AN2359

Application note

Double output Buck-Boost converter with VIPerX2A

Introduction

This paper introduces two off-line non-insulated SMPS double outputs in Buck–Boost configuration based on VIPerX2A family The power supplies are operated in wide input voltage range, i.e. 88 to 265VAC. They can supply small loads, such as a microcontroller, triacs, display and peripherals in the industrial segment and home appliance. In the

applications where a double output is required, two different solutions can be used. The first one regards an insulated converter topology, with second output generated by means of one winding on the magnetic core of the inductor with a proper turns ratio. Nevertheless, this solution is expensive in terms of transformer and can be used for medium and high current or insulated applications. For low power and low cost applications, a non-insulated converter topology can be used. The proposed topology, based on Buck-Boost converter, is used to supply negative output voltage referred to neutral in all those applications where the galvanic insulation is not required. The principle schematic is shown in figure below.

Proposed double output Buck-Boost topology

VOUT1 is provided using the classic Buck-Boost configurations, while VOUT2 is obtained thanks to an intermediate tap on the inductor.

Compared to other already proposed solutions, the second output is obtained thanks to an intermediate tap on a low cost inductor. This configuration limits the parasitic capacitive effect between the two winding and improves the regulation at open load.

Further advantage is related to the regulation feedback connected on VOUT2. Thanks to this regulation, it is possible to cover those applications where a low tolerance and low voltage is required (i.e. a microcontroller) and a high tolerance and high voltage is required for the auxiliary circuit (drivers, relays…).

December 2006Rev 1 1/18

www.st.com

ContentsAN2359

Contents

123

VIPerX2 description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Output voltage selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Application example nº 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3.13.2

Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Thermal measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4Application example nº 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.1

Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

567

Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

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AN2359List 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.

Converter schematic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Typical waveforms at 88VAC: open load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Typical waveforms at 88VAC: full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Typical waveforms at 265VAC: open load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Typical waveforms at 265VAC: full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Commutation at full load: 88VAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Commutation at full load: 265VAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Output ripple voltage at full load: 88VAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Output ripple voltage at full load: 265VAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Turn on losses measurement at full load: 88VAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Turn on losses measurement at full load: 265VAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11VIPer22A Thermal profile: at VIN= 88VAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11VIPer22A Thermal profile: at VIN= 265VAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11VIPer22A temperature at maximum load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Converter schematic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Typical waveforms at 300VDC and full load: commutation . . . . . . . . . . . . . . . . . . . . . . . . . 15Typical waveforms at 300VDC and full load: detail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15PCB Layout (not in scale). Option nº 1: -12V output voltage . . . . . . . . . . . . . . . . . . . . . . . 16PCB Layout (not in scale). Option nº 2: -24V output voltage . . . . . . . . . . . . . . . . . . . . . . . 16

3/18

List of tablesAN2359

List of tables

Table 1.Table 2.Table 3.Table 4.Table 5.Table 6.Table 7.Table 8.Table 9.Table 10.Table 11.Table 12.

Proposed converters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5SMPS specifications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Component list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Circuit characterization - VIN = 120VDC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Circuit characterization - VIN = 320VDC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Circuit characterization - VIN = 374VDC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9SMPS specifications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12Component list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Experimental results - VIN=120VDC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Experimental results - VIN=320VDC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Experimental results - VIN=374VDC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

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AN2359VIPerX2 description

1 VIPerX2 description

The proposed converters are based on The VIPerX2A family, which is a range of smart

power devices with current mode PWM controller, start-up circuit and protections integrated in a monolithic chip using VIPower M0 Technology.The VIPerX2A family includes:

––

VIPer12, with a 0.4A peak drain current limitation and 730V breakdown voltage;VIPer22, with a 0.7A peak drain current limitation and 730V breakdown voltage.

The switching frequency is internally fixed at 60kHz by the integrated oscillator of the VIPerX2.

The internal control circuit offers the following benefits:

–––

Large input voltage range on the V DD pin accommodates changes in supply voltage;

Automatic burst mode in low load condition;Overload protection in hiccup mode.

