MOSFET due to discharging this energy

increases proportionately, which

may

become

a

limiting

factor

in

hard

switching topologies. Figure 4 shows the

loss distribution for various types of

power MOSFETs in a typical 200W off

line single transistor forward converter

operating at 100kHz.

Figure

5

shows

the

same

loss

distribution as in Figure 4

but at

200kHz. operating frequency.

FET Type1 FET Type2 FET Type 3 FET Type 4 FET Type 5 FET Type 6

Pcond

3.65

2.54

2.63

2.63

1.86

3.93

Psw

6.3

6.24

5.9

5.82

5.60

6.64

Pcoss

0.96

1.06

1.06

1.04

1.44

1.2

Ptotal

10.91

9.84

9.59

9.49

8.90

11.77

FET Power Dissipation In A Typical 200KHz. Offline Hard

Switching RCD Reset Forward Converter At 200W, 100C

0

2

4

6

8

10

12

FET

Type1

FET

Type2

FET

Type 3

FET

Type 4

FET

Type 5

FET

Type 6

Devices ( Power MOSFET)

Pcond

Psw

Pcoss

Ptotal

From Figure 4 and 5, one can observe

that as the switching frequency doubles

the conduction loss remains the same,

but the switching loss and loss due to

Coss doubles. The loss due to Coss is

77.4% that of conduction loss ( I

Rds on ) and 16% of the total losses for

the type 5 FET at 200kHz.

Summary:

Coss at 80% of Vdss and Coss effective

to 80% of Vdss specified are of different

values, and have different implications.

Coss is used only for the purpose of

calculating its loss’ contribution as this

loss can’t be instrumentally measured.

Coss

effective

is

used

solely

for

calculating any resonant circuit that

involves the output capacitance of the

MOSFET. For all practical purpose,

these specified values are sufficed to use

for operating Vds voltages in the range

of 70% to 90% of Vdss with about 10%

or less error. If more accuracy is

required, the associated test circuits

above can be used to measure to the

exact operating voltage .

Conclusion:

Historically the energy loss due output

capacitance in a MOSFET has been

deemed to be insignificant. With the

general increase in application operating

frequency and proliferation of differing

topologies this design tip demonstrates

how important this capacitance can be

and

shown

a

meaningful

way

of

calculating

its

contribution

to

the

switching energy loss.

FET Type1 FET Type2 FET Type 3 FET Type 4 FET Type 5 FET Type 6

Pcond

3.65

2.54

2.63

2.63

1.86

3.93

Psw

3.15

3.12

2.95

2.91

2.80

3.32

Pcoss

0.48

0.53

0.53

0.52

0.72

0.6

Ptotal

7.28

6.19

6.11

6.06

5.38

7.85

FET Power Dissipation In A Typical 100 KHz. Offline Hard

Switching RCD Reset Forward Converter At 200W, 100C

0

1

2

3

4

5

6

7

8

FET Type1

FET Type2

FET Type 3

FET Type 4

FET Type 5

FET Type 6

Devices ( Power MOSFET )

Pcond

Psw

Pcoss

Ptotal