1 Introduction

The transformer has been power supply equipment and devices, small size and high power density, a stumbling block to realize the modular. Although the high frequency conversion technology into the power supply, you can get rid of the huge volume of power frequency transformer, but also need to use a ferrite core of the high-frequency transformer. Ferrite core high-frequency transformer is smaller than the power frequency transformer, but it is far from the requirements of the modular. It is not only the volume is too large, and it is the amount of heat, electricity leakage is not small. So in recent years, many experts, scholars and engineers have been studying ways to solve this problem. The successful development of high frequency flat plate transformer makes a leap in the transformer technology. It not only can reduce the size of the transformer a lot, but also make the transformer internal temperature rise is very low, leakage inductance is very small, the efficiency can be achieved 99.6%, the cost is lower than the average power of the transformer. It can be a single end forward and flyback, used for AC/DC and half bridge, full bridge DC/DC converter and push-pull converter. It is especially suitable for low voltage and high current converter. So it is particularly appropriate to use it as a modern computer power source.

2 the problem of conventional transformer in the operation of high frequency

(1) leakage inductance (referred to as leakage)

The ideal transformer (fully coupled transformer) primary side winding generated by the magnetic flux should be all through the secondary side winding, without any loss and leakage. But in fact the conventional transformer can not be achieved without any loss and leakage. The magnetic flux generated by the primary winding can not all pass through the secondary side winding. The non coupling part of the magnetic flux in the winding or conductor has its own inductance, stored in this "inductance" of the energy and the main power transformer circuit coupling. This inductance we call "leakage"". The requirements of an ideal transformer for insulation and to obtain a low electromagnetic interference (EMI) and the need for a very tight electromagnetic coupling to reduce the leakage inductance of the requirements, is mutually contradictory.

When the transformer is switched off (when the power is switched off or the switch is off), the energy of the leakage is released to form a distinct noise. Can see the noise of high frequency spike pulse waveform on the oscilloscope. The amplitude of the high frequency spike pulse waveform is proportional to the product of the relative time variation rate of the Uspike and the leakage inductance Lleak. That is:

|Uspike|=Lleakdi/dt (1)

When the operating frequency is increased, the change rate of the current relative time is also increased. The effect of leakage will be more serious. The influence of leakage inductance is proportional to the switching speed of the converter. Leakage of high peak pulse will be damaged by the transformation

Fig. 1 Schematic diagram of conventional transformer and flat transformer

(a) conventional transformer (b) flat transformer

Power devices in the device and the formation of a significant electromagnetic interference (EMI). Buffer network must be added in the converter circuit to reduce the peak pulse amplitude of Uspike. But adding the buffer network will increase the loss of the converter circuit. The converter circuit is improved with the operating frequency, the loss is increased, and the efficiency is reduced.

(2) inter winding capacitance

When the winding of the transformer is a multi-layer winding, there is a potential difference between the top layer and the bottom layer. There is a potential difference between the two conductors. This capacitor is called "inter winding capacitance". When working at high frequency, the capacitor charges and discharges at an alarming rate. Capacitor charging and discharging process will produce loss. In a given period of time, the number of its charge and discharge, the greater the loss.

(3) skin effect

(4) proximity effect

(5) local hot spots

Conventional transformer operating at high frequency, the core of the central will have a local hot spots. Therefore, in order to reduce the thermal effect, when the operating frequency of the transformer is increased, the magnetic flux density must be reduced accordingly, and the volume of the transformer is increased. This makes it impossible to use it to make high power density.

For the low output voltage ideal converter, it is very high. When a conventional transformer is used, the 1 turns of the output winding is usually 32 turns. In this way, the primary winding is required to be arranged in a multi layer, and the leakage inductance and the capacitance between the windings, the skin effect and the proximity effect are all in the transformer.

3 Comparison of conventional transformer and flat transformer

Conventional transformer is usually composed of a single core multi primary winding, and the flat transformer is composed of a single turn (or a few turns) primary winding and a multi core. These cores are equipped with a single turn of the secondary winding and packaged into modules, as shown in Figure 1.

(1) conventional transformers because of its primary winding, so the leakage inductance is relatively large, flat transformer single turn (or turn) secondary windings coupled and single turn primary winding is very tight, so the leakage inductance is very small. 30A flat transformer leakage inductance is only 2.0nH. So when it is used in the fast switching circuit, not only the loss is very small, but also can reduce the stress of other parts in the circuit.

(2) the frequency characteristic of the flat transformer is better than that of the conventional transformer. Plate transformer can work in (100 ~ 500) kHz frequency between. (3) the flat panel transformer can be fixed directly to the bottom plate, so the heat dissipation condition is very good. The special transformer is a small size and large surface area. So it does not have the problem of local hot spots.

(4) because the plate transformer can improve the heat dissipation problem. So it can achieve high magnetic flux density,