ST AN2300 Application note
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
AC input voltage V
Outputs
Total out put power 0.6W
IN
185÷265V
V
out1
V
out2
=12V; I
=5V; I
AC
out2
out1
=40mA
=30mA
January 2006 Rev1 1/21
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Contents AN2300
Contents
1 Capacitive converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2 Modified Buck converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1 Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.2 EMI measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.3 Higher output power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.4 Efficiency comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
2.5 Different output voltages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2/21 Rev1
AN2300 Figures
Figures
Figure 1. Capacitive converter schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 2. Buck converter modified schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 3. Buck basic operation during the switch TON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 4. Buck basic operation during the switch TOFF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figu r e 5 . Modified Buc k current flow a t Io ut2 = 0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figu r e 6 . Modified Buc k current flow a t Io ut2¼0 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figu r e 7 . Buck conver te r with VI P e r12A, s chemati c A (Vout 1 referred to GND) . . . . . . . . . . . . . . . . . 8
Figure 8. Buck with VIPer12A, schematic B (Vout1 referred to -5V) . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 9. PCB layout based on schematic B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Figure 10. Buck waveforms (schematic A) @230VAC, full load; Ch1=VS, Ch2=Vout2, Ch3=Vout1,
Ch4=IL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 11. Buck waveforms (schematic B) @230VAC, full load; Ch1=VS, Ch2=Vout2, Ch3-Ch2=Vout1,
Ch4=IL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 12. Capacitive converter line regulation, at full load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Figure 13. Buck converter line regulation (schematic A), at full load. . . . . . . . . . . . . . . . . . . . . . . . . . 11
Figure 14. Buck converter line regulation (schematic B), at full load. . . . . . . . . . . . . . . . . . . . . . . . . . 11
Figure 15. Efficiency vs Vin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Figure 16. Capacitive converter: conducted emission @ 230VAC, full load: Phase . . . . . . . . . . . . . . 12
Figure 17. Capacitive converter: conducted emission @ 230VAC, full load: Neutral . . . . . . . . . . . . . 12
Figure 18. Buck converter: conducted emission @ 230VAC, full load: Phase . . . . . . . . . . . . . . . . . . 13
Figure 19. Buck converter: conducted emission @ 230VAC, full load: Neutral. . . . . . . . . . . . . . . . . . 13
Figure 20. Line regulation @full load (Buck converter, schematic A) . . . . . . . . . . . . . . . . . . . . . . . . . 14
Figure 21. Line regulation @full load (Buck converter, schematic B) . . . . . . . . . . . . . . . . . . . . . . . . . 14
Figure 22. Out1 load regulation @ Iout2 = 0 (Buck converter, schematic A) . . . . . . . . . . . . . . . . . . . . 14
Figure 23. Out2 load regulation @ Iout1 = 20mA (Buck converter, schematic A) . . . . . . . . . . . . . . . . 14
Figure 24. Out1 load regulation @ Iout2 = 0 (Buck converter, schematic B) . . . . . . . . . . . . . . . . . . . . 15
Figure 25. Out2 load regulation @ Iout1 = 20mA (Buck converter, schematic B) . . . . . . . . . . . . . . . . 15
Figure 26. Efficiency vs Vin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Figure 27. Efficiency comparison between schematics A and B for IDz2 = 30mA . . . . . . . . . . . . . . . 16
Figure 28. Schematic A modifications for 4V < Vout1 < 11V (Vout2 @ 5V) . . . . . . . . . . . . . . . . . . . . 17
Figure 29. Schematic B modifications for 9V < Vout1 < 16V (Vout2 @ 5V) . . . . . . . . . . . . . . . . . . . . 17
Rev1 3/21
Tables AN2300
Tables
Table 1. Power supplies main specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Table 2. Capacitive converter part list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Table 3. Buck converter part list (schematic A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Table 4. Buck converter part list (schematic B) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Table 5. Higher output power requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Table 6. Schematics A and B part list modification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Table 7. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4/21 Rev1
AN2300 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
R2
C2
D1
D2
C3
C5
C6
Dz1
Dz2
Vin
R1
Neutral
C1 C4
Line
Table 2. Capacitive converter part list
Reference Value Part type
R1 10Ω 1/4Ω Resistor
R2 150KΩ 1/4Ω Resistor
C1 47nF X2 Capacitor
C2 2.2
C3 82nF Ceramic cap acitor
C4 10002
µF X2 Capacitor
µF, 25V Elec troly tic Capa c i tor
Vout1
Vout2
C5 82nF Ceramic cap acitor
C6 4700
µF, 25V Elec troly tic Capa c i tor
D1 Diode 1N4007
D2 Diode 1N4007
Dz1 12V Zener Diode 1N5349B730
Dz2 5.1V Zener Diode BZX85C5V1
Rev1 5/21
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, V
Figure 2. Buck converter modified schematic
= 12V and V
out1
out2
= -5V.
S
+
Vin
-
The second complementary output, V
1
, is generated charging the capacitor C2 during the
out2
D
Dz
L
Vout1
C1
GND
C2
Vout2
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,
D
, clamps such a voltage to (VD+VZ), where VD is the voltage drop across the diode D, and
Z
V
is the zener voltage. If a capacitor is connected across the anode of the zener and the
Z
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 D
shown in F igure 4.), until it zeroes and the output capacitor C1 feeds the load.
6/21 Rev1
, as
1
AN2300 Modified Buck converter
Figure 3. Buck basic operation during the
switch TON
Vin
S
D1
Vout2
L
Rload
+
C1
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. I n fact, if there is no load on
V
, the whole free-wheeling current will flow through both diodes, D1 and DZ, as shown in
out2
Figure 5.).
Figure 5. Modified Buck current flow at Iout2 = 0
Figure 4. Buck basic op eration during the
switch T
S
Vin
Figure 6. Modified Buck current flow at I
D1
Vout2
OFF
L
Rload
+
C1
out2
≠0
Vin
S
D1
DZ
L
+
C1
+
C2
As the current drawn from V
Rload1
Vout1
Vin Rload1
Iout1
Iout2 = 0
Vout2
increases, the free-wheeling current flows through a
out2
S
D1
DZ
L
+
C1
+
C2
Vout1
Iout1
Rload2
Iout2
Vout2
different path, splitting in two components as shown in Figure 6. In this way the power
dissipation in D
performs better if the complementary output is loaded, for a given output current I
In order to guarantee the proper operation of the converter when V
is reduced and the efficiency is increased accordingly. Thus, the converter
Z
is in open load
out1
out1
.
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.
Rev1 7/21
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