ST AN3105 APPLICATION NOTE

AN3105

Application note

48 V - 130 W high efficiency converter with PFC for LED street lighting applications - European version

1 Introduction

Nowadays, LEDs are becoming ever more popular, thanks to their particular characteristics, such as high efficiency and long life, and therefore they are pushing the innovation of current lamp types and strongly contributing to reducing the energy consumption for internal or external lighting. This is also the case in street lighting applications, where higher efficiency and long life are vital for reducing costs.

For these reasons a street lighting power supply designed to power an LED lamp must have high efficiency and at least a similar lifetime, in order to guarantee the maintenance free operation required by these kinds of applications.

This application note describes the characteristics and features of a 130 W demonstration board (EVL130W-SL-EU), tailored on an LED power supply specification for street lighting. The circuit is composed of two stages; a front-end PFC using the L6562AT and an LLC resonant converter based on the L6599AT.

The peculiarities of this design are; very high efficiency, extended European input mains range (177-277 VAC) operation, and long term reliability.

Because reliability (MTBF - “Mean Time Between Failures”) in power supplies is typically affected by electrolytic capacitors and their typical high failure rate, unless using very expensive types, this board offers a very innovative design approach as the board doesn't implement any electrolytic capacitors, which are replaced by film capacitors from EPCOS. Component de-rating has also been carefully applied during the design phase, decreasing the component stress as recommended by MIL-HDBK-217D. The number of components, thanks to the use of the new L6562AT and L6599AT devices, has also been minimized, therefore increasing the MTBF and optimizing the total component cost. Thanks to the high efficiency achieved no heatsinks are required. The resonant stage power components are SMT, like most of the passive components, therefore decreasing production costs.

The board also has protections in case of overload or short-circuit, open-loop by each stage, or input overvoltage. Because of the particular application, all protections, in the case of intervention, are auto-restart.

Figure 1. EVL130W-SL-EU: 130 W SMPS for LED street lighting applications

September 2010

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Contents	AN3105

Contents

1	Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 1
2	Main characteristics and circuit description . . . . . . . . . . . . . . . . . . . . .		5
	2.1	Power Factor corrector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
	2.2	Resonant power stage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
	2.3	Startup sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
	2.4	Output voltage feedback loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
	2.5	L6599AT overload and short-circuit protection . . . . . . . . . . . . . . . . . . . . . .	7
	2.6	Overvoltage and open-loop protection . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
3	Efficiency measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		9
4	Input current harmonics measurement . . . . . . . . . . . . . . . . . . . . . . . . .		10
5	Functional check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		12
	5.1	PFC circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	12
	5.2	Half-bridge resonant LLC circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	12
	5.3	Dynamic load operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
	5.4	Overcurrent and overvoltage protection . . . . . . . . . . . . . . . . . . . . . . . . . .	15
	5.5	Converter startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	17
	5.6	Thermal map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	18
6	Conducted emission pre-compliance test: peak measurement . . . . . 20
7	Bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		21
8	PFC coil specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		26
9	Transformer specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		28
10	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		30

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AN3105	List of tables

List of tables

Table 1. EVL130W-SL-EU demonstration board: overall efficiency vs. load . . . . . . . . . . . . . . . . . . . 9 Table 2. Thermal maps reference points - PCB top side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Table 3. Thermal maps reference points - PCB bottom side . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Table 4. EVL130W-SL-EU demonstration board: bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Table 5. PFC coil winding data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Table 6. Transformer winding data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Table 7. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

