JAJSE40A October 2017 – February 2018 UCC28780
RBLEED is used to discharge the clamp capacitor voltage to a residual voltage (VRESIDUAL) during the 1.5-s fault delay recovery time (tFDR). After the converter recovers from the fault mode, lower VRESIDUAL reduces the maximum current stress (ISHORT(MAX)) flowing through the switching devices within their respective safe operating areas, even if the output voltage is shorted. VRESIDUAL can be determined by the target ISHORT(MAX) multiplied with the characteristic impedance between the leakage inductance (LK) and the clamp capacitor (CCLAMP). ISHORT(MAX) is based on the de-rated maximum pulse current of QH or the output-rectifier current reflected to the primary side, whichever is lower. This design guide can be applied to both primary and secondary resonance ACF converters. An excessively low value of RBLEED results in over-discharging of CCLAMP, and introduces excess continuous power loss which affects standby power.