One of the challenges of LED light designers is to create lighting systems that produce a constant light output without significant flicker. In systems using single-stage LED drivers, illumination without flicker is difficult to achieve because random distortion of the line voltage causes the LED current to be unstable and produce flicker. The circuit feedback response is usually the root cause of the problem because it may not react quickly enough to the distortion.
To solve this problem, we designed a 7.8W LED system and tested it using a power factor correction (PFC) control circuit. We found that the combination of the two - using internally generated sinusoidal references and discontinuous conduction mode (DCM) operation - can improve performance, reduce component count and eliminate flicker.
Internally generated digital sinusoidal reference
In most designs that use current mode operation to control the PFC, a sinusoidal reference is obtained by sensing the input voltage using a resistor divider. An internally generated sinusoidal reference that is generated using a digital map to make the LED current of the buck-boost topology more stable. The internally generated sinusoidal reference also eliminates the resistor divider, resulting in fewer component counts and a more compact design. Figure 1 provides a block diagram of the circuit.
Figure 1. LED driver configuration
The power supply voltage (Vcc) is supplied by a high voltage (HV) device built into the controller. Because the input voltage zero-crossing trigger is detected by the Vcc and HV blocks, the internal sinusoidal reference and the zero-crossing trigger signal are synchronized. The output signal from the on-chip digital-to-analog converter (DAC) produces a digital sinusoidal reference using internally mapped sinusoidal information and a synchronized zero-crossing trigger signal. When the Vcc voltage is less than 15.5V, the zero-crossing trigger detector (ZCD) has a low voltage and the DAC and internal clock automatically provide a digital reference signal with a digital step of 32 bits.
CRM and DCM operations
In converters that use a boost topology, the input current is configured by the inductor current. This optimizes the PFC in critical conduction mode (CRM) operation due to the presence of a constant on-time and variable off-time. On the other hand, due to the buck-boost topology, the input current is proportional to the switching current. As a result, the PFC is lowered in the CRM operation, and the line peak voltage becomes flatter. The input current is determined by the inductor current associated with the MOSFET turn-on. Equation 1 shows how to calculate the input current in a buck-boost converter.
Based on Equation 1, the input current is proportional to the input voltage by controlling the constant on-time and the constant switching period in the buck-boost converter. This means that the best mode is a discontinuous conduction mode (DCM) operation with a fixed on-time.
Constant current line regulation
As mentioned above, to improve LED current ripple and line voltage distortion, our design uses DCM operation and fixed internal sinusoidal references in buck-boost converters. This method does a better job of adjusting the LED current. It has no noticeable flicker, even with line voltage transients between 90Vac and 265Vac. Figure 2 shows the current slope of the inductor current.
Figure 2. Current slope of the inductor current
Based on the results of Figure 2, the output current can be calculated using Equation 2.
In Equation 2, since the fixed sinusoidal I peak is controlled by an internal reference, the output current is not a function of the input voltage. Figure 3 shows the line regulation for constant current.
Figure 3. Constant current line regulation
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