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
-
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
7.3.1.3 VS (Voltage-Sense)
The VS pin connects to a resistor-divider from the auxiliary winding to ground. The auxiliary voltage waveform is sampled at the end of the transformer secondary-current demagnetization time. The waveform on the VS pin determines (1) the timing information to achieve valley-switching, (2) the timing to control the duty-cycle of the transformer secondary current, and (3) the output voltage over-voltage. Avoid placing a filter capacitor on this input which interferes with accurate sensing of this waveform.
Besides, the VS pin also has these two functions: (4) senses the bulk capacitor input voltage to provide for ac-input run and stop thresholds, and (5) to compensate the current-sense threshold across the AC-input range. This information is sensed by monitoring the current pulled out of the VS pin during the MOSFET on-time. During this time the voltage on the VS pin is clamped to about 250 mV below GND. As a result, the current out of the pin is determined by the upper VS divider resistor, the auxiliary to primary turns-ratio and the bulk input voltage level. For the AC-input run/stop function, the run threshold on VS is IVSL(run) (typical 210 µA) and the stop threshold is IVSL(stop) (typical 75 µA). The values for the auxiliary voltage divider upper-resistor RS1 and lower-resistor RS2 can be determined by the equations below.
where
- NPA is the transformer primary-to-auxiliary turns ratio,
- VIN(run) is the AC rms voltage to enable turn-on of the flyback converter (run),
- VBULK(run) is the DC bulk voltage to enable turn-on of the flyback converter (run),
- IVSL(run) is the run-threshold for the current pulled out of the VS pin during the primary MOSFET on-time. (see the Electrical Characteristics table).
where
- VOV is the maximum allowable peak voltage at the converter output,
- VF is the output rectifier forward voltage drop at near-zero current,
- NAS is the transformer auxiliary to secondary turns ratio,
- RS1 is the VS divider upper-resistor resistance,
- VOVP is the overvoltage detection threshold at the VS input (see the Electrical Characteristics table).
Notice that VS pin absolute maximum current IVS in its negative clamping is 1.2 mA. After determined RS1 it is required to check if VS pin current stays ≤ 1.2 mA. The check is to determine the input voltage ratio in this design and make VIN(max) / VIN(run) ≤ IVS / IVSL(run) = 1.2 mA / 0.25 mA = 4.8, i.e., VIN(max) / VIN(run) ≤ 4.8. If the design cannot meet this criterion, external circuit is needed to add in to make sure VS pin current ≤ 1.2 mA, for example, to use a zener type of device to clamp the transformer aux-winding negative voltage to achieve VIN(max) / VIN(run) ≤ 4.8.