ST AN1722 Application note

AN1722

APPLICATION NOTE

Design and Realization of a CCFL Application Using TSM108,

STN790A, or STS3DPFS30, and STSA1805

1. ABSTRACT

This technical document shows how to use the integrated circuit TSM108, the PNP power bipolar transistor STN790A, or the P channel power MOSFET STS3DPFS30, the NPN power bipolar transistor STSA1805 and the diode 1N5821 in order to design and realize a CCFL application. Such work allows STMicroelectronics’ customers to choose an alternative design and STMicroelectronics itself to supply all devices concerning the power transistor part and also the control and driver part for these applications (KIT approach). In the application block diagram below, the several STMicroelectronics’ power devices are inserted in the related block.

Figure 1: Block diagram of the application

BUCK SECTION

+12V

STN790A

1N5821

STS3DPFS30

STSA1805

PUSH-PULL SECTION

PWM SECTION

2. TSM108 DESCRIPTION

TSM108 is a PNP power bipolar or P channel power MOSFET controller. TSM108 includes a PWM generator (AMP3 in fig. 2), voltage and current control loops (AMP1 and AMP2 respectively in fig. 2) and it also includes safety functions that lock the PNP power bipolar or P channel power MOSFET in off state. The TSM108 can sustain 60V on V

device) and I

(min value) and 30 mA (max value).

June 2004

(base or gate drive source current to switch off the device) are respectively 15 mA

source

TSM108

and the I

(base or gate drive sink current to switch on the

sink

LAMPS

OUTPUT SECTI ON

Rev. 2

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AN1722 - APPLICATION NOTE

Figure 2: TSM108 schematic circuit

As exposed above, the safety functions UV and OV can switch off the power transistor (PNP power bipolar or P channel power MOSFET) when the V

is under a definite min voltage or when the Vcc overcomes a

definite max voltage. In fact, in these cases the output signal of the AMP 5, or the AMP 6, is low and the NAND output is high. Considering the UV function, fig. 3 shows the circuit part concerning it.

Figure 3: UV schematic circuit detail

184kΩ

76.5kΩ

2.52V

The V

voltage (the one in the non-inverting pin of the AMP5) is:

5.76

V+=

5.76184

(1.1)

and considering:

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=2.52V (1.2)

AN1722 - APPLICATION NOTE

the minimum V

under which the application will switch off is:

52.2

5.76

≈+=

(1.3)

V5.8)5.76184(

Considering the OV function, fig. 4 shows the circuit part concerning it.

Figure 4: OV schematic circuit detail

275kΩ

23.2kΩ

2.52V

The V

voltage (the voltage in the inverting pin of the AMP6) is:

2.23

V+=

−

2.23275

(1.4)

and considering:

the maximum V

over which the board will switch off is:

52.2

2.23

= 2.52V (1.5)

(1.6)

≈+=

V4.32)2.23275(

In order to adjust the UV and OV voltages it is necessary to insert suitable resistances as showed later in this paper.

It is important to highlight that, normally, the max δ (duty cycle) of the base drive, or gate drive, is around 95 %.

3. STN790A DESCRIPTION

The STMicroelectronics’ power bipolar transistor device STN790A is housed in the SOT-223 package. Such device is manufactured in PNP planar technology using a 'Base Island' layout that involves a very high gain performance and a very low saturation voltage.

The main characteristics of the STN790A device are:

1) V

2) V

3) V

4) I

5) I

≥ 30V

eco

≥ 40V

ecs

≥ 5V

beo

= -3 A (continuous current)

= 1 A (continuous current)

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6) V

7) H

= 1.2 mV (typ) @ Ib = -20 mA @ Ic = -2 A (typical conditions)

ec(sat)

= 100 (min) @ Ic = -2.5 A @ Vec = 3V (typical conditions)

4. 1N5821 DESCRIPTION

The STMicroelectronics’ SCHOTTKY diode is integrated in the package DO-201AD and has very small conduction losses, negligible switching losses and extremely fast switching.

