AN1484 Application note
A 3.6 W travel adaptor using VIPer12A-E
Introduction
The VIPer12A-E is an integrated PWM and MOSFET circuit for low power applications in the 5 W range, typically in cellular phone adapters. It is housed in surface mount SO-8 and DIP8 packages. With the availability of VIPer12A-E in the SO-8 package and the limited number of external components for a real PWM operation, building a compact and performant power supply becomes simple. The travel adaptor design presented here has been designed to minimize overall cost for a secondary voltage and current regulated adapter, topology widely used in cellular phone adapters. The VIPer12A-E is an excellent solution for lower consumption in standby, in home appliances, for example, where it will be required to limit non negligible "off use" consumption, as recommended by the "European Commission of Energy". Thanks to the low power consumption of VIPer12A-E, it is possible to achieve 100 mW standby power in a wide range of operations. Table 1. System performances
Parameters Standby power Efficiency at 3.6 W Short circuit power Load regulation 100 VDC 90 mW 62% 1W 3% 380 VDC 119 mW 66% 1.3 W 2%
Figure 1.
Travel adaptor
October 2007
Rev 4
1/18
www.st.com
Contents
AN1484
Contents
1 Principle of operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 The VIPer12A-E . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.1 1.1.2 1.1.3 1.1.4 1.1.5 Star t-up phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Auxiliary supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Burst mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Compensation and duty cycle control . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Primar y drive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2
Secondar y regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.2.1 1.2.2 Voltage regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Current regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2
The transformer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1 2.2 2.3 2.4 2.5 Primar y inductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Transformer structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Peak drain voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 EMC compatibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Transformer specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3
System performances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.1 3.2 3.3 Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Standby consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
4
Design material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1 4.2 4.3 PCB solder side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Silk screen solder side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Component list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
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AN1484
List of figures
List of figures
Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure 10. Figure 11. Figure 12. Figure 13. Figure 14. Figure 15. Figure 16. Figure 17. Figure 18. Figure 19. Figure 20. Figure 21. Travel adaptor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 VIPer12A-E internal structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Transformer structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 At full load at 100 V (sandwich) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 At full load at 100 V (on top) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 In short circuit at 100 V (sandwich). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 In short circuit at 100 V (on top) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Hiccup mode at 100 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Hiccup mode at 380 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 EMC compensation technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Efficiency at 100 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Efficiency at 380 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Vout vs. Iout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Standby consumption at 100 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Standby consumption at 380 V . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Bottom view of charger board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 PCB layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 SMD components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Top view of the charger board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Through-hole components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
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Principle of operation
AN1484
1
Principle of operation
The circuit is a standard flyback converter with secondary current and voltage regulation driving the VIPer12A-E feedback pin through an optocoupler.
1.1
1.1.1
The VIPer12A-E
Start-up phase
VIPer12A-E, as any product of the VIPer family, has an integrated high voltage current source to charge the C3 Vdd capacitor until it reaches its startup level (15 V). When Vdd reaches 15 V, the VIPer switches, supplied by the energy stored in C3, until it is supplied through the auxiliary winding.
1.1.2
Auxiliary supply
VIPer12A-E has a wide operating voltage range from 9 V to 40 V, respectively maximum and minimum values for undervoltage and overvoltage protections. This wide voltage supply range simplifies the design of the VIPer12A-E supply but, to ensure proper operation of the application in any case, it is advised:
in normal operation, to supply VIPer12A-E within its operating range; in standby mode, to minimize auxiliary consumption to achieve very low standby power; in short circuit, to limit output power by going into hiccup mode; in constant current mode, to ensure current regulation below 2 V before going into hiccup mode.
1.1.3
Burst mode
The VIPer12A-E integrates a current mode PWM with a Power MOSFET and includes the leading edge blanking function. The burst mode is a feature which allows VIPer12A-E to reduce its average switching frequency when the energy drained by the output load goes below: Equation 1
fs w 2 E = ( t b V i n ) --------2 Lp
tb blanking time Vin DC input voltage, fsw Switching frequency, Lp primary inductance).
This is obtained with a small ripple current around shut down current of feedback pin and maintaining the Vdd voltage above 9 V. If Vdd falls below 9 V there is "bad burst mode" in which VIPer12A-E repeats the restart cycle continuously, with a worse standby consumption and a higher secondary ripple voltage.
