# Innovative Flyback Transformer Design: Enhancing Efficiency and Reducing Harmonic Current

Wai Keung Mo, Kasper M Paasch, Thomas Ebel

► To cite this version:

Wai Keung Mo, Kasper M Paasch, Thomas Ebel. Innovative Flyback Transformer Design: Enhancing Efficiency and Reducing Harmonic Current. The 26th European Conference on Power Electronics and Applications, GDR SEEDS France & EPE Association, Mar 2025, Paris, France. 10.34746/epe2025-0256. hal-05087183

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Submitted on 27 May 2025

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# Innovative Flyback Transformer Design: Enhancing Efficiency and Reducing Harmonic Current

Wai Keung Mo¹, Kasper M. Paasch², Thomas Ebel³

Centre of Industrial Electronics/ Department of Mechanical and Electrical Engineering

University of Southern Denmark / Alsion 2, 6400

Sønderborg, Denmark

E-Mail: wkmo@sdu.dk¹, Paasch@sdu.dk², ebel@sdu.dk³

URL: http://www.sdu.dk

## Acknowledgements

This work was supported by interreg Deutschland-Danmark via the Smart Power Conversion project (ref.16-2,1-221). Find further information on Interreg Deutschland-Danmark on www.interreg5a.eu.

## Keywords

« Active filters », « Harmonics », « Magnetic device », « Simulation », « Winding topology ».

## Abstract

A novel flyback transformer design reduces harmonics and improves efficiency. It uses two auxiliary windings and a curved air gap. Simulations and experiments show significant improvements compared to traditional designs. Future research will focus on optimizing the design for various applications and power levels.

## Introduction

Stringent harmonic current regulations, such as IEC 61000-3-2-2019 [1], pose a significant challenge for AC-DC converters due to their nonlinear behaviour beyond peak power levels, unlike Class C equipment [2]. Traditional designs generate substantial harmonic pollution, negatively impacting power quality and equipment reliability. Power factor correction (PFC) offers effective solutions, categorized as two-stage and single-stage. While two-stage PFC achieves high performance, it suffers from increased complexity and cost. Single-stage PFC circuits (e.g., [3]) address these limitations with a single switch but may experience higher conduction losses due to exceeding DC-link voltage limits. Certain designs overcome this challenge, achieving size and cost reduction [4]. This paper introduces an innovative flyback

transformer design utilizing integrated magnetics for enhanced size and cost efficiency [5]. However, conventional layouts employing separate windings on different core legs suffer from magnetic interference and losses. To address these limitations and improve harmonic current performance, some previous research works propose a "smart winding configuration" and a curved-air gap (CAG) solution shown in fig.1 [6] [7].

<figure><img src="image_2.png" /><figcaption>Fig.1: Single-stage single switch curved air gap PFC flyback converter</figcaption></figure>

The initial version of the integrated PFC flyback converter, depicted in Figure 1, exhibited commendable performance to improve low harmonic current significantly and to achieve reasonable efficiency (85%).

Fig. 2 presents the simulated voltage waveforms across the main switch M1 and Winding N1. While the converter demonstrated satisfactory results, the voltage across M1 exceeded 800V with a peak of 900V, which could potentially contribute to efficiency losses. Additionally, the voltage across Winding N1 reached 1.4kV during the off state of the main switch.
<figure><img src="image_3.png" /><figcaption>Fig. 2: Voltage waveforms across Winding N1 and main switch M1 (Simulation result)</figcaption></figure>

To address the technical challenges encountered in the Curved-air Gap PFC integrated flyback transformer design [7], we propose a novel flyback transformer configuration. Additionally, a comprehensive thermal simulation was conducted to evaluate the thermal performance of this innovative design.

It is important to note that this paper exclusively focuses on the flyback transformer design itself and does not delve into the complete single-stage, single-switch PFC flyback converter topology. A separate paper will be dedicated to a thorough analysis of the converter and its harmonic current performance, supported by experimental validation.

## Advanced version of Single-stage, single-switch (ASS SS) curved air gap PFC flyback converter operation

Figure 3 illustrates an advanced single-stage, single-switch power factor correction (PFC) flyback converter with a curved air gap. This innovative topology incorporates a specialized flyback transformer and high-speed diodes (D7, D8, and D13) to optimize performance. An additional winding (SS), highlighted in red in Figure 3, is connected between winding CC and diode D7. Together, these elements function as an integrated boost converter, effectively increasing the voltage across the primary winding (A and AA) and enhancing the input harmonic current performance.

<figure><img src="image_4.png" /><figcaption>Fig. 3: Advanced version of single-stage single-switch(ASS SS) curved air gap PFC flyback converter</figcaption></figure>

## Integrated flyback transformer

As illustrated in Fig. 4, we have implemented a design enhancement by incorporating an additional winding (SS) between Winding C and Winding AA. This supplementary winding acts as a current snubber, effectively dissipating excessive energy and safeguarding the integrity of the primary windings (A and AA).

