New AC LED Converter Topology Design

The traditional constant current control of LED lamps is through AC/DC, and then through the DC/DC converter for constant current control. In the AC/DC converter, a filter capacitor is usually used after the rectifier circuit to smooth the output voltage, but the presence of large capacitors causes the input current waveform at the AC end to become a sharp pulse instead of a sine function (reducing the power factor). Based on the above defects in LED control, this article uses ACLED converter control. In the DC LED converter, since 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 in value and uses an electrolytic capacitor. However, the life of the high-value electrolytic capacitor is much shorter than the life of the LED, resulting in a reduction in the life of the overall converter.

If AC LED is used, the input and output power are both pulsating, and the required energy storage capacitor value is smaller, which will increase the life of the overall converter. The existing AC LED lamp circuit structures include series structure, ladder structure and bridge structure. In the AC LED circuit structure, when the input AC source is a grid voltage of 220 V, 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 turn-on voltage is increased, resulting in a very low power factor. 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 improved, but the efficiency will be reduced due to the current limiting resistor. In addition, when the LED operating frequency is 50 Hz, the light source will obviously flicker; and when the operating frequency is 100 Hz, most people cannot feel the flicker of the light. In order to solve the low power factor and efficiency of AC LED, and to increase the frequency shown in the load structure, this paper proposes a new AC LED converter topology circuit, and analyzes and studies the topology circuit and control method.

1 Transformation structure of AC LED lamp

Figure 1 is the structure diagram of the proposed AC LED converter. vdc is the pulsating voltage of 220 V AC voltage after uncontrolled rectification. In order to improve the power factor, this paper adopts Buck PFC circuit to correct the current waveform. The output voltage of Buck PFC circuit is converted into AC signal through half-bridge converter, which is provided to the load LED lamp as the input voltage of the subsequent LCC circuit. Therefore, the topology circuit mainly includes rectification circuit, Buck PFC, half-bridge converter and LCC resonant circuit. As can be seen from Figure 1, the values ​​of C1 and C2 are very small, so the capacitor has no effect on the life of the entire topology circuit; the fundamental frequency of the input and output AC voltage of LCC resonant circuit is 100 Hz, so the voltage frequency of load LED is also 100 Hz, and the voltage flicker cannot be seen at this frequency.

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 effective value of the sinusoidal input current. From the relationship between the input voltage and output voltage of the converter, we can know that:

Substituting (2) into (3), we can get the expression of d(t):

Combining equations (2), (3) and (4), we can obtain:

So the effective value of the capacitor ripple current is:

For the dual-tube forward circuit, the relationship between the primary current and the secondary current of the transformer is as follows:

First, the main switch frequency is set to fs1 = 16.6 kHZ. Because the electrolytic capacitor affects the service life of the converter, the energy storage capacitors C1 and C2 are as small as possible, and C1 = C2 = 1 mF. When the inductor current works in continuous conduction mode (CCM mode), the current ripple size is:

Where: D is the duty cycle of switch S1. In CCM mode, D = Vce/Vdc. Considering that the maximum value of Vdc is 310 V, according to the output voltage limit of Buck PFC circuit, the peak value of Vce should not exceed 220 V. The maximum value of inductor current ripple ΔI L1 is set to not exceed 4A. The optimal reference value of L1 is:

Considering that Buck PFC can operate in discontinuous conduction mode (DCM), the value of inductor L1 can be 0.865 mH.

1.2 Selection of LCC parameters

Next, we analyze the calculation method of LCC parameters. By complementary conduction of switch tubes S2 and S3, the input of the LCC resonant circuit is AC, and the working state is shown in Figure 2.

Figure 2 LCC equivalent circuit

According to its working state, the LCC equivalent circuit can be obtained, where C=2 C1=2 C2, and the LED is replaced by its equivalent model.

Von is the series turn-on voltage of 23.5V, and Re is the series equivalent resistance of 7Ω. When the absolute value of the output voltage Vo is less than the turn-on voltage Von, io=0, which is equivalent to no-load condition, and the input-output transfer function is:

When the absolute value of the output voltage Vo is greater than the turn-on voltage Von, the load is equivalent to Ro, and the input-output transfer function is:

According to the load characteristics, when |Vo| < Von, |Vo| should be raised to Von as quickly as possible so that the LED can be lit as soon as possible. This can make the resonant frequency of equation (12) equal to the switching frequency of S2 fs = 45kHz, that is:

When the absolute value of the output voltage Vo is greater than the turn-on voltage Von, and it is hoped that the change speed of Vo will slow down so that the LED will continue to be turned on, the gain of the input-output transfer function is also required. The gain value must be high enough so that the output voltage can reach the maximum voltage value that the LED can withstand. That is:

Where: Vomax is the peak value of the output voltage, Vimax is the maximum value of |Vcc/2|. Combining equations (10) to (12), it can be obtained that under the condition of satisfying equation (13), according to the restriction condition of equation S95H, the inductor L2 can be selected to be less than 0.45mH.

Our design goal is to output power of 20W and the maximum output peak voltage does not exceed 34V. The turn-on voltage of each LED string load is 23.5V, the equivalent internal resistance is 7Ω, and the maximum peak current does not exceed 1.5A. Therefore, the inductor r6 values ​​are selected as 50, 100, 150, and 200uH to obtain the simulation waveform of Figure 3. According to the gain and resonant frequency constraints, L2=100uH is selected, and C2=133nF can be obtained from formula (12).

Figure 3 Bode plot of input-output transfer function under load

Through the above theoretical analysis, we have conducted simulation analysis on the circuit without PFC correction circuit and the circuit with PFC correction circuit in Figure 1, as shown in Figure 4 and Figure 5. Through simulation analysis, it can be seen that the topological circuit structure and parameter selection of Figure 1 are feasible.

Figure 4: Simulated waveforms of input AC voltage and AC current without PFC

Figure 5: Input AC voltage and AC current simulation waveforms with PFC

2 Experimental verification

According to the previous theoretical analysis, we made the control module of AC LED lamp, and its design parameters are based on simulation data. The input AC voltage Vin is 220V, the switching frequency of S1 is 20 kHz, and the switching frequency of S2 and S3 is 30 kHz; the main power inductor L1 is 0.865 mH; the resonant inductor L2 is 0.1 mH, the energy storage capacitors C1 and C2 are both 1.2 mF; the resonant capacitor C3 is 133 nF, and the PWM signal of the switch tube is generated by DSP. Figure 6 shows the AC voltage and AC current test waveforms at the power input end when there is no PFC circuit in Figure 1, and Figure 7 shows the AC voltage and AC current test waveforms at the power input end when there is a PFC circuit.

Figure 6 Input voltage Vin and input current iin waveforms without PFC circuit

Figure 7 Input voltage Vin and input current iin waveforms with PFC circuit

3 Conclusion

This paper proposes a new AC LED converter topology circuit to solve the problem that the filter capacitor of the traditional LED lamp rectifier circuit turns the AC input current waveform at the front end of the rectifier into a sharp pulse, resulting in low power factor and increased harmonic components. Through theoretical analysis of the new topology circuit in Figure 1, the capacitance value in the LCC circuit is small, which solves the problem of short service life of LED caused by the large capacitance of the traditional rectifier circuit. This paper also theoretically analyzes the selection of Buck PFC circuit parameter values, simulates and analyzes the phase relationship between the input voltage and current with and without power factor correction circuits, and finally verifies the feasibility of the new circuit topology structure proposed in this paper through experiments.