The main characteristics of the 1N5821 device are:

1) V

2) I

RRM

= 3A

≥ 30V

5. STS3DPFS30 DESCRIPTION

The STS3DPFS30 device is mounted inside a P channel power MOSFET, using the STripFET layout that allows a lower Rds(on) and a SCHOTTKY diode. It is housed in the SO-8 package.

The main characteristics of the STS3DPFS30L device are:

1) V

≥ 30V

≥ 20V

2) V

3) R

ds(on)_max

= 3A (integrated diode);

4) I

5) V

F_max

6) V

RRM

= 0.09 Ohm @ Id = 1.5 A @ Vsg=10V

= 0.51 V (integrated diode)

= 30V (integrated diode)

6. STSA1805 DESCRIPTION

The STMicroelectronics’ power bipolar transistor device STSA1805 is housed in the TO-92 package. Such device is manufactured in NPN planar technology using a 'Base Island' layout that involves a very high gain performance and a very low saturation voltage.

The main characteristics of the STSA1805 device are:

1) V

2) V

3) V

4) I

5) I

6) V

7) H

≥ 60V

ceo

≥ 150V

ces

≥ 7V

ebo

= 5A (continuous current)

= 1A (continuuous current)

= 140 mV (typ) @ Ib = 50 mA @ Ic = 2A (typical conditions)

ce(sat)

= 270 (typ) @ Ic = 2A @ Vce = 1V (typical conditions)

7. APPLICATION INTRODUCTION

The CCFL applications (Cold Cathode Fluorescent Lamp) are generally used for the monitor back lighting which is often used to illuminate the signs.

The part of the circuit driving the CCFL lamps is composed of a DC-AC converter. The CCFL applications use special compact fluorescent lamps. The lamps number can be 1, 2, 4, or 6 and the output power can be in the range of 2-24W. The DC-AC converters transform the low DC in input voltage in necessary high AC output voltage for the fluorescent tubes. The CCFL are usually powered with a 12 V

voltage.

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Today, two topologies are available for driving the above-mentioned special tubes: the 'ROYER' and the FULL BRIDGE solutions.

The 'ROYER' solution uses a Push-Pull current fed converter where the current source is due to an inductor and where it is possible to regulate the lamps brightness. Such a regulation is carried out by means of the inductor current controls, through a PNP power bipolar or P channel power MOSFET transistors (STN790A or STS3DPFS30 respectively), working in PWM mode, and a free wheeling diode. The diode, the transistor and the inductor make a BUCK converter stage before of the PUSH-PULL converter stage. The PUSH-PULL converter uses two NPN power bipolar transistors (STSA1805). The other solution, the FULL BRIDGE topology, uses four power MOSFET transistors, two pair of complementary power MOSFET transistors, driven by a suitable IC.

The design described in this paper uses the 'ROYER' topology, thus, only such topology will be studied.

In the graph below a schematic circuit of a CCFL application, using two paralleled 6W lamps and only one transformer, is shown (this is one of the several possible output stage configurations).

Figure 5: 'ROYER' converter schematic circuit

8. FLUORESCENT TUBES CHARACTERISTICS

Fluorescent lamps are generally made with tubes filled with a gas mixture at low pressure. The inner sides of the tubes are covered with fluorescent elements. During the start-up, before the tube lights on, the lamp has a very high resistance. Usually, in the common fluorescent lamps, the electrodes voltage increases up to around 500V and starts to warm up and emit ions, but in the CCFL tubes the voltage between the lamp electrodes reaches up to 1300V. Fig. 6 shows CCFL lamp characteristics before the striking.

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Figure 6: Lamp voltage before striking

When the fluorescent lamp lights on, the gas mixture inside is fully ionized, and an arc across the two electrodes occurs. In this new condition, the lamp resistance drops to 60 KOhm and the voltage across the lamps drops to about 800V (Fig. 7 and Fig. 8 show the lamp characteristics and the V-I characteristic respectively after the striking).

