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Need source resistor for current mode forward converter?

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eem2am

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Hello,

I am doing an offline Two Transistor Forward Converter with LT1241 PWM controller.

I am doing it using current-mode, with a resistor in the source of the lo-side primary-side Mosfet.

Now, this is a bit strange, because the current sensed by this current sense resistor is composed of ….

1. the primary-referred secondary current.
2. the primary magnetising current.

Number 1 above is fine,
Number 2 above throws a “spanner in the works”

The magnetising current is not directly related to power being passed to the output, but we have to put up with it.

It creates an extra ramp on the primary current sense signal.

And please may I put the following three questions, any answers gratefully appreciated………………..

A. Does this additional ramp, caused by the magnetising current, act like “slope compensation”. ?

B. And does it just mean that the converter is really a “hybrid” converter, -part current-mode and part-voltage mode ?


C. If instead, I were doing this converter using pure voltage-mode control with a voltage mode PWM controller, would I definitely NOT have any resistor in the source of the lo-side primary mosfet ?
-That is, would I definitely not bother sensing the primary current when using pure voltage mode control?
 

willi65

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Hi...
the LT1241 like the same as UC3842/44 ec.. this Controller are Currentmode controllers .. the Ramp are the current trough the Prim Winding of the Transformer...U sense the Output Voltage this are compare with the ramp.and the PWM switch of...
U done the layout already? there are some for check very well..
Ur source Resisitor are low value can use MPC resistor no inductance...
U need an slope compesation when u have an with range of output current..this reduce the jitter..because the Current mode need an ramp..ur ramp is not changeable with the resistor ..only with the Inductor.....
 

FvM

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Current mode means, that the inductor respectively switch peak current is controlled by an inner control loop. It doesn't necessarily involve a particular relation of this value and other process values, e.g. output current, secondary transformer current, whatever.

A voltage mode controller doesn't basically need a inductor respectively switch current measurement. But it may utilize it e.g. for short circuit protection.

There are also ways to measure the switch current (at least roughly) by indirect methods as MOSFET saturation voltage.
 

ahoss2000

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The difference between voltage mode and current mode is that voltage mode uses an artificial ramp ( usually the timing cap sawtooth) for the PWM, and current mode uses the ramp from the inductor current.

The ramp in current mode when primay current is sensed is made up from Primay magnetizing current as well as secondary inductor current ramp. current mode can also use the secondary inductor current if a current sense transoformer is used to detect the secondary current.
 

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