LLC or Flyback Topology? It all depends on the end-user requirements

Many high-efficiency power supplies can be designed using either an active clamp flyback (ACF) converter or an LLC switcher IC to achieve their design goals. Which one should be chosen in actual design? Some design engineers will make corresponding choices based on personal preference, familiarity, and historical experience in some special applications. However, when faced with two or more possible solutions, the selection of the best solution depends on sound engineering reasoning, design requirements, and the priority of product efficiency, size, BOM, power density, design simplicity, and other factors that affect the design.

Power Integrations (PI) offers a full range of highly integrated flyback converters in ultra-thin packages for TV, monitor and high-power charger applications. We also offer a new family of LLC switcher ICs that, when paired with our new HiperPFS™-5 power factor correction (PFC) front-stage ICs, can deliver up to 240W of power output in the same application at very high efficiency.

But for a specific design application, which solution is more suitable, ACF or LLC?

The Ultimate Flyback Solution

The isolated flyback topology is the easiest to implement. Power Integrations offers many flyback controller IC product lines, including the InnoSwitch platform. The key innovation of the InnoSwitch IC is the use of Power Integrations' unique FluxLink™ magnetic inductive coupling technology, which not only enables precise, high-performance secondary feedback control, but also has the advantages of simple circuits and a small number of components that are usually only available in primary feedback control. The application of efficient synchronous rectification technology can ensure extremely high efficiency over the entire load range while having extremely low no-load power consumption. Due to the use of FluxLink, no optocoupler is required in the line, which can ensure that the primary and secondary synchronous rectification switches work together without cross-conduction, thereby greatly increasing reliability. .

Power Integrations offers a wide range of InnoSwitch flyback ICs with integrated silicon, gallium nitride (GaN) and silicon carbide (SiC) power switches for automotive applications at various voltages. For designers who need to achieve the best efficiency and smallest size through flyback solutions, InnoSwitch4-CZ and ClampZero™ active clamp ICs (both use GaN switches) and HiperPFS-5 PFC stages can be used to design USB PD mobile device chargers that are extremely small and can be mass-produced (see Figure 2). For details, see Power Integrations' reference design DER-957.

Figure 2: Ultra-compact and high-efficiency PFC + flyback power conversion solution using HiperPFS-5 with InnoSwitch4-CZ and MinE-CAP technology

The InnoSwitch4-CZ product family uses a thin InSOP-24D package, which integrates a 750V PowiGaN switch, primary and secondary controllers, ClampZero interface, synchronous rectification, and a feedback link that complies with safety standards. The steady-state switching frequency of up to 140kHz reduces the size of the transformer and further improves the power density. Compared with other active clamp flyback solutions, the InnoSwitch4-CZ and ClampZero chipsets can provide up to 95% efficiency and maintain extremely high efficiency under different input voltages, system loads and selected output voltages. This is achieved by controlling the main power switch with zero voltage switching characteristics and the active clamp switch in a variable frequency non-complementary mode. This control method supports both continuous and discontinuous conduction operating modes, which greatly improves design flexibility and maximizes efficiency under all operating conditions. These flyback switch ICs have excellent constant voltage/constant current accuracy and are not affected by changes in peripheral component parameters. While ensuring input voltage detection, safety and protection functions, its no-load power consumption is less than 30mW.

In 2021, Anker, a global mobile device charging expert, announced that its Nano II series USB C chargers are designed based on InnoSwitch4-CZ and ClampZero ICs, and said that the chipset "has extremely excellent integration and efficiency levels, which is the key to the Nano II series' ultra-compact design."

LLC solution that can achieve extremely high efficiency

The above designs are already very compact and sophisticated, so what higher requirements will customers put forward? How to meet these requirements? The answer is a higher level of efficiency, which can be achieved with the help of Power Integrations' HiperLCS™-2 chipset, a half-bridge resonant switch or LLC topology architecture. This two-chip solution allows design engineers to easily create extremely efficient and ultra-compact power supply and adapter designs to gain a key advantage in market competition.

