Design and application of ultra-low standby power adapter with power less than 75 W

Power adapters are widely used in laptops, game consoles, printers, DSL modems, and mobile phones, and the application scale is very large. Judging from people's usage habits, these devices are also in light load or standby (no-load) working mode for a considerable proportion of the time. Therefore, while "Energy Star" and other standards are committed to improving the working efficiency of power adapters used in these devices, they also focus on improving light load energy efficiency and reducing standby energy consumption.

For example, the U.S. Environmental Protection Agency (EPA) version 2.0 of the Energy Star external power supply specification (EPA 2.0 for short) further improves the energy efficiency requirements based on version 1.1 (see Table 1), where Ln is the natural logarithm of the rated output power.

Table 1: EPA Energy Star External Power Supply Version 1.1 and 2.0 Specifications

The power levels of different adapters vary greatly. According to the requirements of standards such as IEC61000-3-2, power supply with power greater than 75 W needs to add power factor correction (PFC), while there is no such requirement for power below 75 W. This article focuses on the features required for adapters with power below 75 W to meet the new EPA 2.0 regulations, as well as the high-performance, energy-efficient controllers from ON Semiconductor that can provide these required features.

Ways to meet energy efficiency regulations

To meet the above-mentioned requirements for external power supply working efficiency and standby energy consumption, we first need to analyze the source of loss. In fact, the loss during operation mainly includes two aspects, namely switching loss and loss caused by leakage inductance. These two types of losses can be quantified by equations (1) and (2) respectively:

From these two equations, we can see that there are two ways to improve working efficiency: one is to reduce the switching frequency (FSW), that is, to use the frequency foldback technology when the load is light; the other is to reduce the drain voltage (VDRAIN (turn-off)) when turning off, and accordingly, the valley switching technology can be used.

As for standby mode, a significant loss comes from the static loss of the startup circuit , that is, the startup resistor continuously consumes current from the large capacitor , causing power loss. There are many ways to reduce the startup circuit loss, such as using a controller with very low startup current, using an integrated startup current source with very low leakage current when turned off, and connecting the startup circuit to a half-wave rectified AC input.

Key Features of the NCP1237/38/87/88 Controllers

The NCP1237, NCP1238, NCP1287 and NCP1288 are the next generation of fixed frequency pulse width modulation (PWM) controllers from ON Semiconductor for applications that require cost-effectiveness, reliability, design flexibility and low standby power consumption, such as AC-DC adapters for notebooks, LCD monitors, game consoles and printers, as well as consumer electronics applications such as DVDs and set-top boxes (STBs).

This family of devices includes a number of key features that help improve adapter efficiency and reduce standby power consumption. For example, conventional controllers require a startup resistor to start the controller from the rectified AC line voltage, and this startup resistor continues to consume power during normal operation. In contrast, the NCP1237/38/87/88 series controllers have a built-in startup field-effect transistor (FET) that acts as a high-voltage current source. When the input AC voltage is applied to the adapter, this current source powers the controller's VCC capacitor. This high-voltage startup circuit is turned off under normal operating conditions (when the flyback auxiliary winding provides bias voltage to save power), consuming very little power; at the same time, the controller does not require a startup resistor (see Figure 1), helping to reduce standby energy consumption, reduce component count and save board space.

Figure 1: Comparison between with startup resistor and without startup resistor (built-in voltage startup current source)

This series of controllers also uses frequency anti-walking technology and skip cycle mode at light load to reduce the switching frequency at light load, thereby improving energy efficiency; at the same time, the switching frequency is clamped at 25 kHz to eliminate audible noise. In addition, this series of devices provides a variety of protection features, such as dual startup current levels, input undervoltage and main power overvoltage protection, overload protection, dual transition protection thresholds, soft start and latch protection. This series of devices also provides an optional dynamic self-powered (DSS) function, which eliminates the need for auxiliary windings; and has built-in slope compensation, which does not require external settings. Taking NCP1238 as an example, the typical application circuit diagram of this device is shown in Figure 2.

Figure 2: Typical application circuit diagram of NCP1238

Application design steps and key points

1) Power supply Segment design

To apply the NCP1237/38/87/88 series controllers in a design, the power stage must be designed first. Since the power is less than 75 W, a flyback converter is commonly used at this power level. Accordingly, the parameters of the flyback converter components need to be calculated and the appropriate components need to be selected. For example, the output power can be calculated based on the output voltage and output current , and the energy efficiency can be estimated based on the EPA standards. The input power can be estimated by combining the output power and energy efficiency. The average input current can then be calculated, and the maximum capacitance value can be calculated. For detailed calculations of parameters such as capacitors, transformers, inductors, and MOSFETs in the power stage design , see references (1) or (2).

