Optimizing Adapter Power Supply Design

Jul 1, 2010 12:00 PM
Jason Sun, Fairchild Semiconductor, China

TURNS RATIO (N)

Finally, we discuss how the design is influenced when the turns ratio (n) of the flyback DC/DC transformer is changed. It can be seen from the VRO calculation that when the turn ratio (n) rises, the reflected voltage VRO will rise, the primary MOSFET maximum voltage Vds_max also rises, but the secondary rectifier maximum voltage Vsd_max drops. Therefore, the turn ratio (n) must be a proper value that is trade-off between Vsd_max and Vds_max. As with the calculations made earlier in this design, we must first determine the primary/secondary side voltage, then determine the turn ratio (n), and finally select the proper current specifications of the MOSFET and rectifier based on the currents that we have calculated.

Figure 12 shows the performance and cost comparisons among several MOSFETs (sourced from Fairchild's website for reference only). It can be seen from this figure that the 13A/600V and 16A/600V MOSFETS (Parts FCPF13N65NT and FCPF16N65NT respectively) are very similar in their performance and cost. While the 13A/800V MOSFET (Part FQA13N80) is more expensive than the 12A/600V MOSFET (Part FQPF12N60NC), it exhibits poorer RDS(ON) performance. So, it is better to calculate the MOSFET voltage specification first, and then follow by calculating the current. Of course, the voltage of the secondary side rectifier is related to the output voltage of the power supply; when the output voltage is very high, a secondary side rectifier with a higher voltage is desirable.

Of course, the example design still requires modification before it can be turned into a highly reliable and safe power supply product. The power adapter must be able to survive the extreme temperature, humidity, and vibration of harsh application environments, as well as be able to meet strict performance requirements and the stringent government regulations surrounding safety and energy-efficiency.

REFERENCES

1. Fairchild Semiconductor: Design Guidelines for Quasi-Resonant Converter using FSCQ-series Fairchild Power Switch (FPS),AN-4146,Fairchid semiconductor application note, 2005.

2. Fairchild Semiconductor: FAN6300-Highly Integrated Quasi-Resonant PWM Controller, AN-6300,Fairchid semiconductor application note, 2008.

3. Fairchild Semiconductor:Design of Power Factor Correction Circuit using FAN7529, AN6026, Fairchild Semiconductor Application note, 2008.

4. Fairchild Semiconductor: Secondary Synchronous Rectifier for Quasi-Resonant Controllers, AN-6085, Fairchild Semiconductor application note, 2009.

5. Lloyd H.Dixon,“Magnetics design for switching power supplies”,2000

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