Testing the board

AN2929

20/32

Doc ID 15333 Rev 2

Figure 20.

Output power vs. input voltage with input power of 1 W

If brownout protection is required, for the same load condition the input power increases.

The brownout external circuit (R16, R17 and R18 see schematic in Figure 2) is connected in

parallel with a bulk capacitor (C3 in the schematic) and some power is dissipated on it

according to the voltage across the bulk capacitor. The following formula gives the additional

power dissipation in the brownout circuit:

Equation 1

In light-load condition the voltage across bulk capacitor can be considered constant and

equal to the peak of the AC input voltage. Considering the worst case of maximum input

voltage, the dissipation in the brownout circuit is:

Equation 2

Table 16.

Output power when the input power is 1 W

(mW)

(mW)

Efficiency

(%)

Pin-Pout

(mW)

90

1000

767.1

76.71

232.853

115

1000

762.1

76.21

237.910

230

1000

752.0

75.20

248.024

265

1000

726.2

72.62

273.815

AM02174v1

720.0

730.0

740.0

750.0

760.0

770.0

80

120

160

200

240

280

VINAC (VRMS)

POUT (mW)

)

18

R

17

R

16

R

(

V

P

2

RMS

_

BULK

LOSS

_

BR

+

+

=

mW

43

k

3222

)

V

265

2

(

)

18

R

17

R

16

R

(

V

P

2

2

PK

_

ACMAX

LOSS

_

BR

≅

Ω

⋅

⋅

⋅

=

+

+

=