AND8042/D Implementing Constant Current Constant Voltage AC Adapter by NCP1200 and NCP4300A http://onsemi.com Prepared by: Hector Ng ON Semiconductor APPLICATION NOTE Circuit Description Circuit and BOM of the AC adapter is shown in Figure 1 and Table 1. This design can accept universal AC input from 90 V to 264 VAC. Bulk capacitors C5 and C6 are split by inductor L1 and L2 to form the EMI filter as well as to provide energy storage for the remaining DC to DC converter circuit. Thanks to dynamic self supply of NCP1200 (please refer to NCP1200 data sheet), Vcc capacitor C7 is charged to startup voltage 11.4 V and the power MOSFET MTD1N60E starts switching. To reduce power consumption of NCP1200, HV pin (pin 8) is supplied by half wave rectification through a parallel combination of diode D6 and resistor R13. A small signal diode 1N4148 is enough for this function because diode D6 just has to withstand one diode drop during negative half cycle. R13 is to equilibrate the voltages on the 1N4148 when both diodes and high volt current source of NCP1200 are in the off state. R12 is to set the power level at which NCP1200 goes into pulse skipping, please refer to below section for more details. RCD snubber R1, C1 and D3 provides the necessary snubbing function to prevent drain voltage of MTD1N60E to exceed 600 V. Choosing suitable value for the sensing resistor R7 is very important as it limits the primary peak current during power up. If its value is too low, the system cannot deliver enough power during full load low AC input. On the contrary, the transformer may go into saturation and damages Q1 and NCP1200. Information on how to determine value of R7 is elaborated in latter paragraph. Introduction This paper describes a compact design of constant current constant voltage (CCCV) AC adapter based on the current mode PWM controller NCP1200 and the secondary side feedback IC NCP4300A. By these two ICs from ON Semiconductor, circuit design is much simplified. These devices enable users to meet ever increasing demand of smaller dimension and more sophisticated protection feature of AC adapter. On the primary side, NCP1200 is used as the PWM controller. This current mode controller requires very few external components and no auxiliary winding is needed to supply this IC. In addition, NCP1200 can fulfill IEA recommendation easily because it features a pulse skipping low power consumption mode. NCP4300A is a general purpose device which consists of two operational amplifiers and a high precision voltage reference. One of the operational amplifiers is capable of rail to rail operation. NCP4300A is employed to provide voltage as well as current feedback to NCP1200. Output of the AC adapter is maintained at 5.2 V from no load to 600 mA. Further increase in load enters constant current output portion and output is kept at 600 mA down to zero volt. This output characteristic assures a basic protection against battery overcharge which is needed by a lot of applications, for instance cellular phone AC adapter. Semiconductor Components Industries, LLC, 2001 February, 2001 – Rev. 1 1 Publication Order Number: AND8042/D D1 MUR120 L1 470 µH 0.2 A 470p 250 V 100 k 1 W C1 + C5 4.7 µ 400 V 90–264 VAC + DF06S – V4 1 U2 4 + C6 4.7 µ 400 V 2 R1 U1 1 2 3 4 3 8 7 6 5 D3 1N4937 + C2 10 µ Q1 MTD1N60E R10 68 k 3 1 In1– 2 In1+ 3 Ground 4 U4 D4 1N4148 D5 1N4148 Figure 1. Circuit Description VCC 8 Out2 7 In2– 6 In2+ 5 NCP4300AD C9 0.047 µF + R5 10 k 1% U3 2 R12 10 k R3 10 k 1% C4 47 µ Out1 R7 3.3 0.6 W SFH6156–3 4 1 L2 470 µH 0.2 A R2 3.3 k R4 1.5 k C3 330 µF C10 1 nF 250 VAC Y1 D6 1N4148 + C7 47 µF + 5.2 V, 600 mA R6 0.15 R8 2.7 k 1% R9 470 R11 75 k 1% C8 0.1 µ AND8042/D 2 http://onsemi.com NCP1200 NCP1200D60 R13 220 k L3 4.7 µH 1A D2 1N5819 AND8042/D Table 1. Reference Part Quantity Manufacturer U1 NCP1200D60 1 ON Semiconductor U2 DF06S 1 General Semi or IR U3 NCP4300AD 1 ON Semiconductor U4 SFH6156–3 1 Infineon Q1 MTD1N60E 1 ON Semiconductor C1 470 p, 250 V 1 C2, C7 10 F, 25 V 2 C3 330 F, 35 V 1 Panasonic FC Series or Rubycon JXA Series C4 47 F, 16 V 1 Panasonic FC Series or Rubycon