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AND8042/D Implementing Constant Current Constant Voltage AC Adapter by NCP1200 and NCP4300A
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APPLICATION NOTE
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.
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.
(c) Semiconductor Components Industries, LLC, 2001
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Publication Order Number: AND8042/D
L1 470 H 0.2 A
D1 MUR120 470p 250 V 100 k 1 W D2 1N5819 L3 4.7 H 1A 5.2 V, 600 mA
C1 90-264 VAC + - V4 4 DF06S 1 U2 3 + C5 4.7 400 V + C6 4.7 400 V 1 2 3 4
R1
U1 8 7 6 5 D3 1N4937 Q1 C2 10 + C3 330 F +
R4 1.5 k
R2 3.3 k
R3 10 k 1% R5 10 k 1%
+ C4 47
2
NCP1200 NCP1200D60 D6 1N4148
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MTD1N60E
C10 1 nF 250 VAC Y1 R7 3.3 0.6 W SFH6156-3 4 1 2 R10 68 k
U3 Out1 1 In1- 2 In1+ 3 Ground 4 8 Out2 7 In2- 6 In2+ 5 VCC
R6 0.15
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2
+ C7 47 F R13 220 k
NCP4300AD C9 0.047 F R11 75 k 1%
R8 2.7 k 1%
R9 470
L2 470 H 0.2 A
R12 10 k
C8 0.1
3
U4
D4 1N4148
D5 1N4148
Figure 1. Circuit Description
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Table 1.
Reference U1 U2 U3 U4 Q1 C1 C2, C7 C3 C4 C5, C6 C8 C9 R1 R2 R3, R5 R4 R6 R7 R8 R9 R10 R11 R12 R13 D1 D4, D5, D6 D2 D3 L1, L2 L3 T1 C10 Part NCP1200D60 DF06S NCP4300AD SFH6156-3 MTD1N60E 470 p, 250 V 10 mF, 25 V 330 mF, 35 V 47 mF, 16 V 4.7 mF, 400 V 0.1 mF 0.047 mF 100 KW, 1.0 W 3.3 K 10 K, 1% 1.5 K 0.15 W, 0.1 W SMT 3.3 W, 0.6 W 2.7 KW, 1% 470 W 68 K 75 KW, 1% 10 KW 220 KW MUR120 1N4148 1N5819 1N4937 470 mH, 0.2 A 4.7 mH, 1.0 A Transformer 1.0 nF, 250 VAC, Y1 Cap Quantity 1 1 1 1 1 1 2 1 1 2 1 1 1 1 2 1 1 1 1 1 1 1 1 1 1 3 1 1 2 1 1 1 ON Semiconductor ON Semiconductor ON Semiconductor Panasonic FC Series or Rubycon JXA Series Panasonic FC Series or Rubycon JXA Series Manufacturer ON Semiconductor General Semi or IR ON Semiconductor Infineon ON Semiconductor
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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:
VCC
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. 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 W. 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.
Out1 OP1 In1- + + OP2
Out2
In2-
GND In1+ In2+
Figure 2. Vcurrent * reference + 2.7 K * 2.6 + 0.09 V 2.7 K ) 75 K
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
D2 1N5819 VO(SC) R6 0.15
L3 47 H 1A Short Circuit
Figure 3.
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Table 2. NCP1200 DISCONTINUOUS MODE DESIGN WORKSHEET
System Parameters Vmax Vmin Fline Vmin(DC) Fs(max) Fs(typ) Fs(min) Vo Io h Vbd Vd PI Iin(pk) Vo Vpwr_sw(max) Dmax Iin(av) Ratio N1/N2 Recommended Lp Lp RDS(ON) Pdls(pwr_sw) Input Filter Capacitor Recommended Cin Cin Output Diode Selection Io(pk) Vro 2.40 A 32.20 V Output Peak Current Output Maximum Reverse Voltage 14 mF 9.4 mF Recommended Input Filter Capacitance Input Filter Capacitance 264 V 90 V 50 Hz 85.73 V 69 KHz 60 KHz 51 KHz 5.2 V 0.6 A 75% 600 V 1V 4.16 W 0.21 A 85.72 V 459.07 V 0.50 0.05 A 13.83 4.650 mH 3.200 mH 16 ohm 0.12 W Maximum AC Input Voltage Minimum AC Input Voltage Line Frequency Minimum DC Voltage Maximum Switching Frequency Typical Switching Frequency Minimum Switching Frequency Output Voltage Maximum Output Current Efficiency Power MOSFET Breakdown Voltage Output Diode Voltage Drop Input Power Maximum Primary Peak Current Reflected Output Voltage Maximum Voltage across the Power Switch Circuit (Less Leakage Spike) Maximum Turn On Duty (Full Load, Low Line) Maximum Input Average Current Turn Ratio Between Primary and Secondary Recommended Primary Inductance Primary Inductance Maximum RDS(ON) of Power MOSFET Maximum Conduction Loss of Power MOSFET Selected Device 60 KHz User Input Cells Results
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Table 2. (continued) NCP1200 DISCONTINUOUS MODE DESIGN WORKSHEET
