9014 Datasheet, PDF (1/16 Pages) Wing Shing Computer Components – NPN SILICON TRANSISTOR

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 R_DS(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 spe-
cially 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 sup-
ply (DC-DC converter) of computer microprocessors. Chapter 2 describes QFET’s organiza-
tional 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 micro-
processors, 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
drop of the rectifier diode to the MOSFET’s low R_DS(on) loss, is explained in detail in the main
body of this note. Lastly, Chapter 4 describes an example of a synchronous rectification DC-
DC converter design that uses the Fairchild PWM control IC RC5058, and the loss and effi-
ciency associated when FQP60N03L is used.
2. QFET
Poly
Poly
N⁺ _P+
P⁺
P⁺
N^- epi.
N⁺ substrate
Conventional (Cellular)
P⁺
N⁺_P+
N^- epi.
N⁺ substrate
QFET (Stripe)
Figure 2-1. Comparison of a QFET and a Conventional MOSFET Structure
1
Rev. B, July 2000