4/18
AN1484
Principle of operation
1.1.4
Compensation and duty cycle control
The internal structure of VIPer12A-E feedback and compensation pin (FB pin 3) is shown in Figure 2. The current injected on the FB pin is added to the current coming from the SenseFet in R2 and then compared to an internal 0.23 V Vref. When FB voltage is closed to ground, the VIPer12A-E delivers its full power. On the other hand, when FB voltage is above 0.23 * (R1+R2)/R2, the VIPer12A-E stops switching. The FB pin is directly driven by the emitter of the optocoupler, behaving as a current source. This current is filtered by a small 47nF capacitor C5 to guarantee cycle-to-cycle stability.
Warning:
C5 must be kept very close to the VIPer12A-E feedback pin to avoid high frequency instability on the compensation loop.
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Principle of operation
AN1484
1.1.5
Primary drive
In a flyback power supply, the transformer is used as an energy tank fuelled during the ON time of the Power MOSFET. When the Power MOSFET turns off, its drain voltage rises from low value to the Input Voltage + Reflected Voltage when the secondary diode conducts, regaining on the secondary the magnetic energy stored in the transformer. As primary and secondary windings are not perfectly magnetically coupled, there is a serial leakage inductance that behaves like an open inductor charged at Ipeak that makes the Mosfet drain voltage reach higher values. If the peak voltage is higher than the Vdss of the VIPer12A-E Power MOSFET, the device will be destroyed. So the drain voltage must be kept below its avalanche voltage of 730 V. A clamper based on an RCD network or a diode with a transil to clamp the rise of the drain voltage is frequently used. The presence of the clamper is an extra consumption in standby mode, especially the RCD clamper with respect to the transil clamper. Because the power consumption is manageable with the transil clamper, this solution has been chosen here. Figure 2. VIPer12A-E internal structure
1.2
1.2.1
Secondary regulation
Voltage regulation
Voltage regulation is achieved with a zener diode D6 directly driving the optocoupler. The resistor R3 limits the current in both the zener and the opto in case of overvoltage. The VIPer12A-E feedback pin is current controlled and its requirement goes from a few A at full load to 1 mA in standby. The same current change is experienced by the regulating zener on the secondary side of the converter leading to around 5% load regulation. It is possible to improve the load regulation by connecting a resistor between the zener and the Vout. Of course, this degrades standby power consumption.
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AN1484
Principle of operation
1.2.2
Current regulation
The current regulation uses the drop voltage across a shunt resistor R6/R7/R8 to bias the T1 transistor base-emitter junction. The T1 collector drives the optocoupler limiting the output power. Figure 3. Application schematic
+VOUT 6V/600mA 47uF 16V GND
C7
TP4
D6 5.1V
R5
R3
56
C8 100nF
1k
R6
2.7 R7
2.7 R8 TP6
TP7
D5 SMBYW01-200
470uF 25V
T1 BC847B
TP5
TR1 TRANSFORMER
C5 1.5nF 2KV R2 22
C6
2.7 BAV103 D2
D3 SMAJ188A
D4 BGY20G
V DD
CONTROL
SOURCE FB
C4 47nF VIPer12A
IC1 SFH517 TP3 INDUCTOR 1mH I1 TP2 D1 S1ZB6D TP1 R1 AC IN 10 0.6A, 600V + AC IN C1 4.7uF 400V C2 4.7uF 400V C3 10uF 63V
The accuracy of this circuit is especially limited in temperature but is unrivalled in terms of cost. The addition of the R5 base resistor is necessary in short circuit to avoid destroying T1
IC2
DRAIN
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The transformer
AN1484
Base-Emitter junction. R3 is also necessary in short circuit otherwise T1 collector current directly flows through D6 and the optocoupler is not driven anymore, leading to an increase of short circuit power consumption. C8 limits the gain in frequency of T1, stabilizing the loop.
2
The transformer
An important part of an SMPS design lies in the transformer. Its performances are key to the system performances. The requirements for this application are small size and limited voltage on the drain. Table 2. Transformer target specification
Parameters Power Saturation current Primary inductance Reflected voltage Leakage inductance Primary capacitance Value 5W > 400 mA 2.5 mH 50 V < 100 H or <3% > 20 pF
2.1
Primary inductance
A simple calculation gives the range of values of primary inductance suitable for this application. VIPer12A-E has a drain current limitation of 360 mA min. The energy transferred is Equation 2
1 2 E = -- L p l p f s w 2
in discontinuous mode. Emin=5 W, Ip=360 mA and fsw=50 kHz giving Lp>1.54 mH The transition mode is when Equation 3
To n Vi n = To f f Vr ( To n Vi n = Lp Ip )
The expression of Lp is: Equation 4
fs w 1 2 L p = -- ( T o n V i n ) ------E 2
With Vin=150 V, Vr=50 V, Ton=5 s, E=5 W give Lp=2.8 mH Vin was chosen to reach the continuous mode at low input voltage level. Vr is low to limit the drain peak voltage.
