Efficient Safety Circuit for Electronic Ballast

AN1601/D
Efficient Safety Circuit for
Electronic Ballast
Prepared by: Michael Bairanzade
ON Semiconductor
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APPLICATION NOTE
INTRODUCTION
The self−oscillating circuit, commonly used in the low
cost half−bridge converter, is prone to thermal runaway
when the fluorescent lamp does not strike. As a
consequence, either the switches are oversized 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.
SAFETY NETWORK DESCRIPTION
The schematic given in Figure 4, partially reproduced in
Figure 2, includes a safety circuit built with R8, D10, Q4, the
sense network C16, D5, C10, R17, R16, and D11 being
shown in Figure 4.
Basically, the strike voltage is scaled down by the resistor
divider R16/R17 and rectified by diode D11. The capacitors
C10 and C16 give a time constant to delay the action,
allowing the start up of a normal lamp for about 5 seconds.
Capacitor C18 filters the gate voltage, making sure that
noise will not trigger the thyristor. When the voltage across
C10 exceeds the zener value of D5, the thyristor Q4 is
triggered, pulling the low side of the winding T1d to ground.
The Vaux voltage, supplied by the PFC, is applied across
D10/R8/N4 and the DC current Is forces the toroid into the
saturation region by the extra flux coming from Is*T1d.
Consequently, the output to base positive coupling of each
transistor becomes negligible, the μr being now equal to 1,
and the converter stops immediately. Since the value of Is is
made larger than the holding current IH, the SCR stays ON
until the line is switched OFF: the fault is memorized and the
module is fully protected.
On the other hand, Is shunts to ground all of the energy
coming from the pre−charge resistor R3 (see Figure 4) and
the Vaux winding connected across the PFC output inductor:
the front end stage is switched off, since the Vaux drops
below the low voltage threshold of IC; and the power
dissipated by Joule’s effect in R8 is negligible.
Since a 10 mm toroid is large enough to accommodate 20
turns for T1d (AWG 32 or lower), one needs only 50 mA of
DC current to saturate the toroid. These numbers must be
recalculated for different toroid size and ferrite material.
Eventually, the start−up network can be deactivated when
the safety circuit is triggered, by using two extra diodes to
clamp the voltage below the trig point of the DIAC as
depicted Figure 3.
PROBLEM DESCRIPTION
The schematic diagram of the evaluation board given in
Figure 1 is built around a standard half−bridge
self−oscillating converter to feed the lamp, together with a
Power Factor Correction circuit in the front end.
This topology makes profit of the RLC series resonant
network. It can indefinitely sustain the open load condition
(i.e., broken filament) since there are neither a current flow
nor voltage spikes in the circuit under this mode.
When the lamp runs in steady state, the current is limited
essentially by the impedance of the series inductor L1 and,
thanks to the free wheeling diodes connected collector to
emitter, 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
resonance built with L1/C11/C12/R18, yielding large peak
collector current and high voltage at the L1/C11 node, hence
across the lamp. Usually, the lamp strikes rapidly, depending
upon the temperature and the peak voltage applied across the
electrodes. A typical four−foot long tube needs 800 V to
strike, with a preheating time of around 500 ms for the
filaments. However, at the end of life, or under worst case
conditions (low line voltage, negative ambient temperature,
etc.), the lamp may not strike and the circuit will continuously
operate in the start−up mode, yielding maximum losses in the
power transistors. Such level of losses generate heat which,
unless the devices are heavily heatsunk, will increase the die
temperature above the maximum rating in a few seconds. At
this moment, the transistors are exposed to a high thermal
runaway risk and TO220 packaged parts may blow up in less
than two minutes. This time is shorter for smaller packages
like the DPAK or the TO92.
