SMPS- Current mode controller

[kanishuw]

Could someone please clearly explain the operation of a current mode controller in an SMPS?
Comparison with the voltage mode controller would be greatly appreciated.

thanks !
Kanishuw

 

[eem2am]

my excellent friend.

it is simple...

voltage mode, the ramp is from inside the chip..........current mode, the ramp is from the current in the current sense resistor.

 

[svhb]

With a current mode controller, you make a two-step control loop. The inner loop is the current-controller, the outer loop the voltage controller.

The voltage loop gives a certain setpoint to the current loop. So from the point of the voltage loop, the converter is a current source.

Some advantages:
- easier to make stable control loops at higher bandwidths
- easy to implement overcurrent protection
- input voltage changes have no effect on the (slower) voltage loop, but are taken care of by the (faster) current loop. With voltage control, when the input voltage changes, it takes some time before the output changes, and only then the control loop can start minimizing the error. With current control, there should be no changes in output voltage at all (theoretically) because the duty cycle is adjusted very fast to maintain the load current.

 

[eem2am]

and also,

if you do current mode control, but using too much slope compensation.....then you end up with voltage mode control.