AN4007 datasheet(20/33 Pages) STMICROELECTRONICS

20 / 33 page

Protection features

AN4007

20/33

Doc ID 022455 Rev 2

Protection features

The VIPER37LE has several protection features that considerably increase end-product

safety and reliability: overload protection, overvoltage protection, shorted secondary rectifier

detection and transformer saturation protection. In the following paragraphs all protections

are tested and the results are presented.

2.1

Overload and short-circuit protection

If the load power demand increases, the output voltage decreases and consequently the

feedback loop reacts, increasing the voltage on the FEEDBACK pin.

The FEEDBACK pin voltage increase leads to the PWM current set point increase, with the

rise of the power delivered to the output. This process ends when the delivered power

equals the load power requested.

If the load power demand exceeds the power capability (that can be adjusted using RLIM),

the voltage on the FEEDBACK pin continuously rises, but the drain current is limited to the

fixed current limitation value.

When the FEEDBACK pin voltage exceeds VFB_lin (3.3 V typ), the VIPER37LE takes it as a

warning status of an output overload condition. Before stopping the system, the device waits

for a time fixed by the FB capacitor. When the voltage on the FEEDBACK pin exceeds

VFB_lin, an internal pull-up circuit is disconnected and the pin starts sourcing a 3 A current

that charges the capacitor connected to the FEEDBACK pin itself. As the FEEDBACK pin's

voltage reaches the VFB_olp threshold (4.8 V typ.), the power MOSFET stops switching and

is not allowed to switch again until the VDD voltage falls below VDD_RESTART (4.5 V typ.).

If the short-circuit is not removed, the system starts to work in auto-restart mode: in this

case the MOSFET switches for a short period of time and the converter tries to deliver to the

output as much power as it can, and for a longer period where the device is not switching

and no power is processed.

As the duty cycle of power delivery is very low (around 4%), the average power throughput

is also very low, resulting in a very safe operation.

Figure 21 and 22 show the triggering of the overload and the operation with continuous

overload.

AN4007 Datenblatt(PDF) 20 Page - STMicroelectronics