Improving peak current-mode control: Page 3 of 5

May 10, 2013 //By Terry Allinder
Improving peak current-mode control
Terry Allinder of Texas Instruments focuses on a novel method to improve the performance of peak current-mode control.
is ( Equation 6 ):

     (6)
 
 

Forced to over design
The analysis shows that the input power, which is calculated at the minimum input line voltage, increases from 6.81W to 8.12W, which is a 16.1% increase. This forces power-supply designers to over design the power stage. Figure 3 is a plot of V OUT versus the output current (I OUT) of a power supply operating with peak current-mode control.


Click on image to enlarge.
Figure 3. Peak current-mode control V OUT vs. I OUT.

Novel implementation of peak current-mode control
To improve the performance of peak current-mode control, a novel method of biasing up the peak primary current as seen across the current sense resistor was developed, line current limit feed-forward. In the flyback converter topology we can take advantage of the transformer auxiliary winding, which is used to provide bias power to the controller. Referring to Figure 4 , while the main flyback switch is on, Q1, the voltage on the auxiliary winding is negative and proportional to the rectified line voltage ( Equation 7 ).

     (7)

Where: naux is the number of turns on the flyback primary (Np) divided by the number of turns on the transformer auxiliary (naux) winding.


Click on image to enlarge.
Figure 4. Flyback converter with feed forward.

The input to the PWM QR pin of the controller is a current mirror. As the voltage on the auxiliary winding goes negative, a current (IQR) proportional to the input rectified line voltage is mirrored by a gain of 100 ( Equation 8 ), and injected on to the current sense resistor to offset the current sense voltage (VCS OFFSET). The current into the QR pin is set with a resistor RFF.

     (8)

The first step is to force the peak current at the maximum V IN to produce the same input power as we calculated at the

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