Abstract: Aiming at the problem that the filter capacitor of the traditional LED lamp rectifier circuit makes the AC input current waveform of the rectifier front end become a sharp pulse, resulting in low power factor and harmonic component increase, a novel AC LED converter topology circuit is proposed. The Buck PFC power factor correction circuit and LCC circuit were analyzed in detail. The working principle of the Buck PFC circuit, the working frequency of the switching tube and the capacitance value are analyzed. At the same time, the working principle and parameter value of the half-bridge circuit are analyzed theoretically and simulated. Finally, experiments verify that the circuit topology can improve the power factor. The circuit topology can overcome the problems of the traditional rectifier circuit due to the back-end capacitance caused by the controller's service life and harmonic components.
The traditional LED lamp constant current control is through AC/DC, and then through the DC/DC converter for constant current control. In the AC/DC converter, the output voltage is usually smoothed by the filter capacitor behind the rectifier circuit, but the large capacitor The presence of the input current waveform at the AC becomes a sharp pulse and is no longer a sinusoidal function (reduced power factor). Based on the above defects in LED control, this paper uses ACLED converter control. In the DC LED converter, since the input power is pulsating, the output power is constant, and the intermediate storage capacitor is needed to balance the difference between the two. Therefore, the storage capacitor has a large value and uses an electrolytic capacitor, but the value is high. The life of electrolytic capacitors is much less than the lifetime of LEDs, resulting in a reduction in the life of the overall converter.
If the AC LED is used, the input and output power are both pulsating, and the required storage capacitor value is small, which will increase the life of the overall converter. The existing AC LED lamp circuit structure has a series structure, a ladder structure and a bridge structure. In the AC LED circuit structure, when the input AC source is 220 V, 50 Hz, if the current limiting resistor is not connected in series, a large number of LED lamps are required in series to limit the LED current. At this time, the total turn-on voltage is increased to cause power. The factor is very low. When the current limiting resistor is connected in series, the number of LEDs required to be connected in series is reduced, and the power factor is increased, but the current limiting resistor will cause the efficiency to decrease. In addition, when the LED operating frequency is 50 Hz, the light source will obviously flicker; when the operating frequency is 100 Hz, most people will not feel the flashing of the light. In order to solve the power factor and low efficiency of AC LED and improve the frequency of the load structure, this paper proposes a new AC LED converter topology circuit, and analyzes and studies the topology circuit and control method.
1 AC LED lamp conversion structure
Figure 1 is a block diagram of the proposed AC LED converter. The vdc is 220 V AC voltage through the uncontrolled rectification ripple voltage. In order to improve the power factor, the Buck PFC circuit is used to correct the current waveform. The Buck PFC circuit output voltage is converted to an AC signal by a half-bridge converter as an input voltage to the downstream LCC circuit and supplied to the load LED. Therefore, the topology circuit mainly includes a rectifier circuit, a Buck PFC, a half bridge converter, and an LCC resonance circuit. It can be seen from Figure 1 that the values ​​of C1 and C2 are very small, so the capacitance has no effect on the lifetime of the entire topology circuit; the fundamental frequency of the input and output AC voltages of the LCC resonant circuit is 100 Hz, thus the load The voltage frequency of the LED is also 100 Hz, and the frequency does not see the flashing of the voltage.
Figure 1 New AC LED converter
1.1 Buck PFC circuit parameter setting
In Figure 1, if the rectified input voltage is:
V is the effective value of the sinusoidal input voltage, then the input current is:
I is the rms value of the sinusoidal input current. The relationship between the input voltage and the output voltage of the converter shows that:
Bring (2) into (3), the expression of d(t) is:
Lianli (2), (3), (4) can be obtained:
Therefore, the effective value of the capacitor ripple current is:
For a two-switch forward circuit, the relationship between the primary current of the transformer and the secondary current is as follows:
First set the frequency of the main switch tube to fs1=16.6 kHZ. Because the electrolytic capacitor affects the service life of the converter, try to make the storage capacitors C1 and C2 small, take C1=C2=1 mF. When the inductor current works in continuous conduction mode When in (CCM mode), the current ripple size is:
Where: D is the duty cycle of switch S1, D = Vce/Vdc in CCM mode. Considering Vdc maximum is 310 V, according to Buck PFC circuit output voltage limit Vce peak should not exceed 220 V, set inductor current pattern The maximum value of the wave ΔI L1 does not exceed 4A, then the best reference value for L1 is:
Consider that Buck PFC can work in discontinuous conduction mode (DCM), the inductance L1 value can be 0.865 mH.
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