<figure><img src="image_5.png" /><figcaption>Fig. 4: Enhanced CAG PFC flyback transformer</figcaption></figure>

The integrated flyback transformer features an innovative design incorporating two auxiliary windings (N1 and C) strategically positioned around the primary boost inductor windings (N and CC). This configuration ensures a stable input current and effectively mitigates harmonic distortion. The carefully optimized turn ratio among N1, C, CC, N, and SS contributes to enhanced performance and efficiency.
<figure><img src="image_6.png" /><figcaption>Fig. 5: Voltage waveforms across new version of innovative flyback transformer (Winding N1 and main switch M1) (Simulation result)</figcaption></figure>

Fig. 5 indicates that the voltage across the main switch M1 slightly exceeds 600V. This suggests potential for reducing power losses associated with switch M1. The voltage across the Winding N1 reaches 880V.

The single-stage, single-switch CAG PFC flyback converter comprises a boost PFC stage, a flyback stage, and an integrated flyback transformer (as illustrated in Figure 3). The integrated transformer features two boost inductors (L1 [Winding N] and L2 [Winding CC] in Figure 4) and a flyback transformer (T1 [Winding A and AA on the primary side, and Winding B on the secondary side] in Figure 4), along with L4 [Winding N1] and L3 [Winding C], shared by both stages. The boost PFC stage draws current from the AC mains and stores it in two boost inductors, improving the power factor by aligning current draw with the AC voltage source. The flyback stage operates similarly to a standard flyback converter, with a single power switch and a transformer combining the two boost inductors. This reduces the overall circuit size and cost.

## 1. Integrated transformer and CAG

The integrated flyback transformer incorporates a CAG, which enables control of the flux density in the ferrite core, creating a variable inductance that maintains CCM operation. The curved magnetic core increases the magnetic path length and reduces core flux density. This CAG solution effectively prevents core saturation and enhances transformer efficiency. All windings are placed on the center leg of a PQ-type core, and the CAG acts as a copper shield to improve EMI noise reduction. Additionally, two auxiliary windings are included: the first auxiliary winding (L4 [Winding N1] and L3 [Winding CC] in Figure 3) is designed to reset the energy stored in the flyback transformer [8]. The secondary auxiliary

windings (L1 [Winding N] and L5 [Winding SS] in Figure 4) not only further reset stored energy but also function as a clamped winding to reduce leakage energy during the flyback primary switch's off period.

## 2. Energy flow and current snubbers

Fig. 6 depicts the energy flow from the parasitic output capacitance of the power MOSFET during the off state to the current snubber inductor (Winding N1). The inductor L4, L5, and diodes D13, D8 can be used in combination to create two current snubbers that help suppress voltage spikes and reduce electromagnetic interference (EMI) caused by sudden current changes within the circuit.

<figure><img src="image_7.png" /><figcaption>Fig. 6: Energy flow between two current snubbers and Power Switch (off state)</figcaption></figure>

The inductor windings, N1 and SS, are designed with appropriate inductance values to determine the energy storage capacity within the magnetic field. Diodes D13 and D8 are typically selected as ultrafast diodes, their choice influenced by the circuit's switching speed. These diodes must respond promptly to voltage spikes, providing an effective discharge path for the inductor's stored energy.

The formula to calculate the inductance of WindingN1 is as follows:

$$L_{N1} = \frac{C_{oss}((k_1(V_{in} + \frac{N_p}{N_s}(V_o + V_f)))^2 - V_{in}^2)}{I^2} \quad (1)$$

Where, $L_{N1}$ is the inductance of WindingN1 in Henry (H), $V_{in}$ is the input voltage in volts (V), $N_p$ is the number of primary winding turns, $N_s$ is the number of secondary winding turns, $V_o$ is the output voltage, $V_f$ is the forward voltage of the output rectifier, $k_1$ is the voltage ratio of $V_{in}$ and Voltage spike, $C_{oss}$ is the output capacitance of the
power Mosfet at off state, I is the current passing through from C<sub>oss</sub> to Winding N1.

## Optimal inductance

<figure><img src="image_8.png" /><figcaption>Fig.7: Choosing L<sub>p</sub> for harmonic current requirements by a single switch, single stage CAG PFC flyback transformer.</figcaption></figure>

Fig.7 shows the process of selecting L<sub>p</sub> for harmonic current requirements in the single switch, single stage CAG PFC flyback transformer.

