Electronic ballast on the UBA2021 chip. Scheme, description

ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Electronic ballasts. Electronic ballast on the UBA2021 chip. Encyclopedia of radio electronics and electrical engineering

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Consider an electronic ballast implemented on a microcircuit UBA2021. Schematic diagram of the electronic ballast for a fluorescent lamp with a power of 58 W is shown in fig. 3.31.

The "heart" of the electronic ballast is the UBA2021 chip. This specialized IC is designed to work with both conventional and compact fluorescent lamps. The UBA2021 includes a high-voltage driver with a trigger circuit, a generator and a timer that provide control during the stages of starting, heating, ignition and burning of the lamp, as well as protection against capacitive mode.

The IC withstands voltages up to 390 V and short-term voltage spikes (t < 0,5 s) up to 570-600 V. The low-voltage supply voltage is internally fixed, which eliminates the need to install an external zener diode. Fixation is carried out at currents up to 14 mA with short-term (t < 0,5 s) bursts up to 35 mA. The block diagram of UBA2021 is shown in fig. 3.32.

The microcircuit is made in a plastic case with 14 pins (either DIP-14 or SO-14). Pin assignment of the UBA2021 chip are given in table. 3.5.

Table 3.5. Pin assignment of the UBA2021 chip

Electronic ballast on UBA2021 chip

Rice. 3.31. Scheme of electronic ballast on the UBA2021 chip (click to enlarge)

Electronic ballast on UBA2021 chip
Rice. 3.32. Structural diagram of the UBA2021 chip

The electronic ballast is operable in the mains voltage range of 185-265 V at a frequency of 50-60 Hz. Automatic control maintains the lamp burning power within 47,6-50,3 W when the mains voltage changes within 200-260 V. The UBA2021 controls the operation of powerful PHX3N50E MOSFETs, which are the keys of a half-bridge inverter, which is powered from the mains with a rated voltage of 23 In i1 with a frequency of 50-60 Hz. This provides the necessary shift in the power levels of field-effect transistors, which provides protection against capacitive operation.

The main advantages of this product are a small number of external components and low cost due to the use of the UBA2021 IC, which is able to provide maximum design flexibility with a minimum number of peripheral elements.

Consider the operation of the circuit in more detail. The AC mains voltage is converted by a four-diode bridge rectifier (or diode bridge) and a smoothing capacitor into a DC voltage (310 V) that feeds the half-bridge inverter. The noise suppression mains filter prevents the penetration of interference into the network.

The half-bridge inverter belongs to a group of high-frequency resonant voltage converters, which are convenient for driving gas discharge lamps. The applied principle of zero-voltage switching of two powerful MOSFETs allows to reduce their switching losses and ensures high efficiency of the ballast.

After the mains voltage is applied, the fluorescent lamp is first preheated. This is called soft start and ensures reliable and durable lamp operation. The value of the heating current is regulated by the UBA2021 chip. This current passing through the filaments of the lamp heats the electrodes of the lamp to a temperature sufficient for the emission of electrons. Warming up reduces the ignition voltage of the lamp, which reduces shock electrical loads on the circuit elements.

After switching on, the rectified mains voltage is supplied to the buffer capacitor C4 through the resistor R1 (Fig. 3.31), which limits the current surge. The capacitor smooths out voltage ripples at twice the mains frequency. The resulting high-voltage voltage VHV (310 V) DC is the power supply for a half-bridge inverter, the power components of which include transistors VT1, VT2, coil L1, capacitors C5, C6, C7 and a lamp EL1.

At the start-up stage, the current from the high-voltage capacitor C4 passes through the resistor R2, the lamp filament, the resistor R7, terminals 13 and 5 of the UBA2021 chip, connected to each other during the start-up period by an internal key, and charges the low-voltage power capacitors C9, C11 and C13. As soon as the supply voltage VS on C13 reaches a value of 5,5 V, the UBA2021 switches, as a result of which the transistor VT2 opens and the transistor VT1 closes.

This allows the starting capacitor C12 to be charged through the internal circuit of the microcircuit. The supply voltage VS continues to increase, and at VS > 12 V, the internal oscillator of the microcircuit begins to generate. The current consumption of the IC is internally fixed at about 14 mA.

Next comes the transition to heating stage. In the absence of a lamp, the start is automatically blocked, because in this case the charging circuit of the starting capacitor is broken.

