# FlyBack design using Relfected Voltage VOR Method

1. ## FlyBack design using Relfected Voltage VOR Method

Hi,

I need design steps for the design of a universal (85-265VAC) input, 12v 30 WATT "Discontinuous Mode" Flyback converter using VOR method. Is there any comprehensive document available.

http://micro.rohm.com/en/techweb/kno...pwm01/940/#p01

i need explanation of this method. (web link). How can i compare it with the method used in the attached file.

Regards

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2. ## Re: FlyBack design using Relfected Voltage VOR Method

All methods are the same because the theory is the same. Just apply standard circuit analysis for the Flyback converter in CCM and to see when it enters DCM, just use the usual way of saying Average current < ripple current.

If you need to implement this practically, just find a suitable Flyback IC and follow their design procedure.

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3. ## Re: FlyBack design using Relfected Voltage VOR Method

i have implemented a flyback using uc3845. Transformer is recovered from an old supply with air gap of 0.5mm. 10 turns gives 16uH @ 100KHz. The supply becomes unstable at high load and high input voltage 250AC (rms).

I also need help in dot convention.

Kindly explain "Average current < ripple current."

regards

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I want to design the supply by setting the peak primary current at 1A. The sense resistor is 1 ohm.

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4. ## Re: FlyBack design using Relfected Voltage VOR Method

I also need help in dot convention.
Dot convention represents the way windings are wound. Standard circuit theory books explain this in detail with examples.

"Average current < ripple current."
If the average current of the magnetizing inductance is less than the ripple current you add to it, the flyback enters in DCM because the diode prevents the magnetizing current to go negative.

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5. ## Re: FlyBack design using Relfected Voltage VOR Method

is there any difference in inductance and "magnetizing inductance" of a core ?

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I need explanation of the equations. see attached files

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First we decide turn ratio.
Second We calculate secondary side inductance. and then primary inductance is calculated.
Can i find derivation of these equations?

6. ## Re: FlyBack design using Relfected Voltage VOR Method

First we decide turn ratio.
That equation comes out from the volt second balance on the magnetizing inductance.
Second We calculate secondary side inductance. and then primary inductance is calculated.
PrimaryPeak=Vin*Dmax/Lprimary*fsw => IsecondaryPeak=Vout(1-Dmax)/Lsecondary*fsw
Iout=Average current of the secondary (i.e. area of a triangle)=1/2 * IsecondaryPeak*(1-Dmax) => IsecondaryPeak=2*Iout/(1-Dmax)

Insert the 2nd equation in the 1st and you have the boundary which is what they have in their equations. Make it be an inequality to fulfill the DCM condition.

is there any difference in inductance and "magnetizing inductance" of a core ?
Usually it is called magnetizing inductance because xformers have more inductances, so that you do not get confused.

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7. ## Re: FlyBack design using Relfected Voltage VOR Method

http://micro.rohm.com/en/techweb/kno...pwm01/940/#p01

"To provide for a margin, such as an over-load protection point, the maximum load current should be 1.2 times the Iout. Since specifications for Iout are 3A, Iomax should be 3.6A. In terms of specifications, Vout should be equal to 12V, and the VF and Duty, values calculated in Step (1) should be used."

What does it mean?

How can i add this margin on primary side using current sense resistor? May be by allowing slightly large peak current through primary winding?

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http://micro.rohm.com/en/techweb/kno...pwm01/940/#p01

"To provide for a margin, such as an over-load protection point, the maximum load current should be 1.2 times the Iout. Since specifications for Iout are 3A, Iomax should be 3.6A. In terms of specifications, Vout should be equal to 12V, and the VF and Duty, values calculated in Step (1) should be used."

What does it mean?

How can i add this margin on primary side using current sense resistor? May be by allowing slightly large peak current through primary winding?

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8. ## Re: FlyBack design using Relfected Voltage VOR Method

How can i add this margin on primary side using current sense resistor? May be by allowing slightly large peak current through primary winding?
Yes, calculate secondary peak current with the formula they have and then calculate the primary peak.

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