Must know the LED drive design 5 key points
1. Chip Heating
This issue typically occurs with high-voltage driver chips that include internal power modulators. For example, if a chip consumes 2 mA at 300 V, the power dissipation is 0.6 W, which naturally causes the chip to heat up. The main source of current in these chips comes from the power MOSFETs used for driving. A basic formula for calculating the current is I = CVF, but in practice, due to the charging resistance, the actual current is usually around I = 2CVF, where C represents the gate-source capacitance (Cgs) of the MOSFET, V is the gate voltage, and F is the operating frequency. To reduce the chip's power consumption, you should aim to minimize C, V, or F. If those parameters can’t be changed, consider off-chip solutions that distribute the power load without adding extra power consumption. Another simple approach is to improve the cooling system.
2. Power MOSFET Heating
This is a common problem discussed on various power electronics forums. The power loss in a power MOSFET is divided into two parts: switching loss and conduction loss. In most cases, especially in LED power supply applications, switching loss dominates over conduction loss. Switching loss is influenced by the gate-drain (Cgd) and gate-source (Cgs) capacitances of the MOSFET, as well as the driver capability and switching frequency of the chip. To address this heating issue, several strategies can be considered:
- Don’t choose MOSFETs based solely on low on-resistance, as smaller on-resistance often means larger Cgs and Cgd values. For instance, the Cgs of a 1N60 is about 250 pF, while that of a 5N60 can reach up to 1200 pF. This difference can significantly impact performance.
- Optimize the operating frequency and driver strength. Higher frequencies increase both conduction and switching losses. If the MOSFET is overheating, consider reducing the frequency, but be cautious—lowering the frequency may require increasing the peak current or using a larger inductor, which could lead to inductor saturation.
- If saturation is a concern, switching from Continuous Conduction Mode (CCM) to Discontinuous Conduction Mode (DCM) might help, though it may require an additional output capacitor.
3. Working Frequency Dropping
This is a common issue during debugging. The frequency drop is usually caused by two factors: a low input-to-output voltage ratio and excessive system interference. To avoid this, don’t set the output voltage too high, even though higher voltage can improve efficiency. For interference issues, try the following:
- Adjust the minimum current threshold.
- Ensure clean wiring, especially for the feedback path.
- Choose an inductor with a small current rating or one with a closed magnetic circuit.
- Add an RC low-pass filter, though its consistency may not be perfect, it’s generally acceptable for lighting applications.
Regardless of how bad the frequency instability is, it’s always a negative factor, so it must be resolved.
4. Inductor vs Transformer Selection
This is a critical point in many designs. Some users report that the same driver works fine with one inductor but not another. This could be due to differences in inductance, core material, or saturation. If the current waveform looks different, don't just adjust the sense resistor or frequency—this can affect the LED’s lifespan. Before finalizing the design, perform accurate calculations. If the theoretical and measured results differ significantly, check for frequency reduction or transformer saturation. When a transformer saturates, inductance decreases, causing a sharp rise in peak current, which can damage the LED, even if the average current remains the same.
5. LED Current Control
Everyone knows that excessive ripple in the LED current can shorten its lifespan. However, there’s no clear consensus on how much ripple is acceptable. Some LED manufacturers suggest up to 30%, but this hasn’t been fully validated. It’s best to keep the ripple as low as possible. If thermal management is poor, derating the LED is necessary. More precise guidelines from experts would help promote better LED usage and reliability.
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