AN-6099

APPLICATION NOTE

© 2013 Fairchild Semiconductor Corporation

www.fairchildsemi.com

Rev. 1.0.1 • 3/12/13

3

channel width-to-length ratio. The other concept originally

developed for high-voltage devices, but now being used for

low-voltage devices as well; is the use of charge balance or

super-junction device structures. With the use of the charge

balance approach, two-dimensional charge coupling in the

drift region can be obtained. The latest middle-voltage

power MOSFETs from Fairchild employ this shielded-gate

structure, where the shield electrode is connected to the

source, as shown in Figure 4. The shield electrode, along

with the thicker oxide between electrode and drift region,

provides charge balance for drift region. This enables higher

doping in the drift region, resulting in reduced drift

resistance. The specific resistance of these new medium-

voltage power MOSFETs has been significantly improved

over the previous generation, while improving on the

now becoming more relevant in synchronous rectification.

The importance of these loss components rises at higher

switching

frequencies

and

higher

output

currents.

Fairchild’s new medium-voltage MOSFETs are being

optimized to minimize the diode reverse recovery as well as

the output capacitance. The latest PowerTrench

FDP045N10A,

employs

shielded-gate

structure

that

provides charge balance. By utilizing this advanced

than the previous generation and competitor MOSFETs, as

shown in Figure 5.

Power Losses in Synchronous

Rectification

Conduction Loss

Power losses in secondary rectification are very critical,

especially in low-voltage and high-current applications, as

shown in Figure 6. Therefore, secondary-side synchronous

rectification is an excellent solution to improve system

efficiency. As shown in Figure 7, the conduction loss of

diode rectifier contributes significantly to the overall power

loss in a power supply. The rectifier conduction loss is

presents a resistive V-I characteristic. The forward-voltage

drop of synchronous rectification can be lower than that of a

diode rectifier and, consequently, dramatically reduces the

rectifier conduction loss. Conduction loss can be obtained

through below equation:

2

(1)

For high-voltage MOSFETs, the resistance of packages has

TO-220 standard package, depending on the voltage rating,

by using modern medium voltage MOSFETs technology.

Unlike

high-voltage

MOSFETs,

the

package

itself

contributes a significant portion of the total resistance for

medium-voltage MOSFETs due to wire bonding, lead, and

is accounted for by the package resistance in a 75 V/2.3 

MOSFET, as shown in Figure 8. SO-8 packages were

popular before upgraded power package Power56. Total on

resistance of medium-voltage MOSFET can be dramatically

reduced by using an SMD package, such as Power56. It can

also reduce package inductance that causes undesirable

replacing lower voltage rating MOSFETs.

Figure 6. Power Losses Analysis in ATX Power Supply

Figure 7. Power Losses Comparison between Diode

Rectification and Synchronous Rectification