ST AN1894 Application note

		AN1894
®		- APPLICATION NOTE
	VIPower: VIPer12A 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 building an auxiliary power supply based on the VIPer12AS monolithic device with rather low output power required.

1. INTRODUCTION

There are some applications, where an off-line power supply without isolation between input and output can be tolerated and rather low output current is required. In this case the converter should be simple and low cost. These requirements can be satisfied by a step-down converter based on monolithic device VIPer12AS that incorporates the PWM controller together with the Vertical power MOSFET switch in a SO8 package. The presented power supply has two variants. The first is a buck (step-down) converter with a positive output voltage referenced to the common ground. The second one with negative output voltage is a buck-boost converter. The presented reference board incorporates both variants by different assembly options.

2. CIRCUIT DESCRIPTION

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

2.1.1 Operating Conditions

Input Voltage range	90-264 VAC
Input Voltage Frequency range	50/60 Hz
Main Output	15V / 100mA
Second Output (through linear regulator)	5V / 60 or 20mA
Total Maximum Output Power	1.6W

2.1.2 Circuit Operation

The total schematic of the power supply 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 line via one way rectifier 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 forms an EMI filter. The DC voltage at C2 is then applied to the VIPer12 that works as a high side switch. It means the IC and corresponding 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. The voltage feedback loop is provided via zener diode D3, resistor R3 and capacitor C6.

The diode D7 between capacitor C7 and C4 ensures the proper start-up of the converter. Thanks to this diode the feedback loop circuit is separated from supply circuit. The internal start-up current source of the VIPer12 charges the IC supply capacitor C4 to a specified start-up threshold voltage of about 16V.

March 2004

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As soon as C4 voltage reaches the start-up threshold the internal 60 kHz oscillator sets the internal flipflop and through output driver turns-on the internal high voltage power MOSFET. The power MOSFET 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 output capacitor C3 connected to the inductor’s second terminal is much lower than input bulk capacitor voltage the inductor current will ramp-up. As soon as the inductor current ramp reaches the VIPer’s internal set-point defined by feedback loop, the internal power switch turns off. The inductor keeps the direction of the current flowing and it reverses the 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 source terminal of the VIPer12 sees a negative level of the forward biased D5 (when referenced to ground) so it can be considered as grounded. This allows the inductor current to flow also through D4 and supply the VIPer12 and give the feedback information about output voltage.

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

R1

D1

D2

L1 BC

10R

GL1M

GL1M

1.5mH 100mA

90...264V~

3W

1000V

1000V

CON1

1A

1A

1

L

L

N

2

N

clamp

+

C1

+

C2

4.7uF

4.7uF

400V

400V

KMG

KMG

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.

8	7	6	5		U1
Drain4 Drain3			Drain2 Drain1		VDD		4
	Source2		Source1				3
					FB
2		1		VIPer12AS

D5

RGL34J

600V

0.5A

R5

0R

		D7
VDD
C4		LL4148
	+
10uF		D3
50V		ZMM13
KME
		R3		D4
		1k		RGL34J
C6			C7	600V
22nF			100nF	0.5A
				R6
L2				0R	U3	L78M05CDT (1a)
1mH					VIN	VOUT
200mA
						GND
LBC
			C3
		+ 56uF
			35V			(1b)
			LXY		U2	L78L05CD
			D6	8	VIN	1
			ZMM18			VOUT
					NC1 NC2 GND1 GND2 GND3 GND4
					4 5 2 3 6 7

+15V

	CON2
+5V	3
	2
	1
C8	clamp
100nF

The output voltage of the converter at the 15V terminal is determined by the voltage drop across zener diode D3 together with voltage drop across the resistor 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 can change from 0mA (full output power) to about 0.9mA at no output load. The R3 voltage variation is 0.9V and FB pin 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 regulator. Regulator U2 accommodated 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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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 Isolated 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 KMG 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 KME 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	GL1M	Diotec GL1M diode, 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	LL4148	LL4148 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 inductor, 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	STMicroelectronics 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	STMicroelectronics L78M05CDT positive voltage regulator 5V 0.5A 5%, for
			variant 1a
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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 contains both SMD and through hole components. The board incorporates both a buck and buck-boost variant of the converter. The outline dimensions are 38x29mm. Assembly top side (trough-hole components) and solder bottom (SMD components) side can be seen in Figure 2 and Figure 3.

Figure 2: Assembly Top (not in scale)

Figure 3: Assembly Solder Side (not in scale)

Figure 4: PCB Layout (not in scale)

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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.

Figure 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

Figure 6: Output Regulation Characteristics (Parameter is Vin)

	19
	18
[V]	17					125VDC
						150VDC
Voltage
						200VDC
						250VDC
						300VDC
Output
	16					350VDC
						375VDC
	15
	14
	0	0.02	0.04	0.06	0.08	0.1

Output Current [A]

Using the same measured values, Figure 6 can be redrawn using a different parameter. Figure 7 shows the variation of the output voltage with input DC voltage change. The output current is the parameter in this case.

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