Testing the board

AN2929

24/32

Doc ID 15333 Rev 2

2.7

Output overvoltage protection

An output overvoltage protection is implemented by monitoring the voltage across the

auxiliary winding during the MOSFET off-time, through the D2 diode and the resistor divider

R3 and R14 (see schematic of Figure 2) connected to the CONT pin of the VIPER27LN. If

assumed and the power section is no longer allowed to switch on. To re-enable operation,

spikes erroneously trip the protection, a digital filter was implemented so the CONT pin has

Figure 25.

OVP circuit

The value of the output voltage when the protection has to be tripped can be set by properly

selecting the resistor divider R3 and R14. R3 is selected according to the maximum power

that the converter has to provide to the output, and R14 is selected according to Equation 3:

Equation 3

The protection was tested by disconnecting the opto-coupler from the feedback pin and

applying a minimum load to the converter. In this way the converter operates in open loop

and delivers the maximum power that it can to the output. The excess of power with respect

to the load charges the output capacitance, increasing the output voltage since the OVP is

tripped and the converter stops working.

In Figure 26 it is possible to see that the output voltage increases and, as it reaches the

value of 5.58 V, the converter stops switching. In the same figure the CONT pin voltage

(Ch1, yellow waveform) is shown. The crest value of the CONT pin voltage tracks the output

voltage.

AM02175v1

Current Limit Comparator

OVP DETECTION

LOGIC

Rlim

Dov p (D2)

Auxiliary

w inding

From SenseFET

To OVP Protection

To PWM Logic

Curr. Lim.

BLOCK

(R3)

CONT PIN

Rov p (R14)

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

−

⋅

⋅

=

OVP

D

_

drop

OVP

_

OUT

s

AUX

OVP

)

3

R

_(

LIM

)

14

R

_(

OVP

V

V

V

N

N

V

R

R

)

2

D

_(

ovp