LLC or flyback topology? Totally depends on end needs

By: Aditya Kulkarni, Senior Product Marketing Manager, Power Integrations

Many high-efficiency power supplies can be designed using active clamp flyback (ACF) converters or LLC switch ICs to achieve their design goals. In the actual design, which one should you choose? Some design engineers make choices based on personal preference, familiarity, and historical experience of past usage in a particular application. However, when faced with two or more possible solutions, the choice of the best solution depends on sound engineering reasoning, design requirements, and priorities for product efficiency, size, BOM, power density, design simplicity, and other Some factors that affect the design.

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

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

Power factor correction

Many countries and regions require a power supply with a rated input power exceeding 75W, and the input current must be corrected so that its waveform and phase relationship are similar to the sinusoidal input voltage. This regulation is to prevent power loss from transmission lines and interference with other equipment connected to the AC power source. This adjustment is called power factor correction (PFC).

At this year’s APEC conference in Houston, Texas, Power Integrations introduced a new power factor correction (PFC) IC, the HiperPFS-5 family of quasi-resonant (QR) DCM PFC ICs. It integrates a 750V PowiGaN™ gallium nitride switch, which can deliver up to 240W of output power without the use of a heat sink, achieving a power factor of better than 0.98. Its highest efficiency can reach 98.3%. This pre-chip supports 110/220VAC input and converts the input to 400V DC output bus voltage.

Figure 1: Using HiperPFS-5, a QC DCM boost PFC circuit can be designed with a very high level of integration,

And HiperPFS-5 is the industry’s first IC with built-in PowiGaN boost switch

Power Integrations’ HiperPFS-5 PFC IC uses an innovative quasi-resonant (QR) discontinuous conduction mode (DCM) control technique to adjust the switching frequency for different loads, input voltages and each line cycle. DCM control in QR mode reduces switching losses and allows the use of smaller PFC inductors and lower cost boost diodes. Compared with the traditional critical conduction mode (CRM) boost PFC circuit, the variable frequency engine can reduce the size of the boost inductor by more than 50%. Low switching and conduction losses (further reduced due to PowiGaN switching) coupled with lossless current sensing enable the HiperPFS-5 IC to deliver very high conversion efficiency over the entire load range. And no-load power consumption is only 38mW.

Rugged 750V GaN switches are also useful in regions around the world where input voltage surges may occur due to unstable mains supply. When using conventional silicon MOSFETs, rising input voltage can cause component failure. The HiperPFS-5 IC can maintain high power factor at different input voltages (up to 305VAC) and can operate continuously during input voltage dips up to 460VAC. In addition, the HiperPFS-5 IC integrates Power Integrations’ X-capacitor auto-discharge (CAPZero™) capability, including redundant pins required to meet safety regulations, and high-voltage auto-start – all on a low-profile InSOP™-T28F surface mounted in a power package.

By pairing the HiperPFS-5 IC with Power Integrations’ flyback or LLC switch ICs, the DC bus voltage can be converted to the desired output of 24V, 19V, 12V, 5V, etc. As a result, design engineers can easily meet the most stringent efficiency standards and eliminate heat sinks, while cutting the bill of materials in half, creating compact ultra-fast chargers and High power USB PD adapter.

Ultimate flyback solution

An isolated flyback topology is the easiest to implement. Power Integrations offers many product families of flyback controller ICs, including the InnoSwitch platform. A key innovation in InnoSwitch ICs is the use of Power Integrations’ unique FluxLink™ magnetic inductive coupling technology, which enables accurate, high-performance secondary feedback control with the simplicity of wiring and components normally associated with primary feedback control. advantage of small numbers. The application of high-efficiency synchronous rectification technology can ensure extremely high efficiency in the entire load range, and at the same time, it has extremely low no-load power consumption. Due to the use of FluxLink, there is no need for optocouplers in the circuit, which can ensure that the primary and secondary synchronous rectifier switches work together, and no cross-conduction phenomenon occurs, thereby greatly increasing reliability. .

Power Integrations offers a wide variety of InnoSwitch flyback ICs that integrate silicon, gallium nitride (GaN), and silicon carbide (SiC) power switches for automotive applications at various voltages. For design engineers who need the best efficiency and smallest size from a flyback scheme, the InnoSwitch4-CZ and ClampZero™ active clamp ICs (both GaN switches) and the HiperPFS-5 PFC stage can be used to design a small footprint of USB PD mobile device chargers that can be sold in mass production (see Figure 2). See Power Integrations’ reference design DER-957 for details.

