al_value

Stan Zurek, AL value, Encyclopedia-Magnetica.com, {accessed on 2019-11-21} |

*A _{L}*

A ferrite core RM8 gapped by the manufacturer so that the inductance factor *A*_{L}=250nH/turn^{2} (the *A250* inscription on the core)

^{by S. Zurek, E. Magnetica, CC-BY-3.0}

The *A _{L}* value is used widely with relation to magnetic cores made of soft ferrite. The name permeance is physically and mathematically synonymous with

Mathematically, the *A _{L}* has the SI unit henry (H), but the the relationship to inductance is non-linear and the practical unit is nanohenry per square turn or nH/turn

Therefore, to calculate inductance the *A _{L}* value must be multiplied by the square of the number of turns

$A_L = \frac{L}{N^2}$ | (H/turn^{2}) = (H) |

So the *A _{L}* value for a given core can be calculated if the number of turns is known and the inductance can be measured.

If A_{L} value is known then the inductance can be calculated as:

$L = A_L ⋅ N^2$ | (H) |

(See also the calculator of AL value from inductance and number of turns).

In the design of transformers and inductors for switch mode power supplies the switching parameters and power level dictate the values of inductance required for such component.

Therefore, the value of inductance is known for the next design step. Using the *A _{L}* value allows for a quick calculation of the required number of turns for a given core size.

It should be noted that the *A _{L}* value is often given in the units of (nH) or similar, with the “per square turn” implied. It is important to remember that the relationship between the

The *A _{L}* value is especially useful when designing with gapped cores, for instance for gapped inductors or flyback transformers. Under normal conditions the air gap stores all the energy and dictates the effective permeability of the magnetic core.

For a simplified case of a uniform magnetic circuit the inductance can be calculated from the following equation:^{12)}

$L = \frac{N^2 ⋅ \mu_0 ⋅ \mu_r ⋅ A}{l}$ | (H) |

where: *N* - number of turns, *μ*_{0} - magnetic permeability of free space (H/m), *μ _{r}* - relative permeability of the material (unitless),

The above equation can be rewritten as:

$L=N^2 ⋅ C_x$ | (W) |

where:

$C_x = \frac{\mu_0 ⋅ \mu_r ⋅ A}{l}$ | (W) |

And by comparing the equations it can be seen that the value $C_x = A_L$ and it is a constant for a given magnetic core of fixed parameters, as long as the effective magnetic permeability is not affected (e.g. saturation is avoided).

Therefore, if the manufacturer provides the *A _{L}* this simplifies the calculations.

A typical notation *A _{L}=160 nH ±3%* means that the core is gapped with such an air gap that

The tight tolerance of ±3% is possible to attain for proportionally larger gaps. In the example above 150 μm is a relatively large values for the magnetic path of the core, which is 19 mm. This reduces the effective permeability from over 1000 to around 137 (see also the calculator of effective permeability).

For smaller gaps the influence of the core is increased and the tolerance could be as wide as ±25%. The same applies for ungapped cores.^{13)}

An example of data sheet giving the *A _{L}* value.

ER14.5-3-7, Planar ER cores and accessories, Ferroxcube

^{by Ferroxcube, Copyright ©}

al_value.txt · Last modified: 2019/06/03 18:28 (external edit)