SLUS458I July 2000 – June 2024 UCC28C40 , UCC28C41 , UCC28C42 , UCC28C43 , UCC28C44 , UCC28C45 , UCC38C40 , UCC38C41 , UCC38C42 , UCC38C43 , UCC38C44 , UCC38C45
PRODUCTION DATA
The current sensing network consists of the primary-side current sensing resistor (RCS), filtering components RCSF and CCSF, and optional RP. Typically, the direct current sense signal contains a large amplitude leading edge spike associated with the turnon of the main power MOSFET, reverse recovery of the output rectifier, and other factors including charging and discharging of parasitic capacitances. Therefore, CCSF and RCSF form a low-pass filter that provides immunity to suppress the leading edge spike. For this converter, CCSF is chosen to be 100pF.
Without RP, RCS sets the maximum peak current in the transformer primary based on the maximum amplitude of the CS pin, which is specified to be 1V. To achieve 1.36A primary side peak current, a 0.75Ω resistor is chosen for RCS.
The high current sense threshold of CS helps to provide better noise immunity to the system but also results in higher losses in the current sense resistor. These current sense losses can be minimized by injecting an offset voltage into the current sense signal using RP. RP and RCSF form a resistor divider network from the current sense signal to the reference voltage of the controller (VVREF) which adds an offset to the current sense voltage. This technique still achieves current mode control with cycle-by-cycle over-current protection. To calculate required offset value (VOFFSET), use Equation 16.
After adding the RP resistance, adjust the RCS value accordingly.