Low voltage halogen lamp electronic transformer based on IR2161

Halogen lamps are also called tungsten halogen lamps. The filament of the halogen lamp is tungsten filament. In addition to filling the appropriate amount of argon gas, the lamp tube is also filled with trace amounts of bromine or iodine halogen, so it gets its name. Halogen lamps, like incandescent lamps, are thermal radiation light sources. The luminous efficiency, lifespan and color rendering index of halogen lamps are better than ordinary incandescent lamps. Therefore, they are widely used in lighting occasions such as showrooms, commercial kitchen windows, photography, videography, theaters, and urban lighting. Their function is to replace other light sources. irreplacable. In the global electronic ballast market, low-voltage lighting electronic transformers account for 10%.

The rated operating voltage of low-voltage halogen lamps is mostly only 12V. If 220V commercial power supply is used, a bulky iron core (step-down) transformer must be used. Electronic transformers are used to replace old-fashioned iron core transformers, which have the advantages of small size, light weight, material saving, and power saving.

Low-voltage halogen lamp electronic transformers, like fluorescent lamp electronic ballasts, convert power frequency into high frequencies of tens of kHz. The two are very similar in circuit structure, and most of them use half-bridge inverter topology. However, since halogen lamps are a resistive load, there are some differences between halogen lamp electronic transformers and fluorescent lamp electronic ballasts, which are specifically reflected in the following aspects.
1) The halogen lamp electronic transformer does not require preheating and triggering start sequence;
2) The DD bus voltage of the halogen lamp electronic transformer is a full-wave rectified AC line voltage, which does not require a large-capacity filter capacitor;
3) The line power factor of the halogen lamp electronic transformer is close to 1. And there is no need to use power factor correction (PFC);
4) The output is isolated high frequency and low voltage, which is safe.

Using discrete components to design and produce electronic transformers, if it has protection and dimming functions, many components need to be added. The electronic transformer based on the intelligent half-bridge driver chip IR216l produced by IR Company does not require a saturable magnetic ring transformer and only requires a small number of components to achieve high performance, high reliability and dimming functions.

1 Basic low-voltage lighting electronic transformer
circuit 1 shows the basic circuit of a 100W/12 V halogen lamp electronic transformer based on IR216l.

After the industrial frequency mains power is turned on, the current through Rs of the bridge rectifier flows into the starting current on the IC pin VCC, and most of the remaining current charges the capacitor CVCC1. When the voltage on the IC pin VCC reaches the startup threshold (11.8 V), the IC starts to work. Once the IC is powered on, the charge pump circuit consisting of CSNUB, DCP1 and DCP2 feeds current to the IC pin VCC. Bootstrap diode DB and capacitor CB power the IC's high-side driver circuit. Zener diode DZ is used to shunt excess IC current to prevent IC damage.

The filament resistance of halogen lamps has a positive temperature coefficient, and the "cold resistance" at room temperature is much smaller than the "hot resistance" when the lamp is working. When the lamp is started, a large surge current will be generated, which will affect the lamp life. However, IR2161 provides soft-start operation to avoid surge current. During lamp start-up, the IR2161 outputs a high frequency of 125 kHz. Since the primary leakage inductance of the output high-frequency transformer T1 in the system is fixed, it exhibits higher impedance at higher frequencies and the voltage on the primary winding is lower. As a result, the output voltage of the transformer is lower, the lamp current is smaller, and the protection circuit is prevented from being triggered. After about 1 s, the circuit runs at a lower frequency. During this process, the voltage on the external capacitor CSD at IC pin 3 increases from OV to 5V.

When no-load, VCSD=OV, the oscillator frequency is about 60kHz. Under maximum load, VCSD=5V, the oscillator frequency is about 30kHz. When the output is short-circuited, a large current flows through the half-bridge and is sensed by RCS. As long as the voltage on IC pin 4 (CS) exceeds the threshold level of 1V for more than 50ms, the system will shut down. If the load exceeds 50% of the maximum load, the voltage on IC pin 4 will exceed the lower threshold voltage of O 5 V. After 0.5S, the system will shut down. Whether it is short circuit protection or overload protection, it can be automatically reset. The IR2161 also provides an overtemperature shutdown feature. When the chip junction temperature exceeds the over-temperature limit of 135°C, the half-bridge switch will stop working to avoid MOSFET burnout.

Figure 2 shows the output stage circuit for calculating RCS. The voltage at the midpoint of the half-bridge is 1/2 of the DC bus voltage. The DC bus RMS voltage is the same as the AC line RMS voltage, the RMS voltage on the load is 50% of the AC line RMS voltage, and the current through RCS is half the load current. Since the load is resistive, the current waveform is a sinusoidal envelope, the half-bridge output duty cycle is 50%, and the peak voltage on RCS at the maximum load is VCS(PK)=O. 4 V, the RMS current through RCS is

For a PLOAD=100W electronic transformer operating at VAC(RMS)=220 V AC line voltage, the current sensing resistor RCS value is

The selection procedure for output transformer T1 is as follows.
1) At 30 kHz and maximum load, select the correct core size and determine the maximum allowable power based on the core data provided by the manufacturer.

