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# (Dry goods exchange) Talking about air gap and leakage inductance

2023-11-07【Archive】

Saturation of magnetic core is equivalent to an air-core coil on primary side of transformer (equivalent to a short circuit) and its current will be very large, increasing until transformer or fuse blows.

Magnetic core air gap is abbreviation for magnetic core air gap. As a rule, ferrite and silicon steel magnetic cores are not a solid closed body. Air core air gap so people can use toroidal transformers when they don't need an air core. When a core air gap is used, butt gap is deliberately increased or non-magnetic materials such as high temperature paper are inserted into gap.

The air gap only opens in high-frequency transformer to prevent magnetic saturation of iron core. Because there are high-order waves in UPS, air gap must be opened, but principle of opening air gap in transformer is different from inductance gap. Transformers are made of silicon steel sheets, and gap between two facing silicon steel sheets is called air gap. The larger air gap, greater resistance. The air gap of transformer is designed to prevent magnetic saturation during operation! The air gap is gap left at intersection of iron core! It has nothing to do with winding. The presence of an air gap increases resistance, but it is beneficial! The role of air gap is to reduce magnetic permeability so that characteristics of line are less dependent on original magnetic permeability of core material. The air gap avoids magnetic saturation when there is a large AC signal or DC bias, and better control of inductance. However, in case of an air gap that reduces permeability, more coil turns are required and associated copper loss increases, so a compromise is required.

Common reverse power supply, when air gap is small, smaller air gap, smaller power, larger air gap, more power, generally air gap can be adjusted to achieve maximum output power. , of course, under no circumstances can it enter saturation region, that is, input current cannot go off scale. When sanding air gap, you can use a small strip of water sandpaper (adding water will make sanding speed faster and flatter), and put glass on bottom. If you want a large air gap, sand middle, and if you want to reduce air gap, sand both sides.

The leakage inductance of flyback power supply transformer is a very important parameter. Since flyback power supply requires transformer to store energy in order toWe make full use of transformer core, an air gap is usually required in magnetic part. circuit. The purpose is to replace iron core. The slope of hysteresis loop of core allows transformer to withstand high surge currents without iron core becoming saturated with a non-linear state. Close magnetic circuit.

The coupling between primary poles of a transformer is also a key factor in determining leakage inductance. To get coils of primary pole as close as possible, multilayer winding method can be used, but this will increase distributed capacitance of transformer. Choose an iron core with a relatively long window as much as possible to reduce leakage inductance. For example, effect of using EE, EF, EER, and PQ type magnetic cores is better than that of EI type.

The magnetic core of a flyback power transformer operates in a state of unidirectional magnetization, so magnetic circuit must open an air gap, like a pulsating DC inductor. Part of magnetic core is connected through an air gap. I understand principle why air gap is open: since power ferrite also has a work characteristic (hysteresis loop) that is like a rectangle, Y-axis on work characteristic represents intensity of magnetic induction (B), and current production process is generally point saturation above 400 mT. Typically, this value should be 200-300 mT in design. The x-axis indicates magnetic field strength (H). This value is proportional to strength of magnetizing current. Opening an air gap in magnetic circuit is equivalent to tilting magnet's hysteresis loop to X axis. With same magnetic induction strength, it can withstand a larger magnetization current, which is equivalent to accumulation of more energy in magnetic circuit. .This energy is stored in switch tube. The air gap of flyback power core has two functions, one is to transmit more energy, and other is to prevent core from going into saturation.

The flyback power supply transformer operates in a state of unidirectional magnetization, which not only transfers power through magnetic coupling, but also performs multiple input and output voltage conversion functions. Therefore, handling of air gap must be very careful. If air gap is too large, leakage inductance will increase, hysteresis loss will increase, and iron and copper losses will increase, which will affect overall performance. power source. Too small an air gap can saturate transformer core and damage power supply.

When there is an air gap in iron core of transformer, since magnetic permeability of air is only a few thousandths of magnetomotive force of iron core, magnetomotive force is almostapplies to air gap. Therefore, average magnetic permeability of air-gap transformer core will be greatly reduced, not only will residual magnetic induction decrease, but maximum magnetic induction Bm can reach saturation magnetic induction Bs, thereby increasing magnetic flux increment, transformer core is no longer subject to magnetic saturation.

Add air gap:

1. Reduced inductance;

2. The excitation current increases;

3. Decrease Br;

4. Increase energy storage capacity, good anti-saturation (Le is greatly increased, and B corresponding to same NI is much smaller).

After opening air gap, due to increase in leakage inductance, flux linkage surrounded by secondary coil is reduced. In fact, most fundamental problem is two,

One: excitation current is connected in parallel with output current, an increase in excitation current means a decrease in efficiency;

Second: Leakage inductance increases, so harm from this will not be mentioned. (subject to verification)

The reason for leakage inductance will cause magnetic flux of primary and secondary coils can not be fully compensated, and magnetic core will be magnetized after a long time, so strong direct excitation needs to add a small air gap, so that DC component of transformer can enter into air gap. I have also heard about this theory that magnetic flux is not completely cancelled, and I think it makes sense.

