How to perform offline LED driver circuit design
As a new type of energy-saving and environmentally friendly green light source products, LED has broad market prospects. At present, there are thousands of LED driver ICs in the market. Among them, we have a lot of single-chip circuit structures (Figure 1a).
According to the IC's data sheet, this type of IC is a high-efficiency LED drive control circuit that operates in PWM mode. With an external circuit, it can accommodate a wide input voltage range from 8V to 450V. The external power MOS transistor can be set by a fixed resistor (or capacitor) to reliably drive the LED string in a constant current manner. The LED current can be set by selecting the appropriate current limiting resistor. It also provides a linear dimming function that supports digital pulse (PWM) dimming with a low frequency variable duty cycle.
According to the application and according to different standards, the driving scheme can be divided into three types.
According to the PWM adjustment method, it can be divided into two types: constant frequency and constant off time (Fig. 1).
The voltage applied across the inductor in the steady state multiplied by the on-time is equal to the turn-off time. The inductor voltage is multiplied by the turn-off time: Von*Ton=Voff*Toff, ie (Vin-Vo)*Ton=Vo*Toff.
From the calculation formula of the inductance, the difference between the two can be seen. The inductance calculation in Fig. 1a is L = (Vin–Vo) * Ton / ΔI, and the inductance calculation in Fig. 1b is L=Vo*Toff. /ΔI.Io=Ip-ΔI /2. As shown in the current waveform diagram in Figure 2, after the inductance is determined, the input voltage Vin changes in Figure 2a causes the ripple current ΔI to change, so that the output current changes; the ripple current in Figure 2b ΔI is independent of the input supply voltage. Therefore, in a wide voltage application environment where voltage fluctuations are large, a constant off time circuit mode is used.
According to whether it is isolated, it can be divided into two categories: isolated and non-isolated.
The isolated and non-isolated drive modes are primarily for utility input. When using the non-isolated mode of Figure 3a, it is recommended to work in the current continuous mode; when using the isolation mode of Figure 3b, it is recommended that the transformer operate in a discontinuous mode (ie, at the end of each cycle, the transformer has no residual magnetism), which ensures Each time the switching cycle, the primary side of the transformer is delivered to the secondary side with the same energy (independent of the supply voltage).
The isolation mode of Figure 3b is a flyback circuit structure. When Q1 is turned on, the primary winding current increases, the secondary winding has no current, and the load continues to flow through C2. When Q1 is turned off, the secondary winding is turned on, and the energy stored in the transformer is released as a load through the secondary winding. Transformer transfer power P = 1/2 * Imax * Imax * L * Fosc. When using this isolation method, it must be noted that the secondary side is not in the loop control, there may be a large current and damage the LED. The protection circuit limit must be increased when using flow.
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