This approach uses resonant switching to eliminate losses during switching transitions, and can even achieve efficiencies that are at least 2% higher than those achieved by the best flyback designs . The HiperLCS-2 two-chip solution consists of an isolation device and an independent half-bridge power device. The isolation device integrates a high-bandwidth LLC controller, synchronous rectification driver, and FluxLink isolation control link; while the independent half-bridge power device uses Power Integrations' unique 600V FREDFET (fast recovery epitaxial diode FET) to provide lossless current sensing and integrate high- and low-side drive circuits. Both devices are packaged in a thin InSOP-24. Compared to discrete LLC designs, this highly integrated, high-efficiency architecture does not require a heat sink, has negligible parameter tolerances between different devices, and can reduce the number of components by up to 40%.

The power supply design based on the new HiperLCS-2 chipset can provide 250W output power with a conversion efficiency of over 98%. It can achieve no-load input power of less than 50mW at 400VDC input, and can provide continuous high-precision output even when no-load, easily meeting the world's most stringent no-load and standby efficiency standards. HiperLCS-2 devices can maintain constant high-efficiency performance over the entire load range, and the power consumption is extremely low. It only needs to be directly conducted through the FR4 PCB board to dissipate heat. In the adapter design with a continuous output power of up to 220W, no heat sink is required, and it can provide 170% of the peak output power for a short time. All HiperLCS-2 series devices have self-powered startup function, and can also provide startup bias power for the PFC power stage implemented using the company's HiperPFS IC. The secondary side detection method can ensure that the adjustment accuracy is less than 1% under different input voltages, over the entire load range, and in mass production. Compared with traditional optocouplers, the safe isolation high-speed digital feedback control using Power Integrations' FluxLink technology can provide faster dynamic response and better long-term reliability.

The specific circuit diagram is shown in Figure 3. DER-672 is a corresponding reference design.

Figure 3: Efficient LLC design using the Power Integrations HiperLCS-2 chipset

The HiperLCS-2 control engine also provides progressive burst mode and control techniques without using more output filtering components to suppress higher output ripple during standby.

If achieving extremely high efficiency is the ultimate goal without considering other factors such as design ease, manufacturability and specific operation, then the LLC resonant converter is undoubtedly the best choice.

The trade-off between most compact design and highest efficiency

250W is just the starting power for LLC converters, which can be used in applications up to several kW. On the other hand, flyback designs are close to their limits at 250W. Output powers above 250W may require a different kind of flyback solution in terms of transformer size and primary switch current (which is mitigated by the ultra-low RDS(ON) of GaN switches). The need for subsequent design and design portability is another factor that engineers must consider when deciding which platform to adopt.

Power Integrations' flyback and LLC solutions are highly efficient and have low BOM count.

For ultimate miniaturization and lowest BOM count, the flyback InnoSwitch4-CZ design is the way to go. For USB PD charger and adapter applications with very wide output ranges (5V, 9V, 12V, 20V, and even 28V), the flyback solution remains the most common choice.

To obtain the best efficiency, you can choose the LLC solution, which is the combination of HiperLCS-2 and HiperPFS-5.

In short, if you want a solution with extremely high efficiency, choose HiperLCS-2 and HiperPFS-5; if you want the easiest to design, most compact and lowest BOM solution, choose InnoSwitch4-CZ, HiperPFS-5 and ClampZero.

Additional content:

Power Factor Correction

Many countries and regions require that power supplies with a rated input power of more than 75W must correct the input current so that its waveform and phase relationship are close to the sinusoidal input voltage. This regulation is to prevent power loss in the transmission line and interference with other devices connected to the AC power supply. This adjustment is called power factor correction (PFC).

At this year's APEC conference in Houston, Texas, Power Integrations launched a new power factor correction (PFC) IC, the HiperPFS-5 series quasi-resonant (QR) DCM PFC IC . It integrates a 750V PowiGaN™ gallium nitride switch, which can provide up to 240W of output power without using a heat sink and achieve a power factor better than 0.98 . Its maximum efficiency can reach 98.3% . This front-end chip supports 110/220VAC input and converts the input into a 400V DC output bus voltage.