Characteristics and principle of PT4107_DB03 full voltage 20W fluorescent lamp switch constant current source

Electrical parameters and BOM

The main electrical parameters of this constant current source are shown in Table 1. The parameters in the table are obtained by testing in CCM mode. It is designed for 85~245V AC power supply. In fact, it can work in a wider voltage range, such as 60~270V, but the output current will change. The output voltage of driving LEDs from different manufacturers will change slightly. This is caused by the different forward voltage drops of LEDs and will not affect the constant current accuracy. Changing the oscillation frequency and component parameters will cause the circuit to change its working state. For example, reducing the frequency or reducing the inductance of L3 will cause the circuit to enter DCM mode, and the electrical parameters of the circuit will change. The components of the circuit have made a compromise in terms of cost and reliability, and the number of components has been reduced to a minimum. Table 2 is a detailed material list. In order to ensure quality, try to use components from recommended manufacturers.

Test waveform

Figure 3 is the waveform of the emitter of the electronic filter T1. The output voltage is 16V DC. The input voltage is in the range of 70V to 245V, and this voltage is stable.

Figure 3: Base voltage

Figure 4 is the waveform of the MOS tube gate, which is a typical gate drive pulse waveform. The frequency is basically fixed, and the duty cycle of the pulse changes with the load current and input voltage. When the load is fixed, the duty cycle increases as the input voltage decreases, and the duty cycle at the lowest operating voltage is 0.48. The pulse amplitude is fixed at 14.8V and should not increase with the increase of input voltage. In the measurement, it can be seen that the pulse jitters in the horizontal direction. This is not a fault, but a spread spectrum function has been added to the chip to reduce EMI.

Figure 4: Gate signal waveform

Figure 5 is the drain voltage waveform of the MOS tube. The waveform frequency is the same as the gate, but the polarity is opposite. When the constant current source is unloaded, the drain voltage is 1.4 times the AC input voltage, and when loaded, it is 1.2 to 1.3 times the AC input voltage. Due to the use of ultra-high-speed recovery diodes for freewheeling, the reverse electromotive force generated by the inductor is damped, so the waveform is very clean. Note that when using an oscilloscope to test the drain voltage, a special high-voltage probe must be used, otherwise the oscilloscope will be damaged.

Figure 5: Drain signal waveform

Figure 6 is the source voltage of the MOS tube. This voltage is the voltage drop of the working current of the MOS tube on the sensing resistor. Its amplitude is proportional to the working current of the MOS tube. This voltage is sent to the chip as a control signal in a single cycle to control the duty cycle of the gate pulse of the MOS tube so that the current flowing through the LED is constant. The biggest difference between the source voltage and the gate voltage is that there are spikes on the front and back edges of the pulse. The spikes are generated by the output ballast inductor and the parasitic inductance of the MOS tube. These spikes are the source of switching losses. The slope at the top of the waveform is generated by conduction loss. Conduction loss and switching loss are the main causes of MOS tube heating.

Figure 6: Source sampling signal waveform

Figure 7 The two waveforms on the upper left are the voltage waveforms of LED+ and LED- respectively (with 240 ohm load), and the waveform of (LED+)-(LED-) on the lower left is the output voltage. The right figure is the output current ripple measured by the current induction loop. Since the high-frequency response of the current loop is very good, it shows a peak current of tens of millivolts. They are caused by the reverse electromotive force generated by the parasitic inductance of the loop, and the filter capacitor can do nothing about it. Note that a special current probe or current induction loop must be used to measure the current with an oscilloscope.

Figure 7: Output current waveform

Figure 8 shows the output current corresponding to different input voltages in a 27OC room temperature environment, that is, the input voltage regulation rate characteristics.

Figure 8: Input voltage regulation

Figure 9 shows the effect of ambient temperature change on output current. This curve is fitted using test data in Origin software. The test data comes from 10 circuit boards shown in Figure 2 in an aging box, working under load at -15OC to +75OC, with a step length of 5OC.

Figure 9: Temperature-output current characteristics

Instructions

Note: This constant current source is a non-isolated structure. The circuit board and LED pins are energized. Strictly abide by the live safety operation rules to avoid electric shock accidents!

First, check the series and parallel structure of the LED on the LED board. Each string of LEDs must be within the range of 12 to 28, 10 to 15 series and parallel, the total current is controlled within 260mA, and the total power should not exceed 20W. The constant current source board uses a 2-wire power cord to connect to the 220V mains, L to the live wire, and N to the ground wire. The mains is allowed to fluctuate by ±15%. After connecting the LED, turn on the power. It is not recommended to power on first and then connect the LED, as this will damage the LED and shorten its service life. When the LED is lit, if the current deviates from the design value, connect an ammeter with a range greater than 2A in series in the output circuit, and adjust the potentiometer on the circuit board to fine-tune the output current. After the current is adjusted, drip silicone on the potentiometer screw to fix it to prevent vibration from affecting the potentiometer. If the required current value cannot be obtained by adjusting the potentiometer, resistors R6 to R9 can also be changed. Since the heat dissipation setting is designed based on a maximum output power of 20W, do not increase the output power at will. The circuit board can be used directly in production. The Gerber file of the PCB board can be downloaded directly from the PowThch website or requested from the Application System Department, saving design time and cost.