Implementation of the primary-side regulation in flyback converters (Part 2 of 2)
(Part 1 looked at the Fundamentals of the topology and configuration; to read it, click here.)
Circuit design
We now present a real PSR implementation in a 5W battery charger (see Figure 9). Listed in Table 1 are the main parameters of this charger. The PSR controller uses an internal 600V high voltage MOSFET that delivers reduced interference between the MOSFET circuit and the PCB routing. To reduce standby power consumption, satisfying the energy-saving regulations for power supplies, the PSR controller is designed with energy-save mode, which can linearly decrease the PWM frequency as the charger moves into light load condition.
The PSR controller also implements frequency-hopping technology (spread spectrum) to enhance EMI performance and internal compensation to address output voltage drop, which can be caused by a long output cable, further enhancing the output voltage regulation capabilities.
Table 1: Circuit Parameters
Figure 7 and Figure 8 show the results from this experiment. The output voltage-current curve in Figure 7 shows that under commercial AC power supply, the output voltage regulation accuracy can reach up to 2.88%, and when the fold-back voltage is 1.5V, the output current regulation accuracy can reach up to 1.75%. Additionally, this level of current regulation accuracy can be held by controlling the VDD within 5V~28V and the constant current can still be achieved even when the output voltage gets lower.
As shown in Figure 8, the average efficiency can reach up to 72.3% @115V and 71.5& @230V, surpassing the standard average efficiency (68.17%) required by Energy Star 2.0, Efficiency level V. With frequency-hopping added into PWM switching, EMI performance is further enhanced by distributing energy in multiple modulated frequencies instead of a single frequency.
Figure 7: The output voltage/current curves of a 5W charger implementing PSR
Figure 8: The power efficiencies of a 5W charger implementing PSR under different load conditions
Figure 9: A real circuit implementing the 5W charger
LED drivers generally use a secondary-side circuit to control the output current for applications such as LED lighting power. In general, LED forward voltage (VF) increases with higher temperature. Therefore, attention must be paid to the setting of fold-back voltage in circuit design.
If the voltage setting is too high, the driver will not be able to light the LED. The fold-back voltage should be designed according to the forward voltage. In our solution, a 600V high-voltage MOSFET is used in the PSR controller, which helps to reduce the size of the driver.
Figure 10 shows the performance of a LED driver circuit implementing PSR. Table 2 lists the main specification parameters of the driver circuit. Table 3 lists the efficiencies of the driver circuit working at different input voltages. Figure 11 and Figure 12 are photos of the driver. Figure 13 shows the circuit implementing the LED driver.
Table 2: Circuit Parameters
Figure 10: The output voltage/current curves of a 4.2W LED driver implementing PSR
Table 3: The driver efficiencies at different input voltages
Figure 11: A real driver photo (front view)
Figure 12: A real driver photo (side view)
Figure 13: An actual circuit implementing a 12V/0.35A LED driver
Conclusion
With Green energy becoming the world’s focus, great attention is being paid to power-supply efficiency. Power-supply control ICs are playing an important role in enhancing efficiency. These IC power-supply products reduce total cost, switching loss, size, and enhance EMI performance. This article presented a battery charger and a LED driver that implement PSR technology, which achieve constant voltage and current by using the sample voltage signal that is picked up from the auxiliary winding on the transformer.
This technology eliminates the need to use a secondary-side feedback circuit, a phototransistor, and current-detecting resistors, all of which are used in traditional solutions. Thus, deploying PSR controller ICs in battery chargers and LED drivers is an optimal solution that enhances power efficiency and reduces overall product cost.
About the authors
Sean Chen is a technical marketing engineer in the Asia Pacific region for Fairchild Semiconductor. He received his M.S. degree from Chung Yuan Christian University. Prior to his current position, Chen was a field application engineer for Fairchild in Taiwan. His research interests include AC/DC converters and the AC-DC converter market.
Eric Lan is the Vice President, Technical Marketing and Applications, Asia Pacific for Fairchild Semiconductor. He holds an M.S. Degree in Electrical Engineering, from the National Taiwan University, Taipei, Taiwan. He has a broad base of experience in power supplies, power-management ICs, power devices, magnetics, and EMI/EMC applications.
Lawrence Lin is a field application engineer for Fairchild in Taiwan. He graduated from the National Taiwan University of Science and Technology. His research interests are in high-efficiency and low standby-power consumption technologies of AC/DC power supplies.
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