AND8038 Implementing the NCP1200 in a 10 W AC/DC Wall Adapter Prepared by: Christophe Basso ON Semiconductor http://onsemi.com APPLICATION NOTE INTRODUCTION rugged 10 W adapter. This adapter is designed to operate from a universal mains (90–260 VAC) while providing a good standby power at no load. The NCP1200 implements a standard current mode architecture where the switch–off time is dictated by the peak primary current setpoint. By combining fixed frequency and skip cycle operation in a single integrated circuit, ON Semiconductor NCP1200 represents an excellent solution where cost and ease of implementation are premium: low–cost AC/DC adapters, auxiliary supplies, etc. Furthermore, the device does not require any auxiliary winding to operate and thus offers a real breakthrough alternative to UC384X based supplies. This application note details how to build an efficient and + C2 47 F 400 V 1 Adj HV 8 2 FB 7 3 CS VCC 6 B1 SMD Universal Input The Electrical Schematic Driving an external MOSFET, the NCP1200P60, only requires a sense element and a Vcc capacitor. Working together with an internal high–voltage current source, this Vcc capacitor provides the NCP1200 with an average DC level of 11 V typically while it also controls the short–circuit time out. All these parameters are detailed in the application note AND8023 available to download at www.onsemi.com. The electrical schematic appears in Figure 1: C8 10 nF 400 V R7 22 k 2W 1:01 D2 MBR360T3 Lp 1.8 mH + T1 D3 MUR160 L2 22 H + C6b 470 F 16 V 12 V @ 0.85 A + C7 100 F/16 V Ground C6a 470 F 16 V R3 560 R5 3.9 k M1 MTP2N60E 4 Gnd Drv 5 C1 R2 1 Meg R1 10 C4 100 nF 100 nF X2 L1 2 x 27 mH CM Schaffner RN1140–08/2 + C3 22 F 16 V R4 1.8 1W C5 2.2 nF Y Type IC2 TL431 R6 1k C9 1 nF Figure 1. A 10 W AC/DC adapter built with the NCP1200 Semiconductor Components Industries, LLC, 2001 April, 2001 – Rev. 3 1 Publication Order Number: AND8038/D AND8038 protection activates again. If the short–circuit has gone, the IC resumes its operation and delivers its normal level. To check the correct value of the calculated Vcc capacitor, you need to monitor both output voltage and Vcc level on an oscilloscope. A shot as proposed by Figure 2 confirms the validity of a 22 µF choice. We can see that the internal error flag goes high first but as soon as Vout reaches its target level, the flag goes back to zero, confirming the normal controller behavior at the UVLOLow checkpoint. This experiment should be carried in the worse case conditions, e.g. low mains and maximum output load. As stated in AND8023, the Vcc capacitor needs to be evaluated taking into account the startup sequence (actually seen as a transient short–circuit by the controller). An internal error flag is raised within the NCP1200 when an output overload occurs. If this error flag is still asserted when the Vcc capacitor reaches UVLOLow (around 10 V typical), then the IC goes into the latch–off phase: the output drive is locked and the internal consumption falls down to 350 µA typical. When another Vcc breakpoint is reached (around 6.0 V), then the internal current source turns on again and the IC tries to restart. If the error is still present, the Error flag Ok, flag = 0 Figure 2. The startup sequence shows a Vout establishment before UVLOLow is reached Feedback Loop In this application, a precise output voltage is obtained through the use of a TL431. Since we target a 12 V output, you calculate the upper and lower voltage sense elements by applying the following formula: Vout (R6) = 1.0 kΩ. This network ensures a bridge current flow of 2.0 mA which is good for the noise immunity. As any closed