AN805 VISHAY [Vishay Siliconix], AN805 Datasheet - Page 6

• Manufacturer Part Number: AN805
• Description: PWM Optimized Power MOSFETs for Low-Voltage DC/DC Conversion
• Manufacturer: VISHAY [Vishay Siliconix]

AN805
Vishay Siliconix
Figure of Merit for the PWM Optimized MOSFET
Technology
Normalized gate charge serves as a quick figure of merit for
comparing the high-frequency, conventional, and low-threshold
MOSFETs. This was calculated by normalizing the on-resistance
and gate charge of the n-channel MOSFET to 100 m :
Similar performance advantages will be seen for the p-channel
process as well.
Application Areas
Ideal applications for Vishay Siliconix’s PWM optimized MOSFETs
include mobile communication equipment and other hand-held
battery-operated systems, where dc-to-dc converters are
becoming essential, and any other application where small size
and high efficiency are design criteria. A good example is the
demonstration board used as an example in this application note.
The Si9160BQ and Si6801DQ chip set is targeted for the cellular
phone market where single-cell lithium ion batteries are becoming
more popular and high-efficiency boost converters are required.
The buck converter in notebook computers is another key
application for Vishay Siliconix’s PWM optimized MOSFETs. Most
buck converter controller ICs today support synchronous
operation and require all n-channel MOSFETs. A typical
synchronous buck converter is shown in Figure 10.
In addition to non-isolated buck and boost topologies,
Vishay Siliconix’s PWM optimized MOSFETs are also very useful
in the application of synchronous rectification for isolated
converters (Figure 11). The replacement of Schottky diodes with
MOSFETs on the output of isolated topologies is becoming more
popular and even a necessity as output voltages drop below the
3-V level. This makes Schottky diodes impractical for efficiency
reasons. The biggest disadvantage to MOSFETs in isolated
synchronous rectification is that MOSFETs have to be driven and
Schottkies don’t. Vishay Siliconix’s PWM optimized MOSFETs
www.vishay.com FaxBack 408-970-5600
6
PWM Optimized
Conventional
Low- Threshold
MOSFET
Type of
Typical On-Resistance
at 4.5 V (m )
120
100
73
Gate Charge
Specific
have a lower gate charge per unit ohm, making it a lot easier and
more efficient to implement a given drive scheme.
Conclusions
Vishay Siliconix’s PWM optimized MOSFET technology goes
beyond the traditional improvements in on-resistance that have
been the standard benchmark for MOSFETs. This technology
addresses gate, crossover and conduction losses giving the
dc-to-dc converter designer several valuable advantages,
including faster switching times, lower gate losses, and higher
overall converter efficiency with a minimum footprint.
References
Spice Analysis of low loss control/Power MOSFET Chip set for
High Frequency DC-DC Converter Design. Jeff Berwick,
John Huang,
Siliconix Inc. Charles Hymowitz and Steve Sandler. Intusoft Inc.
HFPC Power Conversion, September 1996 Proceedings.
Jerry Fennel, “DO Cross Talk”, Advance Power Supplies, Bishop’s
Stortford, England.
Andrew
Communications, May 1996.
FIGURE 11. Implementation for synchronous rectification in a
16.0
1.7
4.0
N- and P-Control IC,
Si9140, Si9145
FIGURE 10. All n-channel synchronous buck converter.
All N-Channel or
Cowell,
resonant reset forward converter.
eg.,
Normalized Gate Charge
Wayne Grabowski
per 100 m
Si9160
22.0
1.4
4.0
Boost
(nc)
and
Converter
Document Number: 70649
Richard K. Williams.
for
January 1997
Mobile