Analysis of the High-Frequency Circuit of Flyback Converter

Abstract：frequency loop of Flyback Convertor, the author discusses how to select wiring strategies and proper components to reduce the negative effect of high-frequency loop towards the function of convertors. The author also probes into ways to balance the adopted wiring strategies and other design strategies.

Keyword：Convertor, High-frequency loop, Wiring strategy

1. Introduction

The noise problem of switching power supply has always been a headache for engineers, such as prototype not working, audio noise, output oscillation or instability, subharmonic oscillation, protection circuit always malfunctioning, abnormal output ripple and abnormal noise. To solve these noise problems, engineers generally need to spend a lot of time testing and debugging, modifying the design, or rewiring, which will greatly delay the product development process. If some general methods can be found to prevent or avoid noise problems during the design process, it can greatly speed up the design process.

This article takes the most widely used Flyback converter as an example. Through in-depth analysis of the high-frequency circuit, it explores how to select appropriate devices and wiring techniques to reduce the impact of the high-frequency circuit on the converter performance, and how to balance the conflicts between the wiring measures taken and other design requirements.

2. In-depth analysis of high-frequency circuits

Figure 1 is a simplified topology of the Flyback converter, which mainly includes six high-frequency circuits: (a) is the primary power circuit A, (b) is the secondary high-frequency power circuit B, (c) is the clamping absorption circuit C, (d) is the drive circuit D, (e) is the auxiliary winding circuit E, and (f) is the primary control circuit F.

Figure 1 High-frequency circuits in a flyback converter: (a) primary high-frequency power circuit A; (b) secondary high-frequency power circuit B;

(c) Clamping absorption circuit C; (d) Driving circuit D; (e) Auxiliary winding circuit E; (f) Primary side control circuit F.

2.1 Primary power circuit A

As shown in Figure 1(a), the primary power circuit A consists of capacitor C2, the primary side of transformer T1, transistor Q1, and sampling resistor RS. When the primary transistor Q1 is turned on, the transformer energy is stored.

The components of the high-frequency current ip of loop A are very complex, especially the current spike at the time of turning on (as shown in Figure 2), which couples the high-frequency signals of multiple other loops. What is easily overlooked is that when the primary transistor Q1 is turned on, the current generated by the charging of the transformer parasitic capacitance and the reverse blocking recovery current of diodes D1, D2, and D3 will be coupled to the high-frequency current ip of loop A. The impact on other loops will be detailed later.

Figure 2 Waveform of high-frequency current ip in loop A

Since the current sampling resistor of the control chip is also in loop A, the fast dv/dt and di/dt signals generated by the opening and closing of the primary transistor will have a key impact on the overall circuit operation and EMI. Therefore, the wiring of loop A is crucial, and the loop area should be as small as possible. On the one hand, it can reduce the impact on other loops, and on the other hand, it is also helpful for EMI performance. Due to the large size of the high-voltage electrolytic capacitor C1, it is difficult to get close to the transformer T1 and the transistor Q1, which affects the size of the area of ​​the primary power loop A. In addition, the high-frequency performance of the high-voltage electrolytic capacitor C1 is not good. Usually, a small film capacitor C2 is used in parallel with the high-voltage electrolytic capacitor C1 to reduce the area of ​​loop A.

2.2 Secondary power loop B

As shown in Figure 1(b), the secondary high-frequency power circuit B consists of the secondary side of transformer T1, rectifier diode D2, high-frequency capacitor C5 and electrolytic capacitor C6. When the primary transistor Q1 is turned off, the energy storage of transformer T1 releases energy to the secondary side through its secondary winding and rectifier diode D2, charging capacitors C5 and C6.

What needs attention in loop B is the wiring loop area of ​​transformer T1 secondary side, rectifier diode D2, high frequency capacitor C5 and electrolytic capacitor C6, especially the loop area between T1 secondary side, D2 and C5 should be as small as possible, which is helpful for the turn-off voltage overshoot and EMI performance of diode D2. Therefore, C5 generally adopts ceramic or film capacitors with good high frequency performance to overcome the influence of parasitic parameters of electrolytic capacitors.