loop systems, a compensation network needs to be tailored to stabilize the loop. In this aspect, the NCP1200 average SPICE model will save you a tremendous amount of time. The simulation template appears in Figure 3 on the following page, showing how to wire the NCP1200 average model with INTUSOFT’s IsSpice4. Rupper 1 · Vref Rlower Depending on the TL431 type, Vref can be 2.5 V or 1.25 V. With a 2.5 V reference, Rupper (R5) = 3.9 kΩ and Rlower http://onsemi.com 2 AND8038 Iout NCP1200 Averaged IN CTRL 120 LoL 1 kH + CoL 1 kF 0 Vin out1 L1 22 µF 4 Rs out2 10 m 12.2 + 6 D1 MBR140P X1 NCP1200_Av FS = 66 k L = 1.8 m RI = 1.5 12 Vin 126 2 FB GND 2.38 1 127 OUT X1 XFMR RATIO = 0.1 R4 100 m R5 100 m 12.2 12.2 15 14 + C1 470 µF Vstim AC = 1 out1 R17 300 m 12.2 7 9 C5 470 µF Rload 14 C2 10 µF out2 R15 560 2.38 Vout 11.8 11 5 Cf 100 nF 11.1 10 Rupp 3.9 k 2.50 X3 TL431 13 Rlow 1k Figure 3. The average model of the NCP1200 when used in AC analysis the AC stimuli to allow Bode plot generation. Figure 4 portrays the simulated results with a 100 nF feedback capacitor, while Figure 5 offers the true measurement curves. The loop is kept opened in AC thanks to LoL which exhibits a fairly high value. However, during its bias point calculation, SPICE opens all capacitors and shorts all inductors. Therefore, LoL closes the loop in DC but blocks Mag (dB) Phase Gain BW = 600 Hz 0 Y = 20 dB/div 10 100 Y = 45°/div 1k 10 k Phase (deg) 80.00 180.00 60.00 135.00 40.00 90.00 20.00 45.00 0.00 0.00 –20.00 –45.00 –40.00 –90.00 –60.00 –135.00 –80.00 –180.00 100 k 10 100 1k 10 k 100 k Figure 5. confirmed by a network analyzer measurement Figure 4. Bode plot obtained using SPICE http://onsemi.com 3 AND8038 switching cycles in standby operation. By default, skip cycle takes place at 1/3rd of the maximum peak current: 200 mA in our case. Because skip cycle frequency will naturally enter into the audible range, it is important that the skip current value does not engender noise. Fortunately, if that would be the case, you could still wire a resistor bridge on pin 4 to fix a DC point different than the default one (1.4 V). As a result, you can force skip operation to happen at less than 1/3rd of the maximum peak current. However, keep in mind that the highest peak currents in skip mode offer the best standby power. This is because of the switching cycles population within the bursts: less cycles mean less switching losses and better efficiency at no load. A quick method to assess the RMS current in the MOSFET consists in simulating the whole AC adapter with SPICE. This has already been presented in AND8029 and the schematic will not be reproduced here. The simulated results are given below through Figure 6 and Figure 7 while the supply is delivering 10 W: As you can see, curves are in good agreement, despite the small DC gain error which predicts a slightly lower bandwidth in the case of SPICE. In both cases, the phase and gain margins confirm the good stability of the design, but also the validity of the SPICE model (based on Ben–Gurion University GSIM approach). The NCP1200 FB pin being a high impedance path, a 1.0 nF placed between this pin and ground will prevent any noise picking during operation. Transient Results Using the NCP1200 design aid spreadsheet lead us to a transformer offering the following specs: Lprim = 1.8 mH, Np:Ns = 1:0.1, RM8 or E25 core. For ease of implementation, this transformer will be available from Coilcraft, as referenced in the bill of material. The maximum peak current has been fixed to 600 mA. This value essentially defines the air gap requirement in the transformer but also the final potential