Figure 7: Voltage and current Lamp after striking

Figure 8: V-I Characteristic after striking

After the striking, the gas mixture emits radiations able to excite the fluorescent elements inside the tube producing the light in the visible spectrum. In this example, after the striking, the maximum electrodes voltage falls from 1300V down to 770V with a peak current of 12 mA. In the common fluorescent lamp

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the voltage between the tube terminals drops from about 500V, before the striking, to about 220V after the striking. Usually, after the striking, in order to increase the light efficiency, the tube works with a frequency around 25-50KHz, in fact, in this frequency range, the light output can increase up to 15 % for the same input energy. Generally, the common fluorescent lamps can be considered only as a resistive load. In the CCFL lamps, instead, even if the tubes show a resistive behavior, a small but evident capacitive behavior is observed as it is shown in fig.7. In fig. 7 it is evident that the V-I Characteristic is linear until the established voltage value is reached (in this case about 500V). After reaching this voltage value, the characteristic starts to become flat because no ion can emit other radiations.

9. TRANSFORMER DESCRIPTION

The transformer named T

connected to the collectors of the Q

has a central terminal where the inductor L

shown in fig. 5 has three windings. The primary winding terminals are

and Q3 NPN power bipolar transistors. The same primary winding

output is connected. The secondary winding terminals are

connected to the loads. The third winding terminals are connected to the base of the transistors Q

while the second is off and vice versa. During the Q

the related half primary winding, instead, when Q

on state the current flows through the device and

is on the current passes through this second device

and the other half primary winding. Usually the LT primary inductance of the transformer T

compared to the inductance L

LT inductance. In the design, N

ratio is around 80-90 while, N

12 V

input voltage, the v

. The resonance frequency of the PUSH-PULL converter is also due to the

(number of secondary turns) and N1/2 (number of half primary turns)

/2 and N3 (number of third turns) ratio, is around 4-5. In fact, considering a

voltage (the max voltage between the terminal of the central point of the

1/2max

and Q3 so that the first is on

is much lower

primary winding and the reference when the PNP power bipolar or the P channel power MOSFET transistors are always on) is:

as then demonstrated around 19V, the v

Vv ⋅=

max2/1

(the max voltage between the secondary terminals of the

2max

(9.1)

transformer) is:

max2

⋅=

(

(9.2)

)

around 1500-1700V and the v

(the max voltage between the terminals of the third transformer

3max

winding) is:

)(

max3

⋅=

(9.3)

As exposed above, the N

/2 value and not N1 is highlighted. In order to understand the reason of it, it is

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∆

necessary to consider the graph below.

Figure 9: Detail of the transformer T

As already exposed, when the Q

where the T

and generates a magnetic force (Hopkinson law):

switch is on, the I current passes through the 'b' half primary winding of the transformer T

transistor is on, the other is off and vice versa. Now, considering fig. 9

Φ⋅ℜ=⋅I

(9.4)

where Φ is the magnetic flux and ℜ is the magnetic reluctance of the T

⋅=ΦI

(9.5)

core, thus Φ is:

ℜ

The magnetic reluctance ℜ is:

=ℜ

where µ is the core permeability, A is the core section and l is the core length. When T

switches on, the current flows through the other half primary winding 'a' of the transformer T1 and the

flux Φ inverts its direction. Such a flux flows in the magnetic core T

windings N

voltages v

and N3, and also with the other half of the primary windings N1/2, and generating the

and v3 (magnetic law-Lenz law):

∆Φ

−=

∆

−=

(9.6)

Al⋅

switches off and

creating a link respectively with the

;;

∆

−=

2/13322

(9.7)

∆

thus:

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2/1

===

2/1

(9.8)

AN1722 - APPLICATION NOTE

Furthermore, the current i2 (the current flowing through the secondary winding of T1) is:

(9.9)

Ii =

in fact, the apparent input power is:

IVA

in 2/1

(9.10)

while the apparent output power is:

(9.11)

iVA

out

and considering an ideal transformer:

IViV

(9.12)

2/122

and thus:

i 1

2/12

(9.13)

