ZHCSHY7A April 2018 – May 2018 UCC28742
PRODUCTION DATA.
- 1 特性
- 2 应用
- 3 说明
- 4 修订历史记录
- 5 Pin Configuration and Functions
- 6 Specifications
-
7 Detailed Description
- 7.1 Overview
- 7.2 Functional Block Diagram
- 7.3 Feature Description
- 7.4 Device Functional Modes
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8 Applications and Implementation
- 8.1 Application Information
- 8.2
Typical Application
- 8.2.1 Design Requirements
- 8.2.2
Detailed Design Procedure
- 8.2.2.1 Custom Design With WEBENCH® Tools
- 8.2.2.2 VDD Capacitance, CDD
- 8.2.2.3 VDD Start-Up Resistance, RSTR
- 8.2.2.4 Input Bulk Capacitance and Minimum Bulk Voltage
- 8.2.2.5 Transformer Turns Ratio, Inductance, Primary-Peak Current
- 8.2.2.6 Transformer Parameter Verification
- 8.2.2.7 VS Resistor Divider and Line Compensation
- 8.2.2.8 Standby Power Estimate
- 8.2.2.9 Output Capacitance
- 8.2.2.10 Feedback Loop Design Consideration
- 8.2.3 Application Curves
- 8.3 Do's and Don'ts
- 9 Power Supply Recommendations
- 10Layout
- 11器件和文档支持
- 12机械、封装和可订购信息
8.2.2.6 Transformer Parameter Verification
The transformer turns-ratio selected affects the MOSFET VDS and secondary rectifier reverse voltage so these should be reviewed. The UCC28742 controller requires a minimum on time of the MOSFET (tON) and minimum DMAG time (tDMAG(min)) of the secondary rectifier in the high line, under minimum-load condition. The selection of fMAX, LP and RCS affects the minimum tON and tDMAG.
The secondary rectifier and MOSFET voltage stress can be determined by the equations below.
For the MOSFET VDS voltage stress, an estimated leakage inductance voltage spike (VLK) needs to be included.
The following equations are used to determine for the minimum tON target of 0.35 µs and minimum de-mag time, tDMAG(min), target of 1.7 µs. Notice that the minimum tON target of 0.35 µs is determined by CS pin Leading-edge blanking time, TCSLEB in Electrical Characteristics. The target is to design LP and make tON(min) ≥TCSLEB . But in very worst normal operation condition, during the tON(min) , the CS pin OCP should not be triggered, i.e., the CS pin should not reach near 1.41 V defined by VOCP in Electrical Characteristics.
