ST AN2359 Application note

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 2006

Rev 1

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Contents	AN2359

Contents

1	VIPerX2 description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 5
2	Output voltage selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		5
3	Application example nº 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		6
	3.1	Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8
	3.2	Thermal measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
4	Application example nº 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		12
	4.1	Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
5	Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		15
6	Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		17
7	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		17

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AN2359	List of figures

List of figures

Figure 1. Converter schematic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 2. Typical waveforms at 88VAC: open load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 3. Typical waveforms at 88VAC: full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 4. Typical waveforms at 265VAC: open load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 5. Typical waveforms at 265VAC: full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 6. Commutation at full load: 88VAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 7. Commutation at full load: 265VAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 8. Output ripple voltage at full load: 88VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 9. Output ripple voltage at full load: 265VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 10. Turn on losses measurement at full load: 88VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 11. Turn on losses measurement at full load: 265VAC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 12. VIPer22A Thermal profile: at VIN= 88VAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 13. VIPer22A Thermal profile: at VIN= 265VAC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 14. VIPer22A temperature at maximum load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Figure 15. Converter schematic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 16. Typical waveforms at 300VDC and full load: commutation . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 17. Typical waveforms at 300VDC and full load: detail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 18. PCB Layout (not in scale). Option nº 1: -12V output voltage . . . . . . . . . . . . . . . . . . . . . . . 16

Figure 19. PCB Layout (not in scale). Option nº 2: -24V output voltage . . . . . . . . . . . . . . . . . . . . . . . 16

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List of tables	AN2359

List of tables

Table 1. Proposed converters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Table 2. SMPS specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Table 3. Component list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Table 4. Circuit characterization - VIN = 120VDC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Table 5. Circuit characterization - VIN = 320VDC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Table 6. Circuit characterization - VIN = 374VDC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Table 7. SMPS specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Table 8. Component list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Table 9. Experimental results - VIN=120VDC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Table 10. Experimental results - VIN=320VDC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Table 11. Experimental results - VIN=374VDC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Table 12. Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

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AN2359	VIPerX2 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		Output 2	POUT(MAX)
	-12V/150mA		-5V/300mA	3.3W
	-24V/100mA		-5V/300mA	3.9W

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.

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Application example nº 1	AN2359

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
	Specification	Value
	Input voltage range, VIN	88 - 265VAC
	Output voltage VOUT1	-12V
	Output voltage VOUT2	-5V
	Maximum output current IOUT1	150mA
	Maximum output current IOUT2	300mA
	Maximum output power	3.3W

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 = 420 H;

●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.

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