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Powering efficiency

How a flyback controller can achieve 90% efficiency and reduce component count.

The market for switch mode power supply ics shows increasing demand for efficient and cost effective solutions that enable slim and compact designs. NXP's GreenChip TEA1733 flyback controller allows 90% efficiency and less than 100mW standby power can be obtained with the minimum of external components.

Suitable for most applications with power requirements of up to 75W, typical applications include netbook adapters, lcd monitors and printer adapters.
The controller can be used in discontinuous conduction mode (DCM) and continuous conduction mode (CCM), with the combination of fixed frequency operation at high output power and frequency reduction at low output power providing high efficiency over the total load range. Frequency jitter is implemented to reduce electromagnetic interference and the device has several protection schemes to enhance robustness and reliability.

A flyback converter is the most popular switched mode power supply topology today and the typical configuration of a flyback topology featuring the TEA1733 is shown in Figure 1. The TEA1733 uses peak current control and the output power is regulated by the CTRL pin. The load is measured and transferred back to the CTRL pin via an optocoupler.



The primary current across an external resistor R1 is sensed through the Isense pin. Its peak is compared and adjusted with an internal voltage, which is proportional to the CTRL voltage. By controlling the peak current, the duty cycle is adjusted accordingly. When the duty cycle is more than 50%, slope compensation is activated to avoid sub harmonic distortion. The maximum duty cycle is limited at 74%.

In low power operation, switching losses are reduced by lowering the switching frequency. An internal voltage controlled oscillator gradually reduces the frequency to zero. To prevent audible noise, the peak current is set to 25% of the maximum as the frequency reduces.

The TEA1733 integrates frequency jitter in order to reduce emi. The centre frequency of 66.5kHz is smeared over ±4kHz by a jitter oscillator whose frequency is chosen to be 260Hz to avoid audible noise.

During start up, the supply voltage Vcc is charged by the current through a start up resistor and the ic's current consumption is around 10µA. No high voltage start up circuit is required. As soon as Vcc reaches its start up level of 20.6V and all other conditions are met, the controller starts to switch. From that moment, the supply voltage is taken over by the auxiliary winding of the transformer. The normal operating current is typically 0.5mA excluding load at the driver pin. Low current consumption helps to increase the efficiency.

Standby operation
Because of the low current consumption and frequency reduction, a standby power of less than 100mW can be achieved by using TEA1733. With proper choice of external components, such as resistors and the X-cap, the measured standby power is 48mW at 115V ac and 84mW at 230V ac for a typical 65W, 19.5V power supply.

If there is an external 'power on/down' signal available to indicate standby mode, standby power can be further reduced to less than 30mW by switching off the entire application. This is referred to as 'zero Watt design. The basic application diagram of 'active off' situation is shown in Figure 2, in which the external 'power down' signal turns high during the standby mode. Then transistor Qx conducts and pulls down the Vinsense voltage, which initiates the restart protection. The ic immediately stops switching and enters power down mode, consuming only 10µA. In the meantime, Vcc is clamped to just below the start up level to guarantee a quick restart after a standby situation.



Protections
TEA1733 features various protections, including input over voltage/under voltage protection, output over voltage protection, over power protection, and internal/external over temperature protection. These lead to either safe restart or latched protection. For safe restart protection, the ic goes into power down mode first, waking up when all conditions are satisfied. In power down mode, the OPTIMER pin is charged to 4.5V, then slowly discharged to 1.2V. In latched protection, the ic also goes into power down mode, but Vcc is clamped to around 6V. In order to reset the latched protection, Vcc has to go to less than a certain level by unplugging the mains. The 6V clamp is just higher than that level to enable fast reset after a latched protection.

A special function of the TEA1733 is that the Protect pin can realise external over temperature protection and output over voltage protection. The Protect pin has a current source capability of 32µA and sink capability of 107µA. The internal circuit tries to regulate the Protect pin to 0.68V, but no protection will be set if the Protect voltage is between 0.5V and 0.8V. If Vcc is so high that the maximum sink current is not able to pull the Protect voltage to less than 0.8V, output over voltage is detected.

With increasing temperature, the resistance of the negative temperature coefficient thermistor connected at the Protect pin decreases. If the maximum source current is insufficient to keep the Protect voltage in excess of 0.5V, external over temperature is detected. Both situations lead to latched protection.

Temporary over load situation is allowed if the Optimer pin is connected as Figure 3. When Isense peak voltage is more than 400mV, a current of 11µA flows out of the Optimer pin to charge external capacitor C2 and the over power timer starts. For long over loads, over power protection is activated as soon as the Optimer voltage exceeds 2.5V. For short over loads, if Isense drops below 400mV before the Optimer voltage reaches 2.5V, C2 will be immediately discharged and no protection will happen. The over power time can be adjusted by choosing different values of C2 and R3.



In fixed frequency CCM, the maximum output power depends not only on the primary peak current, but also on the duty cycle and therefore the input voltage. The TEA1733 has built in input voltage compensation to ensure accurate over power protection that is nearly constant over the full mains. The over power compensation circuit measures the mains voltage via the Vinsense pin and converts it to a current flowing out of the Isense pin. This current generates a voltage across the soft start resistor R2 in Figure 1, which limits the maximum current through the sense resistor R1. By proper tuning of R2, the maximum output power becomes independent of the input voltage.

Conclusion
The TEA1733 is the first product in the new GreenChip series of lower power ac/dc control ics launched by NXP. Its high level of integration makes it possible to meet growing demand for low cost and compact power supply designs. With a standby power of less than 30mW achieved using a 'zero Watt' design, the device's power performance – especially at standby mode – distinguishes it from other controllers for low power computing and communication applications.

Author profile:
Yingying Wang is an ic design engineer with NXP Semiconductors' high performance mixed signal business unit.

Author
Yingying Wang

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