Home  Ballast Resistors

Nov 1 2019

Electronic Ballast

Ⅰ Introduction

Electronic ballast, a type of ballast resistor, refers to an electronic device that uses electronic technology to drive an electric light source to produce the desired illumination. Another type of ballast resistor which corresponds to this is an inductive ballast. Nowadays, fluorescent lamps are increasingly using electronic ballasts, which are light and compact, and can even integrate electronic ballasts with lamps. At the same time, electronic ballasts can usually have the function of starter, so a separate starter can be saved. The electronic ballast also have more functions, such as improving or eliminating the flicker of the fluorescent lamp by increasing the current frequency or current waveform (such as becoming a square wave), or enabling the fluorescent lamp to use the DC power supply through the power inverter process. Some of the shortcomings of conventional inductive rectifiers have made it gradually replaced by increasingly sophisticated electronic ballasts.

Electronic energy-saving ballast is made up of some electronic components, which is actually a high-power transistor high-frequency switching oscillator circuit. The transistor switching oscillator circuit has the form of single tube oscillation type, double tube series push-pull oscillation type, double tube parallel push-pull oscillation type, and double tube complementary push-pull oscillation type. At present, most of the electronic energy-saving ballast circuits commonly used in the world are series push-pull oscillation type, and the oscillation frequency is 20 to 60 kHz.

Figure 1. 

Figure 1.

Basic Principle of Electronic Ballast

The electronic ballast is a converter that converts power frequency AC power into high frequency AC power. The basic principle is: the power frequency power supply becomes a DC power supply after a radio frequency interference (RFI) filter, full-wave rectification, and passive (or active) power factor corrector (PPFC or APFC). Through the DC/AC converter, the high-frequency AC power supply of 20K-100KHZ is output, and is applied to the LC series resonant circuit connected to the lamp to heat the filament, and a resonant high voltage is generated on the capacitor, which is applied to both ends of the lamp tube to make the lamp "discharge" and be in a "conducting" state, and then enter the lighting state. At this time, the high-frequency inductor acts to limit the increase of current, ensuring that the lamp tube obtains the lamp voltage and lamp current required for normal operation.

2.1 Basic Circuit of Electronic Energy-saving Ballast

The rectifier diodes VD1, VD2, VD3, and VD4 form a bridge rectifier circuit, which cooperates with the filter capacitor C1 to form a DC power supply for the electronic ballast switching oscillation source circuit. The resistor R1 and the capacitor C2 form an integrating circuit, and it constitutes a starting circuit with the diode VD5 and the trigger diode VD (DB3). The transistors V1 and V2 and the high-frequency transformer T (L1L2L3) wound on the same magnetic ring constitute a transformer feedback series push-pull switching oscillator circuit, also called an inverter circuit or a converter, and the oscillation frequency is 20-60 kHz. The resistor R2 and the capacitor C3 constitute an overvoltage protection circuit of the converter. Resistors R5 and R6 are current-limiting protection circuits, and also serve as buffer protection for V1 and V2. The diodes VD6 and VD7 act as clamps to stabilize the switching oscillation of the two high-power transistors V1 and V2, while the inductor L4, capacitors C4 and C5 form a series resonant output circuit.

Figure 2. 

Figure 2.

2.2 Working Principles of Electronic Energy-saving Ballasts and Principles of Component Selection

When the electronic energy-saving ballast is working, the 220V AC power supply is converted to a DC voltage of about 310V by bridge rectification of VD1~VD4 and filtering of C1, and the working voltage is supplied to the V1, V2 transistor triode inverter circuit. The filter capacitor C1 will cause distortion of the voltage waveform in the power supply circuit during charging and discharging. Based on this problem, the capacity of C1 should be small rather than large. However, if the capacity is too small, the DC power supply may be poorly filtered, the fluorescent tube may be flickering or the brightness may be unstable, and the capacitors C1, V1, and V2 may be burnt due to excessive temperature. For 20~40W electronic energy-saving ballasts, the value of C1 is generally 10~20μF, and the withstand voltage of the electrolytic capacitor is 400V; the rectifier diode usually adopts 1A/1000V 1N4007 rectifier diode. If the withstand voltage is too low, the rectifier diode is in danger of being burned.

