AN-9014| Application Note
Application Note 9014
July, 2000
Fairchild QFET for Synchronous Rectification DC to DC Converters
by K.S. Oh
1. Introduction
Fairchild is currently developing and marketing a new QFET series that has improved RDS(on), gate charge, and switching speed characteristics. The superior features of these QFETs are very useful for increasing the efficiency of low output voltage power supplies, making it specially appropriate for computer microprocessor power supply applications which require high efficiency. This application note describes the use of a Fairchild QFET (FQP60N03L ) in the power supply (DC-DC converter) of computer microprocessors. Chapter 2 describes QFET's organizational features and the improvements in its electrical characteristics based on these features. Chapter 3 outlines the required input voltage and current based on the latest trends in microprocessors, and describes the optimum DC-DC converter circuit configuration that will meet these input conditions. The synchronous rectification DC-DC converter introduced in Section 3-1, which increases the overall efficiency by converting the loss due to the forward voltage d
rop of the rectifier diode to the MOSFET's low RDS(on) loss, is explained in detail in the main body of this note. Lastly, Chapter 4 describes an example of a synchronous rectification DCDC converter design that uses the Fairchild PWM control IC RC5058, and the loss and efficiency associated when FQP60N03L is used.
2. QFET
Poly
Poly
N+
P+
P+
P+ N- epi. N+ substrate
P+
N+ + P N- epi. N+ substrate
Conventional (Cellular)
QFET (Stripe)
Figure 2-1. Comparison of a QFET and a Conventional MOSFET Structure
1
Rev. B, July 2000
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