AN2300 Application Note
An alternative solution to Capacitive power supply using Buck converter based on VIPer12A
Introduction
In this paper three different power supplies with two outputs are introduced: a Capacitive passive network, and two versions of a low cost SMPS Buck converter. The last two are based on VIPer12A, a high voltage Power MOSFET with a dedicated current mode PWM controller, start-up circuit and protection integrated on the same silicon chip by STMicroelectronics. The considered converters are compared in terms of output voltage regulation, efficiency and EMI, under the same output power conditions (about 0.6W). Finally some modifications to the Buck converters are presented, in order to extend the output power level to higher values, up to 1.1W. The main specifications of the converters are listed in Table 1. Table 1. Power supplies main specifications
185÷265VAC Vout1=12V; Iout1=30mA Vout2=5V; Iout2=40mA Total output power 0.6W
AC input voltage VIN Outputs
January 2006
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Contents
AN2300
Contents
1 2 Capacitive converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Modified Buck converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1 2.2 2.3 2.4 2.5 Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 EMI measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Higher output power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Efficiency comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Different output voltages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3 4
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
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Figures
Figures
Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure 10. Ch4=IL Figure 11. Ch4=IL Figure 12. Figure 13. Figure 14. Figure 15. Figure 16. Figure 17. Figure 18. Figure 19. Figure 20. Figure 21. Figure 22. Figure 23. Figure 24. Figure 25. Figure 26. Figure 27. Figure 28. Figure 29. Capacitive converter schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Buck converter modified schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Buck basic operation during the switch TON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Buck basic operation during the switch TOFF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Modified Buck current flow at Iout2 = 0. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Modified Buck current flow at Iout2¼0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Buck converter with VIPer12A, schematic A (Vout1 referred to GND) . . . . . . . . . . . . . . . . . 8 Buck with VIPer12A, schematic B (Vout1 referred to -5V) . . . . . . . . . . . . . . . . . . . . . . . . . . 8 PCB layout based on schematic B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Buck waveforms (schematic A) @230VAC, full load; Ch1=VS, Ch2=Vout2, Ch3=Vout1, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Buck waveforms (schematic B) @230VAC, full load; Ch1=VS, Ch2=Vout2, Ch3-Ch2=Vout1, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Capacitive converter line regulation, at full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Buck converter line regulation (schematic A), at full load . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Buck converter line regulation (schematic B), at full load . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Efficiency vs Vin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Capacitive converter: conducted emission @ 230VAC, full load: Phase . . . . . . . . . . . . . . 12 Capacitive converter: conducted emission @ 230VAC, full load: Neutral . . . . . . . . . . . . . 12 Buck converter: conducted emission @ 230VAC, full load: Phase . . . . . . . . . . . . . . . . . . 13 Buck converter: conducted emission @ 230VAC, full load: Neutral . . . . . . . . . . . . . . . . . . 13 Line regulation @full load (Buck converter, schematic A) . . . . . . . . . . . . . . . . . . . . . . . . . 14 Line regulation @full load (Buck converter, schematic B) . . . . . . . . . . . . . . . . . . . . . . . . . 14 Out1 load regulation @ Iout2 = 0 (Buck converter, schematic A). . . . . . . . . . . . . . . . . . . . 14 Out2 load regulation @ Iout1 = 20mA (Buck converter, schematic A) . . . . . . . . . . . . . . . . 14 Out1 load regulation @ Iout2 = 0 (Buck converter, schematic B). . . . . . . . . . . . . . . . . . . . 15 Out2 load regulation @ Iout1 = 20mA (Buck converter, schematic B) . . . . . . . . . . . . . . . . 15 Efficiency vs Vin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Efficiency comparison between schematics A and B for IDz2 = 30mA . . . . . . . . . . . . . . . 16 Schematic A modifications for 4V < Vout1 < 11V (Vout2 @ 5V) . . . . . . . . . . . . . . . . . . . . 17 Schematic B modifications for 9V < Vout1 < 16V (Vout2 @ 5V) . . . . . . . . . . . . . . . . . . . . 17
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Tables
AN2300
Tables
Table 1. Table 2. Table 3. Table 4. Table 5. Table 6. Table 7. Pow er supplies main specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Capacitive converter part list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Buck converter part list (schematic A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Buck converter part list (schematic B) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Higher output power requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Schematics A and B part list modification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
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Capacitive converter
1
Capacitive converter
The schematic of the Capacitive power supply is shown in Figure 1. The capacitor C2 accommodates the AC mains voltage to a voltage level suitable for the application, while R1 and R2 are connected in order to limit the inrush current of the capacitors. The voltage is then rectified by the diode D1 and regulated by means of zener diodes and electrolytic capacitors. The output capacitor values, C4 and C6, have been chosen in order to keep the output voltages ripples below 5%, at the given output load condition. The part list of the converter is given in Table 2. Figure 1. Capacitive converter schematic
R1 Neutral
R2
D1
Vin
C1 C2 Line
C3
C4
D z1
Vout1
C5
C6
D z2
Vout2
D2
Table 2.
