ST AN2067 Application note

AN2067 APPLICATION NOTE

VIPower: DIMMABLE WHITE LEDS POWER SUPPLY WITH VIPer53

In the same way that LED manufacturers succeed			need for higher efficiency and reduced space
to realize blue LEDs, they now propose white			thanks to a lower power dissipation.
LEDs inside a monolithic chip, or so called “single-
chip white” LEDs.			1.1. General features
A current source is the more appropriate way to			The block diagram is given on Figure 1. An
drive LEDs. For a maximum of flexibility, a large			adjustable oscillator is driving a current controlled
voltage range must be supported at the output due			PWM at a fixed switching frequency. The peak
to the large threshold of these white LEDs, and the			drain current is set for each cycle by the voltage
possible serial arrangement of them.			present on the COMP pin. The useful range of the
Furthermore, if these LEDs have to be dimmable,			COMP pin extends from 0.5V to 4.5V, with a
they must be driven with a PWM (current			corresponding drain current range from 0A to 2A.
generator).			This COMP pin can be either sed as an input
As a consequence, key features for this off-line			when	working	in	secondary	feedback
power supply is a current generator, which can			configuration, or as an			utput when the internal
work as a Pulses Width Modulated mode, with a			error	amplifier connected on the VDD pin is
wide output voltage range, and must also suite any					Product(s)
input voltage standard, and a galvanic isolation.			operating in primary feedback to regulate the VDD
			voltage to 15V.
			The VDD und r voltage comparator drives a high
1. VIPer53 DESCRIPTION			voltage startup current source, which is switched
VIPer53, the first multichip device of the VIPer			off during the normal operation of the device. This
family has been chosen to fulfill the requirements.			feature together with the burst mode capability
It features very low Rdson of 1Ω		Obsolete
	allowing to deliver		allows to reach very low level of input power in
	-		standby mode, when the converter is lightly
a typical power of 35W in wide range in a tandard
DIP-8 package without a heatsink, answering the			loaded.

	Table 1. White LEDs P		wer Supply Specification
		Product(s)
	Parameter			Name	Conditions	Min	Typ	Max	Unit
	Output curr nt			IOUT	VOUT = 20V	200		1000	mA
	Output v ltage			VOUT	IOUT = 1A	5		40	V
	Output Power			POUT				40	W
	Obsolete
	Input Voltage			VIN		82		265	VAC

	Rev. 1
November 2004	1/15

AN2067 - APPLICATION NOTE

Figure 1. VIPer 53 block diagram

			OSC				DRAIN
	ON/OFF
		OSCILLATOR
		PWM
		LATCH
	OVERTEMP.	R1	S		BLANKING TIME
	DETECTOR		FF		SELECTION	1V
				Q
		R2
			R4	R5
		R3
	UVLO					0.5V	HCOMP
	COMPARATOR			150/400ns
VDD
				BLANKING			CURRENT
8.4/
	STANDBY				PWM		AMPLIFIER
11.5V					COMPARATOR
	COMPARATOR

0.5V

4V

8V

125k

15V

ERROR

AMPLIFIER

4.35V

OVERLOAD

OVERVOLTAGE

COMPARATOR

Product(s)

COMPARATOR

4.5V

18V

TOVL

COMP

SOURCE

Obsolete

-

converter, the output power is first limited by the

1.2. Overload protection

current limitation of the device. If this overload is

A threshold of 4.35V typical has been

lasting for more than a time constant defined by a

implemented on the COMP pin. This overload

capacitor connected on the TOVL pin, the device

threshold is 150mV below the clamping voltage of

is reset, and a new restarting sequence is initiated

4.5V which corresponds to the current limitation of

by turning on the startup current source. The

the device. In case of a COMP voltage exceeding

capacitors on the VDD pin and on the TOVL pin

Product(s)

will be defined together in order to insure a correct

the overload threshold, the pull up resistor on the

startup and a low restart duty cycle in overload or

TOVL pin is released and the external capacitor

connected on this pin begins to charge. When

short circuit operation. Here are the typical

reaching a value of 4V typical, the device stops

corresponding formulas:

switching and remains in this state until the VDD

COVL > 12.5 10

–6

tss

voltage reaches VDDoff, or resumes normal

Obsolete

4

1

C

OVL

I

D Dch2

operation if the COMP voltage returns to a value

CVDD >

8

below the overload threshold.

