Design of single-stage dimmable off-line AC/DC controller

CS1615 and CS1616 are high-performance single-stage dimmable offline AC/DC controllers. CS1615/16 is a cost-effective solution that provides unparalleled compatibility of single and parallel dimmers for dimmable LED applications. The CS1615 is specifically designed for 120 Vac line voltage applications, and the CS1616 is specifically designed for 230 Vac line voltage applications.

CS1615 and CS1616 are high-performance single-stage dimmable offline AC/DC controllers. CS1615/16 is a cost-effective solution that provides unparalleled compatibility of single and parallel dimmers for dimmable LED applications. The CS1615 is specifically designed for 120 Vac line voltage applications, and the CS1616 is specifically designed for 230 Vac line voltage applications.

designing process

The design process of a single-stage power converter system can be divided into six circuit blocks (Figure 1). After the AC line voltage is rectified, it passes through an electromagnetic interference (EMI) filter to suppress the conducted interference generated by the circuit on the power line. Then the EMI output is converted into the required DC output through a dual-mode flyback/buck-boost converter. The power converter system includes grid bias, steady-state power supply and active clamp support circuit.

Design of single-stage dimmable off-line AC/DC controller
CS1615/16 Design Block Diagram

The CS1616/16 can be configured as an isolated or non-isolated topology using a flyback transformer or a buck-boost Inductor, respectively. Figure 1 illustrates the dual-mode flyback topology. The CS1615/16 uses primary side control to adjust the output current, eliminating the need for optocoupler feedback.

CS1615/16 controls dual-mode flyback/buck-boost to meet the dimmer holding current requirement in dimmer mode, and provides power factor correction in no dimmer mode. Figure 2 illustrates the dual-mode buck-boost topology. CS1615/16 regulates the output current by controlling the peak current to ensure that the target output power charging is achieved every half line cycle. The dual mode ensures that the average input current is greater than the required dimmer holding current when the dimmer is behind, and the line current is shaped to provide power factor correction when it is not behind the dimmer.

The dual-mode buck-boost uses a transformer with a turns ratio of 1 for simulation. The demagnetization time of the transformer/inductor is detected by the FBAUX pin using the auxiliary winding and used as the input of the control loop.

Design of single-stage dimmable off-line AC/DC controller

Buck-boost model
Flyback design

Figure 3 is a composite timing diagram of primary and secondary winding currents and switching cycles. The flyback design depends on knowing the peak current I_PK flowing through the power FET Q4.

Design of single-stage dimmable off-line AC/DC controller
T1, T2, T3, TT timing diagram

The constant current I_IN (CC) is determined by the larger of the TRIAC holding current I_hold or the dimmer current I_fb required by the full-bright power supply. Most dimmers can provide 180C conduction, but some dimmers will not start conduction before 45°C.

In addition to the primary inductance L_P, the flyback design meets the requirements of the dimming mode, and the primary inductance L_P is used to set the required frequency in the PFC mode. The PFC mode frequency distribution has a local value at the peak voltage of the line that generates EMI energy. The frequency peaks are symmetrically located at the phase corners 25°C and 155°C, and drop to the value near the zero crossing of the AC line. EMI performance can be obtained by setting the frequency in the PFC mode to be lower than the peak line voltage of 75 kHz.

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