AN805 VISHAY [Vishay Siliconix], AN805 Datasheet - Page 5

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

A power MOSFET is made up of many single MOSFET cells
arranged in a parallel combination. In an ideal MOSFET all the
cells will turn on together when activated by a gate signal, and a
minimum switching time transition will be obtained. This does not
happen in a conventional MOSFET layout because the gate signal
has to propagate across the silicon in a turn-on “wave,” where the
cells nearest the gate bus turn on first with the outer cells following.
The PWM optimized MOSFET has symmetrical gate bussing, and
its bonding and layout structures minimize the turn-on “wave,” thus
increasing the switching speed of the device.
Efficiency
How much extra efficiency does the PWM optimized MOSFET
provide? A comparison of the efficiency of the synchronous boost
converter (Figure 4) using three different MOSFET technologies
shows that an improvement on the order of 5% can be made if an
optimized device is used.
Document Number: 70649
January 1997
FIGURE 6. Efficiency comparison between high-frequency,
FIGURE 8. Efficiency comparison between high-frequency,
%
%
100.00
100.00
90.00
80.00
70.00
60.00
90.00
80.00
70.00
60.00
conventional, and low-threshold MOSFETs at a
switching frequency of 300 kHz.
conventional, and low-threshold MOSFETs at a
switching frequency of 1.8 MHz.
0.0
0.0
200.0
Output Current 0 to 1000mA
200.0
Output Current 0 to 1000 mA
h % 6801 300 kHz
h % 6542 300 kHz
h % 6552 300 kHz
400.0
400.0
600.0
600.0
h % 6801 1.8 MHz
h % 6542 1.8 MHz
h % 6552 1.8 MHz
800.0
800.0
1000.0
1000.0
Figures 6, 7, and 8 show efficiency at switching frequencies
ranging from 300 kHz to 1.8 MHz, while Figure 9 summarizes the
efficiencies of the three technologies against switching frequency
at an output current of 400 mA. For all the results shown, the input
voltage for the synchronous boost converter was 3.6 V, with an
output voltage of 5 V.
The PWM optimized MOSFET surpasses all other technologies
while maintaining the highest efficiencies over the broadest load
ranges at all switching frequencies. The conventional MOSFET
technology provides the same breadth of efficiency but at a
reduced value. The low- threshold technology is clearly unsuited to
switching at higher switching frequencies with a gate voltage of
4.5 V.
As summarized in Table 1, at all switching frequencies the PWM
optimized MOSFET technology gives superior performance, both
in highest peak efficiencies and over the broadest load range,
making it the ideal choice for most low-voltage dc-to-dc designs.
FIGURE 7. Efficiency comparison between high-frequency,
FIGURE 9. Efficiency vs. switching frequency comparing the
%
100.00
90.00
80.00
70.00
60.00
%
conventional and low-threshold MOSFETs at a
switching frequency of 300 kHz
0.0
100
95
90
85
80
75
70
PWM optimized MOSFET technology with
conventional and low-threshold technologies
0
200.0
Output Current 0 to 1000mA
h % 6801 400 mA
h % 6542 400 mA
h % 6552 400 mA
Switching Frequency (kHz)
h % 6801 1 kHz
h % 6542 1 kHz
h % 6552 1 kHz
500
www.vishay.com FaxBack 408-970-5600
400.0
1000
Vishay Siliconix
600.0
1500
800.0
AN805
2000
1000.0
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