NCP1271ADAPGEVB 19 V, 3.0 A Universal Input AC-DC Adaptor Using NCP1271 Evaluation Board User s Manual NCP1271ADAPGEVB Table 1. FEATURES OF POWER SUPPLY USING NCP1271 Operation Mode Features Topology CCM/DCM Flyback Fixedfrequency currentmode control with inherent primary current limitation. Frequency jittering to soften the EMI signature. Builtin softstart. Output shortcircuit fault detection independent of the auxiliary winding. Integrated high voltage startup that minimizes standby power loss. Standby Condition SoftSkip Operation Adjustable skip level for optimal standby power consumption. Proprietary SoftSkip to reduce the risk of lowfrequency audible noise. SoftSkip operation is automatically disabled if an abrupt transient load is applied from standby operation. This improves the output response to a transient load. Fault Condition Double Hiccup Double Hiccup operation minimizes the power dissipation in a fault mode and Restart allows the application to autorecover when the fault is removed. Latch Protection Latch Off An internal latch makes it easy to add overtemperature protection (OTP) or Activated overvoltage protection (OVP) to any applications. Latch is reset by unplugging the AC input and allowing V to drop below 4 V (typ). CC THE EVALUATION BOARD SPECIFICATION Based on the results from the NCP1271 design Input 85 to 265 Vac, 50 Hz spreadsheet, the final values for this adapters key flyback Output 19 Vdc, 3.0 A, Isolated parameters were calculated to be: Features < 100 mW Input Power at 230 Vac Np:Ns = 5:1 Excellent Light Load Performance Lp = 180 H No Audible Noise R = 0.3 > 85% Full Load Efficiency CS Short Circuit Protection Activates at < 100W Ipeak(full load) = 3.5 A for Any Input Voltage Switch Rating = 6 A, 800 V Diode Rating = 3 A, 100 V A Discontinuous Conduction Mode (DCM) flyback was Rsnubber = 100 k selected for this application. DCM gives very good stability, Csnubber = 10 nF small inductor size (lower leakage inductance), and good transient response. Setting the Short Circuit Protection Level The current sense resistor (R or R8), provides two CS Flyback Calculations functions. First it senses the primary current for currentmode Several resources are available at www.onsemi.com to PWM operation. Secondly, it provides the maximum primary calculate the necessary component values for a flyback current limitation according to equation 1: supply. In particular, an Excel based design spreadsheet can 1V be found at: I (eq. 1) p(max) R www.onsemi.com/collateral/NCP1271SHEET.xls CS Additionally, most of the other NCP12xx application The short circuit protection activates when the I p(max) notes also apply to the NCP1271. For detailed information current is reached for more than 130 ms (typ). This also on designing a flyback power supply, please visit corresponds to V being greater than or equal to 3 V for FB AND8076/D. Other app notes which may also aid in the 130 ms. Therefore, R must be set large enough to ensure CS design include: that the required peak current can always be delivered, but small enough to meet the short circuit protection AND8069/D Tips and Tricks to Build Efficient Circuits requirements. A DCM flyback converter has the following With the NCP1200 relationship: AND8205/D How to Choose a Switching Controller for 2 P 1 2 L I F (eq. 2) out p p SW Design Therefore, for an assumed efficiency of 80%, a peak AND8023/D Implementing the NCP1200 in LowCost AC/DC Converters current of 4 A should trigger the short circuit protection circuitry at 80 W. This corresponds to an R value of CS AND8032/D Conducted EMI Filter Design for the 0.25 . This change in R may also require that the snubber CS NCP1200 and transformer be recalculated to handle this level of peak AND8076/D A 70 W Low Standby Power Supply with the current during the short circuit fault time. A few iterations NCP12xx Series of the Excel based NCP1271 design spreadsheet should produce a good starting point for the applications design.