At the heating stage, MOSFETs VT1 and VT2 are alternately transferred to a conducting state. This generates a square wave AC voltage about the midpoint of the half-bridge with an amplitude of VHV. The starting oscillation frequency is 98 kHz. Under these conditions, the circuit, consisting of C8, VD5, VD6, C9 and SU, is able to perform the function of a low-voltage power source, which during start-up was provided by current through pin 13 of the IC.

During a time interval approximately equal to 1,8 s (heating time t_PRE), the duration of which is determined by the values of C16 and R8, the system is in heating mode. At the same time, a controlled current* passes through the filaments of the lamp, which allows optimal heating of both electrodes of the lamp. The heated electrodes emit (emit) a large number of electrons into the lamp, and in this state, much lower voltages are required to ignite it, which minimizes shock electrical loads on the circuit elements and the lamp at the time of ignition. The heating of the electrodes is very important for ensuring a long lamp life (about 20 thousand hours).

After the onset of generation, a small alternating current begins to flow from the midpoint of the half-bridge through the lamp filaments, L1 and C7. The oscillation frequency gradually decreases, which leads to a corresponding increase in the magnitude of the current. The frequency reduction rate is determined by the capacitance of the capacitor C14 and the internal current source of the IC. The frequency stops falling as soon as a certain value of the AC voltage is reached across resistors R5 and R6, which are heating current sensors.

During the entire heating phase, the frequency of the half-bridge inverter remains above the resonant frequency of the L1C7 circuit (55,6 kHz), and because of this, the voltage on C7 is still small to ignite the lamp.

Council. It is very important to keep this voltage sufficiently low, because premature, so-called cold, ignition leads to rapid wear of the lamp electrodes.

The value of the inductance of the ballast coil L1 is determined by the required current through the lamp, the capacitance of the ignition capacitor C7 and the operating frequency in the combustion mode. The minimum capacitance C7 is determined by the inductance L1, the voltage on the lamp, which does not lead to ignition, at a given heating current, and the minimum mains voltage. As a result, the value of capacitance C7, equal to 8,2 nF, turns out to be optimal for heating.

After the end of the warm-up phase, the UBA2021 resumes further reduction of the half-bridge switching frequency down to the lowest frequency fb (39 kHz). However, now the frequency reduction is carried out much more slowly than it did in the warm-up stage. The switching frequency is shifted to the resonant frequency of a series circuit consisting of the inductance L1 and the total capacitance of the capacitor C7 and the lamp electrodes (55,6 kHz), and the resistances of the DC-blocking capacitors C5 and C6 are quite small.

The maximum value of the ignition voltage in the worst case (when both the luminaire and the electronic ballast circuit are connected to the protective earth of the network) for a TL-D 58W lamp at low temperatures is approximately 600 V.

The combination of the ballast inductor L1 and the ignition capacitor C7 is selected in such a way that the voltage on the lamp can exceed these 600 V required for reliable ignition. The ignition voltage value determines the maximum value of capacitance C7 for a given inductance L1, selected based on the lower frequency fv UBA2021. The lower frequency fv is set by the values R8, C15. The maximum possible duration of the ignition phase t_IGN equal to 1,7 s (is 15/16 of t_PRE); it is set by selecting C16 and R8.

Let us assume that the lamp is lit in the course of lowering the frequency; then the frequency decreases to the minimum value /v. UBA2021 can make the transition to the combustion phase two ways:

when the frequency drops to f_в;
if the frequency f_в not reached, but the transition occurs after the maximum possible duration of the ignition stage t_IGN.

During the combustion phase, the oscillation frequency in the circuit is usually reduced to f_в (39 kHz), which can be used as the nominal operating frequency. However, due to the use of automatic control in electronic ballasts, the oscillation frequency depends on the amount of current flowing through pin 13 (RHV pin) of the UBA2021 IC. Automatic control begins to function after reaching f_в. Automatic control largely stabilizes the luminous flux emitted by the lamp in a wide range of mains voltage variations.

During the start-up phase, the low voltage supply capacitors C9, C10 and C13 are charged by the current flowing from the high voltage capacitor C4 through R2, the lamp filament, R7 and the internally connected terminals 13 and 5 of the UBA2021.