When the electronic ballast is energized and start to work, the rectified DC working voltage is first added to the starting circuit composed of R1, C2, VD5, and DB3, and the DC power is applied to the capacitor C2 through R1, and C2 starts charging. When the charging on C2 reaches the breakover voltage of the trigger diode DB3, the trigger diode is turned from the off state to the on state. The charge stored in the integrating capacitor C2 is applied to the base of the transistor V2 via the trigger diode to generate a base current, thereby exciting the conduction of the transistor V2. The level of the tripping diode DB3 breakover voltage has a certain influence on the conduction state of V2. The higher the breaking voltage of DB3, the higher the charge stored on the integrating capacitor C2, and the easier it is to excite V2 to conduct; otherwise, V2 is not easy to trigger conduction; however, this breakover voltage cannot be too high.

Because the trigger voltage is increased correspondingly with the breakover voltage, the excessive trigger voltage is a threat to the transistor V2, and the withstand voltage value of the transistor should be increased accordingly. Therefore, this breakover voltage is a suitable voltage value. Trigger diodes with a breakover voltage of 20 to 35V are generally selected. The capacity of the integral capacitor C2 also affects the starting characteristics of the circuit. The larger the C2 capacity, the higher the stored charge, the higher the excitation voltage provided to the base of V2, and the easier the transistor V2 will work. However, if the capacity of C2 is too large and the stored charge is too high, there is a danger of breaking through the DB3 trigger diode. Generally, in an electronic ballast of 20 to 40 W, the value of C2 is between 0.01 and 0.22 μF, and the withstand voltage can be applied as long as it is 63 V.

The starting circuit only works at the moment when the electronic ballast starts working. After the inverter circuit of V1 and V2 enters the normal switching oscillation working state, the trigger voltage of the starting circuit is no longer needed. At this time, only the reverse phase relationship between the L2 and L3 coils of the oscillating transformer T is utilized in the inverter circuit, so that when V1 is turned on, V2 is forced to turn off; when V2 is turned on, V1 is forced to turn off. If the trigger circuit is still working at this time, V2 is also excited by the trigger circuit during the turn-on process of V1, which will cause the two high-power transistors V1 and V2 to exhibit a phenomenon called "common state conduction". And at the same time, a short circuit state will occur, and the current of the whole machine will increase sharply, causing the triode or other components to be burnt.

Therefore, the phenomenon of "common state conduction" is quite dangerous, and this situation should be strictly prohibited.

In order to avoid the above-mentioned "common state conduction", a discharge diode VD5 is provided in the starting circuit. It cooperates with V2. When V2 is turned on, V1 is turned off at this time. The positive terminal potential of VD5 is higher than the negative terminal potential, and VD5 is turned on, so that the charge stored on the integrating capacitor C2 is discharged through VD5 and V2; During the turn-on period of V1 and the turn-off period of V2, the potential of the negative terminal of VD5 is higher than that of the positive terminal, and VD5 is turned off. Although VD5 no longer acts as a discharge, because of the large resistance of R1, the charging speed of C2 is slow, and V2 is turned on and V1 is turned off before the charge on C2 is charged to the breakover voltage of DB3. The diode VD5 is specifically set to discharge the charge on C2.

The oscillating transformer is composed of a high frequency ferrite magnetic ring and three sets of feedback coils. When a DD3 trigger diode is present in the avalanche state and is turned on, a positive potential trigger signal is input to the base of the transistor V2, and V2 is turned on. The output voltage is applied to the series resonant circuit of L1 and L4, C4, C5, and the series resonant circuit obtains the charging effect of V2; while L1 charges the L4, C4, C5 series resonant circuit, part of its signal voltage is fed back to the L3 coil of the V2 base input loop through the mutual inductance cross-connection of L1 and L3. Since the phases of L1 and L3 are opposite, the V2 base potential is turned to a negative potential, and V2 is quickly turned off; at the same time, L1 and L2 also pass through a mutual inductance cross-connection to feed back a part of the signal voltage to the other transistor V1. Since the phases of L1 and L2 are the same, V1 instantaneously obtains a positive potential excitation signal voltage and is quickly turned on.

After V1 is turned on, the oscillating voltage supplied to the series resonant circuit by V2 is short-circuited and discharged, and one oscillation period is completed. This means that V2 is equivalent to a charging circuit of the series resonant circuit; and V1 is equivalent to a discharging circuit of the series resonant circuit. The speed of charging and discharging is done at the natural frequency of the series resonant tank. That is to say, the oscillation frequency of the oscillation circuit is determined by the time constant of the series resonance circuit. At the end of the previous cycle, the core of the oscillating transformer is saturated, and the magnetic lines of force decrease sharply rather than increasing. Due to the action of the L1 self-induced electromotive force, the voltage phase across L1 is reversed, which causes the upper phase of the base input feedback coil L3 of V2 to become positive and the lower to become negative, and V2 is turned on again, entering the next oscillation period. The starting circuit composed of R1, C2, VD5, and DB3 is only used at the moment when the electronic ballast is powered on. When the electronic ballast enters the normal working state, the starting circuit no longer functions.

