Research on high power factor and low no-load loss AC/DC power supply

Many electrical devices are still powered when not in use. From the moment the AC plug is connected, they are constantly consuming power, which creates the problem of "standby loss". The standby loss of an appliance is generally only a few watts, but the total standby loss of a large number of appliances is a number that cannot be ignored.

In addition, reducing the power consumption of the circuit under light load has become a development direction of power electronics technology today. The L5991A used in this article is the latest energy-saving control chip from ST. Under light load, the PFC function is turned off and the switching frequency is reduced to reduce the loss of the circuit. This design can reduce the light load loss to less than 1W.

2 Basic working principle

The overall block diagram of the circuit is shown in Figure 1. In order to reduce the harmonics of the input current and improve the input power factor (PF), the circuit uses a power factor correction (PFC) link with L6561 as the control chip. The PFC circuit can obtain a stable voltage of about 400V at the output end within the input voltage range of 88 to 265V.

Figure 1 Overall circuit block diagram

The schematic diagram of light load variable frequency control is shown in Figure 2. The L5991A chip adopts the current control mode. The value Vcomp of the voltage detection terminal (pin 6) is proportional to the peak current of the switch. The energy is transmitted through the transformer, so the load condition can be determined by detecting the value of Vcomp. If the load becomes lighter and the output power becomes smaller (the voltage remains unchanged), the peak current of the switch will decrease accordingly. When the value of Vcomp decreases to a certain threshold voltage VT1, the internal function of the chip (at high frequency, the chip pin 16 is at a high level, connected to pin 4, and the resonant resistance between pin 2 and pin 4 is RA and VB in parallel; at low frequency, the chip pin 16 is at a low level, disconnected from pin 4, and the resonant resistance becomes RA), so that the switching frequency is reduced from fosc to fSB; if the load is heavier and the output power increases, the peak current of the switch will increase accordingly. When Vcomp rises to another threshold voltage VT2, the switching frequency will rise to fosc through the internal function. L5991A can work normally under normal load and light load. VT1 and VT2 can be determined internally or through external additional circuits. Fosc and fSB can be determined by designing appropriate circuit parameters according to actual needs.

Figure 2 Schematic diagram of light load frequency conversion control

When working under full load, due to the high level of pin 16, Q1 is disconnected, Q2 is turned on, Q3 is turned off (see Figure 3), the voltage end connected to the zero current detection end of L6561 is high, and L6561 works normally; when working under light load, due to the low level of pin 16 (resonating with pin 2), Q1 is turned on, Q2 is turned off, Q3 is turned on, the voltage end connected to the zero current detection end of L6561 is low, and L6561 stops working. In this way, the standby loss (light load loss) is fully reduced.

Figure 3 Main circuit diagram of power adapter

3 Parameter design

The main circuit diagram of the 90W power adapter with power factor correction is shown in Figure 3. Its main circuit parameters are as follows: input voltage AC88~265V, frequency 50Hz; output voltage DC12V, maximum output power 90W, switching frequency 65kHz; switching frequency 20kHz when light load, PFC stops working.

1) Design of resonant resistor and capacitor

According to the design of the switching operating frequency, the values ​​of the resonant resistors RA, RB, and the resonant capacitor CT can be determined. The switching frequency fosc = 65kHz, and the switching frequency fSB = 20kHz when light load.

fSB = (1)

fosc = (2)

Where: RA∥RB=

KT=

From equations (1) and (2), we can take RA = 20kΩ, RB = 10kΩ, and CT = 3.3nF.

2) Design of flyback transformer

The design of the transformer plays an extremely important role in reducing power loss. In order to reduce the leakage inductance of the transformer, the "sandwich" winding method is adopted. That is, first wind half of the primary side, then wind the secondary side, and finally wind the other half of the primary side. This can reduce the leakage inductance by 50%.

Since the output power in normal operation is 90W, the minimum input power PinSB for high-frequency operation is designed to be 30W (that is, when the input power is less than 30W, the switching frequency changes from 65kHz to 20kHz), the detection resistor Rs = 0.28Ω, the output voltage Vo = 12V, and the switching operating frequency fAosc = 65kHz, the primary inductance of the transformer Lp = 540μH can be obtained by formula (3).

PinSB = Lpfosc (3)

Assuming that the primary inductor operates in discontinuous conduction mode (DCM), the peak current Ippk passing through the inductor is

Ippk==2.46A (4)

In the formula: Pin=Po/η, Po=90W, take η=0.85.

Normal switch on time

ton = (5)

Off time toff = (6)

Since the inductor works in DCM mode, it is required

toff＋ton< （7）

Taking the duty cycle D==0.2 and the output diode voltage drop VF=0.7V, the turns ratio of the primary and secondary sides of the transformer can be obtained as n=10.

In addition, due to the existence of leakage inductance, the energy on the primary side of the transformer cannot be completely transmitted to the output end. When the switch is turned off, an RCD clamping circuit is required to release the energy stored in the leakage inductance.

3) Design of power semiconductor devices

Since the flyback transformer has a certain leakage inductance, it may cause a certain peak voltage. In addition, considering that the PFC output voltage may fluctuate, a switch tube with a withstand voltage of more than 800V is selected; according to the maximum output power and the minimum duty cycle, the maximum on-current of the switch is 2.46A. In this way, STP6NC90Z can be selected, which has a withstand voltage of 900V and a maximum on-current of 5.8A.

When the PFC output voltage reaches the maximum allowable value, the maximum reverse voltage of the output rectifier diode will reach about 55V. In order to leave a certain margin, a diode with a reverse withstand voltage of 100V is selected, and the maximum current flowing through the rectifier diode is 7.5A. Therefore, STPS10H100CT can be selected, which has a withstand voltage of 100V and a maximum allowable current of 10A.

4) Others

In order to achieve the required deviation value, the feedback circuit adjusts the output voltage by using the optocoupler PC817. In order to reduce the high-frequency output ripple, a small inductor and capacitor filter is added at the end of the output voltage.

4 Experimental Results

The main advantage of the AC/DC power adapter with PFC designed mainly based on L5991A is that the switching frequency automatically decreases from high frequency to low frequency when the load is reduced, and the operation of the previous PFC stage is turned off, thereby greatly reducing the circuit loss and achieving the effect of light load and low loss. Figure 4 (a) and (b) show the waveforms of pin 16 and pin 2 of L5991A when the output power changes from 90W to 10W and from 10W to 90W. From the figure, it can be seen that when the load changes, the sudden change of the level of pin 16 and the sudden change of the resonant frequency of pin 2. Figure 5 (a) and (b) show the changes of pin 16 and the switch drive waveform of L5991A when the output power changes from 90W to 10W and from 10W to 90W. From the figure, it can be seen that the frequency of the switch is about 65kHz in normal operation and about 20kHz in light load. The experimental results show that the power consumed by the entire circuit is less than 1W when running at no load.

(a) Full load → Light load

(b) Light load → Full load

Figure 4 Test waveforms of pin 16 (upper) and pin 2 (lower)

(a) Full load → Light load

(b) Light load → Full load

Figure 5 Pin 16 (top) and driver (bottom) test waveforms

5 Conclusion

The power adapter with power factor correction designed with L5991A as the core can achieve no-load loss below 1W after corresponding parameter design. It has been applied to notebook computers and related fields with high requirements for power loss. It is believed that it will be more widely used as energy-saving requirements increase.