JXA Series C5, C6 4.7 F, 400 V 2 C8 0.1 F 1 C9 0.047 F 1 R1 100 K, 1.0 W 1 R2 3.3 K 1 R3, R5 10 K, 1% 2 R4 1.5 K 1 R6 0.15 W, 0.1 W SMT 1 R7 3.3 , 0.6 W 1 R8 2.7 K, 1% 1 R9 470 1 R10 68 K 1 R11 75 K, 1% 1 R12 10 K 1 R13 220 K 1 D1 MUR120 1 D4, D5, D6 1N4148 3 D2 1N5819 1 ON Semiconductor D3 1N4937 1 ON Semiconductor L1, L2 470 H, 0.2 A 2 L3 4.7 H, 1.0 A 1 T1 Transformer 1 C10 1.0 nF, 250 VAC, Y1 Cap 1 http://onsemi.com 3 ON Semiconductor AND8042/D OP2 is below ground. Once the output current reaches 600 mA, feedback action is taken over by OP2 and one will see a drop in output voltage if load is further increase but output current remains constant. C9, R10 and C8, R9 provide necessary feedback compensation for voltage and current loop respectively. The secondary side of the transformer consists of 2 windings, the output winding as well as a higher voltage winding which is used to supply power to NCP4300A. As the output may drop to 0 V during constant current operation, turn ratio of this higher voltage winding must be able to sustain minimum Vcc as specify by NCP4300A. Or else, the system will be lost of feedback and the output is not under control anymore. Figure 2 shows the internal block of NCP4300A. A 2.6 V, 1.0% tolerance voltage reference is connected to the non–inverting terminal of OP1. Thus, OP1 gives voltage feedback when its inverting terminal is connected to the potential divider R3 and R5. Characteristic of the voltage reference is similar to industry standard TL431 and a bias current supplied by R2 is needed to guarantee proper operation. This 2.6 V is also divided down by R11 and R8 to provide reference for output current sensing. Voltage developed at the non–inverting terminal of OP2 is: Transformer Design Transformer design involves very tedious calculation. An Excel spreadsheet has been specially designed for NCP1200 to facilitate user with a quick determination of transformer parameters. Table 2 and Table 3 display the results of the spreadsheet after keying in system parameters. Although recommended transformer primary inductance is 4.6 mH, 3.2 mH is chosen instead. A lower primary inductance enables us to have a lower flyback voltage added to the drain of the power MOSFET. This in turn allow us to use a less heavy snubber which implies less power dissipated on the snubber. Disadvantage of a lower primary inductance is the increase in MOSFET conduction loss because of higher primary peak current. However, output of this AC adapter is only 3.0 W and typical RDS(on) of MTD1N60E is merely 5.9 . Increment in conduction loss is not significant in this case. After the primary inductance is determined, we have to decide on the ferrite core. It can be seen from the Excel spreadsheet that E16/8/5 core is big enough for this transformer. Primary (N1) and secondary (N2) number of turns needed are 166 and 12 respectively. However, one more winding N3 is required to supply NCP4300A. It is critical that voltage output of N3 must be higher than minimum operating voltage of NCP4300A even when output has dropped to 0 V. Under this condition, output winding loop can be represented by Figure 3. VCC Out1 Out2 OP1 OP2 - + + In1– In2– GND In1+ In2+ Figure 2. Vcurrent reference 2.7 K2.7K75 K · 2.6 0.09 V D2 1N5819 Since Out1 and Out2 are wired together by diodes D4 and D5, feedback current through the opto–coupler U4 is dominated by whichever op–amp output that has a lower voltage. Thus feedback is dominated by OP1 until voltage developed across R6 reaches 0.09 V and this is equivalent to 600 mA passing through R6. Thanks to the rail to rail capability of OP2 in NCP4300A, current sensing function is guaranteed although voltage of non–inverting terminal of L3 47 µH 1A Short Circuit VO(SC) R6 0.15 Figure 3. http://onsemi.com 4 AND8042/D Table 2. NCP1200 DISCONTINUOUS MODE DESIGN WORKSHEET System Parameters Vmax 264 V Maximum AC Input Voltage User Input Cells Vmin 90 V Minimum AC Input Voltage Results Fline 50 Hz Line Frequency Vmin(DC) 85.73 V Minimum DC Voltage Fs(max) 69 KHz Maximum Switching Frequency Fs(typ) 60 KHz Typical Switching Frequency Fs(min) 