1Motorola Applications Data
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1Motorola Applications Data

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1Motorola Applications Data Prepared by: Michael BAIRANZADE Power Semiconductors Application Engineer MOTOROLA – SPS – TOULOUSE INTRODUCTION The self oscillant circuit, commonly used in the low cost half bridge converter, is prone to thermal runaway when the fluo- rescent lamp does not strike. As a consequence, either the switches are over sized to sustain such a fault condition, or the circuit includes a safety network to avoid this risk. Although several schematics are usable to perform such a function, the one described in this paper is easy to implement and does not influence the normal operation of the converter. PROBLEM DESCRIPTION The schematic diagram of the evaluation board given Fig- ure 1 is built around a standard half bridge self oscillant con- verter to feed the lamp, together with a Power Factor Correction circuit in the front end. This topology makes profit of the RLC series resonant net- work. It can indefinitely sustain the open load condition (i.e. broken filament) since there are neither a current flow nor volt- age spikes in the circuit under this mode. When the lamp runs in steady state, the current is limited es- sentially by the impedance of the series inductor L1 and, thanks to the free wheeling diodes connected collector to emit- ter, there are no voltage spikes across the power transistors. The operation of the ballast is more complex during the start–up sequence, when the circuit operates close to the res- onance built with L1/C11/C12/R18, yielding large peak collec- tor current

  • clamp diode

  • series inductor

  • diode dc

  • improved high

  • start–up clamp

  • safety circuit

  • dc current


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SEMICONDUCTOR APPLICATION NOTE by AN1601/D
Prepared by: Michael BAIRANZADE
Power Semiconductors
Application Engineer
MOTOROLA – SPS – TOULOUSE
not strike and the circuit will continuously operate in the start–INTRODUCTION
up mode, yielding maximum losses in the power transistors.The self oscillant circuit, commonly used in the low cost half
Such level of losses generates heat which, unless the devicesbridge converter, is prone to thermal runaway when the fluo-
are heavily heatsunk, will increase the die temperature aboverescent lamp does not strike. As a consequence, either the
the maximum rating in a few seconds. At this moment, theswitches are over sized to sustain such a fault condition, or the
transistors are exposed to a high thermal run away risk andcircuit includes a safety network to avoid this risk. Although
TO220 packaged parts may blow up in less than two minutes.several schematics are usable to perform such a function, the
This time is shorter for smaller packages like the DPAK or theone described in this paper is easy to implement and does not
TO92.influence the normal operation of the converter.
SAFETY NETWORK DESCRIPTIONPROBLEM DESCRIPTION
The schematic given in Figure 3, partially reproduced in Fig-The schematic diagram of the evaluation board given Fig-
ures 2a, includes a safety circuit built with R8, D10, Q4, theure 1 is built around a standard half bridge self oscillant con-
sense network C16, D5, C10, R17, R16 and D11 being shownverter to feed the lamp, together with a Power Factor
in Figure 3.Correction circuit in the front end.
Basically, the strike voltage is scaled down by the resistorThis topology makes profit of the RLC series resonant net-
divider R16 / R17 and rectified by diode D11. The capacitorswork. It can indefinitely sustain the open load condition (i.e.
C10 and C16 give a time constant to delay the action, allowingbroken filament) since there are neither a current flow nor volt-
the start–up of a normal lamp for about 5 seconds. Capacitorage spikes in the circuit under this mode.
C18 filters the gate voltage, making sure that noise will not trig-When the lamp runs in steady state, the current is limited es-
ger the thyristor. When the voltage across C10 exceeds thesentially by the impedance of the series inductor L1 and,
zener value of D5, the thyristor Q4 is triggered, pulling the lowthanks to the free wheeling diodes connected collector to emit-
side of the winding T1d to ground. The Vaux voltage, suppliedter, there are no voltage spikes across the power transistors.
by the PFC, is applied across D10/R8/N4 and the DC currentThe operation of the ballast is more complex during the
Is forces the toroid into the saturation region by the extra fluxstart–up sequence, when the circuit operates close to the res-
coming from I *T1d.onance built with L1/C11/C12/R18, yielding large peak collec- s
Consequently, the output to base positive coupling of eachtor current and high voltage at the L1/C11 node, hence across
transistor becomes negligible, the r being now equal to 1, andthe lamp. Usually, the lamp strikes rapidly, depending upon the
the converter stops immediately. Since the value of I is madetemperature and the peak voltage applied across the elec- s
larger than the holding current I , the SCR stays ON until thetrodes. A typical four feet long tube needs 800 V to strike, with H
line is switched OFF: the fault is memorized and the modulea pre–heating time of around 500 ms for the filaments. Howev-
is fully protected.er, at the end of life, or under worst case conditions (low line
voltage, negative ambiant temperature, etc...), the lamp may
Motorola Applications Data 1 Motorola, Inc. 1997

