Design of DVD switching power supply with very low standby power consumption

Preface:
　　In the current world where energy supply is becoming increasingly tight and calls for energy conservation are growing, energy conservation of DVD, a large consumer electronic product, is inevitably on the agenda. The DVD power supply solution proposed by ON Semiconductor can meet the most stringent standby power consumption standards and will not bring concerns about cost increases. This article introduces how to use NCP1200P40 to design a DVD power supply with full-range input, simple circuit, few peripheral devices and low cost (as shown in Figure 1)

1. Normal mode
　　When the power is turned on, NCP1200 is powered from the AC side by diode D218, and charges capacitor C207 at a rate of 4mA through IC8 pin. When the Vcc voltage reaches 11.4V, NCP1200 starts to work. (This working state is called dynamic self-powered DSS, which is a patented technology of ON Semiconductor*) After the working state is established, the IC power supply will be supplied by the auxiliary winding through D207 and R203, cutting off the internal startup current source to minimize the power loss. Here, ON Semiconductor provides great flexibility for different customer requirements. The high-voltage power supply can be connected from the AC side to
　　the diode, or directly from the high-voltage end to the eight-pin of NCP1200. Directly connected from the high-voltage end to the eight-pin, the rectified voltage is 400V, the charging current is 4mA, the typical value of Cvcc is 10uF, and the charging time is calculated to be 5ms. The IC loss and the driving MOS tube current are set to 1.5mA,
so the fall time is 13.3ms, The total loss is 400*4mA*5ms/(5ms+13.3ms) =437mW. Connecting a diode from the AC side, the average voltage of the eighth foot becomes 2*Vacmax/(, and the loss is further reduced to 278mW. We can also connect an auxiliary winding to raise the VCC voltage to higher than 11.4V, automatically cut off the internal startup current source, and the integrated circuit will be completely powered by this winding, so that the loss will be further reduced (to ensure that the VCC voltage is not higher than 16V, a resistor can be connected in series at the auxiliary winding to solve the problem, as shown in the application schematic diagram). It should be emphasized here that NCP1200 can work normally without an auxiliary winding. This only depends on the customer's application requirements. The
　　DVD switching power supply designed with NCP1200 requires very few peripheral devices, which not only provides convenience for customers to design smaller DVD power supplies, but also reduces costs. Concentrating more functional modules in the IC will improve the stability of the system. The following figure shows the Vds waveform during normal operation.

2. Standby mode
　　As shown in Figure 1, when the MCU gives a standby control signal, that is, STBY=H, Q202 and Q201 are turned on, and the 60V voltage on capacitor C221 charges C222. Since the feedback of TL431 is connected to the 5V_STBY terminal, the feedback current of the photocoupler increases, and the duty cycle is pulled down, so that the voltage of the 5V_STBY terminal is maintained at 5V. Since the original 60V output voltage is adjusted to 5V output, the duty cycle of the switch becomes very small, so all other output voltages also become very small, compressed by about 12 times, and the power consumption of all outputs becomes smaller.
　　What needs to be noted in this DVD power supply is the selection of capacitor C221. Its capacitance value is recommended not to exceed 6.8uF, otherwise there will be a spike on the capacitor when it is turned on, which may cause damage to the transistor Q201. In this example, 4.7uF is selected.
　　When in standby mode, the feedback of the photocoupler becomes very heavy, directly pulling down the FB voltage of pin 2. When the voltage of pin 2 is lower than the set value, the IC enters a working state called SKIP CYCLE. This reduces the loss of the entire power supply. When the power supply enters SKIP In CYCLE mode, all outputs including IC power supply are reduced, and NCP1200 maintains its own power supply through the internal dynamic self-power supply circuit. The figure below shows the Vds waveform in standby mode.
　　The DVD power standby function designed with NCP1200 has very small loss in standby mode, which fully meets the latest IEA recommendations.
　　IEA latest recommendations:
　　Input power range 0.3W-15W
　　2001.1 / 1W 2002.1 / 0.75W 2003.1 / 0.3W
　　Input power range 15W-50W
　　2001.1 / 1W 2002.1 / 0.75W 2003.1 / 0.5W
　　Input power range 50W-75W
　　2001.1 / 1W 2002.1 / 0.75W 2003.1 / 0.75W

three-way overload short circuit protection
　　When the power supply is working in an overload condition, all outputs will be forced to a low potential, no current will flow through the photocoupler U203, the current setting point reaches the highest, and the 2-pin FB voltage also reaches 4.2V. NCP1200 has an internal overload protection circuit, which will be triggered when the 2-pin FB voltage exceeds 4V, and NCP1200 will force a low duty cycle voltage. Because the output voltage is at a low potential under overload conditions, the auxiliary winding cannot supply power, and the voltage Vcc on C207 will drop linearly. When the Vcc voltage drops to 9.8V, the overload condition still exists. To prevent the dynamic self-power supply (DSS) from being triggered, NCP1200 will terminate the drive pulse and put the circuit into standby mode. In this state, the current consumption is typically 350uA. When the Vcc capacitor continues to discharge to a typical value of 6.3V, NCP1200 turns on the current source. When the Vcc voltage reaches a typical value of 11.4V, the IC will start a new cycle. If the fault condition is eliminated before reaching UVLOh, the circuit will work normally, otherwise it will start the next fault cycle. This situation can also be applied to an open loop. When the optocoupler is open, the FB voltage on pin 2 reaches the maximum value of 4.2V to trigger protection, and the process is the same as overload protection.

　　Conclusion

　　NCP1200 is a highly integrated switching power supply control chip with simple design, which shortens the design cycle of power supply engineers. Moreover, this chip has been widely used in different power supply fields, such as laptop chargers, printer power supplies, etc. The unique dynamic self-power supply and SKIP CYCLE technology make NCP1200 also very suitable for DVD green power supply, making a small contribution to the world's energy.