Capacitive converter part list
Reference Value 10 1/4 150K 1/4 47nF 2.2F 82nF 10002F, 25V 82nF 4700F, 25V Resistor Resistor X2 Capacitor X2 Capacitor Ceramic capacitor Electrolytic Capacitor Ceramic capacitor Electrolytic Capacitor Diode 1N4007 Diode 1N4007 12V 5.1V Zener Diode 1N5349B730 Zener Diode BZX85C5V1 Part type
R1 R2 C1 C2 C3 C4 C5 C6 D1 D2 Dz1 Dz2
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Modified Buck converter
AN2300
2
Modified Buck converter
The considered circuit is based on the modified Buck converter shown in figure 2. It provides two outputs with reversed polarity, Vout1 = 12V and Vout2 = -5V. Figure 2. Buck converter modified schematic
S
1 L
Vout1
+
D C1
Vin
-
Dz GND
C2
Vout2
The second complementary output, Vout2, is generated charging the capacitor C2 during the free-wheeling of the inductor current. The voltage across such a capacitor is regulated by means of a zener diode of suitable value. The power switch, S, operates at high frequency for power conversion. The voltage is then filtered by the LC filter made up by L and C1. In the standard Buck topology, the voltage of the node 1 is clamped by the diode D, allowing the free-wheeling of the inductor current. In the proposed solution, the zener diode, DZ, clamps such a voltage to (VD+VZ), where VD is the voltage drop across the diode D, and VZ is the zener voltage. If a capacitor is connected across the anode of the zener and the ground, a negative voltage source is generated. Of course, due to the principle of operation, the second output cannot supply more current than the first one. The switching cycle can be basically divided in two periods as shown in Figure 3. and Figure 4. Considering discontinuous conduction mode (DCM), during the conduction of the switch S the input DC bus is connected to the output and supplies the load, as shown in Figure 3.). Once the switch is turned off, the inductor current free-wheels through the diode D1, as shown in Figure 4.), until it zeroes and the output capacitor C1 feeds the load.
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Modified Buck converter
Figure 3.
Buck basic operation during the switch TON
Figure 4.
Buck basic operation during the switch TOFF
S
L
S
L
Vi n D1 R lo ad + C1 Vou t 2
V in D1 R lo a d + C1 Vou t 2
The presence of the zener diode in the free-wheeling path does not affect the basic operation of the converter, but it could impact on the efficiency. In fact, if there is no load on Vout2, the whole free-wheeling current will flow through both diodes, D1 and DZ, as shown in Figure 5.).
Modified Buck current flow at Iout20
S
L Vout 1
L V out 1
Figure 5.
Modified Buck current flow at Iout2 = 0 Figure 6.
S
Vin
D1 C1
+
R lo ad1 Iout1
Vin
D1 C1
+
R load1 Iout1
DZ
DZ
+ C2 Iout2 = 0
+ C2
R load2
Iout2
Vout 2
V out 2
As the current drawn from Vout2 increases, the free-wheeling current flows through a different path, splitting in two components as shown in Figure 6. In this way the power dissipation in DZ is reduced and the efficiency is increased accordingly. Thus, the converter performs better if the complementary output is loaded, for a given output current Iout1. In order to guarantee the proper operation of the converter when Vout1 is in open load condition, a bleeder resistor has to be connected. A practical implementation of the circuit is presented in schematic A (see figure Figure 7.), where R1 is the bleeder resistor; D3, C3 and C4 are needed for VIPer12A biasing; L1, C1, D1, C2 make up the input filter for EMI compliance; R0 limits the inrush current of the capacitors.
