Parameters and selection considerations of magnetic ring elements for energy-saving lamps

In the electronic circuit of energy-saving lamps, the magnetic ring is known as the heart of the energy-saving lamp. Whether in the debugging of the electronic circuit of energy-saving lamps or in production, the changes in the parameters of the magnetic ring have a great impact, which can be said to be a domino effect. The parameters affected include: the start-up time of the energy-saving lamp, the switching performance of the transistor, the operating frequency of the ballast, the lamp power, etc. Especially under the voltage condition of 110V, the circuit design does not use a voltage doubling circuit, and is particularly sensitive to the selection of the magnetic ring. Below I will explain the various parameters of the magnetic ring and the selection considerations in two parts.

1. Analysis of various parameter curves

See the figure below:

In the picture:

B is the magnetic induction intensity.

BS is the saturation magnetic induction intensity.

BM is the highest magnetic induction intensity.

H is the magnetic field strength.

Br is the residual magnetic flux when the magnetic field induction intensity H=0.

He and Hc are coercive (magnetic) forces.

In energy-saving lamps, the magnetic ring generally uses a saturable toroidal core. In order to make the half-bridge inverter circuit of the energy-saving lamp have good switching characteristics and produce a good oscillation waveform, the magnetic ring must have a hysteresis loop similar to a rectangle as shown in the figure. In the S-shaped characteristic curve, point a is the starting point, from point a to point b, then to point c and point d, and finally back to the original point a, so that a complete magnetization cycle is obtained. Such a hysteresis loop has obvious saturation points and saturation segments, and has good symmetry. The hysteresis loop similar to a rectangle can make the front and rear edges of the current waveform in the magnetic ring coil steeper, which can better meet the driving requirements of the transistor. If the S-shaped hysteresis loop is not completely symmetrical at each point, it will seriously affect the switching characteristics of the half-bridge inverter circuit of the energy-saving lamp, resulting in increased losses and increased temperature rise of the transistor.

We use another figure to illustrate the temperature characteristic curves of the initial magnetic permeability of several magnetic rings commonly used in energy-saving lamps.

In Figure 2: Curve 1 is the curve of magnetic permeability B and temperature of 3K. It can be seen from the figure that the 3K material reaches the first peak relatively quickly, then quickly drops to the valley point, about 80 degrees, and then slowly rises to the Curie point, about 200 degrees.

Curve 2 is the curve of B and temperature of magnetic permeability 2.5K. It can be seen from the figure that the magnetic permeability of 2.5K material has been rising with temperature, the valley point is extremely short, and the valley point temperature is relatively high, reaching about 180 degrees, and the Curie temperature is about 210 degrees.

Curve 3 is the B-temperature curve of magnetic permeability 2.3K. It can be seen from the figure that the B value of 2.3K material does not change much with temperature, the valley point is about 150 degrees, and the Curie temperature is about 220 degrees. From the temperature curves of the above three materials, it can be seen that the Curie temperatures of the three materials can meet the requirements of energy-saving lamps, and the maximum temperature inside the energy-saving lamp shell generally does not exceed 150 degrees. Comprehensive analysis of the three curves shows that the stability of 3K material is slightly worse, and the valley point temperature of 2.5K material is higher. If the temperature inside the energy-saving lamp shell is too high, reaching a maximum of 150 degrees, the magnetic If the B value does not decrease but keeps increasing at this time, it will inevitably lead to overdrive of the transistor, increase of current, and finally catastrophic consequences. 2.3K material is very popular in energy-saving lamps due to its stable temperature curve. Unless there are special requirements, energy-saving lamps generally use 2.3K or 3K magnetic rings. The perfect temperature curve should be flat, almost invisible, with a long valley point, preferably between 70-150 degrees, and the Curie temperature can be above 200 degrees. Unfortunately, such magnetic rings have not been used in energy-saving lamps so far.

