An offline LED driver circuit design

As a new type of energy-saving, environmentally friendly green light source product, LED has a broad market prospect. At present, there are thousands of LED driver ICs on the market. Among them, the single-chip circuit structure is more common (Figure 1a).

According to the IC data sheet, this type of IC is a high-efficiency LED drive control circuit that works in PWM mode. With the help of external circuits, it can adapt to a wide input voltage range from 8V to 450V. Through external resistors (or capacitors), a fixed frequency can be set to control the external power MOS tube to reliably drive the LED string in a constant current manner. The LED current can be set by selecting an appropriate current limiting resistor. At the same time, it provides linear dimming function and supports low-frequency variable duty cycle digital pulse (PWM) dimming function.

According to the application and different standards, its driving 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 the stable state, the voltage applied across the inductor multiplied by the conduction time is equal to the inductor voltage at the turn-off time multiplied by the turn-off time: Von*Ton=Voff*Toff, that is, (Vin-Vo)*Ton=Vo*Toff.

The difference between the two can be seen from the calculation formula of inductance. The calculation formula of inductance in Figure 1a is L = (Vin–Vo) * Ton /ΔI, and the calculation formula of inductance in Figure 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 change of input voltage Vin in Figure 2a causes the ripple current ΔI to change, resulting in the change of output current; 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 it is isolated or not, it can be divided into two categories: isolation and non-isolation.

The isolated and non-isolated driving modes are mainly for AC input. When the non-isolated mode of Figure 3a is used, it is recommended to work in the current continuous mode; when the isolated mode of Figure 3b is used, it is recommended that the transformer work in the discontinuous mode (that is, at the end of each cycle, the transformer has no residual magnetism), so as to ensure that the energy transmitted from the primary side of the transformer to the secondary side is the same in each switching cycle (regardless of the power supply voltage).

The isolation method in Figure 3b is a flyback circuit structure. When Q1 is turned on, the current in the primary winding 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 to the load through the secondary winding. The 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 damages the LED. When using it, a protection circuit current limit must be added.

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

When AC mains is used as input, it is only necessary to shape and filter the power supply voltage before connecting it to the DC application circuit.

Design of LED drive circuit based on AX2028

LED products have requirements for their driver power supplies including: stable, reliable, efficient, and universal. AX2028 is an LED driver constant current control chip with a system application voltage range of 12VDC to 600VDC, a duty cycle of 0-100%, and a fixed off-time working mode. It supports AC 85V-265V input, non-isolated and isolated application solutions. AX2028 uses unique technology for constant current control and compensation methods, so that the LED current changes within the range of AC 85V-265V less than ±3%. AX2028 uses an optimized system structure, making the system efficiency higher than 92%. It 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 fault occurs, the power supply system enters the protection state until the fault is resolved and the system re-enters the normal working mode.

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

The circuit uses EMI suppression circuit, rectification and filtering circuit, valley filling PFC circuit and AX2028 constant current system circuit to drive the LED.

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

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

Current 220MA±1MA (output current adjustable)

Voltage regulation: 1%

Load regulation: 3%

Power efficiency＞88%

PFC＞0.92

THD＜45

Short circuit protection

Open circuit protection; no-load output voltage limit <48V (voltage limit adjustable)

MOS temperature rise <20℃

Specifications:

Single-Sided PCB Layout

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

It is not easy to cause device solder joints, and the patches are arranged vertically.

Noise-free

Unique circuit structure, no flicker

High cost performance

For non-isolated lamps, the design principle can be extended to the above typical LED fluorescent lamp application design ideas. By changing the arrangement of LED light sources, it can be transformed into various forms of LED lamps. In view of the different requirements of various LED lamps for driving power supplies, the output characteristics of the power supply can be changed to meet different needs. AX2028 can be used to design driving power supplies for isolated and non-isolated LED light source lamps such as bulb lamps, PAR lamps, downlights, recessed lamps, wall washers, table lamps, and thyristor dimming lamps.

Conclusion

The significant design improvements of AX2028 make its performance more perfect. Its fixed Toff working mode, high duty cycle, efficiency up to 92%, high constant current accuracy and other features make it more suitable for driving power supply applications of LED lighting fixtures.