Figure 2: Ultra-compact and high -efficiency PFC + flyback power conversion scheme using

HiperPFS-5 with InnoSwitch4-CZ and MinE-CAP technology

Housed in a thin InSOP-24D package, the InnoSwitch4-CZ product family integrates a 750V PowiGaN switch, primary and secondary controllers, ClampZero interface, synchronous rectification, and a safety-compliant feedback link. Steady-state switching frequencies of up to 140kHz reduce transformer size, further increasing power density. Compared to other active clamp flyback schemes, the InnoSwitch4-CZ and ClampZero chipsets deliver up to 95% efficiency and maintain very high efficiency over varying input voltages, system loads and selected output voltages. This is achieved by controlling the non-complementary mode of frequency conversion for the main power switch with zero voltage switching characteristics and the active clamp switch. This control method supports both continuous and discontinuous conduction modes of operation, greatly increasing design flexibility and maximizing efficiency under all operating conditions. These flyback switch ICs have excellent constant voltage/constant current accuracy and are not affected by changes in the parameters of peripheral components. Under the premise of ensuring input voltage detection, safety and protection functions, its no-load power consumption is less than 30mW.

In 2021, global mobile device charging specialist Anker announced that its Nano II series of USB C chargers are based on the InnoSwitch4-CZ and ClampZero ICs, and said the chipset “has an exceptional level of integration and efficiency that the Nano II series enables. The key to an ultra-compact design.”

A very efficient LLC solution can be achieved

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

This scheme uses resonant switching to eliminate losses that occur during switching transitions, and can even achieve efficiencies that are at least 2 percent 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 isolated device integrates a high-bandwidth LLC controller, synchronous rectifier driver, and FluxLink isolated control link; Lossless current detection, and also integrated upper and lower transistor drive circuits. Both devices are available in a thin InSOP-24 package. Compared to discrete LLC designs, this highly integrated and efficient architecture eliminates the need for heat sinks, has negligible parameter tolerances between different devices, and reduces component count by up to 40%.

The power supply design based on the new HiperLCS-2 chipset can provide an output power of 250W 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 maintain constant high-efficiency performance over the entire load range with extremely low power dissipation, requiring only direct conduction heat dissipation through the FR4 PCB, eliminating the need for heatsinks in adapter designs up to 220W continuous output power, and capable of short-term 170% peak output power is provided. All HiperLCS-2 family devices feature self-powered start-up and also provide start-up bias power for PFC power stages implemented using the company’s HiperPFS ICs. The secondary-side sensing method ensures less than 1% adjustment accuracy at different input voltages, over the entire load range, and in mass production. Safely isolated high-speed digital feedback control using Power Integrations’ FluxLink technology provides faster dynamic response and better long-term reliability than conventional optocouplers.

Figure 3: Efficient LLC Design Using Power Integrations HiperLCS-2 Chipset

The HiperLCS-2 control engine also offers progressive burst mode and control technology, which eliminates the need for more output filtering components to suppress higher output ripple during standby.

If the ultimate goal is to achieve extremely high efficiency, without considering other factors such as design simplicity, producibility and specific operation, the LLC resonant converter is undoubtedly the best choice.

The trade-off between the most compact design and the highest efficiency

250W is just LLC converter starting power, it can be used for applications up to several kW. On the other hand, the flyback design is approaching its limit at 250W. Output power beyond 250W may require a different kind of flyback scheme in terms of transformer size and primary switch current requirements, which are mitigated by the ultra-low R _DS(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.

Both Flyback and LLC solutions from Power Integrations are very efficient and have a low BOM count.

For extreme miniaturization and lowest BOM count, the flyback InnoSwitch4-CZ design is an option. For USB PD charger and adapter applications with very wide output ranges (5V, 9V, 12V, 20V, even 28V), the flyback scheme is still the most common choice.

For optimum efficiency, an LLC solution is an option, that is, a combination of HiperLCS-2 and HiperPFS-5.

In short, choose HiperLCS-2 and HiperPFS-5 for extremely efficient solutions; choose InnoSwitch4-CZ, HiperPFS- 5 and ClampZero.