In this application note "Switching Power Supply Topology Voltage Mode vs. Current Mode", page 2, Current Mode Control section the author claimed as follows:

  1. Since the Error Amplifier is now used to command an output current rather than voltage, the effect of the output inductor is minimized and the filter now offers only a single pole to the feedback loop (at least in the normal region of interest). This allows both simpler compensation and a higher gain bandwidth over a comparable voltage-mode circuit.

Could anyone explain it in more detail? How "the effect of the output inductor is minimized and the filter offers only a single pole to the feedback loop" as
Error Amplifier is used to command an output current rather than voltage?

  • \$\begingroup\$ The current is implied being the voltage across the measuring resistor Rsense. So the comparator changes state depending on the current through Rsense, and the error voltage. Both are used instead of just the error voltage. The current through Rsense is proportional to the output current. By my reasoning, the Rsense feedback makes a low time-constant feedback path. I agree that the app note explains poorly. I would love to see those equations. The improved time-constants can make the circuit more stable in that region. \$\endgroup\$
    – user55924
    Jul 26 '17 at 22:49

With voltage mode control you have a control system measuring the output voltage and modifying a PWM signal feeding an LC filter. So you have the double pole response of the filter inside the loop to compensate.

With current mode control you have an inner current loop. It senses the inductor current (with a resistor in series, or by other methods) and changes the duty cycle of the power stage to keep the current proportional to a control voltage, making it a voltage-controlled current source. (Within BW limits, etc.)

So the outer voltage loop "programs" the current loop, which typically has a higher bandwidth than the voltage loop.

To the outer control loop this "looks" like a voltage controlled current source driving a load capacitor, which is a single pole response. So compensation of the outer loop is easier, typically done with a type II compensator vs. the type III compensator necessary for voltage mode control.

  • \$\begingroup\$ +1 could you give some number to see inductor current is almost like a constant current source in current mode control and not in voltage control? \$\endgroup\$
    – emnha
    Jul 27 '17 at 20:58
  • 1
    \$\begingroup\$ Dr. Ray Ridley is one of the pioneers of analyzing current mode control. You can read his thesis and lots of other information that answers your question in detail on his website, www.ridleyengineering.com \$\endgroup\$
    – John D
    Jul 27 '17 at 22:30
  • \$\begingroup\$ I was to find the article "Current Mode or Voltage Mode?" by Dr. Ray Ridley but I couldn't find it now. Do you know where can I get it? \$\endgroup\$
    – emnha
    Jul 27 '17 at 22:36
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    \$\begingroup\$ I don't know how to find that specific article, but there are many other good resources on Dr. Ridley's site. \$\endgroup\$
    – John D
    Jul 28 '17 at 19:18

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