Problems with conventional transformers at high frequencies

(1) Leakage inductance (abbreviated as leakage inductance)

The magnetic flux generated by the primary winding of an ideal transformer (fully coupled transformer) should all pass through the secondary winding without any loss or leakage. But in reality, it is impossible for a conventional transformer to achieve no loss or leakage. The magnetic flux generated by the primary winding cannot all pass through the secondary winding. The uncoupled magnetic flux has its own inductance in the winding or conductor, and the energy stored in this "inductance" is not coupled with the main power transformer circuit. We call this inductance "leakage inductance". The requirements for insulation of an ideal converter and the requirement for tight electromagnetic coupling to reduce leakage inductance in order to obtain very low electromagnetic interference (EMI) are contradictory.

When the transformer is not powered (turned off from the power supply or the switch is in the off period), the energy stored in the leakage inductance will be released to form obvious noise. The high-frequency spike pulse waveform of this noise can be seen on the oscilloscope. The amplitude of the high-frequency spike pulse waveform Uspike is proportional to the product of the leakage inductance Lleak and the relative time rate of change of the current. That is:

|Uspike|=Lleakdi/dt（1）

When the operating frequency increases, the rate of change of current relative to time also increases. The impact of leakage inductance will be more serious. The impact of leakage inductance is proportional to the switching speed of the converter. Excessively high spike pulses generated by leakage inductance will damage the power devices in the converter and form significant electromagnetic interference (EMI). In order to reduce the peak pulse amplitude Uspike generated by leakage inductance, a snubber network must be added to the converter circuit. However, the addition of the snubber network will increase the loss of the converter circuit. As the operating frequency increases, the converter circuit will increase losses and reduce efficiency.

(2) Interwinding capacitance

When the transformer windings are multi-layered, there is a potential difference between the top winding and the bottom winding. When there is a potential difference between two conductors, there is capacitance. This capacitance is called "inter-winding capacitance". When operating at high frequencies, this capacitance will charge and discharge at an amazing rate. The charging and discharging of the capacitor will generate losses. The more times it charges and discharges in a given time, the greater the losses.

(3) Skin effect

(4) Proximity effect

(5) Local hot spots

When a conventional conversion transformer operates at high frequency, there will be a local hot spot in the middle of its magnetic core. Therefore, in order to reduce the thermal effect, when the operating frequency of the conventional conversion transformer increases, its magnetic flux density must be reduced accordingly and its volume must be increased. This makes it impossible to use it as a high power density power supply.

For low output voltage ideal converter, its step-down ratio is very high. When using conventional conversion transformer, usually 1 turn of output winding requires about 32 turns of primary winding. In this way, the primary winding needs to be arranged in multiple layers, so the leakage inductance and inter-winding capacitance are large, and the skin effect and proximity effect are serious.