# An AC/DC LED Driver with Unity Power Factor and Soft Switching
## 4 Parameters Design and Experimental Results
### Experimental Results  
From Equation (16), it is noted that the switching frequency is used to control the output power. As seen in Figure 2, the LED current is sensed by a series resistor \(R_{s}\). The voltage across \(R_{s}\) which is proportional to the LED current is compared to a reference value \(I_{\textit{ref}}\), and their difference is amplified by an operational amplifier in the error amplifier (EA). The output of the operation amplifier is sent to the half-bridge controller which mainly consists of a double-ended controller (L6599). The outputs of the half-bridge controller are two complementary waveforms (\(v_{GS1}\), \(v_{GS2}\)) of which the frequency is regulated [12]. By this way, the switching frequency of the active switch is controlled to regulate the LED current. Figure 5 shows the measured waveforms of the input voltage and input current. The rms values of the input voltage and current are 112.4 V and 0.57 A, respectively. It can be observed that the input current is sinusoidal and in phase with the input voltage. The power quality in the input line is measured with a power analyzer (PowerLogic PM1000, Schneider Electric, Rueil-Malmaison, France). The measured PF is greater than 0.99, and the THDi is 3.5%. The inductor currents of both converters are shown in Figure 6 to indicate the DCM operation. There are spike currents in both the buck-boost inductor and the buck inductor. Theoretically, it is impossible to have a current spike in an inductor; otherwise, the voltage across the inductor would be tremendous high. The measured spike current in the buck-boost inductor and the buck inductor is believed to result from the interference voltage when one active switch is turned off. Figure 7 shows the waveforms of the output voltage and output current, which are well consistent with the design values. The DC values of the output voltage and current are 198.5 V and 0.304 A, respectively. In addition, the input power and output power are both measured with an oscilloscope. The measured values are 64 W and 60.3 W, respectively. The circuit efficiency is calculated to be 94.2%. Figure 8 shows waveforms of the voltage and current of the active switches. As shown, the switch current always increases from negative to positive. Its intrinsic diode conducts the negative current and clamps the switch voltage at almost zero volts. The active switch would naturally turn on as the switch current becomes positive, leading to ZVS operation. In the prototype circuit, MOSFET IRF840 serves the active switch. The current rating of the source-drain current (i.e., the current rating of the intrinsic diode) is 8 A maximum. It is seen in Figure 8, the maximum value of the diode current is about 3.5 A. Besides, the current in the intrinsic diode decreases linearly to zero. It means that the intrinsic diodes would turn off at zero current. Therefore, there is no faster diode in parallel with the active switch to replace the intrinsic diode. Figure 9 shows the prototype LED driver.  
\begin{table}
\begin{tabular}{l l} \hline \hline The low-pass inductance \(L_{m}\) & 2.0 mH \\ \hline The low-pass capacitance \(C_{m}\) & 0.47 μF \\ \hline Dc-link capacitance \(C_{dc}\) & 100 μF \\ \hline Output capacitance \(C_{o}\) & 100 μF \\ \hline Buck–boost inductor \(L_{p}\) & 0.47 mH \\ \hline Buck inductor \(L_{b}\) & 2.26 mH \\ \hline Active switches \(S_{1}\), \(S_{2}\) & IRF840 \\ \hline Diodes \(D_{1}\)–\(D_{7}\) & MUR460 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Circuit parameters.  
Figure 5: Waveforms of the input voltage and current. (\(v_{in}\): 100 V/div, \(i_{in}\): 1 A/div, time: 5 ms/div).  
Figure 6: Waveforms of inductor currents (a) Buck–boost inductor current \(i_{p}\) (\(i_{p}\): 1 A/div, time: 5 μs/div) and (b) Buck inductor current \(i_{b}\) (\(i_{b}\): 0.5 A/div, time: 5 μs/div).