ST AN1934 Application note

AN1934

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- APPLICATION NOTE

VIPower: VIPer12A NON ISOLATED FLYBACK

CONV ERTE R RE FE R EN CE BO AR D

P. LIDAK - R. HAUSER

ABSTRACT

Presented circuit can be used to produce multiple outputs, non isolated pos itive or negat ive voltage. It is dedicated for building an auxiliary power supply based on the VIPer12AS monolithic device.

1. INTRODUCTION

The aim of the presented reference boards is to propose a solution of the power supply based on an offline discontinuous current mode flyback converter without isolation between input and output. The flyback topology allows to fully exploit current capability of the incorporated monolithic device VIPer12AS when compared with buck converter based power s uppl y. To ensure low cost of the whole power supply the isolation between input and output is not provided. This greatly simplifies the transformer design and production. The VIPer12AS incorporates the PWM controller with 60 kHz internal oscillator and altogether with the vertical power MO SF ET sw itch in a S O-8 package. T he presented power supply has four variants. All these variants have been incorporated in presented reference board by different assembly options.

2. CIRCUIT DESCRIPTION

2.1 NON ISOLATED FLYBACK +5V/500MA, +15V/200MA (VARIANT 1)

2.1.1 Operating Conditions

Input Voltage range Input Voltage Frequency range Main Output (regulated) Second Output Total Maximum Output Power

2.1.2 Circuit Operation

The total schematic of the power supply (Variant 1) can be seen in figure 1. The output of the converter is not isolated from input. For this reason the reference ground is common for an input and output connection terminal. The input capacitor C1 is charged from the mains by single rectification consisting of diodes D1 and D2. Two diodes in series are used for EMI reasons to sustain burst pulses of 2kV. The capacitor C1 together with capacitor C2 and inductor L1 form an EMI filter.

The DC voltage at C2 is then applied to the primary winding of the transformer through the internal MOSFET switch of VIPer12 d uring ON time of the switching period. The s nubber circuit consisting of resistor R3 and capacitor C6 red uces the voltage spike across the primary winding of the transformer due to the parasitic leakage inductance. It al so slows down dV/dt of the primary winding’s voltage a little bit and thus improves EMI.

The power supply provides two out puts from two transformers‘ windings through rectifiers D4, D5 and smoothing capacitors C3 and C4. The VIPer12AS is supplied by 15V output voltage through transistor

90-264 VAC

50/60 Hz 5V / 500mA 15V 200mA

5.5W

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Q2 and diode D7. The diode D7 ensures the proper start-up of the converter by separating the 15V output from the internal start-up current source of the VIPer12AS which will charge the IC supply capacitor C5 to a specified start-up threshold voltage of about 16V. As soon as C5 voltage reaches the start-up thr eshold t he interna l 60 kHz o scillator se ts the int ernal flip- flop and tu rns on the i nternal h igh voltage power MOSFET through the output driver. The power MOSFET applies the bul k capacitor C1 and C2 high voltage to the transformer’s primary winding a nd primary current will ramp-up. As soon as the primary current ramp reaches the VIPer’s internal set-point defined by feedback loop, the internal power switch turns off. The output capacitor C3 or C4 is charged by energy stored in the transformer through rectifier diode D4 or D5. The current loop which charges the 5V output flows through diode D5 only. Because of the D5 location, the 15V output is charged via both diodes D4 and D5. Beside the slight decrease of the conv erter power efficiency, it significantly im proves the cross-regulation of th e outputs which was the main purpose of this arrangement.

The voltage feedback loop senses the 5V ou tput by resistor divider R5, R7. The c ontrol IC U2 compares the resistor divider output voltage with internal reference voltage of 2.5V and changes the cathode voltage accordingly to keep 5V output stable. If the 5V output voltage rises above it’s nominal value, the cathode voltage of U2 g oes down and ca thode current will increase. The cathode current w ill cause a voltage drop across R9 and opens transistor Q1 which will inject the current from Vcc line to FB pin 3 of the VIPer12AS. The FB pin current will decrease the peak primary current to reduce the power delivered to the outputs. Resistor R10 limits the U2 cathode current . Resistor R9 has two roles: it works as pull up for Q1 and ensures bias current of at least 1mA for U2 proper operation.

