetizing current = 15V x 1.666 uS/784 uH = 32 mA, which is quite good. and there should be enough space to wind several 26-turn windings.
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Wich ferite toroid is best for 300KHz full-bridge mosfet driving ??
