Science

Published on January 9th, 2019 | by Sunit Nandi

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# What is Magnetic Saturation in the Transformer?

— OUGE gives you the easy way to know the danger of magnetic saturation

When the manufacturers produce transformers, they often encounter such a customer complaint- magnetic saturation. However, such a negative phenomenon is difficult to find in their production process, because the winding number, inductance, leakage inductance and other items they often test can’t be tested by the comprehensive tester. In fact, the harm of magnetic saturation may result in a large number of products scrapped.

## What is Magnetic Saturation

In simple terms, if we add sugar to a glass of water continuously, the sugar will be dissolved by the water. However, when there is too much sugar, the water can no longer dissolve the added sugar. When the glass of water has reached the maximum amount of sugar that can be dissolved, we say that the sugar in the glass is saturated. Similarly, if a magnetic core coil is added with current, the magnetic field will also be strengthened as the current increases. When the current reaches a certain level, the magnetic field will no longer continue to be strengthened. At this point, the magnetic core and the coil will enter into the saturated region and the magnetic core will be in a saturated state.

Magnetic saturation is a physical property of magnetic materials. After magnetic saturation is produced, it’s harmful on some occasions, but sometimes is beneficial. For example, magnetic saturation voltage stabilizer is to use the magnetic saturation characteristics of the core to achieve the purpose of stable voltage. However, for the power transformer, if the added voltage is much higher than the rated voltage, the current will increase sharply and the transformer will burn out soon. Assumes that there is a magnet, on a unit of electric current, and the magnetic induction intensity is 1, the current increases by 2, magnetic induction strength increases to 2.3, but current to 6, magnetic induction intensity is 7, if further increase the current, the magnetic induction intensity is 7 no longer increases, which means the electromagnet produces magnetic saturation.

When the inductor of the magnetic core has the problem of magnetic saturation, adding ferrite or other magnetic conductive material to the inductor can make use of its characteristics of high magnetic conductivity to increase the inductance, reduce the turns and improve the efficiency. But due to the limitation of the physical structure of magnetic conductive materials, the magnetic flux passing through it cannot be increased indefinitely. Especially in the loop with DC current, if the DC current has saturated the magnetic core, the AC component in the current can no longer cause the change of magnetic flux. Then the inductor is out of action.

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## How it Comes

It is usually because a large DC component flows through the winding, especially when the pulse width is not adjusted properly, it can easily lead to iron core saturation. In order to increase the magnetoresistance of the magnetic circuit, slow down the saturation speed of the iron core, and make the switching power transformer obtain good linearity, the method is to put the appropriate thickness of non-magnetic insulation, usually paper, between the iron core. Thermal saturation, magnetic flux saturation, voltage saturation, and overload saturation will lead to the saturation of switching power transformer, which will lead to too high DC component and partial magnetism. It cannot return to the initial state along the hysteresis loop, so the magnetic flux density drops sharply and the transformer fails. In the PWM waveform, when Fourier decomposition is carried out, it can be found that there are too much DC component. In theory, the smaller the DC flow rate, the better.

E.g.

In the process of magnetization of ferromagnetic materials, magnetic induction intensity B increases with the external magnetic intensity firstly, but when H exceeds a certain value, magnetic induction intensity B tends to a certain fixed value and reaches the magnetic saturation state. The typical magnetization curve is shown in the figure. When B is close to Bp, it enters the critical saturation region, and when B is close to Bo, it reaches the magnetic saturation region. For switching power supply, when the high- frequency transformer magnetic flux (Φ=BS) not changes along with the increase of outside magnetic field intensity significantly, it’s called a magnetic saturation state. Because of the magnetic intensity H changes when the magnetic induction intensity B changes very small, the permeability reduced significantly, permeability u= △B/△H. At this time, the inductance of primary winding Lp is also reduced significantly. As can be seen from the figure, the permeability is equal to the slope of the magnetization curve, but since the magnetization curve is non-linear, u is not a constant.

## The Dangers

Once the magnetic saturation occurs, it will cause great harm to switching power supply. It will not only overheat the components, but also damage the accessories. In the case of magnetic saturation, the inductance Lp of the primary winding decreases significantly, so that the DC resistance of the primary winding and the power consumption of the internal power switch MOSFET increase rapidly, resulting in a sharp increase in the primary current. Maybe TOPswitch’s internal current-limiting circuit has been damaged before the MOSFET can be protected. When the transformer breaks, there are the following phenomena:

1. The transformer heats up and even explodes.
2. The transformer does not work.
3. Transformer burns MOSFET and circuit board due to the open circuit.
4. Due to transformer saturation, the actual power of customer products cannot reach the designed power.

## How to Avoid

1. In order to prevent high-frequency transformer from magnetic saturation, the turns of primary winding should be reduced appropriately. In addition, the larger size of the core as far as possible and to set aside a certain air gap width for the core, also can prevent the core into the magnetic saturation state.
2. Production shall be strictly in accordance with the customer operation instruction letter, and materials shall not be changed without the customer’s consent.
3. Since magnetic saturation is related to the magnetic permeability of magnetic core, use the magnetic core with high magnetic permeability as far as possible. Of course, this will increase the cost.

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