AN75

Issue 1 – January 2011

5

www.diodes.com

© Diodes Incorporated 2010

Maximum drain-source voltage stress on the power MOSFET for this converter is equal to the input

voltage. However, a typical voltage safety margin for the MOSFET defines the maximum reverse

voltage as follows,

V

485

V

373

3

.

1

V

3

.

1

V

(max)

IN

DSS

=

×

=

×

=

which implies that a common 500V MOSFET is suitable.

The power MOSFET losses will be dominated by switching loss. The switching loss depends on the

switching time, frequency, MOSFET drain current and drain-source voltage. The switching rise time

t

AL9910 and layout design. The worse case switching power losses occurs at V

The switching loss is approximately,

()

455mW

2

63.8kHz

65ns

373V

2

63.8kHz

65ns

88mA

297mA

373V

2

f

t

I

V

2

f

t

L

t

V

I

V

P

swi(max)

FALL

pk

IN(max)

swi(max)

RISE

BUCK

OFF

LED(min)

pk

IN(max)

SW

=

×

×

+

×

×

−

×

=

×

×

×

+

×

×

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

×

=

where the switching time t

RISE

and t

FALL

are measured to be 65ns with the 600V MOSFET

SPB03N60S5 as the power MOSFET. As shown in Figure 1, R10 is a series gate resistor that slows

down the MOSFET switching and reduces EMI emission.

The RMS current through the MOSFET at V

mA

89

12

mH

6

.

6

s

μ

9

.

13

V

42

mA

240

V

373

V

42

12

L

t

V

I

V

V

I

BUCK

OFF

(min)

LED

)

nom

(

LED

(max)

IN

(min)

LED

D(RMS)

=

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

+

×

=

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

×

+

×

=

The power MOSFET conduction loss depends on its static drain-source resistance R

MOSFET working temperature. It is possible to calculate the continuous conduction loss:

()

mW

19

5

.

2

mA

89

R

I

P

2

DS(ON)

2

D(RMS)

COND

=

Ω

×

=

×

=

The total power MOSFET loss is:

mW

474

mW

19

mW

455

P

P

P

COND

SW

TOT

=

+

=

+

=

Total MOSFET power loss is dissipated from the SMD package into the PC Board. So it is possible to

calculate the MOSFET working junction temperature can be calculated if the package junction-to-

ambient thermal resistance R

lower then the maximum allowable junction temperature T

C

4

.

109

C

80

W

C

62

mW

474

T

θ

P

T

AMB

thJA

TOT

J

o

o

o

=

+

×

=

+

×

=

The internal ambient temperature within the LED converter, T

W

C

is the thermal resistance for TO-263 with minimum copper area. For practical design, it is

recommended to keep the junction temperature below 110ºC to avoid temperature stress on the

device.