We know that when the series resonant circuit resonates, the voltage of its inductance and capacitance is many times larger than the applied voltage. Electronic ballasts use this principle to ignite fluorescent lamps with relatively high frequency and high voltage at both ends of C5. Because the voltage level at the start of the lamp has a lot to do with the two components C5 and L4. When the Q value of the coil and capacitor is higher, the starting voltage is higher. When the electronic ballast is difficult to start the fluorescent tube, the capacity of the C5 can be appropriately reduced to increase the Q value of the circuit; however, when the Q value is too high, the life of the fluorescent lamp is affected. Therefore, the capacity of C5 should not be too small. In the electronic ballast, the capacity of C5 is generally 0.01 to 0.022 μF. When the leakage fault occurs in the inductor L4, the Q value will also decrease, making the lamp not easy to ignite.

At the moment when the switching oscillating tube V2 is turned off and V1 is turned on, the voltages on the inductor L4 and the capacitor C1 are superimposed together, at which time V2 will withstand a high voltage of nearly several volts, causing V2 to be damaged; The high voltage on the inductor coil is generated due to the superposition of the self-induced electromotive force of the coil itself and the applied voltage during the sudden conduction and interruption of the current flowing through the inductor coil, then we must try to prevent the current in the inductor L4 from being suddenly interrupted, but slowly changing. To achieve the above object, a capacitor C3 is provided on the circuit. Its function is to provide a buffered bleeder current path for the inductor L4 when V2 is turned off, and the resistor R2 constitutes the protection resistor of V1, so that the anti-peak voltage generated by V1 during the off-state is discharged from the resistor R2 to C3, and is buffered and released to the series resonant circuit by C3; R2 also has the function of assisting the circuit to be easily started.

Clamping diodes VD6, VD7 and R5, R6 protect the emission junction of oscillating tube V1 and V2; R5, R6 play a buffering role for the surge current of the feedback coils L2, L3 of the oscillating transformer T. When the magnetic field energy of L4 and L3 is released, the excessive anti-peak voltage can quickly turn on VD6 and VD7, so that reverse breakdown of the V1 and V2 emitter junctions can be avoided. R5, R6, VD6, and VD7 also stabilize the DC operating point of V1 and V2, that is, clamp the base bias of V1 and V2, making the operation of the oscillation source more stable. After the fluorescent lamp is normally started, the internal resistance of the fluorescent tube is lowered, so that the Q value of the series resonant circuit is drastically lowered, and the resonant circuit is detuned. At this time, C5 is equivalent to a high resistance resistor connected in parallel at both ends of the fluorescent tube; and the inductor L4 only acts as a ballast.

 Figure 3..jpg

Figure 3.

 Classification of Electronic Ballasts

3.1 According to the Installation Mode

(1) Freestanding electronic ballast

(2) Built-in electronic ballast

(3) Integrated electronic ballast

3.2 According to the Performance Characteristics

— Ordinary electronic ballasts, 0.6≥120%90%1.4~1.6, high frequency makes it small, light and energy-saving;

— High power factor electronic ballast, H class, ≥0.9≤30%≤18%1.7~2.1, it adopts passive filtering and abnormal protection;

— High-performance electronic ballast, L-class, ≥0.95≤20%≤10%1.4~1.7, it has perfect abnormal protection function and electromagnetic compatibility;

— Cost-effective electronic ballast, L-class, ≥0.97≤10%≤5%1.4~1.7, Integrated technology and constant power circuit design, small impact of voltage fluctuation;

— Dimmable electronic ballasts, ≥0.96≤10%≤5%≤1.7, it adopts integrated technology and active variable frequency resonance technology.

Selection of Electronic Ballasts

Electronic ballasts have obvious advantages in improving the energy efficiency and quality of lighting systems. They are products recommended by new International and are also the trend of future development.

(1) It should be preferred in places where needs continuous intense visual work and highly demanding visual conditions(such as design, drawing, typing, etc.), in places where needs to be extremely quiet(patient rooms, clinics, etc.) and in juvenile viewing places (classrooms, reading rooms, etc.).

(2) In places where dimming is required, a three-color fluorescent lamp with a dimmable digital ballast, which can greatly improve energy efficiency, can be used instead of an incandescent lamp or a halogen lamp.

(3) High-quality, low-harmonic products should be used. We should not simply pursue low prices and these products should meet the technical requirements for use. We need to consider the operation and maintenance effects and make comprehensive comparisons.

