Five Principles of LED Design
Release date: 2017-11-01
Source: Foreign
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Chip Heating
This primarily concerns high-voltage driver chips with built-in power modulators. If the chip consumes 2 mA, and a voltage of 300 V is applied, the power dissipation becomes 0.6 W, leading to heat generation. The maximum current in the driving chip comes from the power MOSFET’s consumption. A simplified formula is I = CVF (considering charging resistance, the actual formula is I = 2CVF, where C is the CGS capacitance of the MOSFET, and V is the gate voltage during conduction). To reduce chip power consumption, it's crucial to minimize C, V, and F. If these cannot be changed, consider transferring the power consumption to off-chip components without introducing additional losses. Alternatively, improve heat dissipation for a more straightforward solution.
Power Transistor Heating
Power transistor heating is often split into switching loss and conduction loss. In most cases, especially in LED AC drive applications, switching loss dominates. Switching loss depends on the CGD and CGS capacitance of the transistor, as well as the driver’s capability and operating frequency. To mitigate this, choose a MOSFET based on its overall performance, not just low on-resistance. Smaller internal resistance means higher capacitance, which increases switching losses. For example, 1N60 has around 250 pF CGS, while 5N60 has about 1200 pF. It’s better to select a transistor that meets the requirements without over-specifying. Also, reducing frequency can help, but it may increase peak current or require larger inductors, possibly causing saturation. Consider switching from Continuous Conduction Mode (CCM) to Discontinuous Conduction Mode (DCM) if necessary.
Operating Frequency Drop
Frequency drop is common during user debugging. It usually stems from two factors: a low input-to-output voltage ratio and system interference. To avoid setting the load voltage too high, which affects efficiency, ensure the ratio is balanced. For interference issues, try adjusting the minimum current setting, cleaning the wiring (especially the sensing path), selecting an appropriate inductor, or adding an RC low-pass filter. Although filters may have some inconsistency, they are generally sufficient for lighting applications. Regardless of how bad the frequency issue is, it must be resolved as it negatively impacts performance.
Inductor or Transformer Selection
Inductors and transformers play a critical role in LED circuits. When using different inductors, the same drive circuit might produce varying results. Some engineers overlook this and adjust the sense resistor or frequency instead, which could harm the LED’s lifespan. Before design, ensure proper calculations. If theoretical and practical values differ significantly, check for frequency reduction or transformer saturation. Saturation reduces inductance, increasing peak current and potentially damaging the LED. Even with stable average current, brightness fluctuations may occur.
LED Current Level
Excessive LED ripple can shorten its life. While many experts suggest a 30% tolerance, there is no verified standard. It’s recommended to keep ripple as low as possible. If cooling is inadequate, derating the LED is necessary. More research is needed to establish clear guidelines for LED manufacturers.
Common Issues in LED Driver Power Supplies
With the promotion of energy-saving LED technology, the quality of the power supply directly affects LED lifespan. This article explores challenges in LED driver design and offers insights for engineers.
1. Drive Circuit Affects LED Life
LED drivers include digital and analog types. Digital drivers use digital circuits for dimming and color control, while analog drivers rely on AC/DC constant current circuits. These components have limited lifespans, and any failure can cause the entire system to fail. LEDs typically last 50,000–100,000 hours, but power supplies rarely match this. Most have a 2–3-year warranty, while military-grade models cost 4–6 times more. Therefore, most LED failures come from the driver circuit.
2. Heat Dissipation Issues
LEDs are cold light sources, so junction temperature must stay within limits. The luminaire must balance aesthetics, installation, light distribution, and heat dissipation. Many manufacturers outsource power supplies, leading to poor heat management. Proper thermal design is essential to ensure both LED and power supply longevity.
3. Power Supply Design Challenges
a. Power Design: Despite high efficiency, LEDs still generate 80–85% heat, raising internal temperatures by 20–30°C. Power supplies must have a 1.5–2x margin to handle this. b. Component Selection: Components like electrolytic capacitors and wires must withstand high temperatures. c. Electrical Function Design: Constant current regulation is critical. LED forward voltage variations require the power supply to accommodate a wide range. d. PCB Layout: Safety spacing, insulation, and heat distribution are vital. e. Certification: No specific standards exist in China, so compliance with CE or UL is necessary.
4. Usage Parameters
When choosing a power supply, select a constant current value matching the LED standard. Voltage should be moderate to avoid unnecessary losses.
Three Protection Methods for LED Drive Circuits
1. Fuse Protection
Fuses are not ideal for finished LED lamps due to slow response and difficulty in automatic reset. They are also bulky and costly.
2. TVS Diode Protection
TVS diodes respond quickly to voltage spikes and protect sensitive components. However, they cannot detect overcurrent and are hard to find with precise voltage ratings.
3. Self-Recovery Fuse (PTC)
PTC fuses automatically reset after overcurrent events. They are compact and reliable. You can use them for branch or full-lamp protection, depending on your needs.
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