Switched-mode power supplies (SMPS) are widely used in modern electronic equipment, providing efficient, reliable and lightweight power conversion. SMPS transformers play a critical role in this process, stepping up or stepping down the input voltage and transferring power from the primary winding to the secondary winding. The design of the transformer, including the position of the lead wire, can significantly affect the performance and reliability of the SMPS. In this article, we will discuss how the lead wire should be positioned when designing an SMPS transformer.
Understanding the Basics of SMPS Transformers
Before we delve into the details of lead wire positioning, let’s review some basics of SMPS transformers. An SMPS transformer consists of a primary winding and one or more secondary windings. The primary winding is connected to the input voltage, and the secondary winding(s) are connected to the load(s) or output voltage(s). The transformer transfers energy from the primary winding to the secondary winding(s) through magnetic coupling.
To achieve high efficiency and power density, SMPS transformers are designed to operate at high frequencies, typically ranging from a few tens of kilohertz to a few megahertz. The high-frequency operation reduces the size and weight of the transformer and other components in the SMPS. However, it also presents some challenges in terms of electrical and magnetic performance and losses.
One of the critical design parameters of an SMPS transformer is the turns ratio, which determines the voltage conversion ratio between the primary and secondary windings. The turns ratio is calculated as the ratio of the number of turns in the secondary winding to the number of turns in the primary winding. For example, a turns ratio of 1:5 means that there are five turns in the secondary winding for every turn in the primary winding, resulting in a step-up voltage conversion ratio of 5:1.
Another important design parameter is the core material and geometry. The core material is typically a magnetic material, such as ferrite or powdered iron, that enhances the magnetic coupling between the windings and reduces core losses. The core geometry, including the shape, size, and cross-sectional area, determines the magnetic flux density and the energy transfer efficiency.
The lead wire, which connects the transformer windings to the external circuit, is also an essential design element. The lead wire position affects the parasitic capacitance and inductance of the transformer, which can affect the high-frequency performance and electromagnetic interference (EMI) emissions. Therefore, it is essential to consider the lead wire position carefully when designing an SMPS.
Factors to Consider for Lead Wire Positioning in SMPS transformers
When designing an SMPS transformer, there are several factors to consider for lead wire positioning, including the following:
1.Separation between primary and secondary leads
One of the critical safety considerations in transformer design is the isolation between the primary and secondary circuits. The lead wire position should ensure that there is sufficient clearance and creepage distance between the primary and secondary leads to prevent electrical arcing and breakdown. The clearance is the shortest air distance between two conductive parts, while the creepage distance is the shortest surface path along the insulating material between two conductive parts. The required clearance and creepage distance depend on the voltage level, the pollution degree, and the material properties.
2.Parasitic capacitance
The lead wire position can affect the parasitic capacitance of the transformer, which is the capacitance that exists between the windings and the core or the external circuit. The parasitic capacitance can cause high-frequency losses, ringing, and EMI emissions. The capacitance is proportional to the surface area of the winding and the distance between the winding and the core or other conductive parts. Therefore, the lead wire should be positioned to minimize the winding surface area and the distance to the core or other conductive.
Principles of positioned earth leads in SMPS transformers
In a switching power supply, the ground wire is the bottom branch of the four current loops. It serves as the common reference point and is used to control interference in the circuit. In SMPS transformer layouts, the placement of ground wiring should be carefully considered, as mixing various grounds will result in unstable operation.
Earth leads positioned principles as follow:
- The wiring direction of the welding surface should follow the circuit diagram. This is because, during the production process, it is common to test various parameters on the welding surface; it simplifies inspection, debugging, and maintenance of the welding surface.
- Wiring diagrams should have as few turns as possible, and the line widths on the printing arc should not change suddenly. The wire corners should be 90 degrees, and the lines should be simple and straightforward.
- A printed circuit cannot have cross circuits, and two methods can be used to avoid them: drilling and wrapping. In special cases, even if the circuit is very complicated, it is also allowed to use wires to simplify the design. Jumpers can be used to solve cross circuit issues. Single-sided boards have in-line components on top and surface-mount devices on bottom, so in-line devices can overlap with surface-mount devices during layout, but overlapping of pads should be avoided.