transformer mechanical noise generated in standby. As explained, the NCP1200 skips 1.030 M 1.040 M 1.050 M 1.060 M 1.070 M Figure 6. Transient results obtained with IsSpice4 Figure 7. Compared to true measurements power in free–air conditions (without a heatsink) of: Worse case conditions (low mains, maximum output current) gives an RMS drain current of 230 mA. Associated with a 6.5 Ω Rds(ON) @ Tj = 100°C, the conduction losses grow up to 340 mW. Using a TO220 package for the MOSFET, offers the ability to dissipate a given amount of Pmax Tj Tamb 1.3 W. Further switching losses Rj a measurements confirm the ability to use this MOSFET without any heatsink up to an ambient of 80°C. http://onsemi.com 4 AND8038 dBµV 90 EN_V_QP 80 60 40 20 0 –20 0.15 1.0 10.0 30.0 MHz Figure 8. The final composite QP plot carried over one line while the other is loaded (230 VAC, Pout = 10 W) Final Performance We have carried some power tests on the 10 W adapters and the below numbers will confirm the pertinence of choosing ON Semiconductor’s NCP1200 for your next designs: Conducting EMI Filtering The 10 W NCP1200 demo board is equipped with a front stage filter who lets you pass the CISPR22 EMI tests in both quasi–peak and average detector methods. The method we used for calculating the filter is described in AND8032 “Conducted EMI Filter Design for the NCP1200’’. The front stage is made of a single common mode (CM) choke whose wiring method gives enough leakage inductance for differential mode (DM) filtering. Figure 8 plots the final CM+DM noise component confirming the test passing. VinDC Pout(W) Pin(W) (%) 126 0 0.245 – 126 10.5 12.6 83.3 356 0 0.462 356 10.5 13.17 79.7 The standby power can be further reduced by implementing one of the method proposed in AND8023 either through an additional diode or an auxiliary winding. Thanks to its inherent protection circuitry, NCP1200 protects the power supply in presence of a permanent output short circuit. When shorted, the average output current was less than 500 mA. http://onsemi.com 5 AND8038 C6a 470 µF/16 V, vertical C6b 470 µF/16 V, vertical C7 100 µF/16 V, vertical C8 10 nF/400 V D1 MUR160, ON Semiconductor D2 MBRS360T3, ON Semiconductor B1 Bridge 1 A/600 V, mini DIP Transformer available from Coilcraft U.S, ref. : Y8848–A Mains connector: Schurter GSF1.1202.31 with fuse 10 W Demoboard, Bill of Material R1 10 Ω, 1 W through holes R2a R2b 2 times 560 kΩ SMD in series R3 560 Ω SMD R4 1.8 Ω, 1W SMD or 1.8 Ω 1 W through holes R5 3.9 kΩ SMD R6 1 kΩ, SMD R7 22 kΩ, 2 W through holes L1 Schaffner RN114–08/2 L2 22 µH, 1 A M1 MTP2N60E, TO–220 through holes, ON Semiconductor IC1 SFH615A–2, SMD (optocoupler) IC2 TL431BC (TO–92), ON Semiconductor IC3 NCP1200P60, DIP8, ON Semiconductor C1 100 nF X2/ 250 VAC C2 47 µF/400 V, snap–in vertical C3 22 µF/16 V, vertical C4 100 nF, SMD C5 1.5 nF Y1 type only Other Available Documents Related to NCP1200: AND8023/D, “Implementing the NCP1200 in Low–Cost AC/DC Adapters” AND8029/D, “Ramp Compensation for the NCP1200” AND8032/D, “Conducted EMI Filter Design for the NCP1200” PSpice, IsSpice4 and Micro–Cap Averaged and Transient models available in ready–to–use templates at www.onsemi.com NCP1200 Design aid spreadsheet with EBNCP1200/D http://onsemi.com 6 AND8038 Printed Circuit Board Details Figure 9. Component Side, Silk Screen, Scale 1 Figure 10. Solder Side, Silk Screen, Scale 1 Figure 11. Copper Traces, Scale 1 http://onsemi.com 7 AND8038 ON Semiconductor and are trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. “Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. 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