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Chapter 4 Experiments

4.4 Experimental Results and Discussion

4.4.2 Conversion Efficiency

To measure conversion efficiencies under different load conditions, we connect the

output of device under test to a DC electronic load. The testing setup is shown in Fig.

4.18. We calculate the conversion efficiency by dividing the power displayed on the DC

(a)

(b)

Fig. 4.19. Measured (a) conversion efficiencies and (b) currents flowing through the optocouplers when VIN = 110 2 V.

electronic load by the power read from the power analyzer. Two groups of experiment

with respectively VIN = 110 2 V and VIN = 220 2 V are conducted, and the results are portrayed in Fig. 4.19 and Fig. 4.20, respectively. In Fig. 4.19(a), we can observe

(a)

(b)

Fig. 4.20. Measured (a) conversion efficiencies and (b) currents flowing through the optocouplers when VIN = 220 2 V.

proposed counterpart does when the output power is larger than 7.2 W. But when the

load gets lighter, the converter with the proposed feedback circuit presents evidently

better efficiencies. For example, there is a 2.2% efficiency improvement under the

1.8-W output power (10% load) and a 3.6% improvement under the 0.9-W output (5%

load). Fig. 4.19(b) gives the measured currents of optocouplers in both of the converters.

Under light-loads conditions, ILED and IFBC in the conventional converter are larger than

ILED and IFBP in the proposed one, and their differences become even larger with the

decrease of the output power. This fact forms the basic reason for the light-load

efficiency improvement. Fig. 4.19(b) also indicates that the current transfer ratio CTR of

the optocoupler continuously degrades with the decrease of the conducting currents.

Therefore, the decrease of ILED is not as much as that of IFBP or IFBC.

Fig. 4.20 shows a very similar situation when a higher input voltage (VIN =

2

220 V) is applied. The conversion efficiencies of the proposed flyback converter shown in Fig. 4.20(a) are much better than that of the conventional one when the load is

lighter than 5.4 W. For instance, there is a 2.9% efficiency improvement under the

1.8-W output power and a 5.1% improvement under the 0.9-W output. Fig. 4.20(b) also

depicts the measured currents of optocouplers in both of the converters. Comparing Fig.

4.20(b) with Fig. 4.19(b), we can see that in Fig. 4.20(b), the two conventional curves

and the two proposed curves are separated even further. It is due to the higher input

voltage such that the inductor current level limit becomes lower under the same output

power condition. Therefore, VFBC becomes lower and VFBP becomes higher, leading to

this phenomenon. This outcome also explains why the proposed converter improves

more efficiency under the same light-load condition when a higher input voltage is

applied.

4.5 Conclusion

The PWM controller and the reverse-type shunt regulator are designed and

fabricated for implementing a flyback converter adopting the proposed feedback scheme.

By integrating both a proposed and a conventional feedback paths, the controller can be

adopted for the conventional feedback topology as well. Two flyback converters with

different feedback schemes are implemented using the fabricated chips for experiments.

The comparisons between the two converters are made as fair as possible by setting

their power stages and their resistor dividers identical, and even in their controllers, all

other building blocks except for the feedback circuits are the same. In the measurements

of the no-load power consumption, the converter adopting the proposed feedback

network is obviously superior to the conventional converter, and the currents of the

optocoupler in the proposed topology are proved to be almost zero under this condition.

Better light-load conversion efficiencies are observed in the converter with the proposed

feedback network, while at heavy loads, there is no significant distinction between the

efficiencies of the two converters.

Chapter 5

Dissertation Conclusion and Future Work

5.1 Dissertation Conclusion

Standby power has resulted in a huge amount of energy waste in recent years,

which makes both industry and academia gradually put concentration on this issue. The

conventional feedback network is widely adopted in isolated offline switch-mode power

supplies owing to the benefits of the simple circuit structure and low cost. However, the

power loss under very light/no-load conditions causes severe standby power problem

because the currents following through the optocoupler are increased with the decrease

of the output power. Previous literature offers some techniques to address the power loss

issue, while all of them bring about more or less disadvantages and are still not very

ideal solutions at all.

This dissertation proposes the phase reversal concept to address the power loss

issue of the feedback network. With this idea, the currents flowing through the

optocoupler and thus the generated power loss are both decreased with the output power.

Also, the supply current of the reverse-type shunt regulator is designed not to flow

through the optocoupler, making this part of power dissipation not reproduced on the

primary side. Following these thoughts, a complete isolated feedback network is

proposed. The power loss analysis of the proposed feedback network shows that the

optocoupler conducts averagely zero currents on both sides, which indicates that the

standby power of the feedback network is minimized.

The PWM controller and the reverse-type shunt regulator are designed and

fabricated for implementing a flyback converter adopting the proposed feedback scheme.

Two flyback converters with different feedback schemes are implemented using the

fabricated chips. Experiments demonstrate a significant improvement in the no-load

power loss over the conventional flyback converter. Better light-load efficiency is also

achieved. Although the experiments are carried out on the basis of flyback converter, we

do believe that it is also applicable to other transformer-isolated topologies. With the

above advantages, the proposed isolated feedback scheme proves to be a promising

solution for future low-standby-power converters.

5.2 Future Work

In this dissertation, we only examine the performance difference between two

converters adopting respectively the proposed and the conventional feedback topology.

It is encouraged to incorporate this proposed feedback method with other techniques,

such as the high voltage start-up, the low-standby-power EMI capacitor discharging

method, and a low-power controller, into a power supply unit to see how actually

low-standby-power it can achieve. For example, a low-power battery charger adopting

all aforementioned techniques would probably meet the strictest standard of 30-mW

standby power.

Another point that deserves improving is the error amplifier inside the reverse-type

shunt regulator. Originally designed with the two-stage operational amplifier structure

for simplicity and large output swing, it can be modified and implemented with an even

lower-quiescent-current circuit, such as a class AB amplifier. The lower quiescent

current is consumed, the lower standby power can be achieved.

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