High frequency transformer is the vital component of switch power supply. The transformer realizes the basic conversion of input and output voltages. On the other hand, it also performs high-voltage isolation at the input and output ends to improve user safety. In order to achieve high-voltage isolation of switching power supply through high frequency transformer, the insulation protection is essential.

The insulation protection of high frequency transformer is generally divided into interturn isolation, inter-layer isolation, winding-to-group isolation and core isolation.

1. Interturn isolation

Here's an example, the isolation between the sixth turn and the fifth turn of transformer. In most cases, conventional interturn isolation can be achieved by using enameled wire. The appropriate enameled wire can be selected based on the V/turn (turn voltage) of the designed transformer.

Of course, the insulation layer of enameled wire is easily damaged by high temperature, so in the design process, in addition to the above V/turn, further selection should be based on winding loss and temperature rise.

2. Interlayer isolation

It refers to the isolation between different winding layers of a single winding of high frequency transformer. For example, winding B is wound in three layers, and the isolation between layer 2 and adjacent layers 1 and 3 is called interlayer isolation. According to the self-insulating layer between each layer of the enameled wire, a certain degree of inter-layer isolation can be achieved, and high-temperature insulating tape can be wrapped between the layers to further strengthen the inter-layer insulation.

During the design process, as mentioned above, a reasonable interlayer insulation method can be selected based on V/layer and operating temperature rise. Of course, in order to save the winding window area of the transformer, it is not recommended to wrap insulating tape between each layer when there's no need. This causes the transformer winding to be bulky and increase the difficulty of winding.

3. Winding-to-group isolation

It's the isolation between different windings of high frequency transformer. For example, a transformer contains three windings A, B and C. The isolation between windings AB, AC and BC is called winding-to-group isolation. This isolation achieves input and output isolation of high frequency transformer. To achieve good inter-group isolation, here are things we should take care of.

Isolation between adjacent contact surfaces

If winding B continues to be wound after winding A, insulation protection should be provided between the winding A layer and the winding layer of the starting winding B layer. It's usually done by wrapping multiple layers of high-temperature insulation tape.

The exact number of layers of insulation tape depends on the insulation level between the windings. In the case of high insulation protection levels, insulation between windings can be achieved physically by retaining walls on matching frames. Commonly used insulation levels are 1.5kV, 2KV, 3KV, etc.

Isolation of end faces of adjacent contact surfaces

It's same as AB winding connection and winding. The two end faces of the A and B windings are generally on the same plane, which is not conducive to the isolation between the two windings. Therefore, during the winding process, the end faces of the two windings should be deliberately offset, and winding B can be retracted inward by a few millimeters to achieve this purpose. Of course, it's also necessary to use insulating tape of corresponding width in the winding area to insulate the windings to achieve smooth subsequent windings.

Isolation of winding outlet

In any case, the front end between adjacent windings must be close to or pass through the insulating surface of the two windings, which is often ignored in the actual winding processing process. An insulating sleeve can be added to the lead-out to achieve isolation of the lead-out. At the same time, be careful not to damage the insulating sleeve during the welding process of the lead end and the frame lead pin.

4. Isolation between winding and core

In most cases, the windings are wound on a bobbin, and the isolation between the windings and the core is initially achieved by the bobbin. However, for a full-winding transformer, the iron core window utilization rate is high, and the distance between the outermost winding layer and the iron core is close, which is not conducive to the isolation of the winding and the iron core.

At this time, it is recommended to re-adjust the calculation transformer and focus on optimizing window utilization. If it is difficult to optimize, the insulation tape can be wrapped around the outer layer of the core side posts or inside the frame winding.