===

10. THE 'ROYER' CONVERTER TOPOLOGY

As previously exposed, the topology solution for CCFL applications used in this paper is the 'ROYER' topology. This topology solution has a current feed PUSH-PULL switching converter stage and also an inductor that together with a PNP power bipolar, or P channel power MOSFET transistor and a free wheeling diode, makes a BUCK converter stage before the PUSH-PULL stage. The PNP power bipolar, or the P channel power MOSFET transistor fixes the output power and thus the lamps brightness. All this is performed through a PWM signal able to drive either the PNP bipolar transistor or P channel MOSFET depending on the kind of device used. In order to implement the PWM of the transistor it is necessary to make an output current sensing (the lamps current) and to compare such a signal to the reference voltage in the AMP1 (see fig. 10 and fig.11 for the schematic circuit designed in this paper using the PNP power bipolar transistor STN790 and the power MOSFET transistor STS3DPFS30 respecticvely). The reference is fixed to 2.52V by the TSM108 internal voltage generator.

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AN1722 - APPLICATION NOTE

Figure 10: CCFL schematic circuit using the PNP power bipolar transistor

Figure 11: CCFL schematic circuit using the P channel power MOSFET transistor

Such a sensing net fixes also the right output power during the voltage net fluctuations.

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The component values for capacitors, resistors, and inductors are selected based on the load power, the operation frequency of the lamp before and after the striking (the operation lamp frequency must be in the range of 25-50Khz), and the current ripple. The PNP power bipolar, or the P channel power MOSFET operation frequency is fixed by means of the 220 pF capacitor C

Before the lamps strike the operation frequency is due to the resonance between the capacitor C

the primary transformer windings inductance LT of the T

transformer (see fig. 12):

(around 90KHz).

and

Figure 12: Resonant schematic circuit before lamps striking

When the lamps are connected, the transformer circuit, considering the ideal transformer, can be represented as in the following graph.

Figure 13: Resonant circuit of the transformer after lamps striking

LTC

⋅

(10.1)

In this condition, the apparent input power is:

IvA

(10.2)

1in

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AN1722 - APPLICATION NOTE

Now it is possible to consider a new equivalent transformer circuit as shown in the graph below and where the apparent power is the same as before.

Figure 14: Equivalent resonant circuit of the transformer after lamps striking

In this equivalent transformer circuit the output impedance is transferred from the secondary winding to the primary winding of the transformer T

(C3=C4=C) and that R

the C

4-Rlamp

net are in parallel configuration, the output impedance can be written as:

is the same for both lamps, considering also that the C3-R

lamp

. In fact, considering that C3 and C4 have the same value

net and

lamp

Lamp

but:

thus:

and thus:

Lamp

( k

−=

where:

is the primary equivalent resistance, while:

⋅⋅

(

iiVIV

2221

)

Lamp

2Ck

−

(

Lamp

⋅⋅

−==

iIi

−==

(10.7)

(10.8)

(10.3)

⋅⋅

(

)

⋅⋅

(10.4)

)

===

)

(10.6)

(10.5)

is the primary equivalent capacitance.

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AN1722 - APPLICATION NOTE

∆

Now, the equivalent primary admittance (Yeq1) is:

−

⋅

where:

is the admittance of the series net:

Lamp

Considering the impedance of the

negligible compared to Ck

possible to find the frequency that maximizes the Y

frequency is the resonance frequency of the application during the lamps on state):

, deriving the Yeq1 with respect to the pulsation ω and equaling to zero, it is

jCR

−

⋅

Lamp

22Ck

+⋅+

Cjk

)1(

Lamp

and, thus, minimizes the Z

eq1

CRj

(10.10)

(10.11)

Cjk

Lamp

(10.9)

)1(

impedance (such

eq1

≅

and thus:

f+⋅≅

When the board is powered, the resistances R

and Q3 and the lamps turn on. After the start-up, during the Q2 on state, the current flowing through

the inductance L

increases with an angular coefficient given by:

After a first instant, the current curve becomes flat and its average value depends on the impedance Z and on the output power. However, when the PNP power bipolar, or P channel power MOSFET is on,

after the lamps start-up, the current oscillates around the average value because the ripple on it depends only on the inductance L

transistor.

