AN3286 datasheet(24/42 Pages) STMICROELECTRONICS

24 / 42 page

Testing the board

AN3286

24/42

Doc ID 17998 Rev 1

Figure 24.

Converter efficiency when the input power is 1 W

With the demonstration board presented here it is possible to have or to not have brownout

protection. All the efficiency and light load performances have been measured with

brownout protection disabled (series RH1, RH2, and RL in Figure 2 disconnected). If

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

because the brownout network is connected in parallel with the bulk capacitor (C3 in the

schematic) and dissipates some power according to the following formula:

Equation 1

In light load condition the voltage across the 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 across the brownout network is:

Equation 2

which is negligible when measuring the efficiency in not too light load condition but becomes

quite important when measuring the standby or light load performance.

2.6

Overload protection

The VIPer25 is a current mode converter. This means that the regulation of the output

voltage is made by increasing or decreasing the primary peak current on a cycle-by-cycle

basis as a consequence of the increase or decrease of the output power demand.

The peak current is internally sensed and converted into a voltage which is compared with

the FB pin voltage. The device is shut down as soon as the two voltages are equal.

When the FB pin voltage reaches VFBlin (3.3 V typical), the drain peak current reaches its

maximum value, IDLIM (which is 0.7 A typical or a lower value according to the value of the

resistor if connected between the ZCD and GND pins).

BR LOSS

BUL K RM S

RH1

RH2

----------------------------------------------

BR LOSS

ACM AX PK

RH1

RH2

----------------------------------------------

265

⋅

()

2.268

⋅

--------------------------------

64mW

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