AN75 データシートの表示（PDF） - Diodes Incorporated.

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AN75

High Power Factor LED Replacement T8 Fluorescent Tube using the AL9910 High Voltage LED Controller

Diodes Incorporated. 

AN75

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,

VDSS = 1.3 × VIN(max) = 1.3 × 373V = 485V

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

tRISE and fall time tFALL is a function of the MOSFET’s gate capacitance, the gate driver capability of the

AL9910 and layout design. The worse case switching power losses occurs at VLED(min) and VIN(max).

The switching loss is approximately,

PSW

=

VIN(max) × ⎜⎜⎝⎛Ipk

−

VLED(min)tOFF

LBUCK

2

⎟⎟⎠⎞ × tRISE × fswi(max)

+

VIN(max) × Ipk

× tFALL × fswi(max)

2

= 373V × (297mA − 88mA )× 65ns × 63.8kHz + 373V × 65ns × 63.8kHz

2

2

= 455mW

where the switching time tRISE and tFALL 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 VLED(min) and VIN(max) is given by,

ID(RMS) =

VLED(min)

VIN(max)

× ⎜⎜⎝⎛ILED(nom)

+

VLED(min)

× tOFF

12

LBUCK ⎟⎟⎠⎞

=

42 V

373V

× ⎜⎜⎝⎛240mA

+

42V

× 13.9μs

12

6.6mH

⎟⎟⎠⎞

= 89mA

The power MOSFET conduction loss depends on its static drain-source resistance RDS(ON) at the

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

( ) PCOND = ID2(RMS) × RDS(ON) = 89mA 2 × 2.5Ω = 19mW

The total power MOSFET loss is:

PTOT = PSW + PCOND = 455mW + 19mW = 474mW

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 RthJA is known. The calculated MOSFET junction temperature, TJ, must be

lower then the maximum allowable junction temperature TJ(MAX):

TJ = PTOT × θthJA + TAMB = 474mW × 62 oC W + 80oC = 109.4o C

The internal ambient temperature within the LED converter, TAMB, is assumed to be 80ºC. θthJA =

62 oC W 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.

Issue 1 – January 2011

5

© Diodes Incorporated 2010

www.diodes.com

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