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Voltage-Mode vs Current-Mode PWM Power Supply Controller

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Advanced Member level 1
Nov 29, 2005
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pwm power supply

I searched for PWM power supply controller from TI and found out some were Voltage-Mode and some were Current-Mode. What's the difference between both of them? Pls advise.

hysteretic current-mode control current sharing

In voltage-mode control ... output of the switching power supply is divided (usually using a voltage divider), is subtracted from a reference and is compensated using and error amplifier. Then the error voltage at the output of the error amplifier is compared to a sawtooth to generate the driving signal for the switching transistor.
So voltage-mode control is a single loop control technique.

Current-mode control is multi-loop control. The outer loop is a voltage-loop -- so you still have the voltage being sensed and subtracted from a reference and compensated -- but now the error amplifier output provides a reference for the inner current loop. In the inner loop a current in the system is sensed (by using a current sense resistor or otherwise) and compared to the reference (from the voltage loop) and this is used to generate the switching signal for the transistor.
Usually the inner current loop is faster than the outer voltage loop. There are also many types of current-mode control: peak current-mode control, average current-mode control, hysteretic current mode control.

If you look at the diagrams for different types of control method you should be able to identitfy the two loops.

Here's some more information drom

In summary, advantages are easy loop compensation, some help with Right-Half-Plane problems (it does not eliminate them), ability to work with both CCM and DCM with good performance, and good line rejection.

Disadvantages are you have to sense current accurately, there is a subharmonic oscillation instability when you approach a 50% duty ratio, and poor signal to noise ratio on the current sense.

Most experienced designers agree that the most important thing to do in current-mode control design is to keep noise off the compensation ramp.

Three advantages of using current mode control which are:

1. Immune to input disturbance
It makes the supply look like a current source to the input therefore voltage changes at the input do not get through to the output.

2. Parallel current sharing
It is easier to parallel current sources into an output capacitor than parallel voltage sources.

3. Current protection
It is often implemented by cycle-by-cycle current limit protection of the power switch, making it immune to over-current damage from short circuited outputs or overloads.

I know this is a lenghy answer but I hope that you find it useful.

Best regards,
voltage mode pwm current source

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