In addition, it is worth mentioning that the reverse recovery current and recovery time of the rectifier diode D2 are usually many times higher than those at normal temperature, even more than 10 times, which will cause high turn-off losses. At the same time, the reverse recovery current will be coupled to the primary A loop during the conduction of the primary transistor Q1, affecting the current sampling signal of the control chip IC and causing abnormal control. Furthermore, high turn-off losses will further increase the temperature rise of the diode, further deteriorating the recovery characteristics of the diode, forming a vicious circle.

2.3 Clamping absorption circuit C

As shown in FIG1(c), the clamping absorption circuit C is composed of the primary side of the transformer T1, the capacitor C3, the resistor R1 and the diode D1, and is used to clamp and absorb the leakage inductance energy of the transformer T1 and reduce the turn-off voltage spike of the transistor Q1.

In the design, the turn-off performance of diode D1 is easily overlooked. When the converter works in the current continuous mode, the turn-off performance of diode D1 has a particularly prominent impact on the converter performance. When transistor Q1 is turned on, the input voltage on C1 and C2 plus the clamping voltage on C3 will be added to diode D1 together, causing a large reverse recovery current (as shown in Figure 2), resulting in a large turn-off loss of D1. If a fast recovery diode with poor recovery characteristics is selected, the temperature rise of D1 will be very high, even at an ambient temperature of 25°C, it can even exceed 125°C. Many manufacturers have learned a lot in this regard.

Therefore, in the current continuous mode, a fast recovery diode with good recovery characteristics must be selected instead of an ordinary fast recovery diode such as FR107. It is recommended to select an ultra-fast recovery diode with a reverse recovery time of less than 75nS.

2.4 Auxiliary winding circuit D

As shown in FIG. 1( d ), the auxiliary winding loop D is composed of the transformer auxiliary winding Na, the diode D3 and the capacitor C7 .

Diode D3 is often a switching diode (1N4148) or a Schottky diode. Due to the fast turn-off characteristics of these diodes, high-frequency oscillations far higher than the switching frequency are easily generated, which will affect the EMI performance of the converter and even couple to the secondary side through the transformer winding to generate additional radiation. Therefore, loop D is required to have a minimum loop area, and sometimes a resistor is required to be connected in series with diode D3 to suppress high-frequency oscillations.

2.5 Primary control loop E

As shown in FIG1( e ), the primary control loop E is composed of a control chip IC, a bypass capacitor C8 and a sampling resistor Rs.

There are two points to note about loop E. C8 must be as close to the control chip IC as possible and form a minimum loop with the Vcc and ground pins of the control chip IC. The loop from the sampling resistor Rs to the chip feedback end needs to avoid coupling with loop A and be connected to loop A using a single-point grounding method.

2.6 Drive circuit F

As shown in FIG. 1f , the driving circuit F is composed of a control chip IC, a gate driving resistor Rgs, a transistor Q1 and a sampling resistor Rs.

When the primary transistor Q1 is turned on, the gate of transistor Q1 needs to be charged, and a large current spike will be generated in the drive loop F. This current spike will be coupled to loop A, such as the peak of the high-frequency current ip in Figure 2, and its size depends on the gate resistor Rgs and the drive impedance of the control chip. Moreover, this current spike will be directly drawn from the capacitor C8, causing the Vcc voltage of the control chip IC to fluctuate instantly, resulting in an impact on the feedback loop operation and the chip being shut down by mistake. Therefore, the gate drive of transistor Q1 often adopts asymmetric drive, that is, it is slow to turn on and fast to turn off.

If the bypass capacitor C8 is not close to the control chip IC, the turn-on peak current required when the transistor Q1 is turned on will cause the voltage of the control chip IC to drop instantly, causing the control chip IC to restart, the transistor to oscillate in the Miller effect region during the turn-on process, or feedback control to be abnormal.

3. High-frequency circuit wiring techniques

The routing of high-frequency loops requires attention to the high-frequency loop area, ground wire and its routing, impedance of vias, and mutual coupling between loops.