10

--------------- – 1

------------------------------------------

DRST

VDD hyst

The drain current that the device is able to deliver

tss

without triggering the overload threshold is called

CVDD >

IDD1

“current capability”, specified as I

in the data

-------------------------

Dmax

VDDhyst

sheet. This value must be used to size correctly

the converter versus its maximum output power.

Where tss and DRST are respectively the time

When an overload occurs on secondary side of the

needed for the output voltages to pass from 0V to

2/15

3/15

AN2067 - APPLICATION NOTE

their nominal values at startup, and the restart duty

results obtained in the corresponding section of

cycle in overload or short circuit condition. A

this document.

typical value of 10 % is generally set for this last

parameter, as it insures that the output diodes and

2. WHITE LEDS POWER SUPPLY

the transformer don’t overheat. The other

2.1. Schematic

parameters can be found in the data sheet of the

The power topology is an off-line fly-back, working

device.

at a fixed frequency of 66KHz.

As

the

VDD

capacitor

has

to

respect

two

The overall schematic is presented on Figure 2.

conditions, the maximum value will be retained to

define its value.

2.1.1. Primary section

1.3. Stand-by operation

On the left hand side of the schematic, there is the

fuse F1, inrush current limiter CTN1, input filter T1,

On the opposite load configuration, the converter

followed

by

the

rectifier

BR1

and

its

bulk

is

lightly

loaded

and

the

COMP

voltage

is

capacitance C3.

decreasing until reaching the shutdown threshold

R4, C4

and

D4

built

the RCD

clamper,

for

at typically

0.5V. At this

point,

the

switching is

discharging

the

leakage

inductance

of

the

disabled

and

no

more

energy

is

passed

on

transformer.

secondary

side.

So,

the

output

voltage

is

D2, R2 and C5 is the rectifier and filtering of the

decreasing and the regulation loop is rising again

primary auxiliary winding, used in forward mode

above the shutdown threshold, thus resuming the

(refer

to

Section

3). This

generates

a

voltage

normal switching

operation. A

burst

mode

with

supply from 21 V up to 80 V, proportional to turns

pulse skipping

is

taking

place,

as

long as

the

ratio between main primary winding and auxiliary

output power is below the one corresponding to

primary winding, and versus input voltage range

the minimum turn on of the device. As the COMP

Product(s)

voltage is working around 0.5V, the peak drain

(110 VAC up to 250 VAC).

A serial voltage regulator is required to supply the

current is very low (it is actually defined by the

VIPer 53 with the correct voltage (around 12 V). It

minimum turn on time of the device, and by the

is built with R14, DZ14, Q1 and C12. Notice that

primary winding of the transformer) and no audible

the VCE0 of this transistor must be higher than

noise is generated.

80 V. This transistor may

also dissipate

0.7 W

Obsolete

In addition, the minimum turn on time depends on

when input voltage is 250 VAC.

the

COMP

voltage. Below

1V

(VCOMPbl),

-

the

The COMP pin filter is done using C8, R9 and C9.

blanking time increases to 400ns, whereas it is

150ns for higher voltages. The minimum turn on

2.1.2. Transformer

time resulting from these values are respectively

By definition, a current generator may have a large

600ns and 350ns, when taking into account the

output voltage variation, according to the output

internal propagation time. This feature brings the

load.

following benefit:

Product(s)

Then, if auxiliary winding is used in fly-back mode,

• This brutal

change

induces

an

hysteresis

there

could

be

a large

voltage

variation

on

between normal operation and burst mode which

auxiliary

winding

as

it

is

proportional

to

the

is

reached

sooner when the

output power is

reflected

voltage. So,

it

will be used

in

forward

decreased.

mode

in

order

to

limit the

voltage

variation

for

• A short value in normal operation insures a good

supplying the VIPer53.

drain current control in case of short circuit on

Obsolete

In order to guaranty the functionality even with low

secondary side.

output

voltage

load,

an

auxiliary

winding

at

• A long value in standby operation reinforces the

secondary side has been added, instead using the

burst mode by

skipping

more switching cycles,

main secondary output to supply the regulation

thus decreasing switching losses.

loop

and

the

voltage

limitation

(using

the

More details regarding the standby operation can

TSM 101).

For

similar

reasons,

this

auxiliary

be found in the data sheet. See also the practical

winding will be also used in forward mode.