The feedback pin FB is sensitive to current and controls the operation of the device.

2 Output voltage selection

Two converters with different output voltage are introduced in this paper. The main

specifications are listed in Table 1.Table 1.

Proposed converters

Output 1-12V/150mA-24V/100mA

Output 2-5V/300mA-5V/300mA

POUT(MAX)3.3W3.9W

As already discussed, VOUT2 is obtained by means of an intermediate tap on the inductor.This imposes, for the two solutions, a different design of the output inductor in terms of turns ratio, i.e. n=1.4 for the –12V solution, against n=3.8 for the –24V solution (even if it could be necessary to tune the turn ratio for proper output voltage).Some disadvantage are related to the –12V solution:

–

The parasitic capacitance effect between the two windings is increased, compared to the second one. This will bring about higher switching losses in turn-on (see Figure 10. and Figure 11.) and, consequently, a worsening in terms of efficiency;A higher voltage diode is needed to supply the VIPer;

The peak current is twice higher, giving less output power margin for a given IDLIM.

––

Therefore, a –24V/-5V solution can be suitably used for all those applications where efficiency and cost are important and, in general, in all the designs where a –24V output voltage does not impact on the cost of the relays and drivers.

5/18

Application example nº 1AN2359

Instead, the -12V/-5V solution can be used all those times where it is not possible to change the auxiliary supply voltage.

3 Application example nº 1

The first application example is a 3.3W double output Buck-Boost converter. The specifications are listed in Table 2.

The schematic of the circuit is shown in Figure 1. and the component list is shown in Table 3.Table 2.

SMPS specifications

SpecificationInput voltage range, VINOutput voltage VOUT1Output voltage VOUT2Maximum output current IOUT1Maximum output current IOUT2

Maximum output power

Value88 - 265VAC

-12V-5V150mA300mA3.3W

The input voltage can range from 88VAC to 265VAC. The input section consists in a resistor as a fuse, a single input rectifier diode and an input C-L-C filter. Such a filter provides both DC voltage stabilization and EMI filtering. The CSN-RSN leg across D1 helps the further reducing of the conducted emissions.

The regulation feedback is connected to VOUT2 by means of the PNP transistor Q1 and zener diode DZ2, in order to provide an output voltage with tight regulation range (the output precision depends on DZ2 tolerance). VOUT1 is obtained thanks to the turns ratio of the transformer.

The output inductor is wound in a TDK drum ferrite core (SRW0913 type), with an intermediate tap for VOUT2. The specifications are the following:

●●●

L1-3 = 420µH;N1-2 = 70 turns;N2-3 = 62 turns.

Optional bleeder resistors, Rb1 and Rb2, can be connected to the outputs in order to improve the regulation.

In particular, Rb1 has to be chosen in order to avoid the overvoltage on VOUT1 when VOUT2 is full loaded and VOUT1 is in no load condition.

6/18

AN2359

Figure 1.

Converter schematic

Application example nº 1

Table 3.Component list

Value22Ω, 1/2W1.2KΩ, 1/4W22Ω, 1/2W68Ω, 1/4W

DescriptionMetallic oxide resistor

ResistorResistorResistorOptional resistor

0.1µF, 400V10µF, 400V10µF, 50V22nF, 35V0.47µF, 50V4.7µF, 50V220µF, 16V470µF, 16V

Polyester capacitorElectrolytic capacitorElectrolytic capacitorCeramic capacitorElectrolytic capacitorElectrolytic capacitorElectrolytic capacitorElectrolytic capacitorDiode 1N4007Diode BA157Diode STTH106 (ultrafast)

Diode Zener 6.8VDiode Zener 4.3VPNP transistor BC558

470µH

Axial inductor

ReferenceRFUSERSN, R1R2R3Rb1, Rb2CSNC1, C2C3C4C5C6C7C8D1D2, D3D4, D5DZ1DZ2Q1L1

7/18

Application example nº 1

Table 3.