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Modified Buck converter Figure 7.
AN2300
Buck converter with VIPer12A, schematic A (Vout1 referred to GND)
D3 DZ 1
16V
+
C4
VDD
FB
SOURCE
A C IN
R0
L1
D1
DRAIN
C3 L
VIPer12A
D2 C1 C2 + C6 DZ 2 A C IN + R1 GN D V out 1
5.1V
V out 2 + C7
Due to the connection of the bleeder resistor, a constant power loss appears in the circuit of Figure 7., given by (1): 2 2V VR1 ou t1 P L = ------------ = ---------------R1 R1 (1)
Referring Vout1 to -5V output, the circuit schematic B shown in Figure 8. can be used: in such a case the voltage drop across the bleeder is only (Vout1 - Vout2) instead of Vout1. Figure 8. Buck with VIPer12A, schematic B (Vout1 referred to -5V)
D3 R2 D4
+ DZ 1 C4 C3
VDD
C5
11V
FB
AC IN
R0
L1
D1
DRAIN SOURCE
L
VIPer12A
D2 + C1 C2 DZ2 AC IN C6 + R1 GND V ou t 1
5.1V
V out 2
+ C7
The part lists of the proposed circuits are given in Table 3. and Table 4. A lab prototype based on schematic B (see Figure 8.) has been built using the layout shown in Figure 9.
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AN2300 Figure 9. PCB layout based on schematic B
Modified Buck converter
Table 3.
Buck converter part list (schematic A)
Reference Value 22 1/2W 330 1/2W 47nF 2.2F, 400V 22nF 4.7F, 63V 33F, 25V 33F, 25V Resistor Resistor (bleeder) X2 capacitor Electrolytic capacitor Ceramic capacitor Electrolytic capacitor Electrolytic capacitor Electrolytic capacitor Diode 1N4004 Diode STTA106 Diode BA157 16V 5.1V, 1/2W 1m H 1.5mH Zener Diode Zener Diode Axial inductor Axial inductor VIPer12A - DIP8 Part type
R0 R1 C1 C2 C3 C4 C6 C7 D1 D2 D3 Dz1 Dz2 L1 L IC
Table 4.
Buck converter part list (schematic B)
Reference Value 22 1/2W 120 1/2W 1k 1/4W Resistor Resistor (bleeder) Resistor Part type
R0 R1 R2
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Modified Buck converter Table 4. Buck converter part list (schematic B)
Reference C1 C2 C3 C4 C5 C6 C7 D1 D2 D3 D4 Dz1 Dz2 L1 L IC 11V 5.1V, 1/2W 1 mH 1.5mH 47nF 2.2F, 400V 22nF 4.7F, 63V 470nF 33F, 25V 33F, 25V Value Part type X2 capacitor Electrolytic capacitor Ceramic capacitor Electrolytic capacitor Ceramic capacitor Electrolytic capacitor Electrolytic capacitor Diode 1N4004 Diode STTA106 Diode BA157 Diode 1N4148 Zener Diode Zener Diode Axial inductor Axial inductor VIPer12A - DIP8
AN2300
2.1
Experimental results
In Figure 10. and Figure 11. the typical waveforms of the Buck converters are shown, at Vin = 230VAC and full load (i.e. Iout1 = 30mA and Iout2 = 40mA).
Figure 10. Buck waveforms (schematic A) @230VAC, full load; Ch1=VS, Ch2=Vout2, Ch3=Vout1, Ch4=IL
Figure 11. Buck waveforms (schematic B) @230VAC, full load; Ch1=VS, Ch2=Vout2, Ch3-Ch2=Vout1, Ch4=IL
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Modified Buck converter Line regulation diagrams are shown in Figure 12., Figure 13. and Figure 14. for the Capacitive and the Buck converters respectively. Figure 12. Capacitive converter line regulation, at full load
Figure 13. Buck converter line regulation (schematic A), at full load
Figure 14. Buck converter line regulation (schematic B), at full load
The efficiency ( = POUT / PIN) of the power supplies has been evaluated at the same output power value (about 0.6W), in the whole input voltage range. The results are shown in Figure 15.
