Aiming at the problems of low power factor and increased harmonic component caused by the filter capacitor of the traditional LED lamp rectifier circuit, a new AC LED converter topology circuit is proposed. The buck PFC power factor correction circuit and LCC circuit are analyzed in detail. The working principle, switching frequency and capacitance of Buck PFC circuit are analyzed. At the same time, the working principle and parameter selection of half bridge circuit are analyzed theoretically and simulated. Finally, experiments show that the circuit topology can improve the power factor. The circuit topology can overcome the problems that the service life and harmonic component of the controller are reduced due to the back-end capacitance of the traditional rectifier circuit.
The traditional constant current control of LED lamp is through AC / DC, and then through DC / DC converter. In AC / DC converter, the filter capacitor is usually used behind the rectifier circuit to smooth the output voltage, but the existence of large capacitor causes the input current waveform at the AC end to become sharp pulse instead of sinusoidal function (reducing power factor). Based on the defects of the above LED control, acled converter is used in this paper. In DC led converter, because the input power is pulsating and the output power is constant, an intermediate energy storage capacitor is required to balance the difference between the two. Therefore, the energy storage capacitor is generally large and electrolytic capacitor is used, but the service life of electrolytic capacitor with high value is far less than that of LED, resulting in the reduction of the service life of the overall converter.
If AC LED is used, the input and output power are pulsating, the required energy storage capacitance is small, which will improve the service life of the overall converter. The existing AC LED lamp circuit structure has series structure, trapezoidal structure and bridge structure. In the AC LED circuit structure, when the input AC source is the grid voltage of 220 V and 50 Hz, if the current limiting resistor is not connected in series, a large number of LED lamps need to be connected in series to limit the LED current. At this time, the total on voltage is increased, resulting in very low power factor. When the current limiting resistor is connected in series, the number of LEDs in series is reduced and the power factor is improved, but the efficiency will be reduced because of the current limiting resistor. In addition, when the LED working frequency is 50 Hz, the light source will obviously flicker; When the working frequency is 100 Hz, most people can’t feel the flashing of light. In order to solve the problems of low power factor and efficiency of AC LED and improve the frequency shown by the load structure, a new topology circuit of AC LED converter is proposed, and the topology circuit and control method are analyzed and studied.
1. Conversion structure of AC LED lamp
Fig. 1 is a structural diagram of the proposed AC LED converter. VDC is the pulsating voltage of 220 V AC voltage through uncontrolled rectification. In order to improve the power factor, Buck PFC circuit is used to correct the current waveform in this paper. The output voltage of Buck PFC circuit is converted into AC signal through half bridge converter, which is used as the input voltage of later stage LCC circuit and supplied to load led lamp. Therefore, the topology circuit mainly includes rectifier circuit, Buck PFC, half bridge converter and LCC resonant circuit. As can be seen from Fig. 1, the values of C1 and C2 are very small and can be achieved. Therefore, the capacitance has no impact on the life of the whole topology circuit; The fundamental frequency of the input and output AC voltage of the LCC resonant circuit is 100 Hz, so the voltage frequency of the load led is also 100 Hz, and the voltage flicker cannot be seen at this frequency.
Figure 1 new AC LED converter
1.1 setting of Buck PFC circuit parameters
In Fig. 1, if the rectified input voltage is:
V is the effective value of sinusoidal input voltage, then the input current is:
I is the effective value of sinusoidal input current. From the relationship between the input voltage and output voltage of the converter:
Bring (2) into (3) and the expression of D (T) is:
Simultaneous equations (2), (3) and (4) can obtain:
Therefore, the effective value of capacitor ripple current is:
For double transistor forward circuit, the relationship between primary side current and secondary side current of transformer is as follows:
First, set the frequency of the main switch as FS1 = 16.6 kHz. Because the electrolytic capacitor affects the service life of the converter, try to make the energy storage capacitors C1 and C2 small, taking C1 = C2 = 1 MF. When the inductive current works in the continuous conduction mode (CCM mode), the current ripple is:
Where: D is the duty cycle of switch S1, and d = VCE / VDC in CCM mode. Considering that the maximum value of VDC is 310 V, the peak value of VCE should not exceed 220 V according to the output voltage limit of Buck PFC circuit, and the inductance current ripple is set Δ If the maximum value of I L1 does not exceed 4a, the best reference value of L1 is:
Considering that Buck PFC can work in discontinuous conduction mode (DCM), the inductance L1 value can be taken as 0.865 MH
1.2 selection of LCC parameters
Next, we analyze the calculation method of LCC parameters. Through the complementary conduction of switching tubes S2 and S3, the input of LCC resonant circuit is AC, and the working state is shown in Figure 2.