Component list (continued)

Value(Read sec. 5)

Description

AN2359

Reference

L2IC

STMicroelectronics VIPer22ADIP

3.1 Experimental results

The power supply has been characterized in terms of line and load regulation. The

efficiency measurements have been taken using a DC power source and a

milliamperometer, in order to have higher accuracy than in AC measurements. In Table 4.,Table 5. and Table 6. the experimental results are shown. It is then possible to observe the efficiency decreases, at same output power, when VOUT2 is more loaded. This can be explained with an increase of the parasitic capacitance effect between the windings. These measurements have been performed without bleeder resistors. Consequently, an

overvoltage occurs on VOUT1 when it is in no load condition and VOUT2 is full loaded. This can be avoided adding a 3.3KW resistor as a bleeder, with only a slight reduction of the efficiency. In Figure 2., Figure 3., Figure 4., Figure 5., Figure 6., and Figure 7. typical waveforms at minimum and maximum input voltage are shown. Figure 8. and Figure 9. shows the output ripple voltage at full load at minimum and maximum input voltage. In Figure 10. and Figure 11. turn-on losses measurements are shown in the same previous conditions. It is important to point out that a lot of power is dissipated in turn-on, due to the parasitic capacitance of the inductor.Table 4.

Circuit characterization - VIN = 120VDC

VOUT1[V]12.0014.7316.0410.5111.1511.3210.4010.9511.14

VOUT2[V]5.014.994.975.004.974.954.994.954.92

IIN [mA]1.089.7118.338.8617.6027.1016.8126.4036.60

PIN [W]0.1291.162.201.062.113.252.023.174.40

POUT[W]

00.7841.490.7881.582.331.562.383.15

IOUT1 [mA]IOUT2 [mA]

000757575150150150

015030001503000150300

η [%]

067.5867.7074.3074.8071.7077.2075.0071.50

Table 5.

Circuit characterization - VIN = 320VDC

VOUT1[V]11.7715.5616.2610.55

VOUT2[V]5.045.025.015.02

IIN [mA]0.473.897.253.55

PIN [W]0.151.242.321.14

POUT[W]0.000.751.500.79

IOUT1 [mA]IOUT2 [mA]

00075

01503000

η [%]

0.0060.4964.7869.65

8/18

AN2359

Table 5.

Circuit characterization - VIN = 320VDC

VOUT1[V]11.2311.3710.411.0112.21

VOUT2[V]5.014.995.0154.98

IIN [mA]7.0110.536.710.3814.44

Application example nº 1

IOUT1 [mA]IOUT2 [mA]

7575150150150

1503000150300

PIN [W]2.243.372.143.324.62

POUT[W]1.592.351.562.403.33

η [%]

71.0569.7372.7672.3071.97

Table 6.

Circuit characterization - VIN = 374VDC

VOUT1[V]11.7415.7716.5110.5411.2611.4110.411.0511.2

VOUT2[V]5.065.0255.025.0155.015.014.98

IIN [mA]0.443.396.383.096.189.365.929.2612.74

PIN [W]0.161.272.391.162.313.502.213.464.76

POUT[W]0.000.751.500.791.602.361.562.413.17

IOUT1 [mA]IOUT2 [mA]

000757575150150150

015030001503000150300

η [%]

0.0059.3962.8668.4069.0567.2970.4669.5666.61

Figure 2.

Typical waveforms at 88VAC: open Figure 3.

load

Typical waveforms at 88VAC: full load

Ch1 Freq - 2.38kHz

Ch1 Freq - 59.61kHz

Ch2 Max - 228mACh2 Max - 572mA

9/18

Application example nº 1AN2359

Figure 4.

Typical waveforms at 265VAC: open Figure 5.load

Typical waveforms at 265VAC: full load

Ch1 Freq - 2.39kHz

Ch1 Freq - 59.65kHz

Ch2 Max - 428mA

Ch2 Max - 532mA

Figure 6.

Commutation at full load: 88VAC

Figure 7.

Commutation at full load: 265VAC

Ch1 Max - 366Vax - 562mCh2 MACh1 Freq - 103V

Figure 8.

Output ripple voltage at full load: 88VAC

Figure 9.

