Single-chip switching power supply high-frequency transformer to reduce its loss and suppress audio noise - Database & Sql Blog Articles

1. Introduction

Monolithic switching power supply ICs are known for their high integration, cost-effectiveness, minimal peripheral circuits, and excellent performance. They can form isolated switching power supplies with high efficiency and no power frequency transformer. From 1994 to 2001, international companies introduced various single-chip switching power supply series such as TOPSwitch, TOPSwitch-II, TOPSwitch-FX, TOPSwitch-GX, TinySwitch, and TinySwitch-II. These have now become preferred choices for medium and small power supplies, precision switching power supplies, and switching power modules.

The high-frequency transformer is a critical component for energy storage and transmission in switching power supplies. Its performance significantly affects the power efficiency, technical specifications, and electromagnetic compatibility (EMC) of the power supply. A high-efficiency high-frequency transformer should have low DC and AC losses, low leakage inductance, minimal winding capacitance, and reduced coupling between windings. The following design considerations are outlined below.

2. Reduce High Frequency Transformer Loss

2.1 DC Loss

DC loss in a high-frequency transformer comes from the copper loss of the coil. To improve efficiency, it's recommended to use thicker wires with a current density of J = 4–10 A/mm².

2.2 AC Loss

AC loss is caused by the skin effect of high-frequency currents and core losses. As high-frequency currents flow through the wire, they tend to concentrate on the surface, reducing the effective cross-sectional area and increasing the AC resistance. The penetration depth decreases with the square root of the switching frequency. To reduce AC impedance, the wire radius must not exceed twice the skin depth. The relationship between wire diameter and switching frequency is shown in Figure 1. For example, at f = 100 kHz, the theoretical maximum wire diameter is φ 0.4 mm. However, using thinner wires wrapped around is more practical to minimize the skin effect.

Figure 1: Conductor wire diameter vs. switching frequency

The core loss also reduces the power supply efficiency. The AC flux density can be estimated by:

B_AC = (0.4πN_PI_PK_RP)/2δ

Where B_AC is the AC flux density in Tesla, N_P and I_P are primary turns and peak current, K_RP is the ratio of pulsating to peak current, and δ is the air gap in cm. For continuous mode operation, B_AC should be around 0.04–0.075 T, and ferrite core loss at 100 kHz should be less than 50 mW/cm³.

3. Reduce Leakage Inductance of High Frequency Transformer

Leakage inductance should be minimized during design. Higher leakage inductance increases peak voltage and drain clamp losses, reducing efficiency. For a transformer meeting insulation standards, leakage inductance should be 1%–3% of the primary inductance. To achieve below 1%, the following measures can help:

1. Reduce primary turns N_P.
2. Increase winding width (e.g., use EE core to increase skeleton width b).
3. Increase height-to-width ratio of the winding.
4. Minimize insulation between windings.
5. Improve coupling between windings.

3.1 Reduce Primary Turns and Increase Height-to-Width Ratio

Choosing an appropriate core shape and reducing primary turns while increasing the height-to-width ratio effectively lowers leakage inductance. The amount of leakage inductance is proportional to the square of the primary turns. The selected core should allow the primary winding to be wound into two or fewer layers to minimize leakage and distributed capacitance. Avoid cores with short windows, as they have poor height-to-width ratios and higher leakage. Use slim cores like EE, ETD, EI, and EC for better performance.

Triple insulated wire is a high-performance insulated wire developed recently. It has three insulating layers with a central conductor, and the insulating layer is a golden yellow polyamide film, known as “gold film.” It can withstand several kV pulse voltages with a total thickness of 20–100 μm. It is ideal for high-frequency transformers in micro-motors and miniaturized power supplies. It offers high dielectric strength, eliminates the need for barrier layers, and allows for a smaller volume compared to enameled wire. Using triple insulated wire for secondary windings and common enameled wire for primary and feedback stages significantly reduces leakage inductance and saves space.

3.2 Winding Arrangement

To reduce leakage inductance, windings should be arranged concentrically, as shown in Figure 2. In Figure 2(a), the secondary uses triple insulated wire; in Figure 2(b), all are enameled wires but include a safety margin and reinforced insulation. For multi-output power supplies, the secondary with the highest output should be close to the primary to improve coupling. When secondary turns are few, multiple strands should be evenly distributed around the skeleton. Foil windings are also an effective way to enhance coupling when possible.

(a) Triple insulated wire for secondary

(b) All enameled wire

Figure 2: Winding arrangement

Distributed capacitance in windings causes repeated charging and discharging, leading to energy absorption. This not only reduces efficiency but also forms an LC oscillator, causing ringing noise. To minimize this, keep lead lengths short, connect the primary winding’s start point to the drain, and use part of the primary winding as a shield to reduce coupling with adjacent windings.

4. Suppress High Frequency Transformer Audio Noise

4.1 Suppressing Audible Noise of High Frequency Transformers

The attractive force between the EE or EI cores of a high-frequency transformer can cause displacement, and the forces between winding currents may shift the coils. Mechanical vibrations can also lead to periodic deformation, resulting in audible noise. For single-chip power supplies under 10 W, the noise frequency is typically 10–20 kHz.

To prevent core displacement, epoxy resin is often used to bond the three contact faces of the two cores. However, this rigid connection is not always effective. Excessive glue can make the core brittle under mechanical stress. Recently, a special "glass beads" adhesive has been used abroad. It consists of glass beads and binder in a 1:9 ratio and is cured at 100°C for one hour. This method acts like ball bearings, allowing slight independent movement of each core without affecting overall positioning, thus reducing noise by about 5 dB.

Fig. 3: Internal structure of high-frequency transformer | Fig. 4: Shielding band of high-frequency transformer

4.2 Shielding of High Frequency Transformers

To prevent the leakage magnetic field from interfering with adjacent circuits, a copper shielding tape can be wrapped around the transformer. This acts like a short-circuit ring, suppressing the leakage field. The shielding should be grounded for optimal performance.

5. Conclusion

The design of a high-frequency transformer for a single-chip switching power supply focuses on three key aspects:

1. Minimizing the loss of the high-frequency transformer.
2. Reducing the leakage inductance of the high-frequency transformer.
3. Effectively suppressing audible noise from the high-frequency transformer.