mC13 100 nF C14 100 nF
AGND
250 V 250 VC12 22 nFR18 PTC
C11 4.7 nF
1200 V
PTUBE = 55 W
T1AL1 1.6 mH
FT063
Q2 Q3
BUL44D2 BUL44D2
R13 R14
2.2 R 2.2 R
R11C9 R12C8
4.7 R2.2 nF 4.7 R2.2 nF
DIAC
C6 10 nF C7 10 nF
D4
T1B R10 T1C
10 R
D3 1N4007
C5 0.22 F
R9
NOTES: * All resistors are ±5%, 0.25 W unless otherwise noted
330 k
NOTES: * All capacitors are Polycarbonat, 63 V,C4 47 F
+ NOTES: * ±10%, unless otherwise noted
450 VR7 1.8 M P1 20 k
C15 100 nF
Q1
D2 MUR180E
R6 1.0 RMTP4N50E
D8 D9
31
C16
630 V
47 nF
2 R5 1.0 R
T2 AGND
D7 D6
R4 22 k 7
5 4
FILTERC3 1.0 F+
2
C2
D1 6330 F C17 47 nF
MUR120 25 V 1
8 630 VR3 3 C1 10 nF
100 k/1.0 W
FUSE
R2 1.2 M
LINE
220 VR1 12 k
Figure 1. Standard Half Bridge Electronic Ballast Schematic Diagram
2 Motorola Applications Data

U1
MC34262On the other hand, I shunts to ground all of the energy com- front end stage is switched off, since the Vaux drops below thes
ing from the pre–charge resistor R3 (see Figure 3) and the low voltage threshold of IC, and the power dissipated by
Vaux winding connected across the PFC output inductor: the Joule’s effect in R8 is negligible.
+VCC
Q2
HIGH EFFICIENCY
R9SAFETY NETWORK
DIAC R10
10 T1A
+Vaux
1N4007R8 T1C R12T1D
D10 5 TURNS180 – 0.25 W
Q3C7
20 TURNS
C8
C5IQ4 S
MCR22–3
NOTE: Partial circuit, see details and references Figure 3
Figure 2a. Low Losses Safety Circuit (MOTFigure 2. Low Losses Safety Circuit (MOTOROLA Patent Pending)
Since a 10 mm toroid is large enough to accommodate 20 Eventually, the start–up network can be deactivated when
turns for T1d (AWG 32 or lower), one needs only 50 mA of DC the safety circuit is triggered, by using two extra diodes to
current to saturate the toroid. These number must be recalcu- clamp the voltage below the trig point of the DIAC as depicted
lated for different toroid size and ferrite material. Figure 2b.
+VCC
Q2
START–UP CLAMP
NETWORK R9
DIAC R10
V = 10 VZ
10 T1A
+Vaux
1N4007R8 R12T1D T1C
D10 5 TURNS180 – 0.25 W
Q3C7
20 TURNS
C8
C5IQ4 S
MCR22–3
NOTE: Partial circuit, see details and references Figure 3
Figure 2b. Deactivation of the Start–up Network (MOTOROLA Patent Pending)
Motorola Applications Data 3
C13 100 nF C14 100 nF
AGND
250 V 250 VC12 22 nFR18 PTC
C11 4.7 nF
1200 V R17 10 k
PTUBE = 55 W
D11 C10
1N4148 10 FR16 1.0 M
D5
ZENER 10 V
T1AL1 1.6 mH
Q2 FT063 Q3
BUL44D2 BUL44D2
R13 R14
2.2 R 2.2 R
R11C9 R12C8
4.7 R2.2 nF 4.7 R2.2 nF
DIAC
C6 10 nF C7 10 nF
D4
T1D
T1B R10 T1C
10 R I C16S
D3 1N4007
4.7 FR9
Q4
D10 1N4148330 k
MCR22–3
NOTES: * All resistors are ±5%, 0.25 W unless otherwise noted
R8 1.0 k
NOTES: * All capacitors are Polycarbonat, 63 V,C4 47 F C5 0.22 F
+ NOTES: * ±10%, unless otherwise noted
450 VR7 1.8 M P1 20 k
C15 100 nF
Q1
D2 MUR180E
MTP4N50E R6 1.0 R
D8 D9
31
C16
630 V
47 nF
2 R5 1.0 R
T2 AGND
D7 D6
R4 22 k 7
5 4
FILTERC3 1.0 F+
2
C2
D1 6330 F C17 47 nF
MUR120 25 V
1
8 630 V
R3 3
100 k/1.0 W
C1 10 nF
FUSE
LINE
R2 1.2 M
220 V
R1 12 k
NOTE: T1A = 1 TURN, T1B = T1C = 5 TURNS, T1D = 20 TURNS
Figure 3. Typical Safety Circuit Application
4 Motorola Applications Data

