FLYBACK CALCULATIONS - PART IV

In all switchmode converters that use a high frequency switching inductor or transformer, such as the typical AC-DC flyback converter shown if Fig. 1, it is of critical importance to avoid saturation of the inductor or transformer. If saturation occurs, the inductance of the transformer or inductor will decrease greatly so that the windings will no longer support the applied voltage. If saturation occurs when the transistor switch is in the on-state, the switch current is theoretically unlimited and this could lead to destruction of the converter. Even when current mode control is used, the initial turn-on spike will be so large that it most likely will cause the switch to fail.

To avoid going into saturation, the transformer must be designed so that at maximum current (set by the current limit), the transformer or inductor will not saturate. Therefore we need to determine the maximum magnetic field density (flux density) that the transformer core material will support (carefully the core manufacturer's data sheet), the effective cross-sectional area of the core, the inductance of the inductor or the primary of the transformer, and the maximum current set by the current limit. Then we must calculate the minimum number of inductor or primary turns that will be required to avoid saturation. The number of turns required is directly proportional to the product of the inductance and the maximum current and inversely proportional to the product of the maximum flux density and the core area.

Npmin = Lm * Ilim * 10^6 / ( Bsat * Ae )

Let

Lm = 0.007 Henry

Ilim = 0.200 Ampere

Ae = 31.5 mm^2

Bsat = 0.25 Tesla ( 2500 Gauss )

Then

Np = 187 turns ( rounded up to the nearest full turn )

The actual design might use 225 turns to provide a safety factor of approximately 20%.

Once we have set the primary turns, we can then calculate the turns ratio that is needed to achieve the required main output (the controlled output) voltage as


n = VRO / ( Vo1 + vf1 )


Let

VRO = 350 volts

Vo1 = 15 volts

Vf1 = 1 volt

where VRO is the reflected voltage from secondary to primary (previously calculated in the post, Flyback Calculations - Part I), Vo1 is the required main output voltage, and Vf1 is the forward voltage of the output rectifier diode at maximum output current. Then

n = 22 (rounded to an integer)

Now it is easy to compute the required turns of the main secondary, as


Ns1 = Np / n = 10 (rounded to the closest integer)

The other secondary output turns can now be computed using the values from the equation

Nox = ( Vox + Vfx ) * Ns1 / ( Vo1 + Vf1 )

where Nox is the required secondary turns, Vfx is its diode forward voltage at the winding's maximum current. For example if Vox is 125 volts and Vfx is 1.2 volts, we have

Nox = 81 turns (rounded up to the nearest integer )

We can also note here that secondaries in this transformer design are roughly 1.5 volts per turn. This is a reasonable value for the design. A practical range for secondary volts per turn is usually 1 to 2 volts per turn.