AN805 VISHAY [Vishay Siliconix], AN805 Datasheet

AN805

Manufacturer Part Number

AN805

Description

PWM Optimized Power MOSFETs for Low-Voltage DC/DC Conversion

Manufacturer

VISHAY [Vishay Siliconix]

Datasheet

1.AN805.pdf (8 pages)

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Designers of low-voltage dc-to-dc converters have two main

concerns: reducing size and reducing losses. As a way of reducing

size, designers are increasing switching frequencies. But the result

has been reduced converter efficiency. To minimize losses,

MOSFET manufacturers have generally focused on lowering

on-resistance. But the results have not been optimal for dc-to-dc

conversion designs, since gate charge and switching speed issues

have been largely ignored. The dominant losses associated with

MOSFETs were once conduction losses, but this is no longer the

case.

Vishay Siliconix’s new family of PWM optimized MOSFETs has

been designed to give the highest efficiency available for a given

on-resistance in switching applications such as dc-to-dc

conversion. These new devices provide a very low gate charge

per unit of on-resistance, in addition to fast switching times. The

result is reduced gate drive and crossover losses, allowing

designers of dc-to-dc converters to simultaneously reduce the

design footprint and increase efficiency.

MOSFET Losses

A simplistic model of power loss in a MOSFET used in a dc-to-dc

converter (Figure 1) can be calculated if we know the RMS, the

current through the MOSFET, the duty cycle, the gate voltage, and

the r

compare the efficiency of designs using Vishay Siliconix’s new

PWM optimized MOSFETs versus conventional and low-threshold

power MOSFETs.

The equation that defines the losses associated only with

on-resistance and the gate drive is:

Document Number: 70649

January 1997

FIGURE 2. Power loss for PWM optimized Si6801 p-channel

DS(on)

RMS

g V

of the MOSFET. This model can then be used to

MOSFET as a function of V

frequency.

DS(on)

Si6801 Power Loss, QG, r

f (Watts) Eq1

T J

for Low-Voltage DC/DC Conversion

PWM Optimized Power MOSFETs

and switching

0.8

0.6

0.4

0.2

[ ] The value of the parameter before the parenthesis is dependent

on the parameter within the parenthesis.

where:

Using Equation 1 we can obtain a plot of power loss (gate loss +

frequencies (Figure 2).

DS(on)

RMS

]

Gate

FIGURE 3. Gate losses and on-resistance losses for PWM

FIGURE 1.

loss) as a function of gate voltage at varying switching

The RMS current in the MOSFET (A)

On-resistance of the device for a given drive voltage

and junction temperature.

The peak driver gate voltage for the MOSFET (V)

Junction temperature of the MOSFET

Duty factor of the MOSFET (Ratio of on time to off

time)

Total gate charge for the MOSFET at a given gate

voltage (C)

Frequency of MOSFET switching (Hz)

Technology Comparison: 1 MHz Power Loss

Crss

optimized power MOSFET (Si6801DQ) versus

conventional (Si6542DQ) and low-threshold

(Si6552DQ) power MOSFETs.

Ciss

Generic MOSFET model with body diode omitted.

[1]

www.vishay.com FaxBack 408-970-5600

DS(on)

RMS

Vishay Siliconix

Drain

Source

oss

AN805

Related parts for AN805

AN805 Summary of contents

Page 1

... V Technology Comparison: 1 MHz Power Loss DS GS 0.8 50 0.6 40 0 FIGURE 3. Gate losses and on-resistance losses for PWM AN805 Vishay Siliconix Drain I RMS Crss oss r Ciss DS(on) Source Generic MOSFET model with body diode omitted. [ ...

Page 2

... AN805 Vishay Siliconix Figure 2 shows the respective contribution of on-resistance and gate charge to overall losses for the p-channel Si6801DQ at three different switching frequencies. At low gate-source voltages, the r of the MOSFET is high and therefore on-resistance losses DS(on) dominate. At higher gate-source voltages, on-resistance becomes almost a constant and the gate charge losses controlled by Q dominate ...

Page 3

... MOSFET capacitance can be charged therefore one of the dominant factors in determining how fast a MOSFET will switch. Vishay Siliconix’s PWM optimized MOSFETs provide a minimum effective gate resistance. AN805 Vishay Siliconix in Figure 1). The effective gate G www.vishay.com FaxBack 408-970-5600 ...

Page 4

... AN805 Vishay Siliconix N-Channel Turn Off 5 ns/dv Si6801 4 ns Si6542 11 ns Si6552 14 ns FIGURE 5. Switching speed comparison between high-frequency, conventional, and low-threshold power MOSFETs. www.vishay.com FaxBack 408-970-5600 4 N-Channel Turn On 5 ns/dv High-Frequency MOSFET Technology Conventional MOSFET Technology Low-Threshold MOSFET Technology Si6801 3 ns Si6542 ...

Page 5

... FIGURE 7. Efficiency comparison between high-frequency, conventional and low-threshold MOSFETs at a switching frequency of 300 kHz 100 % 800.0 1000.0 FIGURE 9. Efficiency vs. switching frequency comparing the AN805 Vishay Siliconix h % 6801 1 kHz h % 6542 1 kHz h % 6552 1 kHz 200.0 400.0 600.0 800.0 1000.0 Output Current 0 to 1000mA ...

Page 6

... AN805 Vishay Siliconix Typical On-Resistance Type of MOSFET PWM Optimized Conventional Low- Threshold 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 ...

Page 7

... Power Dissipation Vds ( Document Number: 70649 January 1997 Resistive Cross over Losses Clamped Inductive Cross over Losses Vds Ids AN805 Vishay Siliconix www.vishay.com FaxBack 408-970-5600 7 ...

Page 8

... This datasheet has been download from: www.datasheetcatalog.com Datasheets for electronics components. ...

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