U1
MC34262If the low voltage Vaux, or similar, is not available (ie: module pated into R becomes high and will generate enough heat to
without a PFC), the current I can be derived from the Vcc line. significantly increase the temperature inside the housing ofs
Obviously, the components must be sized to sustain the high the electronic circuit.
voltage as depicted in Figure 4. In this case, the power dissi-
+VCC
(330 V TYPICAL)
1N4007R
6.8 k / 10 W
20 TURNS
MCR22–8
Figure 4. High Voltage Driven Safety Circuit
To overcome such a problem, the design can be improved DC current, flowing in R associated with R9, becomes tooH
as depicted in Figure 5. The DC current is kept at the I value low to maintain the saturation of the core. The clamp diode D ,H C
by means of R , limiting the losses to less than one watt. The which is mandatory to avoid the re–start of the converter, pro-H
saturation current I is generated by capacitor C which, vides a path for the I′ current. Consequently, the current flow-s s H
associated to the current limiting resistor R , will provide a ing in the start–up resistor R9 is added to the one coming fromT
pulse long enough to switch off the converter when the SCR R , limiting the wattage of that resistor by sharing the holdingH
is switched ON. However, once the capacitor is charged, the current .
+V (330 V TYPICAL)CC
Q2I′H
CLAMP DIODE 330 k – 0.5 W
D DIACC
T1A
1N4007R R T1D T1CH T
5 TURNS
220 k – 0.5 W 1.5 k – 1.0 W
Q1
20 TURNS
CS C
T = C × RS T
T = 200 s MCR22–3
Figure 5. Improved High Voltage Driven Safety Circuit (MOTOROLA Patent Pending)
Motorola Applications Data 5
CONCLUSION BIBLIOGRAPHY:
The high end electronic ballasts can be designed with spe- Michael BAIRANZADE:
cific drivers which include all the requested circuits to perform Electronic Lamp Ballast Design
the safety functions, the extra cost being masked by the over- Motorola AN1543
all complexity. The situation is very different with modules tar-
geted for the low cost market where each extra penny is
valuable. The safety circuits proposed in this paper are easy
to implement and do not need sophisticated and costly com-
ponents to protect the electronic ballasts against the most
common lamp failure mode.
With the galvanic isolation from the base drive of the power
transistor provided by the magnetic circuit, the safety network
is free from uncontrolled feedback from one circuit to the other.
On the other hand, since it dumps the permeability of the mag-
netic core to unity, instead of shunting one base current only,
both transistors are shut off simultaneously, avoiding the risk
of cross conduction during the transient phase.
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AN1601/D◊ 6 Motorola Applications Data