⑴ Loop coupling

Loop coupling is the most important thing to pay attention to during wiring. For example, in the above-mentioned Flyback high-frequency loop, placing the primary control loop E into the primary power loop A will cause obvious coupling interference, thus causing the converter to work abnormally. Therefore, when wiring, coupling between loops should be avoided as much as possible. Single-point grounding is usually a common method to avoid loop coupling.

⑵ Single point grounding (2)

Single-point grounding is also called "Y" type grounding. The single-point grounding method of the high-frequency circuit of the Flyback converter mentioned in this article is shown in Figure 3. However, in actual wiring, there are usually some devices that are shared by multiple circuits. For example, the current sampling resistor RS in Figure 1 is a common device for the primary power circuit A, the primary control circuit E, and the drive circuit F. In this case, a single-point grounding connection can be made through the pad of the sampling resistor RS to minimize the coupling between the circuits, as shown in Figure 4.

Figure 3 Single-point grounding of flyback converter Figure 4 Single-point grounding of shared devices

⑶ Ground wire

The ground wire is the key to high-frequency loop wiring. It not only affects the normal operation and electrical performance indicators of the converter, but also affects the electromagnetic compatibility (EMC) performance of the converter. Therefore, a large area of ​​grounding is usually used to reduce the grounding impedance, which can also play a role in electromagnetic shielding. In the power adapter (Adapter), a whole grounded PCB is often used for shielding, or the power adapter is wrapped with a grounded metal sheet to achieve uniform heat dissipation and electromagnetic shielding. In double-sided boards and multi-layer boards, large-area grounding can be achieved through the entire layer of the ground plane, which will also be of great help to uniform heat dissipation.

When large-area grounding cannot be achieved, try to ensure that the width of the ground wire is >2.54mm. Otherwise, it can only serve as an electrical connection, and the high-frequency grounding impedance of the ground wire will be very high, which will not serve the purpose of grounding.

In addition, the ground line should be avoided as much as possible through holes and jumpers. When the ground line conflicts with other wiring, the ground line should be given priority to avoid single vias and jumpers.

⑷ Loop area

On the basis of ensuring electrical insulation, the loop area should be as small as possible, which can reduce coupling to other loops on the one hand and improve the EMI characteristics of the converter on the other hand. Figures 5 and 6 show examples of high-frequency loop areas on single-sided boards, where the area surrounded by black lines and arrows is the loop area. The components of the two are exactly the same, but the loop area of ​​Figure 4 is much larger than that of Figure 5.

Figure 5 Large loop area Figure 6 Small loop area

⑸ Vias

Vias are often used in multilayer boards, and their parasitic parameters have a great impact on high-frequency ground impedance. Via parasitic parameters include parasitic capacitance and parasitic inductance, and the empirical formula is shown below.

Where: T is the PCB thickness, e is the dielectric constant of the board, D1 is the via pad diameter, D2 is the pad area diameter, h is the via length, and d is the via diameter.

At a frequency of 100MHz, the impedance of a 0.254mm conventional via can reach 0.64Ohm. If it is used for grounding and there is a 1A current, it will produce a 0.64V voltage drop, affecting the grounding effect and even the operation of the converter. If this via is on the ground line, it will have a great impact on the ground impedance of the converter EMI.

Therefore, when routing, try to avoid grounding through vias. If grounding through vias cannot be avoided, multiple vias can be connected in parallel, and the via diameter can be increased to reduce the ground impedance.

4. Weighing conflicts with other design requirements

After in-depth analysis of the high-frequency loops in the Flyback converter, it can be found that wiring is crucial to the performance of the converter. The high-frequency loop area is reduced by single-point grounding to avoid coupling and mutual interference between the above high-frequency loops. However, many other design requirements will be encountered during the actual wiring process, making it difficult to fully implement these measures at the same time. The following are common practical problems and corresponding countermeasures.