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List of figures	AN3105

List of figures

Figure 1.	EVL130W-SL-EU: 130 W SMPS for LED street lighting applications. . . . . . . . . . . . . . . . .	. 1
Figure 2.	EVL130W-SL-EU demonstration board: electrical diagram . . . . . . . . . . . . . . . . . . . . . . . .	. 8
Figure 3.	EVL130W-SL-EU demonstration board efficiency diagrams . . . . . . . . . . . . . . . . . . . . . . .	. 9
Figure 4.	EVL130W-SL-EU demonstration board: compliance to EN61000-3-2 Class-C
	standard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
Figure 5.	EVL130W-SL-EU demonstration board: input current waveform at 230 V - 50 Hz
	- 130 W load and 65 W load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
Figure 6.	EVL130W-SL-EU demonstration board: Power Factor and Total Harmonic Distortion
	vs. load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
Figure 7.	EVL130W-SL-EU demonstration board: PFC stage and L6562AT waveforms
	at 230 V - 50 Hz - full load – detail . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	12
Figure 8.	EVL130W-SL-EU demonstration board: primary and secondary side resonant stage
	waveforms at 230 V - 50 Hz - full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	13
Figure 9.	EVL130W-SL-EU demonstration board: high and low frequency ripple on output voltage
	at 230 V - 50 Hz - full load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
Figure 10.	EVL130W-SL-EU demonstration board: output voltage variation driving
	a CC LED converter - PWM = 90% and PWM = 15% . . . . . . . . . . . . . . . . . . . . . . . . . . . .	15
Figure 11.	EVL130W-SL-EU demonstration board: short-circuit at 230 VAC - 50 Hz - full load
	and open loop protection intervention at 20 W load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	16
Figure 12.	EVL130W-SL-EU demonstration board: startup at 230 VAC - 50 Hz - full load . . . . . . . . .	17
Figure 13.	Thermal map at 230 VAC - 50 Hz - full load - PCB top side . . . . . . . . . . . . . . . . . . . . . . . .	18
Figure 14.	Thermal map at 230 VAC - 50 Hz - full load - PCB bottom side . . . . . . . . . . . . . . . . . . . . .	19
Figure 15.	CE peak measure at 230 VAC and full load - phase wire . . . . . . . . . . . . . . . . . . . . . . . . . .	20
Figure 16.	CE peak measure at 230 VAC and full load - neutral wire . . . . . . . . . . . . . . . . . . . . . . . . .	20
Figure 17.	PFC coil electrical diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	26
Figure 18.	PFC coil mechanical aspect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	27
Figure 19.	Transformer electrical diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	28
Figure 20.	Transformer overall drawing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	29

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AN3105	Main characteristics and circuit description

2 Main characteristics and circuit description

The main features of the SMPS are:

●Extended European input mains range: 177 ÷ 277 VAC - frequency 45 ÷ 55 Hz

●Output voltage: 48 V at 2.7 A

●Long-life electrolytic capacitors are not used

●Mains harmonics: acc. to EN61000-3-2 Class-C

●Efficiency at full load: better than 90%

●EMI: according to EN55022-Class-B

●Safety: double insulation, according to EN60950, SELV

●Dimensions: 75 x 135 mm, 30 mm components maximum height

●No heatsinks needed

●PCB: single side, 35 µm, FR-4, mixed PTH/SMT

2.1Power Factor corrector

The PFC stage, working in transition mode, acts as a pre-regulator and powers the resonant stage with the output voltage of 450 V. The PFC power topology is a conventional boost converter, connected to the output of the rectifier bridge D3. It is completed by the coil L1, manufactured by MAGNETICA, the diode D2 and the capacitors C5, C6, and C7 in parallel. The PFC output capacitors are film type, 5 µF - 800 V, manufactured by EPCOS. Using film capacitors to replace the typical electrolytic capacitors considerably increases the MTBF of the board.

The boost switch is represented by the Power MOSFET Q2. The board is equipped with an input EMI filter necessary to filter the commutation noise coming from the boost stage. The PFC implements the L6562AT controller, a small and inexpensive controller which is guaranteed for operation over a wide temperature range.

At startup, the L6562AT is supplied by the startup resistors R5, R8, and R13 charging the capacitor C13; once the PFC begins switching, a charge pump connected to the auxiliary winding of the PFC inductor L1 supplies both PFC and resonant controllers via a small linear regulator realized by Q1. Once both stages have been activated, the controllers are also supplied by the auxiliary winding of the resonant transformer, assuring correct supply voltage during all load condition operations. The L1 auxiliary winding is also connected to the L6562AT pin #5 (ZCD) through the resistor R18. Its purpose is to provide the information that L1 has demagnetized, needed by the internal logic to trigger a new switching cycle. The PFC boost peak current is sensed by resistor R34 in series to the MOSFET source; the signal is fed into pin #4 (CS) of the L6562AT, via the filter R27 and C16.

The dividers R7, R12, R14, and R22 provide the information on the instantaneous mains voltage to the L6562AT multiplier which is used to modulate the peak current of the boost.

The resistors R2, R6, R9 with R15 and R16 are dedicated to sensing the output voltage and feed, to the L6562AT, the feedback information necessary to keep the output voltage regulated. The components C11 and R20 (C12 is shorted) make up the error amplifier compensation network necessary to keep the required loop stability.

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Main characteristics and circuit description	AN3105

2.2Resonant power stage

The down-stream converter is a resonant LLC half-bridge stage working with 50 percent fixed duty cycle and variable frequency. It implements the ST L6599AT, integrating all functions necessary to properly control the resonant topology.

The resonant transformer, manufactured by MAGNETICA, uses the integrated magnetic approach, so the leakage inductance is used for resonant operation of the circuit. Therefore, no external, additional coil is needed for the resonance. The transformer secondary winding configuration is the typical centre tap, using a couple of type STPS10150CG power Schottky rectifiers. The output capacitors are film type, 4.7 µF - 63 V from EPCOS. Like for the PFC stage, using film capacitors allows to increase considerably the MTBF of the board.

