Saturday, April 6, 2013

Peak current mode Flyback

Peak current mode Flyback converter


One of the most common type of switching converter is the flyback. It is mostly used in AC/DC power supplies because it provides isolation. In order to achieve regulation of its output voltage several techniques may be employed. A robust and simple technique is the peak-current mode control, which advantages are listed below:

Advantages:
- cycle by cycle current protection
- eliminates the inductor from the control loop
- simple and cheap to implement

How it works:
The output voltage signal (VFB) is carried to the negative input of an error amplifier, the positive input is connected to a reference voltage (VREF) instead. The output of the error amplifier (EA) is sent to a comparator where it is compared to the primary inductor current signal (CS). The oscillator begins the switching cycle by triggering the latch which turns the switch M1 on. The current through the primary inductor begins to rise at this point. When the rising current through the primary inductor equals the error amplifier output regulation signal the switch is turned off and this ends the switching cycle. The whole thing repeats at the next oscillator trigger signal.

Observations:
Care should be taken to properly compensate the voltage regulation loop by placing compensation elements across the EA feedback pins, this is necessary to assure the regulation would work stably over a wide band of load and other parameters. Knowledge of control theory is of great help here.

Example:
Below is an example of peak current mode controlled flyback converter, including simplified control circuitry, operating at 5V output voltage. Click on the circuit below to open CircuitLab and then select the ''Open in editor'' button.




Flyback converter current mode control.
Current mode flyback simulation example

Question:
Based on the simulation circuit above and looking at its waveforms, do you think this circuit would work in real world? Please give an explanation to your answer, will come back with the response soon...

Need help in designing your power electronics circuit, LED driver, dc/dc converter, ac/dc power supply?

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1 comment:

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