Laptop AC power adapter design

Laptops are widely used and the market size continues to grow rapidly. Accordingly, the market for laptop power adapters is also very impressive. Users often require high performance, small size or low weight laptops at a reasonable price. For power adapter designers, it is necessary to choose a suitable controller to develop high-efficiency, integrated with rich protection features, and compact adapters.

Fortunately, ON Semiconductor has introduced the new NCP1250/NCP1251 fixed frequency 6-pin pulse width modulation (PWM) flyback controllers, which perfectly meet the needs of designers, enabling them to develop high-performance, high-power density power converters for notebook/netbook power adapters and can be used in applications such as low-power open-frame power supplies for DVD or set-top boxes (STBs).

Laptop power adapter requirements

Judging from the usage of most users, laptops are in light load or standby conditions for a considerable period of time. Compared with improving energy efficiency under 25%, 50%, 75% or 100% load conditions, reducing energy consumption and improving energy efficiency under extremely low load conditions or even standby conditions is more challenging. This requires the power controller to have excellent light load or standby energy consumption performance.

In addition, AC-DC adapters for notebooks are also required to have the following protection features:

.Short circuit protection

(SCP): Must be able to withstand a sustained output short circuit without damage. When the fault disappears, the adapter must be able to recover from protection mode and resume supplying rated power.

.Overvoltage Protection (OVP): In the event that the loop is broken, such as the optocoupler is damaged or the TL431 voltage divider network is affected, the adapter must stop working immediately and remain in this state until the user restarts the adapter.

Over-Temperature Protection (OTP): If the temperature of the adapter exceeds a certain temperature value, the adapter is at risk of being damaged. To avoid this, a thermal sensor is used to continuously monitor the temperature and shut down the adapter continuously if the temperature exceeds the limit set by the designer. When the user restarts the power supply and the temperature drops, the adapter resets.

Over-Power Protection (OPP): For some power supplies, it is important that under worst-case conditions, such as when the load draws too much current, the maximum output current remains under control without actually causing a short circuit.

NCP1250/1 key features and functions analysis

The NCP1250/1 is a fixed frequency PWM controller in a very small 6-pin TSOP package. In addition to its extremely small size, it also offers many advantages that even other higher-end controllers may not have. In the simplest application (5 functional pins), the NCP1250/1 is very suitable for offline power supplies with compact design and minimal protection functions. Since there is a sixth multi-function pin available (see Figure 1), it can also be used for higher-level applications to drive the AC-DC adapter of a notebook or netbook. Unique features such as non-dissipative over-power protection make the NCP1250/1 an excellent choice for high-performance, high-density applications.

Figure 1: NCP1250 typical application example.

The NCP1250/1 operates with peak current mode control, switching at a fixed frequency (65 kHz or 100 kHz) when the converter provides rated power. The NCP1250/1 has a frequency retrace function, and when the power drops to about 20% of the rated value, the frequency usually decreases linearly to 26 kHz, when the load becomes lighter. When the frequency reduction is completed, the device enters the skip cycle mode. This function enables the power adapter to provide high energy efficiency over the entire load range, meeting the needs of designers who focus on energy efficiency performance, especially when the output load is low. It is worth mentioning that the NCP1250/1 still maintains low frequency jitter even when operating in frequency retrace mode, thereby helping to naturally weaken the electromagnetic interference (EMI) signal, even when the converter does not provide full output power. The NCP1250/1 provides almost all the necessary protection functions and perfectly meets the above AC-DC adapter requirements. The only difference between NCP1250 and NCP1251 is that NCP1251 provides integrated OVP function on Vcc pin, while NCP1250 does not provide OVP function on Vcc pin. Below we will discuss the various protection functions of NCP1250/1 and their applications.

1) Short circuit protection

NCP1250/1 ensures short-circuit protection by monitoring the current sense signal on pin 4. If the maximum peak current monitored exceeds the maximum internal current set point (0.8 V/Rsense), the built-in 100 ms timer is started. If the current sense signal drops below the maximum internal current set point, the timer is reset. If the 100 ms timing cycle of the timer is completed, it means that the fault has existed for more than 100 ms, all drive pulses are stopped immediately, and the current consumption of the controller is reduced to about 1 mA. It is worth mentioning that both NCP1250 and NCP1251 include A version and B version, where A is the automatic recovery version, that is, it automatically recovers after 100 ms, and B is the fully latched version.