Output ripple voltage at full load: 265VAC

Ch1 Pk-Pk - 90mV

Ch3 Pk-Pk - 80mV

10/18

Ch2 Max - 530Ch1 Pk-Pk - 108mV

Ch3 Pk-Pk - 82mV

mAAN2359Application example nº 1

Figure 10.Turn on losses measurement at full Figure 11.Turn on losses measurement at full

load: 88VACload: 265VAC

M1 Area 2.92µWs

M1 Area 10.07µWs

3.2 Thermal measurements

In this application, the main thermal issues are related to parasitic capacitance effects that can lead to higher power dissipation in the device and then higher working temperature. In order to evaluate the case temperature of the VIPer in the entire input voltage range, a

thermal mapping by means of an IR Camera was done at ambient temperature and full load. In Figure 12. and Figure 13. the thermal profile of the device at minimum and maximum input voltage range respectively is shown. It is important to highlight that at low line the conduction losses are predominant, instead at high input voltage the switch losses became not negligible, due to parasitic capacitance of the inductor. This is point out in Figure 14.

Figure 12.VIPer22A Thermal profile: at VIN=

88VAC

Figure 13.VIPer22A Thermal profile: at VIN=

265VAC

TCASE(MAX)=53.3°C

TCASE(MAX)=67.3°C

11/18

Application example nº 2

Figure 14.VIPer22A temperature at maximum load

AN2359

4 Application example nº 2

In this second example, the Buck-Boost is modified in order to have –24V/-5Voutputs voltages in a 4W application. In Table 7. the main specifications of the power supply are listed. The schematic of the circuit and the component list are shown in Figure 15. and in Table 8. respectively.Table 7.

SMPS specifications

SpecificationInput voltage range, VINOutput voltage VOUT1Output voltage VOUT2Maximum output current IOUT1Maximum output current IOUT2

Maximum output power

Value88 - 265VAC

-24V-5V100mA300mA4W

The –24V output voltage allows to supply the VIPer directly from the feedback path, saving the cost of a high voltage diode. Even in this case, the feedback regulation is connected to VOUT2 by means of Q1 transistor and DZ1 zener diode. The output inductor, with

intermediate tap for VOUT2, is provided by PULSE (PFM0250 type) with the following features:

●●●●

L1-3 = 510µH ±10%;N1-3 / N2-3 = 3.81 ± 2%;R1-2 = 560mW (max);R2-3 260ΩW (max).

12/18

AN2359Application example nº 2

Also bleeder resistors or zener diodes may be mandatory at no load in order to improve the regulation and avoid output overvoltage.Figure 15.Converter schematic

Table 8.Component list

Value22Ω, 1/2W1.2KΩ, 1/4W22Ω, 1/4W100Ω, 1/4W

DescriptionMetallic oxide resistor

ResistorResistorResistorOptional resistor

0.1µF, 400V10µF, 400V33µF, 25V47nF, 35V22µF, 16V470µF, 25V100µF, 16V

Polyester capacitorElectrolytic capacitorElectrolytic capacitorCeramic capacitorElectrolytic capacitorElectrolytic capacitorElectrolytic capacitorDiode 1N4007

Diode BYT11-400 (ultrafast)

Diode Zener 18VDiode Zener 4.3VPNP transistor BC327

470µH

Axial inductor

ReferenceRFUSERSN, R1R2R3Rb1, Rb2CSNC1, C2C3C4C6C7C8D1D2, D4, D5

DZ1DZ2Q1L1

13/18

Application example nº 2

Table 8.

Component list (continued)

Value(Read sec. 6)

DescriptionPulse PFM0250STMicroelectronics

AN2359

Reference

L2IC

4.1 Experimental results

In Table 9., Table 10. and Table 11. the measures performed on the proposed converter are

listed. In Figure 16. and Figure 17. typical waveforms at 300VDC are shown.

The converter performs well in terms of line and load regulation. The –5V output shows a ±5% of precision. VOUT1, obtained by means of the turns ratio of the inductor, shows good performance too, even if an overvoltage occurs on VOUT1 when it is in no load condition and VOUT2 is full loaded. This can be avoided connecting an appropriate bleeder resistor on VOUT1. The efficiency measurements show a better behavior compared to the –12V

solution. This can be explained because, in this configuration, the turn-on losses are lower compared to the –12V solution.Table 9.

