Power Supply Design

Question: What are the advantages and disadvantages of current-mode control?

Original Question: May I know what are the advantages for using a current-mode-control boost converter instead of voltage mode?

Answer: A good article is Current Mode or Voltage Mode? by Dr. Ray Ridley, in Switching Power Magazine, October 2000, pp. 4,5.

Switching Power Magazine is a free magazine (at least to U. S. subscribers) and you may be able to locate a issue or request a copy of the article. Check out Ridley Engineering's website at www.ridleyengineering.com/. Ridley Engineering has been a sponsor of this website.

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.

Original Question: I read in an article that there are three advantages of using current mode control which are:

1. Immune to input disturbance

2. Parallel current sharing

3. Current protection

without any further details or explanation. Can you explain further?

Answer.

1. 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. It is easier to parallel current sources into an output capacitor than parallel voltage sources.

3. 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.