(4) When using fluorescent lamp less than 25W, as mentioned above, the harmonic limits specified in the GB19625.1-2003 standard are very wide. If it is widely used in a building, it will lead to various adverse consequences. Effective measures should be taken to limit them in the design.

(5) The selected product should not only be investigated for its total input power, but also its output luminous flux. According to regulations, the lumen coefficient (μ) of the ballast should not be lower than 0.95. The EU has specified the energy efficiency rating of the ballast, and the lumen coefficient (μ ≥ 0.96) is also specified accordingly.

Attention:

(1) Pay attention to harmonic content. The newly revised "AC electronic ballast for tubular fluorescent lamps - performance requirements" (GB/T15144-2005) has canceled the polarization of the electronic ballasts specified in the original standard (L and H), and its harmonic limit shall comply with the "electromagnetic compatibility, limit, harmonic, current emission limits" (GB17625·1-2003). Users should pay attention to the harmonic limit of lamps below 25W, which is very loose, such as the massive use of such small-power fluorescent lamps in a building (including 2 feet long T8, T5 lamps and compact fluorescent lamps), will lead to severe waveform distortion , the neutral line current is too large and the power factor is reduced.

(2) Focus on product quality and level. There are many electronic ballasts on the market, and the quality and level are very different. The main performances are: large harmonic content; low lumen coefficient; low reliability; short service life. Although these products are cheap, they must be taken care of, and it is recommended not to use them.

(3) See if the content of the label on the electronic ballast is complete, which mainly includes the Chinese name of the manufacturer or the seller; rated power supply, voltage and frequency; specification, power and number of lamp tubes of the lamp used; wiring circuit diagram; The cross-sectional area of the connecting wire of the connecting terminal; tc value (maximum temperature of the casing); U-OUT value (maximum operating voltage at the output); power factor value and CCC certification mark (it should have the number corresponding to the enterprise certification).

(4) If the tc value of electronic ballast is marked high, it indicates that the high temperature environment resistance is strong. Generally tc ≥ 65 ° C, when tc ≥ 70 ° C or more, it is a better quality product.

(5) The U-OUT value of the electronic ballast (maximum operating voltage at the output) should be properly marked. For electronic ballasts for lamps with T8 36W and below or T5 24W and below, U-OUT ≤ 300V. For electronic ballasts for lamps with T5 28W, T5 35W and T8 58W, the U-OUT value should be between 300V and 450V.

 Figure 4.

Figure 4.

 Technical Terminology for Electronic Ballasts

PF (Power Factor)

The effective use of the input power by combining ballast and lamp is also indicated in some places as Watt/VA or COSΦ. Generally speaking, the PF of the magnetic ballast is 0.5, even after the capacitance correction, it can only reach about 0.8, and the electronic ballast can usually achieve 0.95~0.99. The significance of this is that you make full use of every watt of electricity supplied by the power plant and make an outstanding contribution to environmental protection.

THD (Total Harmonic Distortion)

When the ballast and the lamp are operated under the rated power supply voltage, after the lamp reaches a stable working state, the sum of the odd harmonic components in the power supply current is input. According to Fourier’s definition, a rectangular wave is formed by a series of sinusoids with a common period but different frequencies. The greater the harmonic content, the greater the damage to the input sinusoidal waveform. In the case of using more electronic ballasts, if the THD value is large, it will affect the neutral current of the three-phase AC input, and the neutral current will be too large. Therefore, in general, when we use electronic ballasts on a large scale, it is more appropriate to choose the THD with performance price ratio between 15% and 25%.

CF (Crest Factor)

At the rated supply voltage, the ballast is working with the lamp. When the lamp reaches a stable working state, the ratio of the peak value of the output current flowing through the lamp to the root mean square value is CF=PK/rms. Generally speaking, the smaller the CF value, the more stable the current flowing through the lamp and the longer the lamp life. The IEC / GB standard is CF ≤ 1.7.

EMC (Electromagnetic Compatibility)

A device or system can function properly in an electromagnetic environment and does not constitute an unacceptable electromagnetic disturbance to anything in the environment. There are different implementation standards in Europe and America. A. FCC (USA Standard Class A; Class B) B. CISPR (International Electrotechnical Commission CISPR15) C. EN55015 (European Standard). China will implement according to European standards.

When users use standard-compliant ballasts, their peripheral electronics will not be disturbed, such as electronic computers, wireless phones, and some professional electronic devices.

 Figure 5.

Figure 5.

0 comment

Leave a Reply

Your email address will not be published.

 
 
   
Rating: