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Fairchild QFET for Synchronous Rectification DC to DC Converters

Fairchild Semiconductor 

l

Termination

Active

G

S

P-body pattern

in active area

Discrete Plural Well

Conventional (Cellular)

Unified Singular Well

QFET (Stripe)

Figure 2-2. Comparison of a QFET and a Conventional MOSFET Active Cell

The most noticeable difference between a QFET and a conventional MOSFET is in the P-well

design (Figure 2-1). The conventional MOSFET is composed of a discrete P-well, cellular in

shape, working alone as an active cell. Whereas, a QFET consists of a unified P-well, striped

in shape, which is connected together at the edge of the active area, working in unison (See

Figure 2-2). Unlike the spherical P-well junction in cellular devices such as the conventional

MOSFET, the QFET has a cylindrical P-well junction allowing a higher breakdown voltage for a

given epitaxial layer. This improvement of breakdown voltage efficiency provides lower on

resistance by reducing resistivity and thickness of the epitaxial layer for a given breakdown

voltage. Furthermore, the gate charge and the switching characteristics are improved in a

QFET due to the reduction in the input capacitance by the increase in gate oxide thickness

and optimization of the gate overlap area. RDS(on), gate charge, and switching characteristics

are all important for determining the MOSFET loss in the set (Refer to Section 4-2). Hence, a

QFET is appropriate for computer microprocessor power supply applications which require

high efficiency.

The Fairchild FQP60N03L, a new 30V logic level MOSFET product, is appropriate for low volt-

age DC-DC converters. Figure 2-3 is a graph of RDS(on) as a function of the FQP60N03L's

Junction temperature (TJ) at VGS=5[V] / 10[V] and ID=30[A]. The change in RDS(on) which cor-

responds with the change in temperature is approximately 0.07[mΩ/°C] when VGS=5[V] and

0.05[mΩ/°C] when VGS=10[V].

30

25

20

15

10

5

0

-50

VGS=5[V]

VGS=10[V]

@ I =30[A]

D

0

50

100

150

200

TJ, Junction Temperature [℃ ]

Figure 2-3. Change in RDS(on) as a function of temperature

2

Rev. B, July 2000

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