Design of 10W non-isolated LED driver

summary

    This article introduces a 10W LED driver designed using TI's TPS92210 control chip. TPS92210 has a unique critical mode fixed peak current control function, and the design does not require feedback, making the entire design simple, with fewer components and low cost.

introduction

    Recently, in the LED driver power supply market, non-isolated solutions have become a hot application due to their advantages such as smaller magnetic components, higher energy efficiency, fewer components, lower total bill of materials cost, and the ability to meet safety regulations with mechanical design. This article introduces a 10W LED driver power supply designed using TI's control chip TPS92210. Using the TPS92210's unique critical mode fixed peak current control function, the design does not require feedback, so the entire design is simple, with fewer components, low cost, and high efficiency.

1. Power supply solution introduction

    This solution uses a critical Buck circuit controlled by TPS92210 and sets the TPS92210 to work at a fixed peak current to keep the inductor current peak value fixed. Because the circuit works in critical mode, the average value of the inductor current is equal to half of the peak current, thereby achieving the purpose of outputting a constant current.

    At the same time, this solution does not require additional circuits, and the TPS92210 itself can implement output overcurrent, short circuit, open circuit and other protections. Therefore, the entire solution has fewer components and lower costs.

    The input adopts a valley-fill circuit, which makes the PF value of the whole machine always above 0.7.

    C8, L2, and C5 form a pi-type filter. The whole machine passed the conduction test.

Figure 1: 10W LED power solution

Figure 2: 10W LED power supply picture

1.1 TPS92210 critical mode setting

    The TPS92210 needs to meet three conditions to start a new cycle:

    1) The time since the last opening needs to be greater than the time controlled by the Ifb current.

    2) The time since the last opening needs to be greater than the time limited by the chip's maximum frequency of 7.5us.

    3) The Tze pin must have a zero point crossing from high to low.

    Since the above three conditions need to be met, the design connects the FB pin to Vdd through a resistor to set a fixed DC bias, so that the turn-on of TPS92210 is completely determined by the voltage zero-crossing of the Tze pin, which ensures that the converter always works in the critical current mode.

1.2 Inductor Design

    Calculate the inductance according to the input and output requirements. In this solution, the input is 176V~264Vac, the output is 40V, 0.25A. Since the input uses a valley-fill circuit, the input voltage range can be calculated as follows:

    Based on the minimum and maximum input voltages calculated above, the minimum and maximum duty cycles can be calculated:

    The average output current is 0.25A, and the current works in the critical current mode. The average current on the inductor is the output current. The peak current and effective current of the inductor can be calculated as follows:

    Because the critical mode converter, the higher the input voltage, the higher the operating frequency. Considering the volume and efficiency, the maximum operating frequency is set to 100KHz.

    Then the inductance can be calculated as follows:

    So the inductance is about 700uH.

    Based on the calculated inductance, it can be verified that the minimum switching frequency is:

    Based on the calculated maximum duty cycle and minimum switching frequency, the maximum on-time can be obtained as:

    Tonmax is less than the maximum on-time of 5us allowed by TPS92210, so there is no problem with the inductor design.

    Select the core: Assuming Bmax=2500G, filling factor: k=0.4, current density: j=6A/mm^2, the AP value required for the core can be calculated as:

    According to the AP value, RM5 is selected as the inductor core:

    The Ae area of ​​RM5 is as follows, and the number of turns required for the inductor can be calculated:

    The inductor needs to have approximately 67 turns.

    According to the previously calculated current RMS value and the set current density j, AWG30 is selected to wind the inductor.

    Select 1 strand of AWG30 as the winding.

2. Test results

    Based on the above analysis and design, a prototype was made and its performance was verified. The experimental results are as follows.

2.1 Efficiency Test

2.2 PF value

2.3 Current Accuracy

2.4 Startup

2.5 Output ripple

2.6 Short Circuit Protection

2.7 Open circuit protection

2.8 EMC Testing

3. Conclusion

    This paper analyzes and designs a critical mode buck converter controlled by TPS92210. The feasibility and setting method of TPS92210 critical mode are analyzed. The calculation and design method of inductance are introduced in detail. Finally, a prototype is made to verify the correctness of the analysis and calculation. The feasibility of TPS92210 for non-isolated constant current LED driver power supply is guaranteed.