Current limiting resistor in forward converter

[eem2am]

current limitting

Hello,

Is it correct that a Voltage Mode, Transformer-Isolated, One Transistor Forward Converter would need a current sense resistor in the primary just to limit the primary current from going too high ?

That is, a resistor connected between the source of the switching FET and primary ground, with the voltage across this resistor being monitored by a ‘limiting function’ in the PWM controller ?

For example, during start-up, the inrush current into the secondary inductor and capacitor would surely be excessive unless there was no limiting current sense resistor in the primary circuit ?

Also, if a short circuit appeared on the output, then surely a primary current sense resistor would be the only way of limiting the switch current ?

Is this resistor needed and if so, what are its other purposes ?

 

[neddie]

It depends on the controller , but often the current is sensed with the method you describe. for instance with a uc3842/3 controller chip the current in the switch is turned off when the current sense pin gets to 1V. If you have a 1 ohm sence resistor , the maximum primary current would be 1A. Newer controller chips have a current sense threshold voltage much less than 1V to lower power dissapation in the resistive
sensing element.
The current does not "have" to get to maximum , but the switch will turn off if it does.