2. Selection considerations (in order to improve the reliability and safety of energy-saving lamps, the selection of magnetic rings must adapt to the characteristics and requirements of energy-saving lamps)

1. Choice of shape and size:

The magnetic rings suitable for electronic energy-saving lamps generally have these specifications: ∮10*6*5; ∮10*6*3.5; ∮10*6*3; ∮9*5*3; ∮12*6*4; ∮13*7*4. When the space of the plastic parts of the energy-saving lamp is small, or the PCB area is small, the ∮9*5*3 magnetic ring can be selected. When it is not affected by the space of the plastic parts of the energy-saving lamp and the PCB area, we generally use magnetic rings with specifications of ∮10*6*5; ∮10*6*3.5; ∮10*6*3. When MOSFET is selected as the switch tube in the circuit, we generally use magnetic rings with specifications of ∮12*6*4; ∮13*7*4. Since MOSFET requires a relatively high gate drive voltage, the number of secondary turns of the magnetic ring will be relatively large. For the magnetic ring, it is necessary to have a large enough inner diameter to bypass these secondary coils.

2. Selection of magnetic materials:

Different magnetic materials have different characteristics and different application scopes. Generally speaking, our energy-saving lamps generally use manganese-zinc ferrites. The ferrites suitable for energy-saving lamps are: PC30, PC40 and PC50. In the selection of magnetic materials for magnetic rings, the following requirements should be considered:

(1) The Curie temperature should be high enough. Due to the small space inside the energy-saving lamp and poor heat dissipation, the temperature inside the shell is usually above 80 degrees. If the working environment temperature is too high or the lamp is covered, the temperature inside the shell will be higher, up to 130-150 degrees. In order to ensure that the temperature inside the energy-saving lamp shell is lower than the Curie temperature of the magnetic ring, the magnetic ring should be made of magnetic materials with a Curie temperature higher than 200 degrees.

(2) The initial magnetic permeability should be moderate. Since the initial magnetic permeability of magnetic materials is inversely proportional to the Curie temperature, the higher the initial magnetic permeability, the lower the Curie temperature. Our choice is limited to the range below 4K. Of course, for energy-saving lamps whose shell temperature is not higher than 80 degrees and whose actual lamp power is lower than 70%, or for 110V input without voltage doubling in the circuit, magnetic materials with high initial magnetic permeability can be appropriately selected to obtain a higher driving signal.

(3) The resistivity should be relatively high. When the operating frequency is constant, the eddy current loss of the magnetic material is inversely proportional to the resistivity. In order to reduce the self-loss of the magnetic ring, a magnetic material with a relatively high resistivity should be selected. Although the self-loss of the magnetic ring is negligible in the entire energy-saving lamp circuit loss, the adverse reactions it produces cannot be underestimated.

(4) Appropriate temperature coefficient. For the magnetic ring, we generally require it to have a negative temperature coefficient, that is, its magnetic permeability or the inductance of the magnetic core coil should decrease with the increase of temperature. When the temperature changes from 0 to 100 degrees, the collector current of the transistor increases by about 15%. Within this temperature range, if the magnetic ring has a negative temperature coefficient, it just offsets or mostly offsets the positive temperature coefficient of the transistor, basically maintaining a balance, thus ensuring the stable operation of the electronic energy-saving lamp.

(5) Saturation flux density and hysteresis loop. The magnetic ring in the electronic energy-saving lamp should have a high saturation flux density to ensure that the secondary of the magnetic ring has a sufficiently high driving power and prevent the inductor from easily entering saturation and increasing the temperature rise. Since the hysteresis loss of the magnetic ring is proportional to the area surrounded by the hysteresis loop, a magnetic ring with a relatively narrow hysteresis loop should be selected, which is conducive to reducing power loss. The magnetic ring must have a hysteresis loop that is approximately rectangular as mentioned above. And the hysteresis loop is required to have relatively good symmetry, which can ensure that the two transistors in the circuit produce symmetrical current waveforms and prevent the temperature deviation of the two transistors.

Simple magnetic ring reliability test method, thermal test method: test the magnetic ring in grades (generally test the magnetic permeability and single-turn inductance), record the data, and then place the magnetic ring in an oven at 100 degrees, take it out of the oven after baking for 48 hours and store it naturally for 24 hours. Test the parameters of the magnetic ring again. If the change is not big, it can be used. If the change is too big, it cannot be used. During the test, there must be at least 100 magnetic rings. This method is used to judge whether the consistency of the magnetic ring is up to standard, and whether the parameters of the magnetic ring will change greatly under long-term high temperature, affecting the life of the energy-saving lamp.

Of course, other lighting products similar to energy-saving lamps, such as electronic ballasts and electronic transformers, can also be selected according to this selection method. Except for the working environment, temperature, etc., electronic ballasts and electronic transformers are basically the same as energy-saving lamps.