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Modified Buck converter Figure 15. Efficiency vs Vin
AN2300
Pout/Pin[% ]
100 80 60 40 20 0 180
capacitive schem Abuck . schem B buck .
200
220
240
260
i 280 Vn [V]
2.2
EMI measurements
Conducted EMI measurements have been performed according to EN55022 Class B standard, using a 50 LISN and a spectrum analyzer. In Figure 16., Figure 17., Figure 18. and Figure 19., Phase and Neutral measurement results are shown under full load conditions at nominal 230Vac input voltage.
Figure 16. Capacitive converter: conducted Figure 17. Capacitive converter: conducted emission @ 230VAC, full load: Phase emission @ 230VAC, full load: Neutral
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Modified Buck converter
Figure 19. Buck converter: conducted Figure 18. Buck converter: conducted emission @ 230VAC, full load: emission @ 230VAC, full load: Phase Neutral
2.3
Higher output power
Higher output power levels could be required in some applications. Typical values are 50mA on the 12V output and 100mA on the -5V output, as listed in Table 5. Table 5. Higher output power requirements
185÷265VAC Vout1=12V; Iout1=50mA Vout2=-5V; Iout2=100mA Total output power 1.1W
AC input voltage VIN Outputs
The proposed Buck converters can provide such current values adjusting the value of the bleeder resistor, R1. In fact, in order to maintain the regulation when out1 is in open load condition, (2) has to be verified:
VR 1 --------- > I o u t 2 + I D z 2 R1
(2)
Since Iout2 = 100mA, we can set VR1/R1 = 120mA, resulting in: VR1/R 112/R1=120mA, therefore R1=100 for schematic A (see Figure 7.); VR1/R 17/R1=120mA, therefore R1 = 56 for schematic B (see Figure 8. ). Thus, the R1 value is lower than in the previous case. Of course, this results in higher power dissipation across the bleeder. In Table 6. the part list of the modified components is given.
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Modified Buck converter
AN2300
Table 6.
Schematics A and B part list modification
Reference Value 100 2W 56 2W 47F, 25V 47F, 25V 12V (schematic B) 0 (schematic B) 820 H Radial inductor (schematic A) (schematic B) Part type Resistor (bleeder) Electrolytic capacitor Electrolytic capacitor Zener diode
R1 C6 C7 Dz1 R2 L
The line regulation of the two Buck converters is shown in Figure 20., Figure 21., the load regulation in Figure 22., Figure 23., Figure 24. and Figure 25. the efficiency in Figure 26.
Figure 20. Line regulation @full load (Buck converter, schematic A)
Figure 21. Line regulation @full load (Buck converter, schematic B)
Figure 22. Out1 load regulation @ Iout2 = 0 (Buck converter, schematic A)
Figure 23. Out2 load regulation @ Iout1 = 20mA (Buck converter, schematic A)
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Modified Buck converter
Figure 24. Out1 load regulation @ Iout2 = 0 (Buck converter, schematic B)
Figure 25. Out2 load regulation @ Iout1 = 20mA (Buck converter, schematic B)
Figure 26. Efficiency vs Vin
If a Capacitive network were used to supply such output power, it would require quite big and expensive capacitors. In fact, referring to figure 1, the value of the output capacitors, C4 and C6, can be calculated using equation (3): ( I ou t + ID z ) · T C o u t = ----------------------------------V O U T m a x (3)
where IDz is the current flowing through Dz1 or Dz2 in the circuit of Figure 1., and T is the discharging time of the capacitor. Fixing f = 60Hz for the input voltage frequency and 5% for the maximum output voltage ripple, (3) becomes: Iou t + ID I ut + ID C o u t --------------------------z------ = --o-----------------z f · V O U T m a x f · 5 %VOUT
(4 )
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Modified Buck converter
AN2300
Assuming IDz = 5mA, (4) gives C4 = Cout1 > 1500F for Iout1 = 50mA, Vout1 = 12V, and C6 = Cout2 > 7000F for Iout2 = 100mA, Vout2 = -5V.