It is worth mentioning that on the secondary side of the power supply, synchronous rectification technology can be considered to significantly improve energy efficiency. In this regard, ON Semiconductor 's NCP4302 synchronous rectification controller can be used. Using a synchronous rectification controller such as the NCP4302 in space-sensitive flyback applications such as adapters, chargers and set-top boxes can significantly improve energy efficiency at very low additional cost. The NCP4302 is already available, and the new NCP4303 synchronous rectification controller will also be available in 2010.

2) Set overload compensation

Overload compensation (OPP) affects the primary peak current. The primary peak current can be calculated using the relevant formula, and then the overload compensation resistor value (ROPP) can be calculated. ON Semiconductor has created an overload compensation spreadsheet to facilitate the user to properly select ROPP and its impact on peak current (Ipeak), transient current (ITRAN), output power (Pout) and transient power (PTRAN).

3) Reduce no-load input energy consumption

In terms of reducing no-load (standby) input energy consumption, in addition to the aforementioned design without startup resistors with built-in startup high-voltage current sources and controllers with frequency retracement and skip cycle modes such as NCP1237/38/87/88, many other approaches or tricks can be adopted, such as reducing transformer leakage inductance, not allowing dynamic self-powered operation, reducing VCC clamping resistance value, reducing switching losses, optimizing clamping circuits , reducing eddy currents by feedback resistor dividers, setting stable operation for all load currents, reducing TL431 bias circuit losses, reducing losses in secondary rectifiers and their buffers, and not using output voltage display LEDs.

4) Magnetic design

The magnetic flux density of the magnetic components should be designed for the peak current and provide some margin (5%) to prevent saturation. In addition, it is necessary to combine the specific design requirements to see whether 100% output current is required. If not, reduce the core size. For example, assuming that the maximum output current is 3.5 A, but this high current is only required under transient conditions, its long-term root mean square (RMS) value is only 1.75 A, and the load factor is only 0.5 (not 1). When designers reduce the core size, they can reduce core and copper losses. Calculations such as transformer core size, winding design, and air gap length can also be found in references (1) or (2).

5) Improve electromagnetic interference

In the adapter design, electromagnetic interference (EMI) may occur in AC line filtering, diode buffers, DC output filters, driver clamps, clamp loops, and power switch loops, so improving EMI is also an important task for design engineers . Some design techniques or methods can be adopted accordingly, such as making the area of ​​all switch loops with RF currents small, using two chokes to separate the input AC filter to reduce the influence of parasitic capacitance coupling, and closing the circuit loop that injects RF current through the transformer. For diode buffers, the buffer resistance should be close to the characteristic impedance of the ringing circuit, and the RC (resistance and capacitance) time constant of the buffer should be small relative to the switching period, but should be long compared to the voltage rise time. EMI can also be further improved from the circuit board layout.

Typical 65 W laptop adapter demo board energy efficiency test results

ON Semiconductor built a demonstration board based on the NCP1237 controller for a typical 65 W notebook adapter (output voltage is 19 V) and optimized it for the EPS 2.0 specification. The relevant energy efficiency test results are shown in Table 2.

Table 2: Test results of working energy efficiency and standby energy consumption of 65 W notebook adapter based on NCP1237

It should be pointed out that this energy efficiency test result is measured on a DC cable with a length of 1.05 meters and a copper cross-sectional area of ​​0.75 square millimeters, which is closer to the energy efficiency test results in the real world. The average energy efficiency of this demo board is as high as 87.32 at 115 Vac and 87.21% at 230 Vac, both of which meet the energy efficiency requirements of EPA 2.0 working mode. As can be seen from the right side of Table 2, this demo board has high energy efficiency even at extremely light loads, and its energy consumption in no-load (standby) mode is far better than the EPA 2.0 specification requirement of no more than 0.5 W.

Summarize

The Energy Star 2.0 external power supply specification has put forward higher requirements for the working energy efficiency and standby energy consumption of applications such as notebooks, LCD monitors, printers and set-top boxes. ON Semiconductor's new NCP1237/38/87/88 series controllers have important functions such as frequency reversal and cycle skipping at light loads. Design engineers can use classic flyback converters based on this series of controllers to meet the Energy Star energy efficiency specification requirements. The energy efficiency test results of the 65 W adapter demonstration board based on NCP1237 show that the average energy efficiency is higher than 87%, and it is possible to provide no-load (standby) energy consumption of less than 300 mW. Under the condition of minimizing power waste in the entire power supply design, it can achieve no-load energy consumption of less than 100 mW, meeting and exceeding the requirements of Energy Star.