A small LC filter has been added on the output, in order to filter the high frequency ripple. D21, D22, and R55 implement a voltage controlled bleeder; in the case of no-load operation of the SMPS, this circuit provides a bleeder limiting the increase of output voltage, but not affecting efficiency during normal operation. Please note that the converter has not been designed to work in this condition and therefore its mains consumption is not optimized (~3 W).

2.3Startup sequence

The PFC acts as master and therefore starts first; the resonant stage operates only if the PFC is delivering the nominal output voltage to prevent the resonant converter from working with a too low input voltage which can cause incorrect capacitive mode operation. Therefore both stages are designed to work according to this sequence.

For correct sequencing, the L6599AT makes use of the LINE pin (#7) to sense the PFC output voltage via a resistor divider. The L6599AT LINE pin (#7) has an internal comparator which has a hysteresis allowing the turn-on and turn-off voltage to be set independently. At startup, the LLC stage starts once the PFC output voltage reaches ~ 430 V, while the turnoff threshold has been set to ~ 330 V.

2.4Output voltage feedback loop

The output voltage is kept stable by means of a feedback loop implementing a typical circuit using a TS2431 to modulate the current in the optocoupler diode.

On the primary side, R43 - connecting pin RFMIN (#4) to the optocoupler's phototransistor - allows the L6599AT oscillator frequency to be modulated, therefore keeping the output voltage regulated. It also sets the maximum switching frequency at about 130 kHz. R42, which connects the same pin to ground, sets the minimum switching frequency. The R-C series R37 and C24 sets both soft-start maximum frequency and duration.

All demonstration boards implement the voltage loop circuitry described above but in case

a current loop is also required it can be achieved by implementing the following modifications:

●Replace R30 and R31 0R0 Ω resistors with sensing resistors, 0R033 and 0R039 respectively, both 0805

●Populate on PCB U4 and the relevant components reported on the schematic as N.M.: C36 = 1N0-0805; C37 = 100NF-0805; R51 = 15R-0805; R56 = 1K0-0805; R61 =22K1206; C41 = 2N2-0805; U5 = SEA05TR

●Remove the TS2431AILT

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AN3105	Main characteristics and circuit description

With these modifications the circuit is able to keep the output current constant at 2.7 A down to an output voltage value of around 30 V. This function can be used to optimize the voltage drop and power dissipation in case current linear regulators are used to regulate the current flowing in each LED strip. In case the output current is lower than the current loop setpoint, the voltage loop takes over the operation regulating the output voltage at its nominal value, like using the TS2431AILT.

2.5L6599AT overload and short-circuit protection

The current flowing into the primary winding, proportional to the output load, is sensed by the lossless circuit C34, R53, D19, D18, R57, and C35 and it is fed into the ISEN pin (#6) of L6599AT. In the case of overcurrent, the voltage on the pin overpasses an internal threshold (0.8 V), triggering a protection sequence. The capacitor (C21) connected to the DELAY pin (#2) is charged by an internal 150 µA current generator. If the voltage on the pin reaches

2 V, the soft-start capacitor is completely discharged so that the switching frequency is pushed to its maximum value. As the voltage on the pin exceeds 3.5 V the IC stops switching and the internal generator is turned off, so that the voltage on the DELAY pin decays because of the external resistor connected between the pin and GND. The L6599AT is soft-restarted as the voltage drops below 0.3 V. In this way, under short-circuit conditions, the converter works intermittently with low input average power, limiting the stress of components during shorts.

2.6Overvoltage and open-loop protection

Both circuit stages, PFC and resonant, are equipped with their own overvoltage protections.

The L6562AT PFC controller implements an overvoltage protection against the output voltage variation due to the poor bandwidth of the error amplifier, happening in the case of transients. Unfortunately it cannot protect the circuit in the case of a feedback loop failure such as disconnection or deviation from the nominal value of the feedback loop divider. In the case where a similar failure condition is detected, the L6599AT pin DIS (#8) stops the operation and also stops the PFC operation by means of the L6599AT pin PFC_STOP (#9) connected to the L6562AT pin INV (#1). The converter operation is latched until the VCC capacitors are discharged, then a new startup sequence takes place automatically and the converter resumes operation if the failure is removed or a new sequence is triggered. The same sequence occurs also in the case of input voltage transients which may damage the converter.

The DIS pin is also used to protect the resonant stage against loop failures. The Zener diode D17 detects the auxiliary voltage generated by the LLC transformer. In the case of loop failure, it conducts, and voltage on the DIS pin exceeds the internal threshold, latching off the device. The L6562AT operation is also stopped by the PFC_STOP pin as in the previous case, and after some time has elapsed the circuit restarts.