8/18
AN1484
The transformer The transformer optimization has led to a final value of 2.5 mH partly to reduce the primary turns and their power dissipation with an E12.5 bobbin.
2.2
Transformer structure
A standard transformer structure (by reason of windings order primary/auxiliary/secondary) gives the following results on VIPer12A-E supply: Table 3. VIPer12A-E Vdd with a standard transformer
Conditions Standby Load 6 V/100 mA Load 6V/600 mA Short circuit 100 VDC 10 V 15 V 25 V 9V 380 VDC 8V 15 V 26 V 10 V
Two concerns can be seen from this table: the VIPer12A-E is not going in hiccup mode in short circuit The supply voltage is too low in standby with the risk of a "Bad burst mode" with higher standby consumption and poor regulation (VIPer12A-E undervoltage is at 9 V max with 8 V typical).
The solution, implemented in the demo board, is the optimized structure (by reason of windings order: primary/secondary/auxiliary) shown in Figure 4. Figure 4. Transformer structure
B bn oi
S cond ry ea A xiliary u Pim ry ra
B bin o
A xiliay ur S co day enr Pim ry ra
CR OE
O timsaio p i tn
CR OE
The position of the auxiliary winding on top gives the following benefits:
better coupling of primary and secondary windings thus lower leakage inductance and energy stored in the ringing circuit no coupling between primary / secondary leakage inductance and auxiliary windings less capacitive coupling between primary and auxiliary windings.
Figure 5 and Figure 7 show the drain voltage of the VIPer12-E (Trace 1) and Vdd voltage before R2 (Trace 2) at full load and in short circuit with the auxiliary in sandwich (1) and on top (2) of the windings.
9/18
The transformer
AN1484
Figure 5.
At full load at 100 V (sandwich)
Figure 6.
At full load at 100 V (on top)
Figure 7.
In short circuit at 100 V (sandwich) Figure 8.
In short circuit at 100 V (on top)
The VIPer12A-E auxiliary supply shows the following voltage on the Vdd pin: Table 4. VIPer12A-E Vdd with an optimized transformer
Conditions Standby Load 6 V/100 mA Load 6 V/600 mA Shor t circuit 12 V 18 V 20 V Hiccup 100 V 11 V 18 V 19.5 V Hiccup 380 V
Under these conditions the VIPer12A-E is properly operating. It draws less than 100 mW in standby and the Hiccup mode is safe in short circuit (Figure 9 and Figure 10).
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AN1484
The transformer
Figure 9.
Hiccup mode at 100 V
Figure 10. Hiccup mode at 380 V
2.3
Peak drain voltage
This transformer allows the reduction of Drain peak voltage under any condition. The gain of this structure is 70 V. With the lower reflected voltage (100 V to 50 V), the gain is up to 120 V. There is a good voltage margin at full load under 380 VDC. This means that a standard 200 V transil clamper will not take any energy in normal operation. The clamper is still necessary during start-up and short circuit, if the drain voltage rises above the 730 V VIPer12A-E avalanche voltage. Table 5. Power MOSFET peak voltage at 380 VDC
Conditions Full Reflected voltage Leakage inductance Primary capacitance Primary inductance Full load Ipeak Standard transfo 750 V 100 V 105 H 22 pF 3 mH 260 mA Optimized transfo 630 V 50 V 25 H 26 pF 2.6 mH 275 mA
2.4
EMC compatibility
Most of the EMC performances are due to the "floating" voltage of the secondary winding or to the voltage across the C7 EMC capacitor. This "floating" amplitude is linked to all the parasitic capacitances along the wire between primary and secondary windings. With the optimized transformer, the EMC performances are degraded.
11/18
The transformer
AN1484
It is possible to turn this problem into an advantage. The secondary winding is placed between the primary and the auxiliary. The auxiliary winding is used to compensate the induction from primary to the secondary. Figure 11 shows this compensation. Figure 11. EMC compensation technique
R2
Auxiliary Winding Secondary
Vin
Prim ary Winding
The cold point is wound close to the secondary winding, limiting the voltage swing of the closest one. The voltage variation of the primary and the auxiliary side of the converter must be opposite. In this design, the D3 diode has been placed on the ground so the voltage swing is opposite on the transition.