2.4
Efficiency comparison
The power loss on the bleeder has the main impact on the efficiency of the modified Buck converters. In fact, the output power and the power loss on the zener diode, Dz2, are the same for both converters. The comparison between the circuits of Figure 7. and Figure 8. has shown that: B>A
if
I o u t 2 > --------------------- · I o u t 1 I D z 2 V R 1B 1 -----------V R 1A
1
(5)
where: B>A=efficiency of the schematic A (B) Buck converter; VR1A (VR1B) = voltage across the bleeder resistor R1 in the schematic A (B); IDz2 = current across Dz2. Assuming IDz2 = 30mA, VR1A = 12V, VR1B = 7V, equation (5) becomes: B>A
if
I o u t 2 > 2. 4 I o u t 1 3 0
(6)
where Iout1 and Iout2 are expressed in mA. The efficiency comparison between the two converters, based on (6), is shown in Figure 27. In conclusion, the schematic B (in figure 8) can be used in both cases, although in the lower power case it features a slightly lower efficiency (3÷4%). Figure 27. Efficiency comparison between schematics A and B for IDz2 = 30mA
2.5
Different output voltages
If a lower value of the output Vout1 is desired, the value of the zener diode Dz1 has to be changed. Since Vout1+VDz2 is lower than 16V, the biasing network of the VIPer12A in the
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Modified Buck converter schematic A will also be modified, in order to ensure the start-up of the device. In this way the only difference between the two schematics will be in the reference of the output voltages and in the values of the zener diodes, as can be seen fromFigure 28. and Figure 29.. Figure 28. Schematic A modifications for 4V < Vout1 < 11V (Vout2 5V)
D3 R2 D4
+ DZ1 (Vout1+VD z2)-1 C4 C3
VDD
C5
FB
AC IN
R0
L1
D1
DR AIN SOURC E
L
VIPer12A
D2 + C1 C2 D Z2 5. 1V Vout 2 C6 + R1 GN D Vout 1
AC IN
+ C7
Figure 29. Schematic B modifications for 9V < Vout1 < 16V (Vout2 5V)
D3 R2 D4
+ D Z1 Vout 1-1 C4 C3
VDD
C5
FB
AC IN
R0
L1
D1
DRAIN SOURCE
L
VIPer12A
D2 + C1 C2 DZ 2 5. 1V Vout 2 C6 + R1 GN D Vout 1
AC IN
+ C7
In order to make the VIPer12A properly supplied by the biasing network of the Figure 28. and Figure 29., the formulas (7) and (8) have to be satisfied: Schematic A (Figure 28.) 9 V < V o u t 1 + V D Z 2 < 16 V (7)
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This means that, if Vout2 is fixed at 5V, the allowed range of Vout1 in the schematic A will be about 4V ÷ 11V; if not, these limits will be moved together upwards or downwards depending on the value of VDz2 ( Vout2). Schematic B (Figure 29.) 9 V < V o u t 1 < 16 V (8)
Thus, for the schematic B the minimum allowable value of Vout1 is 9V, quite apart from the value of Vout2. The resistor R2 is optional and can be experimentally fixed between 0 and 1k if a tune of the output voltage is needed.
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Conclusions
3
Conclusions
Two versions of a very low cost Buck converter based on VIPer12A have been proposed and compared with a Capacitive converter in terms of output voltage regulation, input power consumption, EMI and efficiency, in the same output power conditions. As a result of the analysis, it can be pointed out that: the efficiency of both the Buck converters is higher than the efficiency of the Capacitive network; the output capacitors needed in the Capacitive power supply are much bigger and expensive than those required in the Buck converters (1mF and 4.7mF vs 33F); due to the switching operation of the Buck converter, an EMI input filter has to be inserted, as shown in figures 5 and 6; the Buck solution is less expensive than the Capacitive one, with a cost saving of about 10 ÷ 15%.
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Revision history
AN2300
4
Revision history
Table 7.
Date
Document revision history
Revision 1
C hanges First issue
26-Jan-2006
20/21
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Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. The ST logo is a registered trademark of STMicroelectronics. All other names are the property of their respective owners 2006 STMicroelectronics - All rights reserved STMicroelectronics group of companies Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States of America www.st.com
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