LED drive circuit design experience

Photocoupler

1. Power Tube Heating

Many users have discussed this issue on power electronics forums. The power consumption of a power tube is typically divided into switching loss and conduction loss. In most cases, especially in LED driver applications, the switching loss is much more significant than conduction loss. Switching loss depends on the gate-to-source (Cgs) and gate-to-drain (Cgd) capacitance of the power transistor, as well as the driving capability and operating frequency of the chip. To reduce the heat generated by the power tube, you can consider the following solutions:

• A. Selecting the MOSFET properly: Don’t just choose based on low on-resistance. A smaller Rds(on) usually means higher Cgs and Cgd values. For example, the Cgs of 1N60 is about 250pF, 2N60 is around 350pF, and 5N60 reaches up to 1200pF. This difference can significantly impact performance.
• B. Frequency and drive capability: Higher frequency increases both conduction and switching losses. If the power tube gets hot, check if the operating frequency is too high. Lowering the frequency may help, but be aware that it might require a larger inductor or increased peak current, which could cause inductor saturation. If the inductor has sufficient saturation current, consider switching from Continuous Conduction Mode (CCM) to Discontinuous Conduction Mode (DCM), which may require an additional output capacitor.

2. Chip Heating

This issue often occurs with high-voltage driver chips that include built-in power modulators. For instance, if the chip consumes 2mA at 300V, it results in 0.6W of power dissipation, which leads to heating. The main current draw for the driver chip comes from the power MOSFET’s gate charging. The basic formula is I = CVF, but considering the resistance effect, the actual current is approximately I = 2CVF, where C is the Cgs of the MOSFET, V is the gate voltage, and F is the switching frequency. To reduce chip power consumption, try to minimize C, V, and F. If these parameters can't be changed, consider off-chip power dissipation or better thermal management to prevent overheating.

3. Working Frequency Drop

This is a common problem during debugging. Frequency drop can occur due to two main reasons: a low input-to-output voltage ratio or excessive system interference. For the first case, avoid setting the load voltage too high, even though higher voltage may improve efficiency. For the second, consider the following steps:

• a. Adjust the minimum current threshold.
• b. Clean the wiring, especially the feedback path.
• c. Choose a smaller inductor or one with a closed magnetic circuit.
• d. Add an RC low-pass filter—though its consistency isn't perfect, it's still useful for lighting applications.

No matter how bad the frequency is, it always brings negative effects. It must be addressed promptly.

4. Inductor or Transformer Selection

Many users report that the same driver circuit works fine with one inductor but causes issues with another. Check the inductor current waveform. Some engineers overlook this and adjust the sense resistor or frequency instead, which can damage the LED over time. Before design, proper calculation is essential. If theoretical and practical results differ significantly, check for frequency reduction or transformer saturation. When a transformer saturates, inductance decreases, causing a sharp rise in peak current and potentially damaging the LED. Even with constant average current, the light quality will suffer.

5. LED Current Level

Everyone knows that excessive LED ripple can shorten its lifespan. However, no expert has clearly defined how much ripple is acceptable. I once asked an LED manufacturer, and they said 30% was acceptable, but this hasn't been thoroughly verified. It's recommended to keep the ripple as small as possible. If cooling is insufficient, derating the LED may be necessary. I hope some experts can provide clear guidelines to support the widespread adoption of LEDs.

In conclusion, designing an LED driver isn’t overly complex. As long as you perform calculations before testing, measure during the process, and do aging tests afterward, anyone can successfully design an LED driver. It just requires attention to detail and a systematic approach.