One thing can not understand for current amp in Power factor corrector

[simbaliya]

Below is the architecture


"Acting as a control signal to the pre-regulator, I M has
an instantaneous value that follows the shape of the input voltage, and an average
value that is inversely proportional to its rms value. Resistor R 3 converts I M to a
voltage while a voltage proportional to the line current is measured across R s, the
line current sensing resistor.

The difference between these two voltages is applied to the input of the current
amplifier. Under closed loop control, the current amplifier will try to keep this
voltage difference close to zero volts. This forces the voltage produced by the return
current on R s to be equal to the voltage across R 3."

For the last sentence I can not understand why, can someone explain in detail?
thks!

 

[FvM]

I would prefer the term current controller or current control amplifier for the said amplifier. It's a controller with IM as setpoint and Isense as process value. It sets the pwm duty cycle to make Isense equal to the setpoint (as far as possible). IM is a fullwave rectified sine and the current through Rs is controlled to follow this waveform.

 

[simbaliya]

I would prefer the term current controller or current control amplifier for the said amplifier. It's a controller with IM as setpoint and Isense as process value. It sets the pwm duty cycle to make Isense equal to the setpoint (as far as possible). IM is a fullwave rectified sine and the current through Rs is controlled to follow this waveform.

Yes I oso agree with you, as explained in the above paragraph, Voltage drop across R3 and RM will be the same with opposite polarity, thus make IS followed IM, but I just don't understand how is this achieved.

 

[FvM]

but I just don't understand how is this achieved.

By controlling the PWM duty cycle, as said.

 

[simbaliya]

By controlling the PWM duty cycle, as said.

Can you tell me where can I find the tech doc for design of this current control amplifier with the same topology above?

 

[FvM]

The amplifier itself is just an OP implementing a PI characteristic. You can refer to control theory literature about PI controllers for the principle operation. Or do you have difficulties to understand how the multiplier output current IM is converted to a voltage by R3? In my opinion, this is a trivial detail, no very important for the basic device operation. It could be implement in a different way as well.

 

[grizedale]

its a current amplifier and has a bandwidth of about Fsw/12.

It output will be zig-zag, but will have the 100Hz envelope on it, as its inputs are from the current and line voltage.........which are pulsating at 100Hz.

It gets compared to the oscillator via the PWM comparator.

Obviously, the Duty cycle will change slowly, and with again an envelope of 100Hz.

...The voltage amp, current amp, and multiplier , combine together to make the current follow the Bus voltage "humps".



so basically reverse engineer it..
-you know ifs pfc
-so you know the duty cycle will change with a 100Hz envelope
-so you know that the output of the current amplifier will basically be a 100Hz waveform (in steady load condition)

 

[simbaliya]

its a current amplifier and has a bandwidth of about Fsw/12.

It output will be zig-zag, but will have the 100Hz envelope on it, as its inputs are from the current and line voltage.........which are pulsating at 100Hz.

It gets compared to the oscillator via the PWM comparator.

Obviously, the Duty cycle will change slowly, and with again an envelope of 100Hz.

...The voltage amp, current amp, and multiplier , combine together to make the current follow the Bus voltage "humps".



so basically reverse engineer it..
-you know ifs pfc
-so you know the duty cycle will change with a 100Hz envelope
-so you know that the output of the current amplifier will basically be a 100Hz waveform (in steady load condition)

So this is not a normal voltage amplifier but a current amplifier, the output of it is a current. My understanding is that to drive the comparator stage we need to convert the output current to voltage, is that right?

 

[FvM]

So this is not a normal voltage amplifier but a current amplifier, the output of it is a current.

If you refer to the part marked with "I", it's a regular OP and it's output is a voltage that feeds the PWM comparator. I already made a comment about suitable terms.

 

[grizedale]

it just so happens that its input voltage is derived from the smps current, since this current goes through a sense resitor and produces a voltage.

To understand this opamp.

Think what would happen if it was not there.

.........IF ABSEnt, then you would have no way of combining the feedback info from sensed current, bus voltage and output voltage to give the duty cycle.

 

[simbaliya]

it just so happens that its input voltage is derived from the smps current, since this current goes through a sense resitor and produces a voltage.

To understand this opamp.

Think what would happen if it was not there.

.........IF ABSEnt, then you would have no way of combining the feedback info from sensed current, bus voltage and output voltage to give the duty cycle.

Can you explain how the output of the current amp is achieved zig-zag waveform using this configuration? As my understanding is that the output is the amplification of the voltage drop across the resistor in the inverting terminal(which is the voltage difference between RS and R3), if what u said is right, then this voltage drop must oso be zig-zag, but why is so?
.