VIPER12AS-E Ver la hoja de datos (PDF) - STMicroelectronics

Número de pieza

componentes Descripción

Fabricante

VIPER12AS-E

Low power offline switched-mode power supply primary switcher

STMicroelectronics 

Feedback pin principle of operation

VIPER12A-E

The current limitation is obtained with the FB pin shorted to ground (VFB = 0 V). This leads

to a negative current sourced by this pin, and expressed by:

IFB

=

–0----.--2---3----V--

R1

By reporting this expression in the previous one, it is possible to obtain the drain current

limitation IDlim:

IDlim

=

GI

D

⋅

0.23

V

⋅

⎛

⎝

--1----

R2

+

-R-1---1-⎠⎞

In a real application, the FB pin is driven with an optocoupler as shown on Figure 5 which

acts as a pull up. So, it is not possible to really short this pin to ground and the above drain

current value is not achievable. Nevertheless, the capacitor C is averaging the voltage on

the FB pin, and when the optocoupler is off (start up or short circuit), it can be assumed that

the corresponding voltage is very close to 0 V.

For low drain currents, the formula (1) is valid as long as IFB satisfies IFB < IFBsd, where

IFBsd is an internal threshold of the VIPER12A. If IFB exceeds this threshold the device will

stop switching. This is represented on Figure 12 on page 14, and IFBsd value is specified in

the PWM COMPARATOR SECTION. Actually, as soon as the drain current is about 12 % of

Idlim, that is to say 50 mA, the device will enter a burst mode operation by missing switching

cycles. This is especially important when the converter is lightly loaded.

Figure 6. IFB transfer function

It is then possible to build the total DC transfer function between ID and IFB as shown on

Figure 6 on page 10. This figure also takes into account the internal blanking time and its

associated minimum turn on time. This imposes a minimum drain current under which the

device is no more able to control it in a linear way. This drain current depends on the primary

inductance value of the transformer and the input voltage. Two cases may occur, depending

on the value of this current versus the fixed 50 mA value, as described above.

10/21

Doc ID 11977 Rev 2

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