Calculating the saturation current of an inductor?

Question

1. How can I calculate saturation current of a Toroid inductor, with a core?
2. What effect does the current have on the inductivity after crossing the saturation current?

Answer 1

1. Best is to use manufacturers data.

2. Test in an oscillator. (See below)

3. Apply variable DC + AC and monitor effect on AC as DC increased.

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Oscillator method - 2. Above.

Given:

• A flyback converter/oscillator (eg a typical smps boost converter) operating in 'discontinuous mode'

• An oscilloscope

• Variable load.

Iin is a triangle wave plus an off period.
As you increase the load towards saturation the straight portion of the triangle wave will start to assume an upwards kink - ie the rate of rise of current with time will increase as you enter saturation.

Inductance will drop as the core enters saturation. More current further reduces inductance.

From Left and middle and right

Answer 2

How can I calculate saturation current of a Toroid inductor, with a core?

The manufacturer of the cores provides BH curves like this theoretical one: -

The magnetizing force is easily calculated - it is how much current you are feeding into the device multiplied by the number of turns and divided by the length around the toroid: -

Magnetizing force = \$\dfrac{I\times N}{length\space of\space toroid}\$

Once you have that number, look up where on the BH curve your flux density saturation is i.e. is it still on the linear region or is it almost at the horizontally flat region (total saturation).

What effect does the current have on the inductivity after crossing the saturation current?

Inductance is defined as total flux produced per amp of current. As a core saturates you get less and less incremental flux produced for every amp - it is a gradual effect but again the manufacturer's BH curve indicates how the inductance can be expected to decline with increases in current.