ST AN1894 Application note

AN1894

- APPLICATION NOTE

VIPower: VI Per 12A NON ISOLATED BUCK AND

BUCK-BOOST CONVERTER REFERENCE BOARD

P. LIDAK - R. HAUSER

ABSTRACT

Presented circuit can be used to produce a single, non isolated positive or negative voltage. It is dedicated for bu ildi ng an au xiliary pow er sup ply ba sed on the VIPer12A S mo nolithic d evice wi th rath er low output power required.

1. INTRODUCTION

There are some appli cat io ns, wher e an off- line pow e r supp l y wit hout is o lati on be tween i n put an d out put can be tolerated a nd rather l ow output current is required. I n this case the converter s hould be sim ple and low cost. These requirements can be satisfied by a step-down converter based on monolithic device VIPer12AS that inco rporates the PWM contro ller together w ith the Vertical power MOSFET switch in a SO8 package. Th e presen ted pow er supply has two var iants. The f irst is a bu ck (ste p-down) co nvert er with a positive output voltage refer enced to the common grou nd. The second one with negati ve output voltage is a buck-boost converter. The presented reference board incorporates both variants by different assembly options.

2. CIRCUIT DESCRIPT ION

2.1 Buck Converter +15V/100mA, +5V/60mA or 20mA (Variant 1)

2.1.1 Operating Cond itio ns

Input Voltage range Input Voltage Freq uency rang e Main Output Second O utput (thr ough linear r egulator) Total Maximum Output Powe r

2.1.2 Circuit Operation

The total schema tic of the power supply can be seen in Figure 1. The output of the conver ter is not isolated from inpu t. F or t hi s re ason the r efer enc e grou nd is c omm on f or a n inp ut and output conne ct ion terminal. The inp ut capacitor C1 is charg ed from line via one w ay rectifier co nsisting of diodes D1 and D2. Two diodes in series are used for EMI reasons to sustain bur st pulses of 2kV. The capacitor C1 together with capacitor C2 and ind uctor L1 for ms an EMI filter. The DC voltage at C2 is then app lied to the VIPer12 that works as a high side switch . It means the IC and correspon ding supply and feedback loop circuitry is floating. The IC supply circuit consists of the high voltage diode D4, ceramic capacitor C7 low voltage D7 and capacitor C4. T he voltage feedback loop is provided via z ener diode D3 , resistor R 3 and capacitor C6.

The diode D7 between capacito r C7 and C 4 ensure s the prop er s tart-u p of the conve rter. Thanks to this diode the feedback loop circu it is separated fr om supply ci rcuit. The internal start-u p current sourc e of the VIPer12 charges the IC supply capacitor C4 to a specified start-up threshold voltage of about 16V.

March 20 04 1/14

90-264 VAC

50/60 Hz

15V / 100mA

5V / 60 or 20mA

1.6W

AN1894 - APPLICATION NOTE

As soon as C4 voltage rea ches th e start- up t hreshold the inter nal 6 0 kHz o scillator se ts the inte rnal flipflop and th rough output driver tur ns-on the inter nal high vo ltage power M OSFET. Th e power M OSFET applies the bulk capacitor C1 and C2 high voltage to the cathode of the power diode and to one terminal

of the inductor. Since the voltage at the out put capac itor C3 co nnecte d to the induc tor’s second terminal is much lower than input bulk capacitor voltage the inductor current will ramp-up. As soon as the inductor current ra mp reache s the VIPer ’s inter nal set -point defi ned b y feedback loop, t he int ernal pow er swi tch turns off. The inductor keeps the direct ion of the curren t flowing and it reverses th e voltage at C3. The inductor current then flows through the forward biased D5 diode and charges the output capacitor C3. In this switch -off phase the sou rce termi nal of the V IPer1 2 sees a negat ive lev el of the forwa rd biase d D5 (when refere nced to ground) so i t can be considered a s grounded. This all ows the inductor cur rent to flow also through D4 and supply the VIPer12 and give the feedback information about output voltage.

Figur e 1: Schematic diagram of non isolated buck converter with positive output voltage

90...264V~

L N

clamp

CON1

1.5mH 100mA

4.7uF 400V KMG

C4 10uF

VDD

Drain15Drain26Drain37Drain4

4.7uF 400V KMG

Source11Source2

VIPer12AS

50V KME

10R

GL1M 1000V 1A

1 2

VDD

C6 22nF

LL4148

ZMM13

C7 100nF

D4 RGL34J 600V

0.5A

+15V

CON2

+5V

C8 100nF

clamp

Layout Hints: C4, C6 have to be close to VIPer12A

Assembly options: (1a): +5V/60mA, +15V/100mA (1b): +5V/20mA, +15V/100mA

note: all voltages refer to neutral note: sum of currents is 100mA max.