51 KHz Minimum Switching Frequency Vo 5.2 V Output Voltage Selected Device Io 0.6 A Maximum Output Current 60 KHz 75% Efficiency Vbd 600 V Power MOSFET Breakdown Voltage Vd 1V PI 4.16 W Input Power Iin(pk) 0.21 A Maximum Primary Peak Current Vo′ 85.72 V Reflected Output Voltage Vpwr_sw(max) 459.07 V Maximum Voltage across the Power Switch Circuit (Less Leakage Spike) Output Diode Voltage Drop Dmax 0.50 Iin(av) 0.05 A Maximum Input Average Current 13.83 Turn Ratio Between Primary and Secondary Ratio N1/N2 Maximum Turn On Duty (Full Load, Low Line) Recommended Lp 4.650 mH Recommended Primary Inductance Lp 3.200 mH Primary Inductance RDS(ON) 16 ohm Maximum RDS(ON) of Power MOSFET Pdls(pwr_sw) 0.12 W Maximum Conduction Loss of Power MOSFET Input Filter Capacitor Recommended Cin 14 F Recommended Input Filter Capacitance Cin 9.4 F Input Filter Capacitance Io(pk) 2.40 A Output Peak Current Vro 32.20 V Output Maximum Reverse Voltage Output Diode Selection http://onsemi.com 5 AND8042/D Table 2. (continued) NCP1200 DISCONTINUOUS MODE DESIGN WORKSHEET Wire Selection Iin(rms) 0.08 A Maximum Input RMS Current Io(rms) 0.98 A Maximum Output RMS Current Lay_p 1 Layer of Primary Winding Lay_s 1 Layer of Secondary Winding Primary Wire Size AWG 35 Maximum Wire Size Secondary Wire Size AWG 24 AWG 24 RMS Current Density 4.9 (A/mm2) Core Selection Flux Density Safety Factor 0.4 Bobbin Usage Factor 0.4 Core Type Core Type A Core Type B Core Type C Core Type D Core Type E Core Name E 16/8/5 EI28–Z E25/13/7 E 30/15/7 E32/16/9 Ae 20.1 86 52.5 60 83 mm2 Bsat 0.5 0.5 0.5 0.5 0.5 T Aw 22.3 39.4 61 90 108 mm2 Bobbin Winding Window Area Abob 8.92 15.76 24.4 36 43.2 mm2 Usable Area of Bobbin for Winding Gap Length d 0.22 0.05 0.08 0.07 0.05 mm N1 166 39 63 56 40 N2 12 3 5 4 3 Ap 0.02 0.02 0.02 0.02 0.02 1 1 1 1 1 Lay_p Apri As Lay_s Asec 3.98 0.93 1.52 1.33 Area of Single Turn of Primary Wire Layer of Primary Winding Area of Primary Winding Area of a Single Turn of Secondary Wire 0.26 0.26 1 1 1 1 1 1.04 Secondary Number of Turns mm2 mm2 0.26 1.19 Primary Number of Turns 0.96 0.26 0.73 Saturation Magnetic Flux Density mm2 0.26 3.12 Effective Area Layer of Secondary Winding 0.75 mm2 Area of Secondary Winding mm2 Total Winding Area Asum 7.09 1.66 2.72 2.38 1.72 Enough Space? OK OK OK OK OK Maximum Peak Current (Sensing Resistor) Setting DLp 10% Tolerance of Primary Inductance Lp(min) 2.880 mH Lowest Primary Inductance Lp(max) 3.520 mH Highest Primary Inductance Ip(worst) 0.24 A Worst Case Maximum Primary Peak Current (Lowest Switching Frequency and Lowest Primary Inductance Rsense(max) 4.20 ohm Maximum Allowable Sensing Resistance Rsense 3.30 ohm Sensing Resistance Binit 0.32 T Magnetic Flux Density During Startup http://onsemi.com 6 AND8042/D Table 3. Transformer Specification Primary Inductance Lp 3.200 mH Core Type = E 16/8/5 Primary Wire Size = AWG 35 Layer of Primary Winding = 1 Primary Number of Turns N1 166 Secondary Wire Size = AWG 24 Layer of Secondary Winding = 1 Secondary Number of Turns Select Core Type Core Type A N2 12 Gap Length d 0.22 mm Enough Space? = OK Cin 9.4 F Vro 32.20 V Rsense 3.30 ohm Input Filter Capacitor Input Filter Capacitance Output Diode Maximum Reverse Voltage Sensing Resistor Sensing Resistance During flyback cycle, voltage across the output winding Vo(sc) is: N3 N1 Vo(sc) = V(D2) + V(L3) + V(R6) + V(PCB trace) N2 V(D2) = forward voltage drop of 1N5819 ≈ 0.6 V If resistance of L3 is 0.1 , V(L3) = 0.1 × 0.6 A = 0.06 V V(R6) = 0.15 × 0.6 A = 0.09 V If resistance of PCB trace is 0.15 , V(PCB trace) = 0.15 × 0.6 A = 0.09 V N1 = 166T, AWG # 34, : 0.16 mm N2 = 12T, AWG # 24, : 0.51 mm N3 = 40T, AWG # 34, : 0.16 mm Core = E16/8/5 Magnetic Material = PC40 or N67 Air Gap = 0.22 mm (center limb) Primary Inductance (Across N1) = 3.2 mH Therefore Vo(sc) is 0.84 V and volt/turn is 0.84/12 = 0.07. Minimum operating voltage of NCP4300A is 3.0 V. Its supply winding voltage has to be 0.6 V higher if we assume forward drop on MUR120 is 0.6 V. Minimum number of turns required for this winding is 3.6/0.07 ≈ 52 turns. As can be seen from the schematic, these 52 turns