2) Overpower protection

In terms of over-power protection, traditional technology belongs to dissipative OPP, because the OPP circuit still works in standby mode, which affects the light load and standby energy efficiency performance of the adapter. NCP1250/1 uses a unique non-dissipative OPP solution, which only requires the use of 2 resistors and has no effect on light load energy efficiency performance.

Figure 2: NCP1250/1 applies non-dissipative OPP, with no impact on light-load performance.

3) Overvoltage protection

As shown in Figure 1, when the optocoupler in the circuit is disconnected or the TL431 voltage divider network suffers from severe drift (or one of its resistors is missing or has the wrong value), the output voltage may exceed the specified limit value, which is an overvoltage condition. In most cases, the overvoltage condition may be harmful to the downstream load, and the adapter must be completely shut down at this time. The NCP1250/1 also integrates a 3 V reference comparator on the OPP pin, which can be used for various functions such as OVP. If the OPP pin voltage exceeds 3 V four times in a row, the controller IC latches for protection.

In addition, as mentioned above, the NCP1251 differs from the NCP1250 in that it also provides an integrated OVP function on the Vcc pin, which latches off once Vcc exceeds 25.5 V, protecting the controller.

4) Over temperature protection

In many designs, it is necessary to protect the adapter from thermal runaway (i.e., the temperature inside the adapter housing exceeds a certain value). We can apply simple OTP by connecting a negative temperature coefficient (NTC) resistor (R14) in series with a Zener diode (D10) (as shown in Figure 3). As the temperature increases, the NTC resistance begins to decrease and increases the voltage at pin 3. When the voltage level at pin 3 reaches 3 V, the device simply latches and requires a reset and then restart.

Figure 3: Simple implementation of OTP using the OPP network.

Example of 65 W adapter design using NCP1250

ON Semiconductor has designed a 19 V/65 W demonstration board based on the NCP1250 controller. The rated output current in this design is 3.2 A, and it provides over-power protection (3.8 A@100 Vac, 4.1 A@265 Vac), over-temperature protection, and over-voltage protection on the Vcc pin, see Figure 4 (a) and (b).

Figure 4(a): Circuit diagram of a 19 V/65 W adapter based on NCP1250

Figure 4(b): 65 W power adapter demonstration board based on NCP1250

For the detailed design of this 65 W demonstration board, including component selection and related calculations, please refer to Reference 2. In addition to this 65 W demonstration board, ON Semiconductor also provides a 40 W demonstration board with 12 V/3.3 A output, which also provides over-power protection, over-temperature protection, and over-voltage protection on the VCC pin. Energy efficiency test results

The 65 W demo board (with 1.2 m long cable) shows high efficiency under common load conditions such as 25%, 50%, 75% and 100% (see Table 1). For example, the average efficiency under 230 Vrms is as high as 88.67%. In addition, the demo board also has excellent energy efficiency performance when no-load, consuming only 40 mW of power. Due to the use of frequency foldback technology and skip cycle mode, the light load efficiency is also excellent. For example, under 230 Vrms conditions, the input power is only 0.74 W, 0.86 W and 0.98 W when outputting 0.5 W, 0.6 W and 0.7 W respectively.

Table 1: Energy efficiency test results of a 65 W NCP1250-based power adapter demonstration board.

The 40 W demo board has average efficiencies of 87.8% and 87.2% at 115 Vrms and 230 Vrms respectively (with load applied to the board connector), and no-load input powers of 33 mW and 75 mW at 100 Vrms and 230 Vrms respectively, again providing high efficiency and low energy loss.

Summarize:

NCP1250/1 integrates key features required for high-density power adapter applications, such as non-dissipative over-power protection, ability to apply over-temperature protection, small package (TSOP6) and Vcc pin overvoltage protection (OVP) (NCP1251 only). While providing high operating efficiency, NCP1250/1 uses frequency retracement technology and skip cycle mode to achieve extremely low energy consumption in light load or standby mode, thereby providing high energy efficiency over the full load range. In addition, NCP1250/1 uses frequency jitter technology to minimize EMI even during frequency retracement, thereby providing excellent EMI performance. The 65 W adapter demonstration circuit board test based on NCP1250 proves the high energy efficiency performance of this device over the full load range. Customers can use the NCP1250/1 controller to develop high-performance adapters for notebook computers or netbooks.