The minimum L<sub>p</sub> required for this converter can be calculated using equation(2) as follows:

$$L_{req}(\text{min}) \geq \frac{\eta D(1+\frac{k}{2})V_{in}(\text{min})^2}{k f_s P_o} \quad (2)$$

It should be noted that the desired primary inductance must be greater than L<sub>req(min)</sub>, which is inversely proportional to P<sub>o</sub>. Therefore, there are two workable solutions to meet the harmonic requirements.1. For small P<sub>o</sub> (100% dimming), a large L<sub>req(min)</sub> is required, which demands a large inductor volume. 2. For large P<sub>o</sub> (no dimming), a small L<sub>req(min)</sub> is required.

## Transformer winding consideration

To maintain the high efficiency of the innovative flyback converter, the efficiency of the flyback transformer is paramount. While litz wire is often a preferred choice for transformer windings, its suitability in this application warrants further investigation due to the curved air gap within the transformer core. Under heavy load conditions, the majority of the ferrite core may saturate, effectively increasing the air gap. This can significantly elevate winding losses in litz wire, particularly at high frequencies. Consequently, disc-type flat windings emerge as a more optimal solution for high-frequency operation.

# 1. Thermal Simulation

Based on the prototype specifications and subsequent thermal analysis under natural air convection cooling conditions, localized hotspots have been identified within the transformer.

<figure><img src="image_9.png" /><figcaption>Fig. 8 Integrated CAG flyback transformer thermal model: ( coil windings temperature profile)</figcaption></figure>

These hotspots are concentrated primarily in the initial portion of the primary windings (Figure 8) and the upper center pole of the PQ ferrite core (Figure 9).

<figure><img src="image_10.png" /><figcaption>Fig. 9 Integrated CAG flyback transformer thermal model: (ferrite core temperature profile)</figcaption></figure>

# 2. Key Findings

Under natural convection cooling conditions, a substantial portion of the winding temperatures exceed 175°C, surpassing the maximum allowable operating temperature of 180°C for standard enamel solid wire insulation (Class H) as
specified in IEC 61558-1-2019. This non-adherence raises potential safety concerns. Furthermore, core temperatures exceeding 130°C adversely affect the effective initial permeability and contribute to increased overall core losses.

## Experimental results

<figure><img src="image_11.png" /><figcaption>Fig. 10: Harmonic current measurement set-up</figcaption></figure>

Fig. 10 illustrates the harmonic current measurement setup for lighting equipment applications, and the prototype unit of the flyback converter is also depicted in Fig. 11.

<figure><img src="image_12.png" /><figcaption>Fig. 11: ASS_SS curved air gap PFC integrated flyback converter</figcaption></figure>

To validate the proposed CAG PFC integrated flyback transformer solution, multiple transformer prototypes were fabricated and tested in conjunction with the ASS_SS PFC flyback converter, operating under the electrical specifications outlined in Table I.

<table><caption>**Table I: Prototype specifications**</caption><thead><tr><th>Curved air gap PFC integrated flyback transformer</th><th>Single stage single switch PFC flyback converter</th></tr></thead><tbody><tr><td>1. Two identical boost inductor specification: N=18 (n<sub>c</sub>=n<sub>cc</sub>=2, n<sub>al</sub>=14, n<sub>as</sub>=2, n<sub>n</sub>=16 note: N=n<sub>al</sub>+n<sub>cc</sub>+n<sub>as</sub>, N=n<sub>as</sub>+n<sub>n</sub>), copper wire diameter =AWG 23</td><td>1. V<sub>in</sub>=230V<br>2. V<sub>o</sub>=20V@I<sub>o</sub>=3A (P<sub>o</sub>=60W)<br>3. f=100kHz</td></tr><tr><td>2. Flyback transformer specification: N<sub>o</sub>=36, copper wire diameter = AWG 22, N<sub>j</sub>=12, triple insulation wire diameter = AWG 20</td><td>4. PWM controller = UC3843B</td></tr><tr><td>3. Ferrite core specification: PQ40/40, material =3C92</td><td></td></tr></tbody></table>

Figure 12 illustrates the voltage waveform across the main switch M1 (blue) at 550V and the corresponding gate-drive voltage waveform (orange) at 18V.

<figure><img src="image_13.png" /><figcaption>Fig. 12: Voltage Waveforms During 3A Output Current Operation: Main Switch M1 and Gate Drive</figcaption></figure>

Figure 13 presents the voltage waveforms observed across windings N, CC and C with the parameters set as follows: n<sub>n1</sub> = 14, n<sub>c</sub> = n<sub>cc</sub> = 1, and n<sub>n</sub> = 16. Under these conditions, Winding N experiences a peak voltage of 115 V. Windings C and CC both exhibit peak voltages of 18V.