Figure 1: Schematic diagram of non isolated flyback converter (variant 1)

R14

C4 220uF 35V LXY

R10

+15V

CON2

+5V

2 1

clamp

+15V

R9 470R

100nF

C10 1nF

+5V

4.7k

90...264V~

CON1

clamp

10R

GL1M

1000V

1 2

Layout Hints: C5, C8 have to be close to VIPer12A

Assembly options: (1): +5V/500mA, +15V/200mA

note: all voltages refer to neutral

GL1M 1000V 1A

C1 22uF 400V KMG

330uH 190mA

C2 10uF 400V KMG

4 3

VDDVDDVDDVDDVDDVDD

C5 10uF

50V KME

C8 22nF

EF16/4.7 AL = 120nH Gap = 0.22mm

C6 100pF 500V

R16

100R

R17

T1A

3.1mH

160 turns

0.18 CuLL

VDD

Drain15Drain26Drain37Drain4

Sour ce11Source2

VIPer12AS

T1B 33uH

16.5 turns

0.315 CuLL

T1C 10uH 9 turns

0.315 CuLL

STPR120A 200V 1A

D5 STPS1L40A 40V 1A

LL4148

R11

4.7k

BC856B

120uF 35V LXY

TS2431ILT

4.7k

Resistor R11 limits the feedback current to a safe value, which is lower than specified by the maximum rating table in the data sheet. Capacitor C8 improves noise immunity of the FB input against noise.

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2.1.3 Bill of materials

The bill of material presented in Table 1 covers all power supply variants. The components which are specific for a particular variant can be recognized by column named "V ariant". Peak clamp D6 connected across the primary winding is optional and it is not assem bled on the board. In case a precise voltage regulation of the 15V output is required, resistor R6 connected from the 15V output to the control input of U2 can be assembled instead of R5.

Table 1: Bill of material for all variants of non isolated flyback converter

Ref. Q.ty Variant Description

CON1 1 CON2 1

C1 1 22µF Electrolytic capacitor, Nippon Chemi-Con, KMG 400V, 20%

C2 1 10µF Electrolytic capacitor, Nippon Chemi-Con, KMG 400V, 20% C3 1 120µF Electrolytic capacitor, Nippon Chemi-Con, LXY 35V 20% C4 1 (1, 2, 4) 220µF Electrolytic capacitor, Nippon Chemi-Con, LXY 35V 20% C5 1 10µF Electrolytic capacitor, Nippon Chemi-Con, KME 50V 20% C6 1 100pF Ceramic capacitor, X7R, 500V C1206 10% C8 1 22nF Ceramic capacitor, X7R, 50V C0805 10%

C9 1 (1, 4) 100nF Ceramic capacitor, X7R, 50V C0805 10% C10 1 (1, 4) 1nF Ceramic capacitor, X7R, 50V C0805 10% C11 1 (2)

(3)

D1, D2 2 GL1M Diode, Diotec, trr=1.5µs 1000V 1A, MiniMELF

D4 1 STMicroelectronics STPR120A Diode, fast recovery trr=25ns 200V 1A SMA

D5 1 (1, 2, 4)

(3) D6 1 optional STMicroelectronics PKC-136 Diode, Peak clamp, Vbr=160V, 700V, 1.5W DO-15 D7 1 LL4148 Diode 75V 200mA D8 1 (2, 3) ZMM13 Zener diode, 13V 0.5W 5%

L1 1 330µH Inductor, EPCOS, bobbin core, B78108-S1334-J, 190mA 6.4R 10%

Q1, Q2 2 (1, 4) BC856B Bipolar transistor, PNP, 65V 100mA 330mW

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

R2, R5,

R7, R8

R3 1 100R resistor, metal film, 200V 0.25W R1206 1% R4 1 (2, 3) 0R resistor, metal film, R1206 R6 1 optional 24K resistor, metal film, R0805, 100V 0.125W 1% R9 1 (1, 4) 470R resistor, metal film, R0805, 100V 0.125W 1%

R10 1 (1, 4) 1K resistor, metal film, R0805, 100V 0.125W 1% R17 1 0R resistor, metal film, R1206

T1 1 (1, 3, 4)

4 (1, 4) 4.7K resistor, metal film, 100V 0.125W R0805 1%

(2)

Clamp, WECO, 2 pole, horizontal, 1.5mm Clamp, WECO, 3 pole, horizontal, 1.5mm

2.2µF Tantalum capacitor, Size A, B45196E, 10V 7.0R 20% 100nF Ceramic capacitor, X7R, 50V C1206 10%

STMicroelectronics STPS1L40A Diode, Schottky, 40V 1A, SMA 0R Resistor, metal film, R1206

Ns=16/9 turns transformer, Vogt, ferrite Fi324, EF16/4.7, ord. num. 545 23 249 00 Ns=14/11 turns transformer, Vogt, ferrite Fi324, EF16/4.7, ord. num. 545 23 249 00

, 380V, 15A

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U1 1 STMicroelec tronics VIPer12AS, 730V 0.4A, 27R, f=60kHz, SO-8 U2 1 (1, 4) STMicroelectronics TS2431ILT shunt ref. IC, 2.5V 1mA to 100mA 360mW 2% U3 1 (2)

(3b) U4 1 (3a) STMicroelectronics L78M05CDT positive voltage reg., 5V, 0.5A 5%

2.1.4 Transformer Design

Since there is no requirement regarding isolation bet ween primary and sec ondary s ide, the t ransformer construction is easier compared to the isolated version. There is only a single layer of Mylar tape between the primary winding an d secondary windings. Its purpose is n ot to make transformer passing safety regulations but to ensure proper operation of the power s uppl y. Also creepage distances between windings are not that crit ical. T he physical appearance, dimensions and windin gs and pins arrangement can be seen in figure 2.

