Transformers have always been a power supply device and device, reducing the size, increasing power density, and achieving a modular roadblock. Although the high-frequency conversion technology is introduced into the power supply, the bulky power frequency transformer can be removed, but a high-frequency transformer using a ferrite core is also required. Although the volume of the ferrite core high-frequency transformer is smaller than that of the power frequency transformer, the requirements for leaving the modular are still far apart. Not only is it too bulky, but its heat and leakage inductance are not small. Therefore, in recent years, many experts, scholars, and engineers have been studying ways to solve this problem. The successful development of high-frequency flat-panel transformers has made a leap in transformer technology. It not only can make the size of the transformer much smaller, but also can make the internal temperature rise of the transformer is very low, the leakage inductance is small, the efficiency can be 99.6%, and the cost is half lower than the transformer of the same power. It can be used in single-ended positive, flyback, half-bridge, full-bridge and push-pull converters for AC/DC and DC/DC converters. It is especially suitable for low voltage, high current converters. So it is especially suitable to use it as a contemporary computer power supply.
2 problems in the conventional conversion transformer operating at high frequencies(1) Leakage inductance (referred to as leakage inductance)
The ideal transformer (fully coupled transformer) primary flux generated by the primary winding should pass through the secondary winding without any loss or leakage. But in fact, conventional conversion transformers are impossible to achieve without any loss and leakage. It is impossible for the magnetic flux generated by the primary winding to pass through the secondary winding. The uncoupled portion of the flux has its own inductance in the winding or conductor, and the energy stored in this "inductor" is not coupled to the main power transformer circuit. This kind of inductance is called "leakage". The requirements of the ideal converter for insulation and the need for very tight electromagnetic coupling to reduce leakage inductance in order to obtain very low electromagnetic interference (EMI) are contradictory.
When the transformer is not energized (turning off the power supply or the switch is off), the stored energy of the leakage inductance is released to form significant noise. The high frequency spike waveform of this noise can be seen on the oscilloscope. The amplitude of the high-frequency spike waveform and the leakage inductance Lleak are proportional to the product of the current versus time change rate. which is:
|Uspike|=Lleakdi/dt(1)
As the operating frequency increases, the rate of change of current versus time increases. The effect of leakage inductance will be more serious. The effect of the leakage inductance is proportional to the switching speed of the converter. Leakage induced excessive spikes can damage the transformation
Figure 1 Schematic diagram of conventional conversion transformer and flat panel transformer
(a) conventional conversion transformer (b) flat panel transformer
The power devices in the device form significant electromagnetic interference (EMI). In order to reduce the spike amplitude Uspike generated by the leakage inductance, a buffer network must be added to the converter circuit. However, the addition of the buffer network will increase the loss of the converter circuit. The converter circuit is increased with the operating frequency, the loss is increased, and the efficiency is lowered.
(2) Inter-winding capacitance
When the winding of the transformer is a multi-layer winding, there is a potential difference between the top winding and the bottom winding. There is a potential difference between the two conductors, and there is a capacitance. This capacitance is called "inter-winding capacitance." When operating at high frequencies, this capacitor charges and discharges at an alarming rate. Losses are generated during capacitor charging and discharging. The more times it is charged and discharged in a given time, the greater the loss.
(3) Skin effect
(4) Proximity effect
(5) Local hot spots
When a conventional conversion transformer operates at a high frequency, there is a local hot spot in the middle of the core. Therefore, in order to reduce the thermal effect, when the operating frequency of the conventional converter transformer is increased, it is necessary to reduce the magnetic flux density and increase its volume accordingly. This makes it impossible to use it for high power density power supplies.
For a low output voltage ideal converter, its step-down ratio is very high. When using a conventional transformer, usually 1 turns of the output winding, about 32 turns of the primary winding. In this way, the primary winding needs to be arranged in multiple layers, so that the leakage inductance and the large capacitance between the windings, the skin effect and the serious proximity effect are all present in the conversion transformer.
3 conventional conversion transformer and flat panel transformer comparisonA conventional transformer is usually composed of a single core multi-primary winding, and a flat transformer is composed of a single turn (or several turns) of primary windings and multiple cores. These cores are equipped with a single turn of the secondary winding and packaged into a module, as shown in Figure 1.
(1) The conventional converter transformer has a large leakage inductance because of its large number of windings, and the single-turn (or several turns) of the flat-panel transformer has a tight coupling with the primary winding of the single-turn, so the leakage inductance is very high. small. The leakage inductance of the 30A flat panel transformer is only 2.0nH. Therefore, when it is used in a fast switching circuit, not only the loss is small, but also the stress on other components in the circuit can be alleviated.
(2) The frequency characteristics of the flat-panel transformer are better than those of the conventional transformer. Flat panel transformers can operate between (100 and 500) kHz frequencies. (3) The flat-panel transformer can be directly attached to the bottom plate, so its heat dissipation condition is very good. This special transformer is a small component with a large surface area. So it does not have the problem of local hot spots.
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