Direct excitation transfers energy during arousal, and excitation creates a platform for energy transfer. The field current returns energy to power supply or is consumed for magnetic reset during trip period. The lower excitation current, lower excitation energy and less excitation loss. After adding an air gap, magnetic resistance increases, magnetic energy that can be stored in magnetic core increases, remanence Br of magnetic core also decreases, deltB (Bs-Br) increases, and magnetic saturation does not easily occur when conduction period of switch tube increases, but both drive current and leakage inductance increase.

When operating frequency is low, a small air gap can be added. At this time, magnetic saturation is not easy, deltB is increased, number of coil turns can be reduced, and copper loss can be reduced. decreases, but pay attention to peak high voltage during magnetic reset and shutdown.

At a high operating frequency, magnetic energy in core changes greatly per unit time, and losses in core increase sharply. The deltB value of core must be reduced during design. At this time, Bs and Br are not limiting faAt same time, since number of turns is very small, there is no need to add an air gap to increase leakage inductance and excitation current.

1. It is best to add a small air gap for energy storage, which will greatly improve its ability to withstand DC magnetic field displacement;

2. For some applications, a non-energy storage transformer does not require an air gap to be opened and directly fills window area; but generally limited by volume and cost, we always use smallest magnetic core possible to produce maximum power possible. It is better to open a small air gap, so that saturation phenomenon does not easily occur when strength of magnetic field is slightly broken;

3. The current generates a magnetic field, magnetic field transmits energy or stores energy, and power transfer process is: electrical energy-magnetic energy-electrical energy.

Typically, a small air gap can cause transformer to saturate; a large air gap will soften load characteristics of entire power supply, increase change in output voltage between idle and full load, and increase ripple, especially failure.

The magnetic permeability of magnetic core is much greater than that of air. According to H=B/u, it can be seen that smaller u, stronger magnetic field strength H and energy stored in magnetic circuit is proportional to H, W=VuH²/2, V - volume, u permeability, so air gap accumulates most of energy.

H in hysteresis loop represents magnetic field strength H(Io) generated by primary side excitation current, H(I1) generated by primary side anti-secondary current, and H(I2) generated by secondary side total current vector. That is, vector sum of H=H(Io)+H(I1)+H(I2) and H(I1)=-H(I2), i.e. H=H(Io), so H in hysteresis loop is H(Io ), then corresponding current is Io, not Io+I1.

Adding an air gap increases N*I saturation. Most of transformer's energy is stored in air gap. The larger air gap, more energy transformer can store, so be saturated. The energy storage capacity of magnetic core of transformer is E = 0.5 * B * V * H., after adding air gap, H is equivalent of He of magnetic core and air part.

An air-core coil is never saturated, so after adding an air gap, entire magnetic circuit has a small air characteristic. The saturation flux density is still saturation flux density of core material. However, transformers can handle more power (mostly in air gap).

The larger air gap, stronger stray magnetic field in coil, resulting in more eddy currents and more losses!

We can roughly assume that resistance of coreand with an air gap centered in air gap:

Rm≈δ/μ*Ac (1)

where μ=4*π*10^(-7) (H/m) is magnetic permeability of vacuum;

δ——air gap thickness (m);

Ac - air gap area (m^2)

Reverse resistance is permeance or inductance per circle. So air gap core inductance is:

L=μ*Ac*N*N/δ (H)(2)

On other hand, inductance is a flux linkage created by a single current,

L=ψ/I =N*Bm*Ac/Im (H) (3)

(Bm Im - maximum magnetic density (T) and maximum current (A), respectively)

Therefore, combining formulas (2) and (3), we have

δ=1000*µ*N*Im/Bm (mm) (4)

Use type (4) to draw attention

1) Everyone uses international system of units;

2) Formula (4) is sum of air gaps of magnetic circuit. For example, when an air gap is formed with a paper gasket, thickness of paper is half of formula (4);< /p>

3) Since core resistance is omitted in formula (1), calculation result of formula (2) is slightly larger.

Why larger reactor air gap, smaller inductance? Are inductance and impedance directly proportional?

The inductance of coil is proportional to magnetic permeability u of magnetic circuit. The larger air gap, greater resistance of magnetic circuit, that is, lower magnetic permeability, lower inductance. The inductance X of inductor = 2 * ∏ * f * L, so inductance X is proportional to inductance L.

Why do you say that permeability decreases, so inductance is small?

Magnetic permeability is inversely proportional to magnetic resistance: greater magnetic resistance, lower magnetic permeability. What does this have to do with inductance?

The inductance of coil is proportional to magnetic permeability u of magnetic circuit.

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