2.5
Transformer specifications
Lp = 2.5 mH @ 50 KHz Ll =30 mH @ 50 KHz Cp = 35 pF @ 1 MHz Voltages: 55 V-Pri / 7.2 V-sec. / 20.0 V-Aux Isat > 400 mA Pout = 5 W Geometry: E12.5 Winding order: primary / secondary / auxiliary Primary winding: 180 turns AWG 38 Auxiliary winding: 66 turns AWG 38 Secondary winding: 25 triple isolation 0.20 mm
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AN1484
System performances
3
3.1
System performances
Efficiency
The Power losses are distributed at 6 V / 600 mA output power as follows: 400 mW in the output diode 700 mW in the VIPer12A-E 300 mW in the transformer 380 mW in the shunt resistor
Overall efficiency is 3.6 W/ (3.6 W+1.78 W)=67%. If the losses in the shunt resistor are considered as available power, the converter efficiency becomes 3.98 W/(3.98 W+1.4 W) = 74%. This is possible using a secondary controller such as STM's TSM101.
Figure 12. Efficiency at 100 V
Figure 13. Efficiency at 380 V
3.2
Regulation
Figure 14. Vout vs. Iout
13/18
System performances
AN1484
3.3
Standby consumption
The demo board consumes less than 100mW at 100 VDC and 120 mW at 380 VDC. This power level is far below current regulation requirements. The charts in Figure 15/Figure 16 show the details of the charger standby consumption at minimum and maximum input voltage. The major contribution to standby consumption is the VIPer12A-E's own consumption of just 35 mW which is independent of input voltage.
Figure 15. Standby consumption at 100 V
Figure 16. Standby consumption at 380 V
The only change is due to the internal startup current source of 22 A of which the consumption goes from 2 mW at 100 V up to 8 mW at 380 V. Another factor due to the VIPer12A-E is the current used on the feedback pin, regulated around 1mA in standby that leads to a 10mW consumption in the primary and 6 mW in the secondary (with an opto gain of 1). Note that it is necessary to keep a certain level of current in the regulating zener to improve the load regulation. As load increases, the current in the opto and the zener decreases, lowering the output regulated voltage. Overall, VIPer12A-E needs 50 mW to operate in standby. It is possible to spare some m in the auxiliary supply, especially the 22 W serial resistor which is necessary to regulate the transformer ringing voltage peak. In the demo board the transformer voltage has a narrow dynamic so R2 becomes useless. The standby consumption is decreased if the resistor is removed and the transformer is tuned to set 10 V or less on VIPer12A-E Vdd. The standby consumption is less than 60 mW at 100 V and 80 mW at 380 V which is fairly good considering the 50 mW required by VIPer12A-E.
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AN1484
Design material
4
4.1
Design material
PCB solder side
Figure 18. PCB layout
Figure 17. Bottom view of charger board
4.2
Silk screen solder side
Figure 19. SMD components
Figure 20. Top view of the charger board
Figure 21. Through-hole components
15/18
Design material
AN1484
4.3
Table 6.
Ref. R1 R2 R3 R5 R6-R7-R8 C8 C4 C5 C1-C2 C3 C6 C7 D1 D2 D5 D6 I1 IC1 IC2 T1 TR1 JP1
Component list
Component list
Part list description Wirewound res. 10 5% 2 W Chip Res. 22 5% 0.125 W S0805 Chip Res. 56 5% 0.125 W S0805 Chip Res. 1 K 1% 0.125 W S0805 Chip Res. 2.7 5% 0.25 W S1206 Chip Cap. 100 nF 25 V X7R S0805 Chip Cap. 47 nF 50 V X7R S0805 Chip Cap. 1.5 nF Elect. Cap. 4.7 F 400 V 10x12.5 VZ KMG P/N 2222 151 90021 NHG Elect. Cap. 10 F 63 V 5x11 YXG FC PW WD/WL Elect. Cap. 470 F 25 V 10x16 Elect. Cap. 47 F 16 V 6.3x7 Phase Bridge Rectifier Diode BAV103 MINIMELF Diode SMBY01-200 SMA Diode Zener 5.1 V 2% MINIMELF Inductor 1 mH series Optocoupler I.C. VIPer12A-E BC847B SOT23 Transformer Jumper tinned copper wire 0.7 PULSE TDK SIEMENS NEC SHARP TEMIC STMicroelectronics STMicroelectronics SHINDENGEN G.I. MURATA TDK ROEDERSTEIN SAMWHA NICHICON/SANYO NIPPON CHEMI-CON PHILIPS PANASONIC NIPPON CHEMI-CON RUBICON PANASONIC NICHICON SAMWHA Supplier VITROHM TYOHM
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AN1484
Revision history
5
Revision history
Table 7.
Date 04-Jan-2005 18-Oct-2007
Document revision history
Revision 3 4 Minor text changes Document reformatted no content change VIPer12A replaced by VIPer12A-E Changes
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AN1484
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