ZHCSJY8E june 2019 – february 2021 UCC256402 , UCC256403 , UCC256404
PRODUCTION DATA
- 1 特性
- 2 應用
- 3 說明
- 4 Revision History
- 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
- 7.4.1 High Voltage Start-Up
- 7.4.2 X-Capacitor Discharge
- 7.4.3 Burst Mode Control
- 7.4.4
System State Machine
-
Application 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 LLC Power Stage Requirements
- 8.2.2.2 LLC Gain Range
- 8.2.2.3 Select Ln and Qe
- 8.2.2.4 Determine Equivalent Load Resistance
- 8.2.2.5 Determine Component Parameters for LLC Resonant Circuit
- 8.2.2.6 LLC Primary-Side Currents
- 8.2.2.7 LLC Secondary-Side Currents
- 8.2.2.8 LLC Transformer
- 8.2.2.9 LLC Resonant Inductor
- 8.2.2.10 LLC Resonant Capacitor
- 8.2.2.11 LLC Primary-Side MOSFETs
- 8.2.2.12 LLC Rectifier Diodes
- 8.2.2.13 LLC Output Capacitors
- 8.2.2.14 HV Pin Series Resistors
- 8.2.2.15 BLK Pin Voltage Divider
- 8.2.2.16 ISNS Pin Differentiator
- 8.2.2.17 VCR Pin Capacitor Divider
- 8.2.2.18 BW Pin Voltage Divider
- 8.2.2.19 Soft Start and Burst Mode Programming
- 8.2.3 Application Curves
-
Application and Implementation
- 8 Power Supply Recommendations
- 9 Layout
- 10Device and Documentation Support
8.2.2.18 BW Pin Voltage Divider
The BW pin programs the ratio between burst mode entry and exit thresholds as well as senses the output voltage through the bias winding and protects the power stage from over voltage. The nominal output voltage is 12 V. The bias winding has 3 turns, and the secondary side winding has 2 turns. Assuming there is a 0.5-V drop in the rectifier diodes (Vf) and a further 0.5-V drop due to other losses (Vloss), the nominal voltage of the bias winding is given by:
The desired OVP threshold in this design is 140% of the nominal value. The OVP threshold level in UCC25640x device is 4 V, so the nominal BW pin voltage is given by:
The required BW divider ratio is then given by:
In this design, the burst mode threshold ratio is chosen to be 0.6 (Option 6).The target programming resistance is then:
The lower BW resistor can be calculated by:
A standard value of 5.36 kΩ is chosen for the lower BW resistor.
The upper resistor can be calculated by:
A standard value of 30.9 kΩ is chosen for the upper BW resistor.