Figure 2 LCC equivalent circuit
According to its working state, LCC equivalent circuit can be obtained, in which C = 2C1 = 2c2, and LED is replaced by its equivalent model.
Von is the series on voltage of 23.5v, and re is the series equivalent resistance of 7 Ω. When the absolute value of the output voltage Vo is less than the opening voltage von, IO = 0, which is equivalent to no-load condition. The input-output transfer function is:
When the absolute value of the output voltage Vo is greater than the opening voltage von, the load is equivalent to RO, and the input-output transfer function is:
According to the load characteristics, when | VO | < >
When the absolute value of the output voltage Vo is greater than the on voltage von, it is hoped that the change speed of VO will slow down to make the LED continuously conductive. The gain of the input-output transfer function is also required. The gain value must be high enough to make the output voltage reach the maximum withstand voltage value of the LED. Namely:
Where Vomax is the peak value of output voltage and Vimax is the maximum value of | VCC / 2 |. Combined with equations (10) ~ (12), it can be obtained that under the condition of satisfying equation (13), according to the limiting conditions of equation s95h, the optional inductance L2 should be less than 0.45mh
Our design goal is that the output power is 20W, and the maximum peak output voltage does not exceed 34V. The on voltage of each string of LED load is 23.5v, the equivalent internal resistance is 7 Ω, and the maximum peak current flowing through does not exceed 1.5A. Therefore, select the inductance R6 values of 50, 100, 150 and 200uh respectively to obtain the simulation waveform in Figure 3. According to the gain and resonance frequency constraints, L2 = 100uh is taken, and then C2 = 133nf can be obtained from equation (12)
Figure 3 Bode diagram with input-output transfer function under load
Through the above theoretical analysis, we have simulated and analyzed the PFC correction circuit without PFC correction circuit and PFC correction circuit in Fig. 1, as shown in Fig. 4 and Fig. 5. Through simulation analysis, it can be seen that the selection of topological circuit structure and parameters in Figure 1 is feasible.
Fig. 4 simulation waveform of input AC voltage and AC current without PFC
Fig. 5 simulation waveform of input AC voltage and AC current of PFC
2 experimental verification
According to the previous theoretical analysis, we have made the control module of AC LED lamp. Its design parameters are based on the simulation data. The input AC voltage Vin is 220V, the switching frequency of S1 is 20 kHz, and the switching frequencies of S2 and S3 are 30 kHz; The main power inductance L1 is 0.865 MH; The resonant inductance L2 is 0.1 MH and the energy storage capacitors C1 and C2 are 1.2 MF; The resonant capacitance C3 is 133 NF, and the PWM signal of the switch is generated by DSP. Figure 6 shows the AC voltage and AC current test waveforms of the power input tested when there is no PFC circuit in Figure 1, and Figure 7 shows the AC voltage and AC current test waveforms of the power input tested when there is PFC circuit.
Fig. 6 waveform of input voltage Vin and input current iin without PFC circuit
Fig. 7 waveform of input voltage Vin and input current iin with PFC circuit
Aiming at the problems that the filter capacitor of the traditional LED lamp rectifier circuit turns the AC input current waveform of the rectifier front end into sharp pulse, resulting in low power factor and increased harmonic component, a new AC LED converter topology circuit is proposed in this paper. Through the theoretical analysis of the new topology circuit in Figure 1, the capacitance value in LCC circuit is small, which solves the problem of short service life of LED caused by the large capacitance of traditional rectifier circuit. This paper also theoretically analyzes the selection of the parameter value of Buck PFC circuit, simulates and analyzes the phase relationship between the input voltage and current of power factor correction circuit and no power factor correction circuit, and finally verifies the feasibility of the new circuit topology proposed in this paper through experiments.
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