Full voltage 20W LED fluorescent switch constant current source design

Basic characteristics

AC non-isolated high-frequency switching step-down constant current mode AC 85~245V, 50~60Hz working range Series charging, parallel discharge passive power factor correction Can be built in 28mm lamp installation Working environment temperature 0~75oC
Meet the requirements of IEC61000-3-2:2001

Electrical schematic and physical photos

The circuit is shown in Figure 1. The AC mains inlet is connected to a 1A fuse FS1 and a surge-resistant negative temperature coefficient thermistor NTC. This is followed by an EMI filter consisting of L1, L2 and CX1. The BD1 is a rectified full bridge with four high voltage silicon diodes inside. C1, C2, R1, D1~D3 form passive power factor correction. For the working principle, see the article "Optimization Method of Designing LED Fluorescent Lamp with PT4107". PWM control chip U1 and power MOS transistor Q1, ballast inductor L3, and freewheeling diode D5 form a Buck step-down converter. U1 collects the peak current on the sense resistors R6-R9. The internal logic controls the pulse duty of the GATE pin signal. Constant current control. The chip is powered by the electronic filter consisting of T1, D4, C4, and R2~R4. The internal resistance of this filter is very high, the output impedance is very small, and it can provide a stable voltage of about 16V to ensure stable operation of the chip in the full voltage range. . R5 is part of the chip oscillator circuit, changing it will adjust the oscillation frequency. The potentiometer RT is not used for dimming in this circuit, but is used to fine tune the current of the constant current source to make the circuit reach the design power. The parameters of this circuit are 22 LEDs in series, 15 series parallel, driving 330 60 mW white LEDs, each string current is 17.8 mA.

Figure 1: Schematic diagram of constant current source of 20W LED fluorescent lamp

Figure 2 is an actual photo of a constant current source. The 33 components are mounted on a 235 x 25 x 0.8 mm epoxy single-sided printed board. The PCB traces are designed according to electrical and electronic specifications and can be mounted directly on 28 mm. Among the tubes.

Figure 2: 20W LED fluorescent lamp constant current source physical photo

Electrical parameters and BOM

The main electrical parameters of this constant current source are shown in Table 1. The parameters in the table are tested in CCM mode. It is designed for 85-245V AC power supply, and can actually work in a wider voltage range, such as 60 ~ 270V, but the output current will change. The LED output voltage driving different manufacturers will change slightly, which is caused by the difference in forward voltage drop of the LED, which will not affect the constant current accuracy. Changing the oscillation frequency and component parameters will cause the circuit to change its operating 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 are compromised in terms of cost and reliability, and the number of components has been minimized. Table 2 is a detailed material list. In order to ensure the quality, try to use the components of the recommended manufacturer.

Test waveform

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

Figure 3: Base voltage

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

Figure 4: Gate Signal Waveform

Figure 5 shows the drain voltage waveform of a MOS transistor with the same waveform frequency as the gate but with opposite polarities. When the constant current source is idling, the drain voltage is 1.4 times the AC input voltage, and when it is loaded, it is 1.2 to 1.3 times the AC input voltage. Due to the ultra-high speed recovery diode freewheeling, the back electromotive force generated by the inductor is damped and the waveform is clean. Note that the oscilloscope must be used to test the drain voltage with a dedicated high voltage probe, otherwise the oscilloscope will be damaged.

Figure 5: Drain signal waveform

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

Figure 6: Source sampled signal waveform

The top two waveforms in Figure 7 are the voltage waveforms of the LED+ and LED- terminals (with 240 ohm load), and the bottom left is the waveform of (LED+)-(LED-), which is the output voltage. The picture on the right shows the output current ripple measured with a current sense loop. Since the high-frequency response of the current loop is very good, it shows a peak current of several tens of millivolts, which is caused by the back electromotive force generated by the parasitic inductance of the loop, and the filter capacitor is helpless to it. Note that using an oscilloscope to measure current requires a dedicated current probe or current sense loop.

Figure 7: Output Current Waveform

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

Figure 8: Input Voltage Regulation

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

Figure 9: Temperature - Output Current Characteristics

Instructions for use

Note: This constant current source is a non-isolated structure. Both the circuit board and the LED pins are energized. Strictly observe the safe operation rules of the live line to avoid human electric shock accidents!

