Desktop power adapter chooses magnetic core material

　　The efficiency and performance of a saturated single-transformer converter mainly depends on the choice of transformer material. When working at high frequencies, low-loss rectangular hysteresis loop ferrite materials are usually selected. Rectangular hysteresis loops and band-wound toroid materials are also used, because very thin laminated core materials must be used in order to reduce losses, which are more expensive at high frequencies. The recently developed amorphous alloy rectangular hysteresis loop material shows good properties in this application.

　　For some rectangular hysteresis loop materials, the permeability (the slope of the B/H characteristic in the saturation region) after saturation is very low (B is high). The figure shows the characteristics of the H5B2 material. Therefore, the magnetic flux density change between S1 and S2 is very small, and the flyback effect during turn-off is weak, which makes the switch action slow during the conversion period. In the figure shown, it can be seen that H7A or similar core materials have a more powerful switching effect, but because these materials have low Br (remanence) values, core losses are often higher.

　　For toroidal cores, it seems that the designer must use rectangular hysteresis loops, low-loss cores (low core loss but slow switching), or low-permeability and higher-loss cores (switching faster, but the magnetizing current is higher due to the smaller inductance. High, and there is a greater loss) to make a compromise choice. For other core structures, air limitation can be introduced to solve the problem. This can give a powerful switch without increasing the core loss. Suitable rectangular hysteresis loop and metal ribbon winding materials include rectangular permalloy, nickel-iron high-permeability alloy and various HCR materials. Some almost rectangular amorphous alloy materials can also be used. Be careful when choosing the rectangular hysteresis loop material. When the hysteresis loop is too close to the rectangle, it will not cause oscillation because the flyback is too weak.

　　Low-power self-oscillation auxiliary converter

　　Many power converters require a small number of auxiliary chargers as chargers for the control and drive circuits. The required additional charger is often obtained from a 60Hz charger transformer. But this is not always very effective, because the size of the transformer is often determined by meeting the VDE (German Institute of Electrical and Technical Engineers) and UL (Guarantor Laboratory) leakage distance indicators, rather than power requirements. As a result, the size of the transformer is often larger than the size required to meet the power requirements alone.

　　In applications that require uninterrupted chargers (UPs), the backup charger may be a DC battery, and the 60Hz input may be useless. Therefore, a 60Hz transformer cannot be used in this type of system.

　　One solution is to use low-power, high-frequency converters to provide auxiliary chargers.

　　Using self-oscillation technology can get a very effective low-cost converter. Some suitable examples are discussed in this chapter.

　　General working principle

　　In a self-oscillating converter, the switching effect is maintained by the positive feedback taken from the main transformer winding. The frequency is controlled by the saturation of the main transformer or by the saturation of the auxiliary drive transformer, or in some cases by the drive clamping action corresponding to the magnetizing current that increases during the on-time.

　　In a simple system, the frequency is easy to change with changes in the magnetic characteristics of the core, load or applied voltage.

　　Working principle, single transformer converter

　　The figure shows a self-oscillating converter in the form of a single transistor. This kind of converter works in flyback mode and is mostly used in low-power, constant-load applications, such as an auxiliary charger for the control circuit of a large converter (the output given in this example is 12V, 150mA)

　　When it starts to work, the current flowing through R1 turns on Q. As Q turns on, the regenerative positive feedback established by the drive winding P2 is added to the base of Q1 via C1 and D1, so that Q1 is quickly turned on. The collector and emitter currents of Q1 will increase linearly at a rate determined by the primary inductance and the input voltage.

　　As the emitter current increases, the voltage across the emitter resistor R2 (V.) also increases until it approaches the value produced by the feedback winding P2. At this point, Q's base current will be "blocked" and Q1 will start to turn off. Due to normal

　　Flyback, the voltage on all windings will be reversed, and the regeneration shutdown effect generated by the driving winding P2 and capacitor C1 is applied to the base of Q1.

　　The off state continues until all the energy stored in the transformer during the on period is transferred to the output circuit. At this time, the voltage across all windings begins to drop to zero. Now as the voltage on the driving winding P2 becomes zero, the current flowing through R1 charges C1, and the base of Q1 becomes positive again, and then Q1 turns on again, repeating the work cycle.

　　The operating frequency is determined by the primary inductance, the value of R2, the converted load current and voltage, and the selected feedback voltage on P2.

　　In order to minimize the frequency change caused by the load change and keep the flyback voltage constant, the turn-off time must remain nearly constant. To achieve this, sufficient energy must be stored during the conduction period to keep the energy recovery diode D2 conducting during the entire flyback period. In this way, the flyback voltage is kept constant while the output voltage is therefore kept constant. This requires the flyback energy to greatly exceed the load demand, so the excess energy can be returned to the charger during the entire flyback period to maintain D2 conduction.

　　This condition is obtained by adjusting the size of the magnetic core air gap and selecting the transformer inductance. In low-power, constant-load applications, the charger's Zener diode D3 functions to stabilize the charger voltage, ensuring a fixed frequency, thereby stabilizing the output voltage.

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