Designing with a fixed on-time controller to optimize the light-load efficiency of switching power supplies

Due to its high efficiency and high power density, switching power supplies are becoming more and more popular in modern electronic systems. In particular, with the application of control chips, the circuit design of switching power supplies has been greatly simplified. Often, only some peripheral devices need to be added to the pulse width modulation (PWM) control chip to form a switching power supply, which further promotes the design and development of switching power supplies. In terms of types, switching power supplies mainly include two types: AC-DC converters and DC-DC converters. The former converts the input 50/60 Hz AC power into DC voltage through rectification, filtering and other steps, while the latter is widely used to convert and distribute DC power in the system.

Depending on the topology, DC-DC converters include different types such as buck, boost, buck-boost, flyback, forward, push-pull, half-bridge (HB) and full-bridge (FB). Different types of DC-DC converters have different characteristics and often have different application fields. For example, buck, boost, and buck-boost converters are very suitable for low-voltage control applications that do not require electrical isolation, while flyback converters are very suitable for multi-output, high-voltage power applications. The offline switching power supplies used in these applications work from the 110 V/220 V main power supply and achieve electrical isolation by using a transformer to replace the filter inductor.

For offline switching power supplies, low cost is an important goal. For the PWM controller used in it, designers can choose different architectures, such as fixed frequency (FF) and quasi-resonant (QR). For the former, its switching frequency is fixed, its light load efficiency and full load efficiency are both in the normal range, and the operating mode can be continuous conduction mode (CCM) or discontinuous conduction mode (DCM). For the latter, its switching frequency is variable, and its full load efficiency is the best, but at light load, due to the valley jump problem (noise), its operating mode is boundary conduction mode (BCM, also known as critical conduction mode, CRM). In terms of transformer size, the fixed switching frequency architecture is normal, while the quasi-resonant architecture is larger; however, the quasi-resonant architecture has less electromagnetic interference, while the fixed switching frequency architecture is larger. For both architectures, they face the same problem, which is the need to improve energy efficiency under a wider input load range and improve standby energy efficiency.

In addition to these two architectures, the fixed on-time (FON) architecture has attracted more and more attention in the industry in recent years. In this architecture, the peak current remains constant and can be selected by the user; while the switching frequency varies (changing the off time) to provide the required output power, and it provides the maximum output power when the frequency is the highest. The working principle of FON is shown in Figure 1.

Like the fixed switching frequency architecture, the fixed on-time architecture also supports CCM and DCM operating modes. Its output power calculation formula in these two modes is shown in the figure below. As mentioned above, the peak current Ipeak is kept constant by the controller, and the switching frequency Fsw is controlled by the feedback loop. To adapt to different output power requirements, the switching frequency will change to meet the equation in Figure 2. In the absence of loop control (short circuit, start-up), the switching frequency will be clamped.

Under full load conditions, the switching frequency increases until it hits the timing capacitor Ct clamp. Under light load conditions, the peak current decreases and the switching frequency decreases, which limits the problem of audible noise. At light load, due to the decrease in switching frequency, the losses related to switching frequency, such as power MOSFET output capacitance Coss and gate charge loss and leakage inductance loss, will also be reduced. In this way, the energy efficiency of the switching power supply under light load conditions will also be improved. Therefore, we can also conclude that the fixed on-time (FON) controller can significantly improve the energy efficiency of the switching power supply under light load conditions. Figure 3 compares different PWM controller architectures.

NCP1351 fixed on-time controller targeting low-power flyback switching power supply applications

NCP1351 is a high-performance fixed peak current (quasi-fixed on-time), variable off-time PWM controller recently launched by ON Semiconductor, targeting low-power flyback switching power supply applications. Typical end product applications include auxiliary power supplies, printers, game consoles, low-cost adapters and offline battery chargers, which are very cost-sensitive applications.

NCP1351 will reduce the switching frequency when the load is reduced, so that the power supply using NCP1351 can provide excellent no-load energy consumption and optimize power efficiency under other load conditions. When the switching frequency decreases, the peak current will gradually decrease to about 30% of the maximum peak current, thereby preventing the transformer from mechanical resonance, thereby greatly eliminating the risk of audible noise, while maintaining good standby power performance.

NCP1351 includes four different versions: A, B, C and D. The NCP1351 peripheral adjustable timer can continuously monitor feedback activity and protect the power supply in the event of a short circuit or overload. Once the timer expires, the NCP1351 stops switching, with the A version remaining in a latched state and the B version attempting to restart. The C and D versions include dual overcurrent protection limit trip points, allowing the controller to be used in applications where large transient power phenomena occur, such as printers. When a fault is confirmed, the C version latches and the D version automatically recovers.

The internal structure of the NCP1351 reflects an optimized arrangement, with very low startup current, which is a basic parameter when designing low standby power supplies. The negative current sensing technology of the NCP1351 minimizes the impact of switching noise when the controller is operating and allows the user to select the maximum peak voltage flowing through the current sensing resistor. Therefore, its power dissipation can be optimized for specific applications. In addition, the step-down input ripple function ensures a natural frequency tail, making the electromagnetic interference (EMI) signal smoother.