2) Calculate the number of primary winding turns. The minimum number of turns of the primary winding Np (min) can be calculated according to equation (5)

In the formula: VPK is the highest peak bus voltage, which can be 400 V;
Ton(amx) is the maximum conduction time, and its value can be selected as 18μs;
Bmax is the maximum magnetic flux density, in T, and its value can be provided by the manufacturer The curve obtained when the ferrite material is working at 100℃;
Ae is the effective cross-sectional area of ​​the magnetic core, in m2.

3) Calculate the number of turns required for the secondary. The number of secondary winding turns NS can be calculated according to equation (6).

In the formula: VO(RMS)=12V, VAC(RMS)=220V.

IR Company uses a Vogt or Kaschke company toroidal core in the reference design, with a primary winding of 78 turns and a secondary
winding 8 turns. The primary and secondary windings are isolated by a compression molding separator.

2 Electronic transformer circuit with enhanced functions
2.1 Halogen lamp electronic transformer circuit with open circuit protection
Under no-load conditions, high dv/dt on IC pin VS will cause hard switching of the MOSFET, causing the MOSFFT to overheat until it is damaged. To this end, an open circuit protection circuit as shown in Figure 3 can be set up. In the open circuit condition, the peak voltage on ROC is enough to turn on the diode Doc and charge the capacitor CF. As long as the voltage on CF exceeds the short-circuit protection threshold level on IC pin VS, the short-circuit protection circuit is triggered and IR2161 is turned off. To prevent the MOSFET from overheating, the IC will enter an auto-restart mode with 50ms on and 1 s off.

2.2 Electronic transformer circuit that uses frequency to adjust the output voltage
. If the output voltage of the electronic transformer is too low, the light will dim; if the output voltage is too high (more than 20% of the 12V rating), the lamp life will be shortened. Using the circuit shown in Figure 4 provides a better solution. When the output voltage is too high, the voltage on the voltage divider resistor R6 will exceed the internal 2.5V reference value of IC3. The optocoupler transistor in the photocoupler IC2 will be turned on, pulling the potential on the IC1 pin VS low, causing the operation Frequency increases. Due to the primary leakage inductance of transformer T1, the increase in frequency causes the output voltage to decrease. The addition of the frequency adjustment circuit will invalidate the short circuit shutdown protection circuit of IR2161. To solve this problem, a short-circuit protection circuit consisting of VS1, G5, D7 and R3 is added. In the event of a short circuit, thyristor VS1 is turned on, pulling the voltage on the VCC pin of IR2161 below the undervoltage lockout threshold level (10.2V), and the IC is turned off.

2.3 Dimming electronic transformer
circuit Figure 5 shows the dimming electronic transformer circuit. Among them, IC2 is a low-cost open collector or drain output dual comparator. Since the addition of IC2 needs to draw power supply current, the external resistance of IC1 pin VCC should be reduced from 270 kΩ (3W) to 56 kΩ. The unfiltered full-bridge rectified output voltage is divided by R3 and R4 and input to the non-inverting terminal of IC2-1. The voltage on the capacitor CT at the output terminal of IC2-1 is a sawtooth wave signal synchronized with the half cycle of the AC line voltage. This signal is input to the non-inverting terminal of IC2-2, and the other input of IC2-2 is the DC voltage divided by the potentiometer RP1. The output of IC2-2 is a rectangular wave with changing duty cycle. When IC2-2 outputs high level, the voltage on IC1 pin VS is pulled up above the short-circuit protection level through D6, so that IR2161 is turned off, and the secondary of the transformer no longer applies voltage to the lamp. This dimming scheme is trailing edge mode phase cut dimming. By adjusting RP1, the halogen lamp light can be varied from zero to maximum output (i.e. from 0% to 100%) and lamp flicker is eliminated. Figure 6 shows the DC bus voltage and lamp voltage waveforms.

Like fluorescent lamp dimming electronic ballasts, halogen lamp dimming electronic transformers can also use microcontrollers to achieve full digital dimming according to the IEC929 Digital Addressable Lighting Interface (DALI) bus communication protocol, and can use the same slave control system to control multiple electronic transformers.

3 Conclusion
The low-voltage lighting electronic transformer based on the intelligent controller IR2161 only needs a small number of components to achieve short-circuit protection, overload protection, open-circuit protection, overheating protection and dimming functions.