Designing an LED driver power supply isn't difficult, but it's essential to have a clear understanding of its principles. With proper calculations before debugging, measurements during debugging, and post-debugging aging, anyone can successfully design an LED driver.
1. LED Current Magnitude:
Everyone knows that excessive LED ripple will affect LED lifespan, but the extent of the impact is unclear.
2. Chip Heat Generation:
This mainly concerns high-voltage driver chips with built-in power modulators. Reducing chip power consumption, avoiding introducing additional power, and ensuring proper heat dissipation are crucial.
3. Power Transistor Heat Generation:
This issue has been discussed in forums. Power transistor power consumption consists of two parts: switching losses and conduction losses. In LED AC drive applications, switching losses far outweigh conduction losses. Switching losses are related to the power transistor's CGD and CGS, as well as the chip's driving capability and operating frequency. Therefore, addressing power transistor heat generation can be achieved through the following:
A. Don't solely select MOSFETs based on their on-resistance, as lower internal resistance leads to larger CGS and CGD capacitances.
B. The remaining factors are frequency and chip driving capability; here, we'll only discuss the impact of frequency. Frequency is directly proportional to conduction losses, so when a power transistor overheats, the first thing to consider is whether the chosen frequency is too high. When the frequency decreases, to achieve the same load capacity, the peak current must increase, or the inductance must also increase, which can cause the inductor to enter the saturation region. If the inductor saturation current is large enough, consider changing CCM (Continuous Current Mode) to DCM (Discontinuous Current Mode), which requires adding a load capacitor.
4. Frequency Reduction Frequency reduction is mainly caused by two factors
A small input voltage to load voltage ratio and high system interference. For the former, avoid setting the load voltage too high, although a higher load voltage will result in higher efficiency. For the latter, try the following: a) Set the minimum current to a lower value; b) Clean the wiring, especially the critical path, Sense; c) Choose a smaller inductor or use an inductor with a closed magnetic circuit; d) Add an RC low-pass filter. This has some negative effects, as the consistency of capacitance (C) is not good, and the deviation is relatively large, but it should be sufficient for lighting.
5. Inductor or Transformer Selection
Given that high-power LEDs operate at only 3V, converting 220V AC to DC via a full-bridge rectifier results in a voltage drop of approximately 1.8V across the bridge. This means that the energy efficiency for driving a single LED is only 60%. At least three LEDs must be connected in series to achieve an overall energy efficiency exceeding 80%.
Based on the principle of combining three primary colors to create white light, connecting three 1W high-power LEDs (red, green, and blue) in series can achieve the brightness equivalent to a 3W white LED. Furthermore, it allows for the creation of six different colored lights, satisfying people's preference for changing colors.