4/15

Obsolete

.2 Figure

AN2067

Schematic Generator Current

NOTE APPLICATION -

D101

L101

STTH302

10µH

T2

C112

C110

C111

C4

R4

220uF/63V

10uF/63V

100nF

R100

1A OUTPUT

1nF/250V

47K/4W

0.25

BR1

D4

1A/600V

EBR44-600

F1

D122

R122

0.5A

BAS21

3.3

T1

R2

D2

R14

10

BAS21

R106

R102

AC INPUT

C1

C2

3.3K/0.5W

10K

680

R131

100nF/400V

100nF/400V

U2A

2.2K

Q1

PC817

R101

CTN1

COMMON

Product(s)2N5550

12K

MODE

-

FILTER

C131

CT<0

10nF

C3

R108

C114

C113

R107

100uF/400V

R5

VDD

DRAIN

100K

100nF

22nF

6.8K

VIPER53

5.1K

C5

1uF/100V

OSC

C12

15V

U103

6

8

22uF/25V

C11

VCC

4.7nF

TOVL

COMP

SOURCE

C121

TSM101

DZ14

22uF/40V

12V/0.5W

Vref

R109

1K

VREF INPUT

C10

U2B

1

R9

PC817

100nF

6.8K

R103

C8

2

2.2K

4.7nF

J107

C9

470nF

3

7

5

P101

1KA

GND

C YCAP

R105

4

2.2nF/2KV

330

R107

Obsolete

0

Product(s)

5/15

				AN2067 - APPLICATION NOTE
	2.1.3. Secondary section			2.2. Dimming
	On the right hand side of the schematic, the			2.2.1. Dimming purpose
	secondary winding is used in fly-back mode.			The main purpose of this application is to supply
	D101, C112 are respectively the rectifying diode			“single-chip white LEDs”, and to dim the
	and its filtering. L101 and C110 build another low			brightness of these LEDs.
	pass filter.			Because the white color is obtained from two
	The auxiliary secondary winding used in forward			peaks in the spectrum (a blue ray and a yellow
	mode (refer to Section 3) in association with R122,			ray), there is a dependency between the driving
	D122 and C121 built the rectifier and filtering for			current and the white color spectrum.
	the supply of the regulation loop, using a
	dedicated component (TSM101). This supply is			2.2.2. Dimming in the application
	independent from the load voltage.
				This application do not propose any PWM and its
	TSM 101 has been design for voltage and current
				oscillator circuitry, which can be easily found in
	controller, which can be used for the control of a			dedicated literature. The way to proceed is to
	current generator, in association with a voltage			apply an external PWM signal on the node
	limitation. It includes its own reference voltage			VREF_INPUT.
	(bandgap), and two operational amplifiers.			Provided that output impedance of the generator is
				not higher than 50 Ω , the input voltage is forced by
	2.1.4. Current regulation loop			the external generator instead of the DC reference
	The output current is sensed through the shunt			voltage of the TSM 101.
	resistor R100. The shunt voltage is amplified using			The low level voltage of this PWM signal must be
	R101 and R107 in association with one OPamp of			0V, and high level voltage must be around 1 V.
	the TSM 101, building the error amplifier. The			Then, the peak currentProduct(s)of the PWM generator can
	current target is set through	the trimmer P101.		be set using the trimmer P101, as in DC mode,
	R103 and P101 provide a fraction of the reference
				from 0 up to 1 A.
	voltage provided by the TSM 101 (U103).
				The maximum frequency allowed is limited by
	There is also another way to set the current target,
				dynamic behavior of the PWM current generator
	using the connector J107	for dimming (see
				(refer to Section 2.2.4). The best way to use this
	Section 2.2).		Obsolete
			-	power supply, is to set the lowest frequency
				convenient for human eye versus flicker. The
	C113 with R107 is the integrator network of this
	amplifier, in order to cancel the static error of the			highest the period, the highest the dimming range.
	regulation loop.
	Then, the regulation loop continue with the opto-			2.2.3. Audible noise
	coupler U2 (diode and transistor). This set the			When using the power supply with a PWM signal,
	level of the COMP pin filter, which set the peak			some audible noise may be heard, especially if the
	drain current VIPer53’sProduct(s)power cell (current control			frequency of the external signal is inside the
	mode). Thus, the energy stored inside the			audible range.
	transformer during each cycles is transferred on			This noise is emitted by the core of the
	the secondary side, which supply the output			transformer, and is a normal way to work. This
	current.			noise is proportional to the output power
	R131 and C131 is a phase lead network in order			transferred through it.
Obsolete				This noise can be reduced by optimization of the
	to compensate the phase delay due to L101/C110
	filter, for whole loop stability purpose.			transformer.