Could any body please help me with the following problem (You could write in German :-)): Flyback transformer in discontineous mode: "Vin"=11-12V Dc "Ipeak"(primary)=520mA, ton(turn-on time)=5us "Lp"(primary inductance) is calculated by: Lp=(Vin*ton)/Ipeak A EF20 core is used with "Ae"=31mm^2 and "AL"=1200nH. Wire with "d"=0.38mm is used. Air-gap calculated by: "Lg"=[0.4*3.14*Lp*Ipeak^2*10^8]/(Ae*Bmax^2)=0.02mm "Bmax"=2000Gaus selected. Primary number of turns by: "Np"=(Bmax*Lg)(0.4*3.14*Ipeak)=8 turns ------ Surprisingly the primary current taken from the power supply during the turn-on time is too small, about 100mA, expected 540mA!! Questions:Why is the current so small? Why is the inductance not equal to 100uH,it's about 80uH. (In each case the current is too small for Lp=80uH or 100uH) I'll be gratefull if any body can help me with this problem! thank you very much in advance
Thanks for quick reply:-) I'm using this flyback transfprmer to charge a capacitor(my load is capacitive). I saw that my transformer doesn't take the expected current, therefore I removed the secondary winding to see whether the primary works as a normal inductor, what actualy schould be.I recognized the current taken from the power supply is always the same:-(
Hi! I have a flyback configuration as a part of a more complicated low power design. in this design I must disconnect the secondary winding from output capacitor using a switch or a device funktioning as a switch. Since this switch must be placed between the secondary winding(which has floating voltage levels)and output capacitor(which is being charged and has no constant voltage), both sides of the switch have floating potentials, therefore there is no reference potential to activate the switch. Requirements: 1: an absolute disconnection must exist(no leakage current) 2: Maximum current flowing through this switch is about 1A 3: reverse voltage on the switch(off status) about 900V (difference of the voltage on the output capacitor and the secondary winding) 4: switch power dissipation must be very low 5: switch must be very fast, some micro seconds 6: rise and fall time for the switch turn on and off must be in nano second Important: -MOS transistors are not adequate because current leakage through free wheeling diode makes the absolute disconnection impossible! -Relays are too slow and bulky I do appreciate your idea! I'll be gratefull if any body can help me with this problem! thank you very much in advance ---- ----------/ ---- | | switch | ... ... | . . . . --- . . . . --- . . . . | | | | ---- ----------------
Scenario 1: What I do not understand is, do you have a diode between secondary and capacitor? Or should the switch act as a diode (active rectification)? Scenario 2: What I think you want to have is charging the cap and then turn of the flyback, hoping that the cap would hold the charge as long as possible. Then I suggest a semiconductor and a (reed-) relay in series. Plase tell us more about your circuit.
Some more thoughts (I just sav that the first postings are more than 2 weeks old): "AL"=1200nH 8 turns L = 8 8 1,2µH = 76,8µH Why did you expect it should have 100µH? The inductance is not constant, it depends of the field strength / current. Did you succeed in getting an higher primary current? > I saw that my transformer doesn't take the expected current, > therefore I removed the secondary winding to see whether the > primary works as a normal inductor, what actualy schould be. You know that you cannor simply remove the secondary of a flyback? The energy stored in the inductance (air gap) must be "discharged" / zeroed , otherwise the core would saturate. A literature tip: TI / unitrode magnetics design handbook http://focus.ti.com/docs/training/catalog/events/event.jhtml?sku=SEM401014 http://focus.ti.com/lit/ml/slub007/slub007.pdf (many pdfs!) The german word for flyback is Sperrwandler (to use the search function). > Important: -MOS transistors are not adequate because current > leakage through free wheeling diode many IGBTs do not have an intrinsic diode.
In my design I try to charge and discharge a capacitive load, demonstrated as output capacitor. Primary is connected to a low power supply and output capacitor should be charged to a high voltage level. I switch the primary (low side) in order to charge the capacitor and I switch the high side(secondary) to discharge the capacitor. So the diode in the secondary(used to charge the capacitor) is realized by the free wheeling diode of the switch in the secondary side(as depicted in photo) and the diode in primary(used during discharging of capacitor) is realized by the free wheeling diode of the switch in the primary side. Circuit is depicted in the following address. http://www.4shared.com/photo/ElAXGiDe/question_pic.html thanx a lot!
eProfi schrieb: > I suggest a semiconductor and a (reed-) relay in series. may I have more information please?
> Why did you expect it should have 100µH? > > The inductance is not constant, it depends of the field strength / > > current. > > > Did you succeed in getting an higher primary current? Yes I get highet currents, the current meter was not calibrated!! About the inductance: you are right,N*N*AL=L which results in a smaller inductance than expected. The procedure I used to calculate and make the transformer is written in many refrences, but actually it's also confusing.As explained in these refrences, using this procedure results in an expected value of inductance, what indeed doesn't work! **please give me an advice how schold I calculate the number of turns in primary and the Airgap length? **Is this a try and error process in which I calculate the number of turns and the air gap and then I check the behaviour of the transformer and consecuently modify the number of turns and the air gap length? *How is the formel N*N*AL=L for transformer with an air gap?I mean should I make more turns for example 10 and calculate L=N*N*AL=120uH and then try to decrease the L by an air gap?
eProfi schrieb: > many IGBTs do not have an intrinsic diode. do these IGBTs fulfil my riquirements listed above(Current,Voltage,...)? may you please name a few? I could not recognize from the data sheet whether they are with or without an intrinsic diod. thanx in advance
> whether they are with or without an intrinsic diod.
It should be visible in the diagram and in the text.
There are only a few manufacturers: IXYS and IR , to name a few.
You select the core with the correct air-gap, look to the datasheet for
the Al-value and calculate the L.
Siemens has a broad palette of "sifferit" magnetics:
Here is a collection of their products of year 2000:
www.dextermag.com/uploadedFiles/Literature_Ferrite_EPCOS.pdf (4 MB)
Read the slup124.pdf ... slup129.pdf of TexasInstruments.
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