Low standby power adapter solution based on NCP1246 and NCP4354

AC-DC power adapters are used in a wide range of applications, such as notebook computers, LCD monitors, and printers. These power adapters are in idle-nosed mode for a long time and are used in large quantities, resulting in huge power losses. The US "Energy Star" standard has clear requirements for the no-load energy consumption of such external power supplies (EPS), that is, the AC-DC EPS no-load energy consumption below 50 W is less than 0.3 W; the power is between 50 W and 250. W's AC-DC EPS no-load energy consumption should not exceed 0.5 W. At present, the best no-load energy consumption performance of the AC-DC adapter with better performance on the market is about 30 mW. The latest adaptation requirement is no-load energy consumption below 30 mW, which requires lowering the output voltage (Vout) to reduce no-load parasitic energy consumption; it also allows the primary-side controller to be turned off to further reduce energy consumption. In this way, during no-load, the adapter output voltage can be automatically reduced, and the output voltage can jump between 19 V and 2.5 V to reduce the regulation power consumption.

The NCP1246 from ON Semiconductor has a no-load energy consumption of less than 16 mW. The low standby energy solution using the NCP1246 + NCP4354 can easily meet the ENERGY STAR requirements for a 65 W power adapter and can achieve 20 Very low standby power consumption below mW.

Low standby energy consumption scheme consisting of two devices

The NCP1246 is a primary side controller that meets the above requirements. The NCP4354 is a secondary side controller with no-load condition detection and can be turned off by the optocoupler NCP1246. It replaces the TL431 and consumes less than 100 μA. The NCP4354 can detect no-load and the added no-load detection circuit in the device compares the "output voltage" (Vout) with the "no load info". In the high switching frequency or high frequency standby hopping cycle mode, both are equal to the load; in the no-load and very low-frequency hopping cycle modes, the two are different; under small capacitance and constant load conditions, the output voltage remains constant, and "" The no-load information "goes down. This way, no power detection is required, no effect on energy efficiency and safety, and the number of external components is small and easy to adjust.

Figure 1 is the process of entering the shutdown mode. When the output current is gradually reduced from full load, the primary controller will switch from normal operation to skip cycle operation, and the output voltage and the no-load information are basically close; when the input is very low-cycle or off mode, the output no-load voltage will be Below the output voltage, a very low load or no load is detected and the shutdown mode is activated.

Figure 1: The process of entering shutdown mode

Figure 1: The process of entering shutdown mode

When the output voltage is greater than the "no load information", the ON/OFF optocoupler turns off the primary side controller; the output voltage is allowed to drop to 2.5 V before the primary side controller is restarted. This allows for very low frequency skip cycle modes with frequencies as low as 0.01333 Hz (75 s skip cycle time).

Characteristics and applications of NCP1246

The NCP1246 is a fixed frequency, current mode controller optimized for minimizing energy consumption under no-load and light load conditions. The device can be dynamically self-powered, built-in active X2 capacitor discharge; can enter the power-saving sleep state, provide output voltage hiccup mode; reduce the frequency under light load conditions, and adopt the skip cycle mode; save the discharge resistor, thus saving Power and components; significantly reduces power consumption under no-load conditions, maintaining high efficiency over the entire line/load range.

In addition, in the event of a severe fault, the NCP1246 can block the input, allowing direct connection to the NTC; timer-based protection can be automatically restored or locked; with high-voltage start-up circuitry; frequency jitter that softens EMI interference over the entire load range; achieves up to 30 V Vcc operation. The NCP1246 market and applications include: AC adapters (laptops, LCD monitors and printers), open-frame PSUs (DVDs, STBs), offline battery chargers, and consumer electronics power supplies.

In order to comply with safety standards, the AC-DC power supply must have the ability to discharge the electromagnetic interference (EMI) filter X2 input capacitor within less than one second after the AC plug is removed. In order to meet this requirement, a series of high-impedance resistors in parallel with the capacitor are often used. However, the resistor-type drain consumes about 25 mW of input energy. When considering the no-load input energy consumption, the 25 mW energy consumption is relatively Larger. ON Semiconductor's solution uses an active X2 capacitor discharge to detect the disconnected AC connection and discharge the capacitor through the switch. The NCP1246 HV pin contains an AC line pull-out detector. If the timer expires, the input is DC, the discharge circuit is triggered, and the X2 capacitor is discharged, as shown in Figure 2.

Figure 2: NCP1246 AC Line Dial Out Detector

Figure 2: NCP1246 AC Line Dial Out Detector

The device's shutdown mode is triggered by the signal from the secondary side to the NCP1246 feedback pin. The NCP435x uses the same optocoupler to provide this signal for the feedback network; during no load, the NCP435x drives the optocoupler, pulls down the feedback (FB) pin, and turns off the NCP1246. The NCP1246's shutdown mode has two distinctive features, one with a low-power shutdown mode and the other with an optocoupler-triggered shutdown mode.

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