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Main

MKDS 1,5/ 3-5,08

.2 Figure

characteristics

2019.0002

GBU8J

1974.0001

1N4007

2.2 nF - Y1

RV1

300 VAC

STTH3L06U

2.2 nF - Y1

FUSE T4A

description circuit and

470 nF-X2

10 nF

LL4148

R10

177-277 VAC

demonstration EU-SL-EVL130W

C10

LL4148

R24

BC847C

R11

PCB rev. 0.2

R12

R13

VIN

BZV55-B15

C11

R17

R15

R16

R14

220 nF

LL4148

C12

R20

C13

Rev 16774 ID Doc

L6562AT

R18

R19

INV

VCC

R21

LL4148

D22

D21

R55

COMP

2 STF21NM 60N

BZV55-B24

D12

MULT

GND

1860.0013

STPS10150CG

R22

C15

ZCD

R23

R25

R26

15 nF

R27

MKDS 1,5/ 2-5,08

C16

R29

C19

220 pF

R33

R34

N. M.

100 nF

C20

N. M.

15 nF

C17

C18

VCC

R44

VCC

R30

48 V at 2.7 A

board:

R47

R45

D10

N. M.

C21

R32

220 nF

D13

D11

N. M.

R31

C22

L6599AT

LL4148

STPS10150CG

100 nF

R36

BC847C

N. M.

CSS

VBOOT

C24

R37

R38

BC847C

electrical

LL4148

D14

DELAY

HVG

R39

BC847C

VIN

C26

STB8NM60N

C25

OUT

C27

R41

4.7 nF

470 pF

RFMIN

VCC

D16

220 nF

BZV55-B24

N. M.

RX2

LL4148

D15

R42

R43

STBY

VCC

ISEN

C30

LINE

GND

R46

R50

STB8NM60N

SFH617A-2X009

R51

diagram

C33

R49

DIS

PFC-STOP

N. M.

C32

R52

R56

D18

C31

RX1

R53

C34

R54

N. M.

220 nF

LL4148

SEA05 - N. M.

220 nF

10 nF

C35

R57

R58

C37

220 pF

D17

I.sense

N. M.

220 nF

D19

D20

C38

N. M.

VCC

BZV55-B24

LL4148

STPS1L60A

GB6

R62

R60

R61

N. M.

GND

OUT

C36

D23/JPX9

D24

C40

R59

C39

JUMPER

LL4148

470 nF

C41

N. M.

Vctrl

Ictrl

N.M.

REV. 0.9

AN3105

TS2431AILT

AM00868

AN3105	Efficiency measurement

3 Efficiency measurement

Table 1 shows the overall efficiency, measured at 230 VAC - 50 Hz with different loads.

At 230 VAC and full load the overall efficiency is 93.85%, making this design suitable for high efficiency power supplies. The efficiency has been measured at 25%, 50%, 75%, and 100%, the average efficiency calculated according to the ES-2 standard is 91.56%.

Table 1.	EVL130W-SL-EU demonstration board: overall efficiency vs. load
	Load			230 V-50 Hz

		VOUT [V]	IOUT [A]	POUT [W]	PIN [W]	Efficiency [%]
		VOUT [V]	IOUT [A]	POUT [W]	PIN [W]	Efficiency [%]

25% load		47.59	0.682	32.46	37.14	87.39%

50% load		47.55	1.37	65.14	70.89	91.89%

75% load		47.54	2.00	95.08	102.1	93.12%

100% load		47.54	2.74	130.26	138.8	93.85%

Average efficiency						91.56%

The measured output voltage at different load conditions is reported in Table 1. As seen, the voltage is very stable over all the output load range.

The measured efficiency is shown on the lefthand side of the graph in Figure 3, while on the righthand side of Figure 3 the efficiency, at maximum load and at minimum, nominal, and maximum AC input voltage, is reported.

Figure 3. EVL130W-SL-EU demonstration board efficiency diagrams

%FFICIENCY VS LOAD

%FFICIENCY VS 6!#



%FFICIENCY		%FFICIENCY
%FFICIENCY		%FFICIENCY







	,OAD		6!# ;6RMS=

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Input current harmonics measurement	AN3105

4 Input current harmonics measurement

One of the main purposes of a PFC pre-conditioner is the correction of input current distortion, decreasing the harmonic contents below the limits of the relevant regulations. Therefore, this demonstration board has been tested according to the European standard EN61000-3-2 Class-C relevant to lighting equipment, at full load and nominal input voltage mains. Measurement results are in Figure 4 - on the lefthand side.

The circuit shows its ability to reduce the harmonics, also well below the limits of EN61000-3-2 Class-C regulation, not only at full load but also at significant lower load; on the righthand side of Figure 4 the input current harmonics measurement at light load (minimum input power to be compliant with the above mentioned rule is 25 W) shows that even if the power supply is working out of its typical operating region it is still compliant with the EN61000-3-2 Class-C limits.