© Semiconductor Components Industries, LLC, 2001
May, 2001 − Rev. 1
1
Publication Order Number:
AN1601/D
AN1601/D
C13 100 nF
C14 100 nF
250 V
250 V
AGND
R18 PTC C12 22 nF
C11 4.7 nF
1200 V
PTUBE = 55 W
T1A
FT063
L1 1.6 mH
Q2
BUL44D2
Q3
BUL44D2
R14
2.2 R
R13
2.2 R
R11
4.7 R
C9
2.2 nF
R12
4.7 R
C8
2.2 nF
DIAC
C6 10 nF
C7 10 nF
D4
R10
10 R
T1B
T1C
D3 1N4007
C4 47 mF
+
450 V
R7 1.8 M
D2 MUR180E
Q1
MTP4N50E
3
5
+
C2
330 mF
25 V
8
R3
100 k/1.0 W
C15 100 nF
R6 1.0 R
D8
R5 1.0 R
D9
C16
47 nF
630 V
D7
7
D6
4
U1
MC34262
R4 22 k
P1 20 k
1
2
AGND
T2
D1
MUR120
NOTES: * All resistors are ±5%, 0.25 W unless otherwise noted
NOTES: * All capacitors are Polycarbonate, 63 V,
NOTES: * ±10%, unless otherwise noted
C5 0.22 mF
R9
330 k
3
2
FILTER
C3 1.0 mF
6
C17 47 nF
1
630 V
C1 10 nF
FUSE
R2 1.2 M
LINE
220 V
R1 12 k
Figure 1. Standard Half Bridge Electronic Ballast Schematic Diagram
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AN1601/D
+VCC
HIGH EFFICIENCY
SAFETY NETWORK
Q2
R9
DIAC
R10
T1A
10 W
+Vaux
1N4007
R8
180 W − 0.25 W
T1D
D10
20 TURNS
R12
T1C
5 TURNS
C7
Q3
C8
C5
IS
Q4
MCR22−3
NOTE: Partial circuit, see details and references Figure 4
Figure 2. Low Losses Safety Circuit (Patent Pending)
+VCC
START−UP CLAMP
NETWORK
Q2
R9
DIAC
VZ = 10 V
1N4007
180 W − 0.25 W
T1D
D10
R12
T1C
5 TURNS
C7
Q3
20 TURNS
Q4
T1A
10 W
+Vaux
R8
R10
C8
IS
C5
MCR22−3
NOTE: Partial circuit, see details and references Figure 4
Figure 3. Deactivation of the Start−up Network (Patent Pending)
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AN1601/D
C13 100 nF
C14 100 nF
AGND
250 V
R18 PTC
250 V
C12 22 nF
C11 4.7 nF
1200 V
R17 10 k
PTUBE = 55 W
D11 C10
1N4148 10 mF
R16 1.0 M
D5
ZENER 10 V
T1A
FT063 Q3
BUL44D2
L1 1.6 mH
Q2
BUL44D2
R14
2.2 R
R13
2.2 R
R11
C9
4.7 R
2.2 nF
R12
4.7 R
C8
2.2 nF
DIAC
C6 10 nF
C7 10 nF
D4
T1D
R10
10 R
T1B
D3 1N4007
R9
330 k
T1C
IS
4.7 mF
Q4
MCR22−3
D10 1N4148
R8 1.0 k
C4 47 mF
+
450 V
R7 1.8 M
Q1
MTP4N50E
D2 MUR180E
3
P1 20 k
C15 100 nF
R6 1.0 R
D8
R5 1.0 R
C2
330 mF
25 V
R3
100 k/1.0 W
8
630 V
D7
7
4
U1
MC34262
5
+
D9
C16
47 nF
AGND
R4 22 k
D1
MUR120
C5 0.22 mF
NOTES: * All resistors are ±5%, 0.25 W unless otherwise noted
NOTES: * All capacitors are Polycarbonate, 63 V,
NOTES: * ±10%, unless otherwise noted
1
2
T2
C16
2
D6
FILTER
C3 1.0 mF
6
C17 47 nF
1
630 V
3
C1 10 nF
FUSE
R2 1.2 M
R1 12 k
NOTE: T1A = 1 TURN, T1B = T1C = 5 TURNS, T1D = 20 TURNS
Figure 4. Typical Safety Circuit Application
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LINE
220 V
AN1601/D
sustain the high voltage as depicted in Figure 5. In this case,
the power dissipated into R becomes high and will generate
enough heat to significantly increase the temperature inside
the housing of the electronic circuit.
To overcome such a problem, the design can be improved
as depicted in Figure 6. The DC current is kept at the IH value
by means of RH, limiting the losses to less than one watt. The
saturation current Is is generated by capacitor Cs which,
associated to the current limiting resistor RT, will provide a
pulse long enough to switch off the converter when the SCR
is switched ON. However, once the capacitor is charged, the
DC current, flowing in RH associated with R9, becomes too
low to maintain the saturation of the core. The clamp diode
DC, which is mandatory to avoid the restart of the converter,
provides a path for the I′H current. Consequently, the current
flowing in the start−up resistor R9 is added to the one
coming from RH, limiting the wattage of that resistor by
sharing the holding current.
+VCC
(330 V TYPICAL)
1N4007
R
6.8 kW / 10 W
20 TURNS
MCR22−8
Figure 5. High Voltage Driven Safety Circuit
If the low voltage Vaux, or similar, is not available (i.e.,
module without a PFC), the current Is can be derived from
the VCC line. Obviously, the components must be sized to
+VCC (330 V TYPICAL)
I′H
CLAMP DIODE
Q2
330 k − 0.5 W
DC
DIAC
T1A
RH
RT
220 k − 0.5 W
1.5 k − 1.0 W
1N4007
T1C
T1D
5 TURNS
Q1
20 TURNS
CS
C
T = CS × RT
T = 200 ms
MCR22−3
Figure 6. Improved High Voltage Driven Safety Circuit (Patent Pending)
CONCLUSION
The high end electronic ballasts can be designed with
specific drivers which include all the requested circuits to
perform the safety functions, the extra cost being masked by
the overall complexity. The situation is very different with
modules targeted 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 components 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 magnetic 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.
BIBLIOGRAPHY:
Michael Bairanzade:
Electronic Lamp Ballast Design
ON Semiconductor AN1543
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AN1601/D
Notes
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AN1601/D
Notes
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AN1601/D
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