, through the half primary winding of the transformer T1 and through the transistor Q2,

=α

. In PWM mode, instead, it depends also, linearly, on the duty cycle of the

)2(

CkCLT

(10.12)

(10.13)

)2(2

(10.14)

CkCLT +

and R11 (see fig. 15) enable the power bipolar devices

t*v

eq1

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Figure 15 shows the 'ROYER' converter schematic circuit during the start-up considering the current I graph through the inductor.

Figure 15: 'ROYER' converter schematic circuit with inductor current theoretical behavior at star- up

After the striking, the primary current ‘I’ generates the current i transformer T

. At the beginning, the current i2 can be written as:

Lamp

(10.15)

into the secondary winding of the

because the capacitors C

the current i

drops to zero, while vc3 and vc4 reach the maximum voltage. At this time, the current i

inverts its direction and the capacitors C3 and C4 start discharging until the charge inside them becomes

zero and the current i

direction also the voltage v

current ‘I’ starts flowing into the other half winding of the transformer T

and C4 are discharged. Immediately after, these capacitors get charged and

reaches its maximum negative value. Furthermore, when the current i2 inverts the

reverts the polarities so that Q2 switches off, Q3 switches on, and the

t1b2

(see fig. 16).

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Figure 16: 'ROYER' converter schematic circuit with theoretical behavior of v1, v2, and i

The main output electrical parameters v2 (t), I voltages across the capacitors C

and C4) are shown in the following graph under a vectorial

Lamp

(t), V

(t), vc3(t) and vc4(t) (these last two are the

Lamp

representation.

Figure 17: Vectorial representation of v

, i2, v

, vc3 and v

lamp

In fact, assuming that, the lamps have only resistive behavior, the I and the V

The graph also assumes that the vectors V consideration. The I

(the voltage across them) can be written as:

lamp

Lamp

currents flow also through the capacitors C3 and C4 and, thus, the voltages v

Lamp

R*IV

LampLampLamp

and I

(10.16)

are on the real axis at the time taken into

Lamp

currents flowing through them

Lamp

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AN1722 - APPLICATION NOTE

−

and vc4 can be represented as -90 º phase shifted vectors compared to the I is the vectorial sum between the V are very high compared to reactance of C3 and C4, the currents I very much comparable to the voltage v striking, R one, thus making the voltages v after the striking, the max V The voltage v

drops to about 60 KOhm and the reactance C3-C4 becomes higher compared to the first

lamp

must be about 700V across the tubes.

lamp

(voltage across the inductor L1) is the difference between the Vdc and the v

and vc3, or vc4. Before the striking, the resistance lamps R

Lamp

are low and the voltages V

Lamp

across the secondary winding of the transformer T1. After the

and vc4 comparable to v2. However, in order to keep the lamps on,

vectors. The voltage v

Lamp

lamp

voltages

1/2

lamp

are

considering the PNP power bipolar in on-state, or the P channel power MOSFET transistor (see fig. 18).

, v

Figure 18: 'ROYER' converter schematic circuit with the theoretical behavior of v V

, vL1, and

1/2

During the off state of the transistor Q1, the diode D1 turns on and the voltage vL1 becomes:

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(10.17)

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Figure 19: v1, v

off and the diode D1 freewheels

In fact, supposing Q1 always in on state, focusing the attention only on one half-period of the periodic voltage v

voltage v

Figure 20: v

, as showed in fig. 20, the area A2 must be equal to the area A1 because the half-sine wave

1/2

and the voltage Vdc must have the same average value.

1/2

half-sine wave and Vdc graphs

1/2

, and Vdc theoretical behavior when the P channel power MOSFET switches

1/2,vL1

So writing A

and considering A

as:

T 2

max2/12

∫

(10.18)

dc1

(senVA

dt)t

(10.19)

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