D6 ZMM18

U3 L78M05CDT (1a)

VIN VOUT

GND

(1b)

U2 L78L05CD

VIN8VOUT

GND36GND47GND23GND12NC25NC1

L2 1mH 200mA

D5 RGL34J 600V

0.5A

R5 0R

LBC

56uF 35V LXY

The output voltage of the convert er at the 1 5V term inal is de termin ed by the vo ltage dro p across zener diode D3 togeth er with voltage dr op across the re sistor R3 and FB pin voltage. Resistor R3 limits the feedback current to a safe value lower than the maximum rating specified in the data sheet. Capacitor C6 protects the FB input against EMI. One has to take into account the slight variation of the output voltage with the load. It is because the feedback current reacts to the output load change to adopt switching duty cycle. The variable feedback current creates different FB voltage, different voltage drop across the resistor R3 and D3 zener voltage. The feedback current c an change from 0mA (ful l output power) to about 0.9mA a t no outpu t load. The R3 voltage vari atio n i s 0.9 V a nd F B pi n voltage about 1.2V. The D3 voltage variation depends on the diode V-I characteristics. Diode D6 limits the output voltage at light load condition and it also protects the U2 voltage re gulator. Regulator U2 a ccommodated in DPAK or SO-8 package is optional and can be assembled if the power supply for a microcontroller or logic part is required. The DPAK package version of U2 is dedicated for 60mA output current option while U3 in SO-8 can provide max. 20mA.

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AN1894 - APPLICATION NOTE

2.1.3 Bill of Materials

The bill of material that corresponds to the Figure 1 can be seen in Table 1.

Table 1: Bill of Material for Non Iso lated Buck Converter with Positive Output Voltage

Ref. Q.ty Value Description

CON1 1 WECO 10.877.002 - clamp, 2 pole, horizontal, type 94 380V 15A

CON2 1 WECO 10.877.003 - clamp, 3 pole, horizontal, type 94 380V 15A

C1, C2 2 4.7uF Nippon Chemi-Con K M G 400 VB 4R7 M Electrolytic capacitor KMG 400V 20%

C3 1 56uF Nippon Chemi-Con LXY 35 VB 56 M F11 Electrolytic capacitor LXY 35V 273mA

0.35R 20 % -

C4 1 10uF Nippon Chemi-Con KM E 50 VB 10 M Electrolytic capacitor KME 50V 20%

C6 1 22nF Ceramic capacitor X7R 50V 10%

C7 1 100nF Ceramic capacitor X7R 50V 10%

C8 1 100nF Ceramic capacitor X7R 50V 10%

D1, D2 2 GL1 M Diotec GL1M d iode, trr=1.5us 1000V 1A

D3 1 ZMM13 Zener diode 13V 0.5W 5%

D4, D5 2 RGL34J Diotec RGL34J Fast recovery diode trr=250ns 600V 0.5A

D6 1 ZMM18 Zener diode 18V 0.5W 5%

D7 1 LL414 8 L L4148 diode 75V 200mA

L1 1 1.5mH EPCOS B78108-S1155-J inductor, bobbin core BC 100mA 23R 10%

L2 1 1mH EPCOS B82144-A2105-J in ductor, large bobbin core LBC 200mA 3.8R 10%

R1 1 10R Yageo 254-0 10R 5% 1J resistor, wirewound, fusible, TK120 CRF 254-4 3W 5%

R3 1 1k resistor, metal film 100V 0.125W 1%

R5 1 0R resistor, metal film

R6 1 0R resistor, metal film

U1 1 VIPer12AS STMicr oelectroni cs VIPer12AS Off-line SMPS Primary IC 730V 0.4A 27R

U2 1 L78L05CD STMicroelectronics L78L05CD positive voltage regulator 5V 100mA 10%, for

variant 1b

U3 1 L78M05CDT STMicr oelectroni cs L78M05CDT positive voltage regulator 5V 0.5A 5%, for

variant 1a

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AN1894 - APPLICATION NOTE

2.1.4 PCB Layout

The PCB is designed as single sided board made of FR-4 material with 35mm copper plating with solder and silk screen mask. The assembled board co ntains both SMD and through h ole components. The board incor porates both a buck and buck-boost variant of the converter. The outline dimensions are 38x29mm. Assem bly top side (trough-hole components ) and solder botto m (S MD compon ents) sid e can be seen in Figure 2 and Figure 3.

Figur e 2: Assembly T op (not in scale)

Figur e 3: Assembly Solder Side (not in scale)

Figur e 4: PCB Layout (not in scale)

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AN1894 - APPLICATION NOTE

The PCB layout of the copper connections is depicted in Figure 4. The holes for through-hole components are not seen in the picture.

The physical appearance of the converter can be observed from Figure 5.

Figur e 5: Picture of the Converter

2.1.5 Buck Converter Evaluation and Measurements

The output regulation characteristics can be seen in Figure 6. It shows the variation of the output voltage with output current at specific DC input voltage. The zener diode D6 was not connected during the measurements

Figur e 6: Output Regulation Characteristics (Parameter is Vin)

Output Voltage [V]

0 0.020.040.060.080.1

Out put Current [A]

125VDC 150VDC 200VDC 250VDC 300VDC 350VDC 375VDC

Using the sam e me asure d v alues, Figure 6 can be redrawn using a different par ameter. Figure 7 shows the variatio n of the outpu t voltage with inpu t DC voltage change. The output curr ent is the par ameter in this case.

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