Figure 2: Transformer dimensions, windings and bottom view pin arrangement

STMicroelectronics L4931CD50 voltage reg., low drop, with inhibit, 5V, 250mA 4% STMicroelectronics L78L05CD positive voltage reg., 5V, 100mA 10%

The basic parameters of the ferrite core selected from Vogt’s ferrite materials and shapes can be seen in table 2. The gap size was optimised to ensure appropriate current capability and inductance to fully exploit switching frequency and to switch peak current limit of the VIPer12AS to achieve maximum output power.

Table 2: Transformer’s core parameters

Shape Material Gap size [mm] Inductance Factor AL [nH]

EF16/4.7

Vogt Fi 324

0.24 120

An overview of the most important parameters for each winding can be found in table 3. This table is valid for all variants. The only differentiation between the variants is the num ber of turns for the secondary windings. The difference is indicated in the "number of turns" column.

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Table 3: Transformer’s windings parameters

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Order Start Pin End Pin No. of turns Wire

diameter [mm]

13 4 26 5

35 1

2.1.5 PCB Layout

The PCB is designed as a single sided board made of FR-4 material with 35µm copper plating with solder and silk screen mask. The assembled board contains both SMD and through hole components. The board includes all variants of the converter. The o utline dimensions are 59x30mm. Ass embly top side (trough-hole components) and solder bottom (SMD com ponents) side can be s een in figure 3 and fi gure 4.

Figure 3: Assembly Top (not in scale)

160 9 (1, 3, 4)

11 (2)

16.5 (1, 3, 4)

14.5 (2)

0.18 CuLL 3.1mH

0.315 CuLL 10µH

0.315 CuLL 33µH

Wire

material

Inductance

15µH

25µH

Figure 4: Assembly Solder Side (not in scale)

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The PCB layout of the copper connections is depicted in figure 5. The holes for through-hole components are not seen in the picture.

Figure 5: PCB Layout (not in scale)

The physical appearance of the converter can be observed in figure 6.

Figure 6: Picture of the Converter

2.1.6 Evaluation and Measurements

The output regulation characteristics measured on 5V output can be seen in figure 7. It shows the voltage variation of the 5V output when different load is applied to 15V ou tput. Fi gure 8 s hows t he sam e characteristic as figure 7 but measured at 375VDC input voltage.

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Figure 7: Output Regulation Cha racteristics of 5V output at 12 5VDC Input Voltage (Parameter is load

current on 15V output)

5.009

5.007

5.005

5.003

Output Voltage [V]

5.001

4.999

4.997 50 100 150 200 250 300 350 400 450 500

Out p ut Cu r r ent [A ]

20mA 40mA 60mA 80mA 100mA 120mA 140mA 160mA 180mA 200mA

Figure 8: Output Regulation Cha racteristics of 5V output at 37 5VDC Input Voltage (Parameter is load current on 15V output)

5.009

5.007

5.005

5.003

Output Voltage [V]

5.001

4.999

4.997 50 100 150 200 250 300 350 400 450 500

Out p ut Cu r r ent [A ]

20mA 40mA 60mA 80mA 100mA 120mA 140mA 160mA 180mA 200mA

Similarly figure 9 shows the output regul ation characteristics measured on 15V output when different

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load current applied to 5 V output . Figure 10 shows the s ame cha racteristic as figu re 9 but measured at 375VDC input voltage.

Figure 9: Output Regul ation Characteristics of 15V Ou tput at 125VDC Input Voltage (Parameter is load current on 5V output)

15.2

14.8

14.6

Output Voltage [V]

14.4

14.2

20 40 60 80 100 120 140 160 180 200

Output Current [A]

50mA 100mA 150mA 200mA 250mA 300mA 350mA 400mA 450mA 500mA

Figure 10: Output Regulation Characteristics of 15V Output at 375VDC Input Voltage (Parameter is load current on 5V output)

15.2

14.8

14.6

Output Voltage [V]

14.4

14.2

50mA 100mA 150mA 200mA 250mA 300mA 350mA 400mA 450mA 500mA

20 40 60 80 100 120 140 160 180 200

Output Current [A]

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