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 VOUT versus the output current (IOUT) of a power supply operating with peak current-mode control.
Click on image to enlarge.
Figure 3. Peak current-mode control VOUT vs. IOUT.
Novel implementation of peak current-mode control
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
Where: naux is the number of turns on the flyback primary (Np)
divided by the number of turns on the transformer auxiliary (naux)
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 (VCSOFFSET
). The current into the QR pin is set with a resistor RFF.
The first step is to force the peak current at the maximum VIN
to produce the same input power as we calculated at the low-line input (Equation 9
We do that by subtracting the current overshoot as a result of the
propagation delay from the input power at the minimum input line.
Therefore, the required current limit feed-forward as seen across the current sense resistor is (Equation 10
inject a 0.0634V offset voltage onto the current sense resistor, we
need to calculate the voltage on the transformers Auxiliary winding (Equation 7
), selecting IQR in a range of 1 mA to 4 mA. For this example, we used 2 mA. Now we can calculate the required RFF (Equation 10
The current injected on the current sense resistor (Rsense) is the IQR current divided by the current mirror gain of 100 (Equation 12
current is injected onto an external resistor (Equation 13
) that is series with the CS controller pin and the current sense resistor (RSENSE
The result is that the current sense resistor has a biased that is
proportional to the input line voltage. This turns-off the peak current
earlier, forcing the input power at the minimum input line to be
approximately equal to the input power at the maximum input line.
No over design
Peak current-mode control in conjunction
with line current limit feed-forward eliminates the requirement for
power supply designer to over design the power stages. This reduces the
cost, as well as being a low-complexity solution for power supplies and
1. Abraham Pressman, Keith Billings, Taylor Morey, Switching Power Supply Design, Third Edition
, the McGraw-Hill Companies, April 17, 2009.
2. Christoph Basso, "The Over Power Phenomenon,"
How2Power TODAY, October 2010.
For more information visit: www.ti.com/power-ca
Terry Allinder is a principle applications engineer for
Texas Instruments Power Products Division. He has over 30 years of power
electronics experience as an Application Engineer and power supply
designer for military applications. He received his BSEE from California
Polytechnic University, Pomona. Terry can be reached at firstname.lastname@example.org.