First check the string structure of the LED tube on the LED board. Each string of LEDs must be in the range of 12 to 28, 10 to 15 series and parallel, the total current is controlled within 260 mA, and the total power should not exceed 20 W. The constant current source board is connected to 220V mains by a 2-wire power line, L is connected to the live line, and N is the ground line. Allow the mains to fluctuate by ±15%, then connect the LEDs and then turn on the power. It is not recommended to power on the LED first, which will damage the LED and shorten the service life. When the LED is lit, if the current deviates from the design value, a current meter with a range greater than 2A is connected in series with the output circuit, and the potentiometer on the circuit board can be adjusted to fine tune the output current. After the current is adjusted, the silicone is fixed on the potentiometer screw to prevent the vibration from affecting the potentiometer. If the potentiometer still does not get the required current value, the resistors R6 to R9 can also be changed. Since the heat dissipation setting is designed for a maximum output power of 20W, do not increase the output power arbitrarily. The board can be used directly for production, and the Gerber files for the PCB can be downloaded directly from the PowThch website or obtained from the Application Systems department, eliminating the time and expense of design.


Electronic regulator

The circuit that supplies U1 is called a double-capacitor ripple filter. It is an effective power purifier. It has a dual function of a capacitor multiplying low-pass filter and a series regulator, also called an ACR (Amplificatory Capacitance Regulator) circuit. Connect a capacitor C4 to the base of the emitter output. Since the base current is only 1/(1+β) of the emitter current, it is equivalent to a large capacitance of (1+β)C4 at the emitter. This is the capacitance multiplication. The principle of the filter. If a Zener diode is connected to the base to ground, it is a simple series regulator. Therefore, the circuit has the dual functions of voltage regulation and filtering, which can effectively eliminate the high frequency switching ripple. Note that the bipolar transistor is selected to have Vbceo > 500V, Ic = 100mA. Zener diode D4 uses 16~18V, 1/4W small power regulator tube of any type.

Figure 10: Multi-capacity ripple

Power factor correction circuit

Ordinary bridge rectification directly smoothed the AC-DC circuit, the input voltage is sine wave, because the capacitor charge is fast and the discharge is slow, the current is a discontinuous pulse wave, the harmonic distortion is large, and the power factor is low. This circuit uses a low-cost passive power factor compensation circuit, as shown in Figure 11. This circuit is called a balanced half-bridge compensation circuit. C1 and D1 form one arm of the half bridge, C2 and D3 form the other arm of the half bridge, and D2 and R1 form a charging connection path, which is compensated by the valley filling principle. The filter capacitors C1 and C2 are connected in series. The voltage on the capacitor is charged to half of the input voltage (VAC/2). Once the line voltage drops below VAC/2, the diodes D1 and D3 are forward biased, thus making C1 Start parallel discharge with C2. With this circuit, the power factor of the system is increased from 0.6 to 0.88 to 0.9, but it is difficult to exceed 0.92 because there is a dead zone of approximately 60 degrees between the input voltage and current.

Figure 11: Balanced Half-Bridge PFC Circuit

Sampling resistor
Resistors R6, R7, R8, and R9 are connected in parallel as resistors, which can reduce the influence of resistance accuracy and temperature on the output current, and can easily change the resistance of one or several of them to achieve the purpose of modifying the current. It is recommended to use SMD resistors with a precision of one thousandth and a temperature coefficient of 50ppm. If there is a higher requirement for current accuracy and temperature variation, it is recommended to use a constant current sampling resistor for the copper or manganese copper four-terminal.

Electrolytic capacitor

Because the life of aluminum electrolytic capacitors has a great relationship with temperature, the electrolyte loss increases with increasing temperature, and the capacitor life is reduced by half for every 6 OC increase in temperature. Although the life of the LED is as long as 50,000 hours, the life of the electrolytic capacitor is only 4,000 hours. When the temperature inside the tube is relatively high, the life of the electrolytic capacitor is lower. Therefore, the life of the driving circuit depends on the electrolytic capacitor.

Power inductor

The power inductor L3 is a more critical component than the MOS transistor, and requires a high Q value, a large saturation current, and a small resistance. The nominal 3.9 millihenry inductance, Q should be greater than 90 in the frequency range of 40KHz ~ 100KHz, the saturation current is greater than 2 times the working current, here select 500 mA, the winding resistance is less than 2 ohm, the Curie temperature is greater than 400oC Power inductor. The consequences of using a bad value inductor are catastrophic. Once the inductor is saturated, the MOS tube, LED, and control chip will burn out instantly. It is recommended to use the power inductor of the microcrystalline material to ensure that the constant current source operates safely and reliably for a long time.

(Assistant editor: xiaohu)

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