AN2840 datasheet(9/19 Pages) STMICROELECTRONICS

9 / 19 page

AN2840

Short-circuit operation

9/19

Short-circuit operation

During output short-circuit conditions, the converter operates in hiccup mode (see

Figure 11) due to the activation of the overload protection (OLP). Operation at both 115 VAC

and 230 VAC is very similar, since neither the VDD capacitor charge/discharge times nor the

FB pin behavior depend on the bulk capacitor voltage. When a short-circuit is applied, the

FB pin saturates high, and after about 8 ms the capacitors connected to that pin are charged

up to 4.8 V (typ) and the OVL protection is triggered. Once the VDD capacitor is discharged

down to 4.5 V (typ.), the high voltage startup generator is turned on with reduced current

(0.6 mA typ), then when the VDD turn-on threshold is reached, the IC turns on and the cycle

is repeated. Normal operation is restored after the short is removed (auto restart behavior).

As illustrated in figures 13 and 14, even at 230 VAC the circuit behavior remains unchanged,

and even at higher input voltage the working time in short-circuit condition is extremely short

with respect to the off time. This permits limited dissipation for the power components and,

since the components are not subject to excessive thermal stress, reliable operation of the

circuit. Even the drain voltage peak during this condition is well below the VIPER17HN

maximum rating for that pin.

Figure 11.

VIN = 115 VAC - 60 Hz short-

circuit operation

Figure 12.

VIN = 230 VAC - 50 Hz short-

circuit operation detail

CH1: VIPER17HN FB pin (yellow trace)

CH2: VIPER17HN VDD pin (cyan trace)

CH3: VIPER17HN drain pin (purple trace)

CH4: output current (green)

AN2840 数据表(PDF) 9 Page - STMicroelectronics