⑴ Component volume

For example, the volume of transformer T1, the volume of high-voltage electrolytic capacitor C1, and the volume of the heat sink of transistor Q1, and considering electrical insulation, these devices must be kept at a certain distance, resulting in a larger loop area. When the distance between these devices cannot be shortened, you can consider reducing the loop area by paving copper on the PCB, as shown in Figure 5; or use jumpers to achieve the effect of a double-sided board on a single-sided board. For multi-layer boards, you can consider paving the entire layer to reduce the loop area, while reducing the impact of the skin effect and proximity effect.

⑵ Mechanical structure requirements

Usually, products have housings, connectors or cables. In order to match these structural parts, they have some special requirements for the layout of internal components, which results in the components in the high-frequency circuit not being placed according to the minimum circuit area. In this case, you can refer to the similar method of Figure 5 and reduce the circuit area by copper plating on the PCB.

⑶ Consideration of thermal balance

If we only consider the loop area, we need to place many heat-generating devices very close together to generate local hot spots, such as transistor Q1, transformer and secondary diode D2. If we consider the product's temperature rise, we need to place the heat-generating devices in the position where they can dissipate heat the most easily, but that will prevent the devices in the high-frequency loop from being placed according to the minimum loop area. In this case, we can refer to a similar method as Figure 5 and reduce the loop area by copper plating on the PCB, while ensuring the heat dissipation of the heat-generating devices.

⑷ Safety regulations and electrical insulation requirements

Safety regulations and electrical insulation requirements prevent many wirings from being too close together, resulting in the failure to minimize the loop area of ​​the high-frequency loop. Jumpers and trenching can usually be used to resolve the conflict between safety regulations and insulation distance requirements and loop area.

⑸ Requirements for production process

Usually, the input and output wires or sockets of many power supplies often need to be manually soldered. However, in order to reduce the loop area, there are many large areas of copper on the PCB. Although it can help the components to dissipate heat, it will also cause cold soldering of the manual solder joints. For SMT devices, it is also easy to cause cold soldering phenomena such as tombstones. To solve the problem of cold soldering, you can use a flower soldering pad as shown in Figure 7.

Usually, each company uses different production equipment, and has more or less corresponding manufacturability specifications (DFM guidelines). There are many regulations for plug-in, surface mounting and panelization processes, so wiring still needs to refer to these specifications.

Figure 7: Flower pad to prevent cold solder joints

When faced with the above conflicts, it is often impossible to have both, so it is necessary to take solutions step by step according to the priority, or even make some sacrifices in some aspects. Usually, safety regulations and electrical insulation are the first to be met, followed by mechanical structure requirements, and thermal balance and production process are considered later.

5. Conclusion

Through the above in-depth analysis of the high-frequency circuit of the Flyback converter, we can start with the high-frequency circuit and then take corresponding measures, such as wiring techniques and suitable devices, to reduce the impact of the high-frequency circuit on the converter performance.

When the measures taken conflict with each other, by weighing them according to priority, it is possible to design a product with good cost performance and conducive to mass production.

References

[1] Lin Weixun, Modern Power Electronics Technology, January 2006, China Machinery Industry Press.

[2] Qian Zhaoming, Cheng Zhaoji, et al., Fundamentals of Electromagnetic Compatibility Design and Interference Suppression Technology for Power Electronics Systems, Zhejiang University Press, December 2000.

[3] Lang Weimin, “Introduction and Application of Surface Mount Assembly”, September 1, 2007, China Machinery Industry Press.

About the Author

Huang Minchao, male, graduated with a doctorate in power electronics from Zhejiang University in 1998, during which he conducted research on solar micro high-frequency link inverters. He worked as the R&D manager of IBO Power Supply (Hangzhou) Co., Ltd., engaged in the development of DC/DC modules and high-power density energy-saving power adapters; senior engineer of the Power Electronics Laboratory of General Electric Global R&D Center, developing coil drive power supplies for MRI imaging devices; deputy general manager of engineering for the Asia-Pacific region of Shenglang Power Electronics Co., Ltd., engaged in the research and development of medical power supplies. He is currently the general manager and senior consultant of Shanghai Zhengyuan Consulting Co., Ltd., engaged in technical consulting and R&D of difficult issues such as power electronics products, EMC and reliability, and provides R&D design system and team building services. ■