Design of high frequency transformer for 600W dual-switch forward converter

1. Introduction

Magnetic components are widely used in high-frequency switching converters, mainly in two categories: transformers and inductors. When transformers are used, they can play the role of electrical isolation, voltage step-up and step-down, and magnetic coupling energy transfer; when inductors are used, they can play the role of energy storage, wave smoothing and filtering. And the quality of their performance has an important impact on the performance of the converter, especially the efficiency, volume and weight of the entire device. Therefore, the design of magnetic components is an important part of the design of high-frequency switching converters.

The design of magnetic components in high-frequency switching converters usually involves the reasonable selection of core materials based on the working state of the core, and the correct design and calculation of the core and winding parameters of the magnetic components. However, since there are too many parameters involved in magnetic components, their working state is not easy to be thoroughly understood. Therefore, conventional design methods cannot fully reflect their actual working conditions and consider the influence of other factors, making it difficult to achieve the required performance indicators and meet design requirements.

In view of the importance, necessity and complexity of the design of magnetic components in high-frequency switching converters, the author uses the "Magnetics Designer" software of IntuSOFt to calculate and design according to the actual working conditions of magnetic components, and obtains relatively ideal results. This paper first introduces some issues that should be considered and paid attention to in the design of magnetic components, and gives a specific design method and design process for the high-frequency transformer in the 600W dual-tube forward converter , and finally verifies it through simulation.

2. Key points to consider in magnetic component design

2.1 Transient saturation of the core

At the moment of starting the high-frequency switching converter, due to the double flux effect, the core of its magnetic element may reach saturation instantaneously, thereby generating a large surge current, causing damage to the switch device connected to the magnetic element. Therefore, in order to prevent the core from transient saturation, the following methods can be used: first, reduce the working magnetic induction intensity value, but this will reduce the utilization rate of the core; second, increase the soft start link, reduce the conduction pulse width of the power tube at startup, and then gradually increase the magnetic induction intensity to the steady-state value.

2.2 Winding leakage inductance

The leakage inductance of the winding has a great negative effect on the high-frequency switching converter, affecting its normal operation. For example, when the power switch is turned off, the leakage inductance of the winding is released, generating a voltage spike on the main switch, which increases the voltage stress of the power device; in addition, there are multiple magnetic components in a switching converter, and thus multiple parasitic inductances, causing serious electromagnetic interference (EMI). To reduce the leakage inductance of the winding, the following measures can be taken: first, choose a suitable core structure and shape; second, design the winding into a thin and tall type, increase the winding height, and reduce the winding thickness; third, the winding uses twisted copper wire or wide and thin copper foil to increase the copper occupancy factor; fourth, use a layered cross winding method to make the winding tightly coupled.

2.3 Skin effect

When magnetic components work at high frequencies, the alternating current in the wire will produce a skin effect, that is, the current distribution on the wire cross section is uneven, the internal current density is small, and the current density at the edge is large, which reduces the effective cross-sectional area of ​​the wire and increases the resistance. In order to reduce the influence of the skin effect, the wire diameter should not be greater than twice the penetration depth. 3. Design of high-frequency transformer in dual-switch forward converter

Figure 1 is a circuit diagram of a combined dual-tube forward converter. M1, M2, D1, D2 and the secondary side topology form the 1# dual-tube forward converter, and M3, M4, D3, D4 and the secondary side topology form the 2# dual-tube forward converter. During operation, the control pulse of the 2# converter is phase-shifted by 1800 relative to the 1# converter. The dual converters work alternately, transmitting energy to the secondary side, and feeding back energy to the primary side input power supply through diodes D1, D2 or D3, D4 to achieve magnetic reset of the core.

Figure 1 Circuit diagram of combined dual-switch forward converter

The following is a specific analysis and design of the high-frequency transformer in Figure 1. The circuit parameters are as follows: input voltage VCC = 12v, output voltage Vo = 120v, output current Io = 5A, switching frequency f = 100K, duty cycle D = 0.45, filter inductor Lf = 50uH.

3.1 Magnetic Analysis of High Frequency Transformer

Since the excitation voltage applied to the primary side of the transformer is a unidirectional pulse, the magnetic state of the core works on the local hysteresis loop, as shown in Figure 2. When the power tube is turned on, t∈[0, DT], the positive pulse voltage sequence of the primary side of the transformer is excited, and the magnetic induction intensity B in the core is magnetized from Br to Bm along the local hysteresis loop; when it is turned off, t∈[DT, T], the primary voltage of the transformer is zero, the core is magnetically reset through the diode, and the magnetic induction intensity B is demagnetized from Bm to Br along the local hysteresis loop.

Figure 2 Local hysteresis loop of transformer core

As shown in Figure 2, since the core magnetic state only changes in the first quadrant of the BH plane, the core cannot be fully utilized, the utilization rate is low, and it works in a local hysteresis loop, and the magnetic permeability is also low. Therefore, for the high-frequency transformer in the dual-tube forward converter, a magnetic material with high Bs, high permeability, low Br and low loss should be selected.

3.2 Software implementation of high-frequency transformer and SpICe model

According to the design specifications in Table 1, use the Magnetics Designer software to complete the self-design of the high-frequency transformer . First, select the appropriate core and material. After entering the operating frequency value, the Core Wizard software will automatically select the appropriate core size and optimize the size by the program. Then enter the relevant winding voltage value and current value and other design specifications in the transformer window, and make multiple improvements. Finally, the Magnetics Designer will generate a complete output report of the transformer's electrical characteristics and winding specifications to be handed over to the manufacturer to make the high-frequency transformer or provided to the user as a design reference.

In addition, the Magnetics Designer software can establish the Spice model of the designed high-frequency transformer. This model includes all core losses and copper losses, AC and DC resistances, leakage inductance and magnetizing inductance, and winding capacitance, etc., and is implemented in the form of a sub-circuit. It can be easily grafted with the Pspice circuit simulation software to achieve a perfect simulation. 3.3 Simulation Results

In order to verify the design result of the high-frequency transformer , the combined two-switch forward converter shown in Figure 1 was simulated by Pspice. The high-frequency transformer used the Spice model generated by the Magnetics Designer software. The main simulation waveforms are shown in Figure 3.

Figure 3 Main simulation waveforms of the combined two-switch forward converter

FIG3(a) and FIG3(b) are waveforms of the converter output current and output voltage, respectively. The simulation results meet the design requirements and reflect the start-up process of the converter.

4. Conclusion

Magnetic components are essential key components in high-frequency switching converters. They are responsible for the transmission, storage and filtering of magnetic energy, and their performance parameters have a great impact on the performance of switching converters. Therefore, the importance of magnetic component design is self-evident. The method of self-designing magnetic components using Magnetics Designer software proposed in this article not only reflects the actual working conditions of magnetic components, but also proves the practicality of the design scheme. It has certain guiding significance and strong practical value.