Innovative PFC control solution that significantly optimizes the light-load energy efficiency of offline power supplies

Today, power supply designers are faced with many challenges, not only to achieve higher energy efficiency goals, but also to meet the requirements of faster product launch. In terms of achieving higher energy efficiency goals, power supply design must not only take into account full load efficiency, but also need to evaluate the efficiency under conditions such as 10%, 20%, 50% and 75% load. Power supply designers also face many other challenges, such as new power supplies may be more prone to audible noise, reliability and safety must be enhanced, and the launch process and safety certification time must be shortened.

ON Semiconductor's innovative PFC solution to meet the challenge of high energy efficiency

As a global leading supplier of high-performance and energy-efficient silicon solutions, ON Semiconductor continues to develop innovative technologies and products, providing the market with a wealth of power semiconductor solutions, including a powerful PFC product lineup and subsequent products (Figure 1), enabling power designers to continuously develop energy-efficient power solutions. Among them, ON Semiconductor's latest NCP1611 PFC controller uses an innovative current controlled frequency foldback (CCFF) method to drive the PFC boost stage, with a power factor close to 1, a high drive capability of -500 mA / +800 mA, a Vcc range from 9.5 V to 35 V, and features such as non-latching and overvoltage protection, voltage detection, soft start, and overcurrent limiting.

Figure 1: ON Semiconductor’s PFC product lineup.

The NCP1611 active power factor correction (PFC) controller is suitable for boost pre-converters in AC-DC adapters, flat-panel TVs and lighting ballasts, and other medium-power offline applications. The controller uses the patent-pending CCFF architecture. In this mode, the circuit operates in CrM mode when the inductor current exceeds a programmable value. When the current is below this preset level, the current is zero (null), and the NCP1611 can linearly reduce the frequency to approximately 20 kHz. CCFF maximizes rated load and light load efficiency. In particular, standby losses can be minimized. The controller has a series of powerful protection features that can properly handle a variety of power supply operating and fault conditions. The NCP1611 expands the advantages of traditional CrM PFC controllers. Figure 2 is a typical application circuit diagram of the NCP1611.

Figure 2: Typical application circuit diagram of NCP1611.

As an enhanced PFC controller, NCP1611 adopts current control frequency reverse CrM mode and skip cycle mode to optimize efficiency in the entire load range, achieve better light load efficiency, and very powerful safety features.

The unique key features of NCP1611 include: dynamic response enhancer for fast line/load transient response; wide Vcc range up to 35 V with gate voltage clamping function; startup current typical value of 20?A, maximum 50?A (A version Vcc startup voltage 10.5 V; B version is 17 V); line range detection function adjusts and optimizes loop gain; A version provides soft start function, B version can use smaller Vcc capacitor, easy to start; valley conduction function is conducive to providing optimal energy efficiency and generating very low electromagnetic interference (EMI).

In terms of safety, NCP1611 has Vcc undervoltage lockout (UVLO) and line input undervoltage (BO) protection; provides overcurrent protection (OCP) under the condition of inductor saturation or bypass diode short circuit; output overvoltage protection (soft OVP and fast OVP) and undervoltage protection (UVP); feedback open circuit shutdown and ground open circuit fault monitoring; and overheating shutdown. Figure 3 shows the voltage regulation operation of NCP1611.

Figure 3: Voltage regulation operation of NCP1611.

In addition, NCP1611 also has other features, such as fast load transient characteristics, internal 14 V gate voltage clamp with a maximum Vcc of 35 V, smooth startup soft start (version A), powerful open circuit and pin short circuit protection, thermal shutdown, etc. NCP1611 can also achieve voltage regulation, easily solve open circuit and short circuit pin faults, and improve safety; even if the ground pin is open, the component can be protected.

Due to the above excellent characteristics, the market and application of NCP1611 mainly cover large flat-panel TVs, computer power supplies, high-power adapters, LED lighting and ballasts, as well as PFC applications with power greater than 300 W.

Detailed explanation of CCFF architecture and comparison with CrM architecture

As shown in Figure 4, the CCFF architecture timer developed by ON Semiconductor only controls the dead time, and uses the timer to adjust the dead time corresponding to the current level. The retrace frequency is limited to >20 kHz, which has leading performance in the market.

Figure 4: Current-controlled frequency flyback (CCFF) architecture.

Specifically, the CCFF architecture has fixed on-time control and frequency foldback characteristics. At high currents, the circuit operates in critical conduction mode. At low currents (close to the line zero crossing at heavy loads and at full sine square waves at light loads), the next cycle does not start immediately after the core is reset; instead, the timer starts adjusting the dead time; the smaller the current, the longer the timer duration (dead time); the timer duration depends on the size; the timer only controls the dead time (not the switching cycle/off time). Since the dead time is not affected by changes in the current cycle length, valley conduction can be performed without hesitation.

Figure 5: Demonstration board energy efficiency comparison (red: CCFF with skip cycle mode; green: CCFF with skip cycle function disabled; purple: CrM)

The biggest benefit of using CCFF control architecture is to improve energy efficiency. With traditional CrM (critical conduction mode)/BCM (boundary line conduction mode), the switching frequency increases when the load decreases; when the load is extremely low, the controller may enter the "skip cycle mode" and generate audible noise. With CCFF control architecture, the switching frequency can be reduced when the load decreases to reduce power loss; when the load is light, the controller can clamp a lower frequency higher than the audible noise frequency band; when the load is extremely low, the skip cycle mode is used (it can be easily turned off). Therefore, this valley conduction can further improve energy efficiency and reduce electromagnetic interference. Figure 5 compares the energy efficiency of NCP1611CCFF PFC and traditional CrM PFC under different load conditions. As can be seen from the figure, under 10% light load conditions, the energy efficiency of the demonstration board based on NCP1611 with skip cycle mode is as high as nearly 97% (nearly 96% when the skip cycle mode is turned off), while the energy efficiency of the demonstration board based on the traditional CrM architecture is only nearly 87%, which is nearly 10%. It can be seen that NCP1611 performs particularly well in improving the light-load energy efficiency of power supplies.

summary:

The NCP1611 PFC controller uses a novel and patent-pending control technology - current-controlled frequency reversal, operates in critical conduction mode/discontinuous conduction mode (DCM), and has valley switching, which can provide excellent energy efficiency under a wide operating power range, and can provide high power factor and good total harmonic distortion (THD) performance under a wide load range. Compared with traditional CrM PFC controllers, this novel PFC controller has higher fault handling capabilities, better transient response, and can flexibly support different bias situations. It is worth mentioning that the NCP1611 PFC controller is specially optimized, especially for applications such as flat-screen TVs, power adapters, high-efficiency computer power supplies, and LED driver power supplies.