Therefore, the required current limit feed-forward as seen across the current sense resistor is ( Equation 10 ):
To 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 ):
The I CS current is injected onto an external resistor ( Equation 13 ) that is series with the CS controller pin and the current sense resistor (R SENSE). 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 battery chargers.
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