Following the observations from earlier testing, particularly the failure of the bulk capacitor across the BLDC fan supply, the power stage was revisited and refined to improve reliability and robustness.

The initial approach of placing a large capacitor directly across the fan terminals was found to be unsuitable due to the internal driver electronics of the BLDC fan. This prompted a more structured approach to filtering and protection.

Several modifications were implemented as part of this refinement:

🔧 1. Series Inductor in the 24V Supply Path

A power inductor (220 µH) was introduced in series with the 24V supply line feeding the fan.

This serves to:

• Limit high-frequency current spikes
• Reduce switching noise propagation
• Improve overall supply stability

By placing the inductor in series rather than across the load, the filtering action becomes more controlled and avoids stressing the fan’s internal electronics.

⚡ 2. Bulk Capacitor Relocated to Boost Converter Output

The bulk capacitor (470 µF), previously placed across the fan terminals, was moved to the output of the boost converter.

This ensures:

• Proper smoothing at the source
• Reduced stress on downstream components
• Better support for transient load conditions

This placement aligns better with standard power supply design practices.

🛡️ 3. Addition of TVS Diode

A TVS diode was added across the boost converter output.

Purpose:

• Protect against voltage spikes
• Clamp transient overvoltages
• Improve robustness of the power stage

This is particularly useful in systems with inductive elements and switching loads.

🔄 4. Freewheeling Diode Across Fan Supply

A freewheeling diode was introduced across the BLDC fan supply.

Although BLDC fans include internal electronics, this diode provides:

• A path for transient currents
• Additional protection against inductive effects
• Improved reliability during switching events

⚙️ 5. Reduced Local Capacitance at Fan

A smaller capacitor (63 µF) was retained near the fan terminals.

This provides:

• Minimal local filtering
• Support for lower-speed operation (e.g., ~30% duty)
• Avoidance of large surge currents seen with higher capacitance

This value was found to be sufficient without interfering with the fan’s internal control circuitry.

🧠 Key Takeaways

This refinement highlights an important principle:

• Filtering should be applied at the source, not blindly at the load
• Loads with internal electronics require careful consideration
• More capacitance is not always better

Understanding the interaction between power electronics and integrated loads is essential for reliable system design.

🔄 Outcome

After implementing these changes:

• The fan operated reliably
• No further component failures were observed
• Overall system stability improved

💬 Closing Note

This iterative refinement of the power stage significantly improved system robustness. It also reinforced the importance of adapting standard design practices based on the nature of the load.

Would be interested to hear how others approach filtering and protection for BLDC-based systems.