At the combustion stage, a re-switching occurs. Instead of pin 5, pin 13 turns out to be connected to pin 8. Now the current flowing through resistors R2 and R7 is used as an information parameter in the power inverter switching frequency automatic control system, since the strength of this current is proportional to the level of the rectified mains voltage. Ripple with twice the mains frequency (100-120 Hz) is filtered by capacitor C16. As a result, the luminous flux emitted by the lamp remains almost constant when the mains voltage changes in the range from 200 to 260 V.

At frequencies above 10 kHz, the lamp can be considered as a resistive load. The light output of tubular lamps excited at frequencies above 10 kHz is significantly better than when they are powered at a frequency of 50-60 Hz. This means that a TL-D 58W lamp with a 50W high-frequency power supply emits the same luminous flux as a TL-D 58W lamp with a 58W power supply at a frequency of 50-60 Hz. The steady state operating point for a TL-D 58W connected to the ballast is characterized by a lamp voltage of 110 V and a lamp current of 455 mA, which corresponds to a power supply of 50 W. The value of the inductance of the ballast coil L1 is determined by the operating point of the lamp, the capacitance of the ignition capacitor C7 and the operating frequency, which is approximately equal to 45 kHz at a nominal mains voltage of 230 V.

The desired lamp drive power can be achieved with various combinations of inductance L1 and capacitance C7. The choice of a specific combination depends on such factors as the heating mode, the minimum required ignition voltage and the tolerances on the parameters of the circuit components. In most cases, the combination of a choke coil L1 with an inductance of 1 mH and an ignition capacitor C7 with a capacity of 8200 pF is optimal.

To protect the elements of the power circuit from significant overloads, the microcircuit has a built-in protection function from the capacitive mode of operation, which is active at the stages of ignition and combustion. The UBA2021 chip checks the voltage drop across R5 and R6 during the turn on of the transistor VT2 in each cycle of the inverter.

If this voltage is less than 20 mV, which means that the circuit is operating in capacitive mode, the UBA2021 starts to increase the switching frequency at a much higher rate than it lowered it during the warm-up and ignition stages. As a result, the switching frequency will exceed the resonant frequency. When the signs of the capacitive mode disappear, the switching frequency again decreases to the required one.

Lamp removal protection is provided by the low voltage supply method for the UBA2021. When the lamp is removed, the AC voltage on the capacitor C6 becomes zero, which leads to the disappearance of the low-voltage power supply of the IC. After replacing the lamp without switching off the electronic ballast, the operation of the circuit will resume from the start-up phase. And, finally, starting the electronic ballast is impossible in the absence of a lamp - after all, in this case, the starting resistor R7 is disconnected from the high voltage.

The electronic ballast is equipped with an electrolytic capacitor C4 type ASH-ELB 043. These capacitors, specially designed for use in electronic circuits for supplying fluorescent lamps, are characterized by a long service life (15000 hours) at temperatures up to 85 ° C and withstand significant current ripples.

The power switches in the inverter are MOSFETs of the PHX3N50E type (the E index indicates an increased reliability of the device). By using the zero-voltage switching principle, the switching losses of the MOSFETs are minimized. The heating of each of the transistors is caused only by losses in the conductive state, and the degree of temperature increase depends on the resistance of the open channel "drain-source" R_DS on and case thermal resistance R_tn.

The durations of the heating and ignition stages are rather short, due to which the choice of the type of MOSFET was determined by the magnitude of the current flowing through the ballast inductor in the lamp burning mode. The PHX3N50E features a maximum drain-to-source voltage of 500V and an on-resistance of less than 3 ohms.

The design of the L1 ballast coil with an inductance of 1 mH, which can withstand peak ignition currents up to 2,5 A, allows it to be used in circuits without protective earth. The igniter in the electronic ballast is capacitor C7 with a capacity of 8200 pF of the KR / MMKR376 type. This type of capacitor is designed for use in circuits with high slew rates and high repetition rates. The installed capacitor is able to withstand a voltage swing of up to 1700 V (600 V RMS sinusoidal voltage). The capacitor can be replaced with polypropylene K78-2 for 1600 V. Recommended types of electronic ballast components are given in table. 3.6. And in the table 3.7 are given energy characteristics of electronic ballasts on the UBA2021 chip.

Table 3.6. Recommended types of EPR electronic components

Electronic ballast on UBA2021 chip

Table 3.7. Energy characteristics of electronic ballasts

Electronic ballast on UBA2021 chip

Author: Koryakin-Chernyak S.L.

See other articles Section Ballasts for fluorescent lamps.

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