# Engineer Sharing: Offline LED Driver Circuit Design

As a new energy-saving and environmentally friendly green light source product, LED has broad market prospects. At this stage, there are thousands of LED driver ICs in the market. Among them, the one we encounter more is the single-chip circuit structure (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 the help of external circuits, it can adapt to a wide input voltage range from 8V to 450V. Through the external resistor (or capacitor), the fixed frequency can be set to control the external power MOS tube, and the LED string can be reliably driven by a constant current. The LED current can be set by choosing an appropriate current limiting resistor. At the same time, it provides linear dimming function and supports digital pulse (PWM) dimming function with low frequency variable duty cycle.

According to the application and according to different standards, its drive scheme can be divided into three types.

According to the PWM regulation method, it can be divided into two categories: constant frequency and constant off-time (Figure 1).

In a steady state, the voltage applied across the Inductor multiplied by the on-time is equal to the inductor voltage at the turn-off time multiplied by the off-time: Von*Ton=Voff*Toff, ie (Vin-Vo)*Ton=Vo*Toff.

The difference between the two can be seen from the inductance calculation formula. The inductance calculation formula in Figure 1a is L = (Vin–Vo) * Ton /ΔI , and the inductance calculation formula in Figure 1b is L=Vo*Toff /ΔI. Io=Ip-ΔI/2. Figure 2 shows the schematic diagram of the current waveform. After the inductance is determined, the change of the input voltage Vin in Figure 2a leads to the change of the ripple current ΔI, so that the output current changes; the ripple current ΔI in Figure 2b has nothing to do with the input power supply voltage. Therefore, in a wide-voltage application environment with large voltage fluctuations, a constant off-time circuit method is used.

According to whether or not to isolate, it can be divided into two categories: isolated and non-isolated.

The isolated and non-isolated driving methods are mainly for the mains AC input. When using the non-isolation 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 work in the discontinuous mode (that is, at the end of each cycle, the transformer has no remanence), which can ensure The energy transferred from the primary side of the transformer to the secondary side is the same (independent of the supply voltage) for each switching cycle.

Figure 3b is a flyback circuit structure for isolation. 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, and there may be a large current that may damage the LED. The protection circuit must be increased to limit the current when used.

According to the type of power supply, it can be divided into two categories: AC and DC.

When AC mains is used as input, only the power supply voltage needs to be shaped and filtered and then connected to the DC application circuit.

LED driver circuit design based on AX2028″ title=”AX2028″>AX2028

The requirements of LED products for their driving power include: stability, reliability, high efficiency, and universality. AX2028 is an LED driver constant current control chip, the system application voltage range is 12VDC to 600VDC, the duty cycle is 0~100%, and the fixed off-time working mode. It supports AC 85V ~ 265V input, non-isolated and isolated applications. AX2028 adopts unique technology for constant current control and compensation method, which makes the LED current change less than ±3% within the range of AC 85V ~ 265V. The AX2028 adopts an optimized system structure, which makes the system efficiency higher than 92%. Can be widely used in E14, E27, PAR30, PAR38, GU10 and other lamp cups and LED fluorescent lamps.

AX2028 has multiple LED protection functions, including LED open circuit protection, LED short circuit protection, and over temperature protection. When a system failure occurs, the power system enters the protection state, and the system re-enters the normal working mode until the failure is removed.

In the AX2028_TUBE_18W non-isolated application design (Figure 4), the LED light source array is designed as a 0.06W white LED (SMT or straw hat lamp) scheme with 24 series and 12 series and parallel, driving 288 low-power WLEDs with a total power of 18W.

The circuit adopts EMI suppression circuit, rectification filter circuit, valley filling PFC circuit, and AX2028 constant current system circuit to drive LED work.

The performance indicators of the AX2028_TUBE_12W simplified board application scheme are as follows:

Wide voltage AC 85～265v; output 12 series and 12 parallel

Voltage regulation rate: 1%

Power efficiency >88%

PFC＞0.92

THD＜45

Short circuit protection

MOS temperature rise＜20℃

Specifications:

Single Sided PCB Layout

190mm*16mm*11mm, suitable for T8, T10, fluorescent tubes

It is not easy to cause the device to be soldered, and the patches are arranged vertically

no noise

Adopt unique circuit structure, no flicker

Cost-effective

For non-isolated lamps, the design principle can be extended to the above-mentioned typical design ideas of LED fluorescent lamps. After selection, the arrangement of LED light sources can be changed into various forms of LED lamps. According to the different requirements of various LED lamps on the driving power supply, the output characteristics of the power supply can be changed to meet the different needs. AX2028 can be used to design the driving power of LED light source lamps such as isolated and non-isolated bulbs, PAR lamps, downlights, recessed lamps, wall washers, desk lamps, and thyristor dimming lamps.