Wire Selection Iin(rms) Io(rms) Lay_p Lay_s Primary Wire Size Secondary Wire Size RMS Current Density Core Selection Flux Density Safety Factor Bobbin Usage Factor Core Type Core Name Ae Bsat Aw Abob Gap Length d N1 N2 Ap Lay_p Apri As Lay_s Asec Asum Enough Space? 0.4 0.4 Core Type A E 16/8/5 20.1 0.5 22.3 8.92 0.22 166 12 0.02 1 3.98 0.26 1 3.12 7.09 OK Core Type B EI28-Z 86 0.5 39.4 15.76 0.05 39 3 0.02 1 0.93 0.26 1 0.73 1.66 OK Core Type C E25/13/7 52.5 0.5 61 24.4 0.08 63 5 0.02 1 1.52 0.26 1 1.19 2.72 OK Core Type D E 30/15/7 60 0.5 90 36 0.07 56 4 0.02 1 1.33 0.26 1 1.04 2.38 OK Core Type E E32/16/9 83 0.5 108 43.2 0.05 40 3 0.02 1 0.96 0.26 1 0.75 1.72 OK mm2 mm2 mm2 mm2 mm2 mm2 T mm2 mm2 mm Primary Number of Turns Secondary Number of Turns Area of Single Turn of Primary Wire Layer of Primary Winding Area of Primary Winding Area of a Single Turn of Secondary Wire Layer of Secondary Winding Area of Secondary Winding Total Winding Area Effective Area Saturation Magnetic Flux Density Bobbin Winding Window Area Usable Area of Bobbin for Winding 0.08 A 0.98 A 1 1 AWG 35 AWG 24 4.9 (A/mm2) Maximum Input RMS Current Maximum Output RMS Current Layer of Primary Winding Layer of Secondary Winding Maximum Wire Size AWG 24
Maximum Peak Current (Sensing Resistor) Setting DLp Lp(min) Lp(max) Ip(worst) Rsense(max) Rsense Binit 10% 2.880 mH 3.520 mH 0.24 A 4.20 ohm 3.30 ohm 0.32 T Tolerance of Primary Inductance Lowest Primary Inductance Highest Primary Inductance Worst Case Maximum Primary Peak Current (Lowest Switching Frequency and Lowest Primary Inductance Maximum Allowable Sensing Resistance Sensing Resistance Magnetic Flux Density During Startup
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Table 3.
Transformer Specification Primary Inductance Core Type Primary Wire Size Layer of Primary Winding Primary Number of Turns Secondary Wire Size Layer of Secondary Winding Secondary Number of Turns Gap Length Enough Space? Input Filter Capacitor Input Filter Capacitance Output Diode Maximum Reverse Voltage Sensing Resistor Sensing Resistance Rsense 3.30 ohm Vro 32.20 V Cin 9.4 mF Lp = = = N1 = = N2 d = 3.200 mH E 16/8/5 AWG 35 1 166 AWG 24 1 12 0.22 mm OK Select Core Type Core Type A
During flyback cycle, voltage across the output winding Vo(sc) is: Vo(sc) = V(D2) + V(L3) + V(R6) + V(PCB trace) V(D2) = forward voltage drop of 1N5819 0.6 V If resistance of L3 is 0.1 W, V(L3) = 0.1 W x 0.6 A = 0.06 V V(R6) = 0.15 W x 0.6 A = 0.09 V If resistance of PCB trace is 0.15 W, V(PCB trace) = 0.15 W x 0.6 A = 0.09 V 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.
Ip(max) + 1.0 V R7
N1
N3
N2 N1 = 166T, AWG # 34, f : 0.16 mm N2 = 12T, AWG # 24, f : 0.51 mm N3 = 40T, AWG # 34, f : 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
Figure 4.
For discontinuous mode operation, maximum power that can be delivered by the system is:
P max + 1 LpI2 pk(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 W. 3.3 W is chosen to give some headroom during transient response. Before finalizing on this value, one must make
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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 120C, 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 W should give us a safe startup. 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 + 10 K 29 K * 5.2 V + 0.466 V 10 K 29 K ) 75.5 K
Vstby/4 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 Vstby 4R7 f 0.466 4 3.3 2 60000
3.2 E * 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:
Vo(stby) + 0.12 W 50% + 0.1 V 0.6 A
NCP1200
75.5 k + 5.2 Vdc - 29 k
In summary, NCP1200 starts pulse skipping when Io is below 0.01 A or Vo is below 0.1 V.
Adj 10 k
Actual Performance Figure 7 and Table 4 shows the actual performance of the circuit.
6 5 OUTPUT VOLTAGE 4 3 2 1 0 0 0.2 0.4 OUTPUT CURRENT 0.6 0.8
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.
Figure 7. Vo-Io Characteristic @ 110 VAC Input
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Table 4.
Test Line Regulation Load Regulation Output Ripple Efficiency Conditions Vin = 90 to 264 VAC, Io = 0.6 A Vin = 110 VAC, Io = 0 to 0.6 A Vin = 220 VAC, Io = 0 to 0.6 A Vin = 110 VAC, Io = 0.6 A Vin = 220 VAC, Io = 0.6 A Vin = 110 VAC, Vo = 5.2 V, Io = 0.6 A Vin = 220 VAC, Vo = 5.2 V, Io = 0.6 A Results D = 0.5 mV D = 3.0 mV D = 3.0 mV 40 mVpp 40 mVpp 68% 61%
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Notes
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Notes
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