<figure><img src="image_14.png" /><figcaption>Fig. 13: Voltage waveform at different windings (N<sub>1</sub>, CC, C and N, SS) in case of (n<sub>n1</sub>=14, n<sub>c</sub>=n<sub>cc</sub>=1, n<sub>n</sub>=16, n<sub>ss</sub>=1) (Simulation result)</figcaption></figure>
Figure 14 presents the voltage waveforms observed across windings N1 and SS, with the parameters set as follows: $n_n=16$, $n_{ss}=1$. Under these conditions, Winding N1 experiences a peak voltage of 880 V. A peak voltage of 650 V is measured across Winding SS, occurring simultaneously with the minimum voltage level of Winding SS and the maximum voltage level of Winding N1.

<figure><img src="image_15.png" /><figcaption>Fig. 14: Voltage waveform at different windings (N1 and SS) in case of ($n_{n1}=14$, $n_{ss}=1$) (Simulation result)</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>Fig. 15. Power measurement results</figcaption></figure>

The flyback converter unit exhibited an input power level of 66.55W and an output power level of 60W in the absence of an EMI filter solution, resulting in an overall efficiency of 90% as illustrated in Figure 15. However, the input power level increased to 68W when the EMI filter solution was incorporated, leading to a final efficiency of 88%. Additionally, the power factor was measured at 0.957.

<figure><img src="image_17.png" /><figcaption>Fig. 16. Current harmonic bar chart ($P_o=60W$, Power factor = 0.95)</figcaption></figure>

Figure 16 presents the current harmonic bar chart, ranging from the fundamental to the 40th harmonic order. The accompanying diagram illustrates the input voltage and current waveforms for a power output of 60W and a power factor of 0.957.

<figure><img src="image_18.png" /><figcaption>Fig. 17: Input Voltage and Current Waveforms under Output Power = 60W and Power Factor = 0.957 Operating Conditions</figcaption></figure>

Figure 17 illustrates the input voltage ( light blue ) and current ( red ) waveforms. Figure 18 presents the conducted emission measurement results obtained without an EMI filter solution. Despite an overall efficiency of 90%, the conducted emission performance was unsatisfactory. The average emission level across the frequency range of 150kHz to 30MHz exceeded the EMI requirements, reaching levels above 70dBuV in the worst case.
<figure><img src="image_19.png" /><figcaption>Fig. 18: Conducted Emission Measurement Results: No EMI Filter, 60 W Output Power =60W, Input Voltage=230 Vrms</figcaption></figure>

Figure 19 demonstrates substantial improvements in conducted emission measurements achieved through the implementation of the EMI filter solution. However, the net efficiency decreased to 88%.

<figure><img src="image_20.png" /><figcaption>Fig. 19: Conducted emission measurement with EMI filter solution</figcaption></figure>

## Conclusion

The innovative CAG PFC integrated flyback transformer design represents a significant advancement in enhancing harmonic current performance and power efficiency. Moreover, this solution offers advantages in terms of reduced size, weight, and cost compared to conventional approaches employing separate boost and flyback transformers. However, the conducted emissions without an EMI filter solution require further attention to ensure

optimal performance and compliance with regulatory standards.

## References

[1] IEC, EN61000-3-2:2019, EMC-Part3-2: Limits for harmonic current emission(equipment input current ≤16A per phase).

[2] M.Grotzbach, @Line side behavior of uncontrolled rectifier bridges with capacitive DC smoothing," in Proc.Eur.Conf.Power Electron.Appl., 1989,pp.761-764.

[3] R.Redl, L.Balogh, and N.O. Sokal, " A new family of single-stage in isolated power factor correctors with fast regulation of the output voltage," Proc.IEEE Power Electron.Spec.Conf., 1994, pp.1137-1144.

[4] C.Qiao and K.M.Smedley," A topology survey of single-stage power factor corrector with a boost type input-current-shaper," in Proc. IEEE Trans. Power Electron., 2001, vol.16,pp.360-368.

[5] L.Huber and m.M.Jovanovic, " Single-stage, single-switch, isolated power supply technique with inout current-shaping and fast output-votlage regulation for universal input-voltage-range applications.," in Proc.IEEE Appl Power Electron.Conf., 1997, vol.1, pp.272-280

[6] W.K.Mo, KM Paasch, T.Ebel, " Single-stage Single Switch curved air gap PFC integrated flyback transformer design", 2023 17th CPE-POWERENG.

[7] W.K.Mo, KM Paasch, T.Ebel, " Single-stage, single-switch curved air gap power factor correction integrated flyback transformer design for lighting equipment power applications, EPE 2023.

[8] Jong-Jae Lee and Jung-Min Kwon, " Single-Stage Single-Switch PFC Flyback Converter Using a Synchronous Rectifier, " IEEE Transactions on Industrial Electronics, 2008, vol.55, no.3, pp 1252-1364
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