can be added on top of the output winding. Therefore 40 turns is enough for N3. When output is 5.2 V, supply winding voltage of NCP4300A is approximately 24.5 V. Thanks to its wide operating voltage, 24.5 V is below maximum operating voltage of NC4300A (35 V). The final design of the transformer is shown in Figure 4. Another important consideration is the value of sensing resistor R7. Value of R7 control maximum primary peak current by the following equation. Figure 4. For discontinuous mode operation, maximum power that can be delivered by the system is: P max 1 LpI2pk(max) f 2 Where Lp is the primary inductance which we already decided and f is the switching frequency. In other words, Ipk(max) must be high enough to give full load power and this implies that R7 cannot be too high. The Excel spreadsheet has calculated for us that R7 must be lower than 4.2 . 3.3 is chosen to give some headroom during transient response. Before finalizing on this value, one must make Ip(max) 1.0 V R7 http://onsemi.com 7 AND8042/D Vstby/4 sure that transformer does not saturate at power up. During power up when output voltage is much lower than rated value, MTD1N60E is switched off not by PWM action. The power MOSFET is switched off because the primary peak current has reached its maximum allowable value, Ip(max). Ip(max) drives the transformer core up the B–H curve of the magnetic material. B, magnetic flux density must be lower than the saturation value Bsat. For most magnetic material, Bsat equals 0.5 T at room temperature. Nevertheless, Bsat falls as temperature increases and at 120°C, Bsat becomes 0.35 T. Last row in Table 2 shows the magnetic flux density during startup. The value is 0.32 T, thus 3.3 should give us a safe startup. VCS Figure 6. Therefore the input power level Pstby that enters standby mode is given by the following equation. Pstby 1 Lp 2 0.5 3.2 E 3 Pulse Skipping Mode NCP1200 has a pulse skipping standby mode feature and the power level to enter standby mode is adjustable. Figure 5 shows the equivalent circuit of the Adj pin with a 10 K resistor connecting Adj pin to ground. When the voltage at FB pin falls below Adj pin, NCP1200 starts to skip cycle. This voltage Vstby is: Vstby V4Rstby7 f 40.466 2 60000 3.3 0.12 W At light load condition, efficiency should be lower than that of full load. Assume efficiency is 50% when input power is at 0.12 W, load current Io(stby) at that time is: Io(stby) 0.12 W 50% 0.01 A 5.2 V Remember that Vo drops when Io attains 0.6 A. When Vo drops below certain voltage, NCP1200 will also enters pulse skipping mode. Once again, assume efficiency is 50% when input power is at 0.12 W, Vo(stby) at that time is: 10 K29 K · 5.2 V 0.466 V 10 K29 K 75.5 K Vo(stby) 0.12 W 50% 0.1 V 0.6 A NCP1200 75.5 k + In summary, NCP1200 starts pulse skipping when Io is below 0.01 A or Vo is below 0.1 V. Adj 10 k 5.2 Vdc Actual Performance Figure 7 and Table 4 shows the actual performance of the circuit. – 29 k 6 OUTPUT VOLTAGE 5 Figure 5. Since NCP1200 is a current mode device, there is a direct relationship between voltage at the FB pin and the voltage developed by the peak current across the sensing resistor, ie. voltage at CS pin, Vcs. As can be seen from the block diagram of NCP1200 datasheet, Vcs is compared with one fourth of FB pin voltage. Therefore at the verge of entering into pulse skipping mode, we should see a relationship as shown on Figure 6. 4 3 2 1 0 0 0.2 0.4 0.6 0.8 OUTPUT CURRENT Figure 7. Vo–Io Characteristic @ 110 VAC Input http://onsemi.com 8 AND8042/D Table 4. Test Conditions Results Line Regulation Vin = 90 to 264 VAC, Io = 0.6 A = 0.5 mV Load Regulation Vin = 110 VAC, Io = 0 to 0.6 A Vin = 220 VAC, Io = 0 to 0.6 A = 3.0 mV = 3.0 mV Vin = 110 VAC, Io = 0.6 A Vin = 220 VAC, Io = 0.6 A 40 mVpp 40 mVpp Vin = 110 VAC, Vo = 5.2 V, Io = 0.6 A Vin = 220 VAC, Vo = 5.2 V, Io = 0.6 A 68% 61% Output Ripple Efficiency http://onsemi.com 9 AND8042/D Notes http://onsemi.com 10 AND8042/D Notes http://onsemi.com 11 AND8042/D 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. 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