Figure 4 shows a typical application circuit diagram of NCP1351. NCP1351 contains 8 pins, as shown in the figure. Among them, pin 1 is the FB pin, where current is injected to reduce the frequency; pin 2 is the timing capacitor Ct pin, which is responsible for setting the maximum switching frequency when there is no feedback current; pin 3 Cs is responsible for detecting the primary current; pin 4 is the ground pin; pin 5 DRV drives pulses to the power MOSFET; pin 6 is the Vcc pin, which provides a voltage of up to 28 V for the controller; pin 7 is the latch pin, where a positive voltage above 5 V completely latches the controller; pin 8 is the timer pin, which sets the duration before the fault is confirmed. GreenPoint ^TM 40 W Printer Power Supply Reference Design

Based on NCP1351

As mentioned above, the NCP1351 controller is very suitable for applications such as printer power supplies. ON Semiconductor provides a wealth of design resources for NCP1351, including application notes such as "40W Printer Power Supply Design AND8278", "50W Adapter Power Supply Design AND8263", "12W Adapter Power Supply Design", "Modeling Using PWM Switching Technology", and evaluation boards such as "40W Rated/80W Peak Power Printer Board" and "57W Adapter Board". ON Semiconductor also provides GreenPointTM 40W Printer Power Supply Reference Design. In addition, ON Semiconductor also provides some design and development tools, such as NCP1351 inductance calculation data sheet, and simulation tools such as Spice models (PSPICE and ISPICE).

This article will discuss the application design of NCP1351 in 40W printer power supply, analyze the requirements faced by printer power supply today, and how NCP1351 meets these requirements.

As we all know, with global warming becoming a daily topic and oil prices soaring, the world has begun to understand that the current energy use is not conducive to sustainable development. Many initiatives have emerged around the world around different fields (such as external power supplies, home appliances, etc.). Printers are a natural area for government agencies to get involved in, because they are so widely used and consume so much power, to improve the efficiency of power supplies. There are many initiatives and organizations, located in Japan, South Korea, Germany, Europe and the United States. One of the most active standards initiatives is Energy Star, which has started work on specifications for photocopying equipment. Printers that meet Energy Star requirements should automatically enter a low-power "sleep" mode after a period of inactivity. There are also different Energy Star specifications based on the paper size and color capabilities of individual printers. Keeping the printer in a low-power sleep mode for a large part of the time not only saves power, but also allows the printing device to run cooler and last longer. Energy Star version 1.0 specifications for printers and other related equipment were implemented on April 1, 2007, and the second phase of the specifications will be implemented on April 1, 2009.

The problem is that few existing printer power adapters can meet these current light-load efficiency requirements and no-load standby power requirements, not to mention that more stringent requirements are emerging. In addition, the total cost of printer power adapters must also be extremely low because this is a highly competitive market. Therefore, meeting these energy efficiency and power requirements while maintaining the reliability and performance level of printer power adapters becomes a challenge.

Fortunately, the NCP1351C controller can meet the above challenges. Benefiting from its fixed peak current/variable off time architecture, the power adapter using NCP1351C has high energy efficiency from rated load to light load conditions (including different printer sleep modes) and has extremely low no-load energy consumption. It provides transient peak power while also providing a variety of effective protection functions, such as latch overload, short circuit and overvoltage protection. In addition, compared with the current high-level printer power adapter, the unique architecture of NCP1351C also makes the high-voltage input capacitor used in the printer power adapter designed with it 1/3 lower, thereby saving solution cost and size while providing the same performance. Figure 5 shows the ON Semiconductor 40 W GreenPointTM printer power reference design based on NCP1351C. The specifications of the reference design are as follows:

Input voltage: universal input 85 Vac to 265 Vac, 47-63 Hz

Power supply output voltage:

• 32 V / 1 A
• 16 V / 0.625 A

Peak Power:

• 80 W (32 V / 2.5 A and 16 V / 0 A) for 40 ms
• 62 W (32 V / 1.94 A and 16 V / 0 A) for 400 ms

Energy efficiency requirements:

• > 80 % at full load (40 W)
• Sleep mode (2 W and 4 W) > 70 %
• Input power Pin < 0.3 W under no-load condition

[Summary]: In flyback switching power supplies, there are different architectures for PWM controllers, such as fixed switching frequency and quasi-resonance. These two architectures have their own characteristics, but they both need to improve energy efficiency in a wider power range, especially under light load conditions. In this regard, the fixed on-time (FON) architecture has its unique advantages. ON Semiconductor's NCP1351 is a high-performance current mode PWM controller. It is based on fixed peak current (quasi-fixed on-time) and variable off-time technology. When the load is reduced, the switching frequency can be reduced, so that the power supply using NCP1351 can provide excellent no-load energy consumption and provide higher energy efficiency under light load conditions. It is very suitable for cost-sensitive terminal product applications such as auxiliary power supplies, printers, game consoles, low-cost adapters and offline battery chargers. ON Semiconductor also provides rich design resources for NCP1351, including a high-efficiency 40 W GreenPointTM printer power adapter reference design, to help customers meet increasingly stringent energy efficiency regulations, shorten product development time, and accelerate product launch.