Experimental results - VIN=120VDC

VOUT1[V]24.0923.8723.9724.0524.0641.5421.82

VOUT2[V]5.0965.0985.0875.0755.0695.025.083

IIN[mA]0.9863.64313.3320.525.6812.7413.74

PIN[W]0.19720.72862.6664.15.1362.552.75

POUT[W]

00.391641.961553.1423.92671.512.18

η[%]0.0053.7573.5876.6376.4559.1179.40

0301502403003000

IOUT1[mA]IOUT2[mA]

01050801000100

Table 10.

Experimental results - VIN=320VDC

VOUT1[V]24.123.923.9624.0324.0341.1421.89

VOUT2[V]5.0985.0945.0915.0795.0685.0255.096

IIN[mA]0.6662.4359.18213.8517.178.6849.41

PIN[W]0.200.732.754.165.152.612.82

POUT[W]0.000.391.963.143.921.512.19

η[%]0.0053.6471.2175.6076.1757.8777.54

0301502403003000

IOUT1[mA]IOUT2[mA]

01050801000100

14/18

AN2359Layout considerations

Table 11.

Experimental results - VIN=374VDC

VOUT1[V]24.1623.992424.0324.0284121.93

VOUT2[V]5.0985.0945.0945.0855.0715.035.1

IIN[mA]0.5131.8527.0410.6613.146.687.221

PIN[W]0.210.742.824.265.262.672.89

POUT[W]0.000.391.963.143.921.512.19

η[%]0.0053.0169.7573.7174.6656.4775.92

0301502403003000

IOUT1[mA]IOUT2[mA]

01050801000100

Figure 16.Typical waveforms at 300VDC and

full load: commutation

Figure 17.Typical waveforms at 300VDC and

full load: detail

5 Layout considerations

A proper PCB layout is essential for correct operation of any switch-mode converter and the

same basic rules have to be taken into account in order to optimize the current path, especially in high current path routing.

Since EMI issues are related to layout, the current loop area has to be minimized. Moreover, the control ground path has to be separated from power ground, in order to avoid any noise interference between the control section and the power section.

All the traces carrying high currents have to be as short as possible, in order to minimize the resistive and inductive effect.

A particular care has to be taken into account regarding the optimal routing of the input EMI filter path and the correct placement of any single component (L1 –R1 very close to input bulk capacitors, trace as short as possible…).

15/18

Layout considerationsAN2359

Finally, dissipating copper area on the VIPer drain and diodes pins have to be provided, in order to increase the power dissipation capability and, consequently, reduce the devices temperature.

The circuit layout is shown in figure 12 for the –12V configuration and in figure 13 for the –24V configuration. The PCB is the same and includes the options for the two configurations.Figure 18.PCB Layout (not in scale). Option nº 1: -12V output voltage

Figure 19.PCB Layout (not in scale). Option nº 2: -24V output voltage

16/18

AN2359Conclusions

6 Conclusions

Two low cost double outputs Buck-Boost converters have been proposed based on STMicroelectronics VIPer22A.

Thanks to the regulation feedback connected to –5V output, the converters can be suitably used to supply a microcontroller or applications where a high output voltage tolerance is required.

Instead, the -12V or-24V output voltage, achieved by the output inductor turns ratio, can be used for the auxiliary circuits where a lower tolerance can be accepted.

In particular, the –24V option can be preferred because it guarantees a higher efficiency (due to lower turn- on losses) and allows to save the cost of a high voltage diode compared to the –12V solution.

On the other side, the –12V solution has to be used in many applications when it is not possible to change the auxiliary supply voltage from –12V to –24V.

The same topology can be used for lower power range, replacing the VIPer22 with the VIPer12.

In this case the device can deliver up to about 2.2W.

7 Revision history

Table 12.

Date04-Dec-2006

Revision history

Revision

1

Initial release

Changes

17/18

AN2359

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