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I'm designing an ESC for a bldc motor, I want to prevent inrush currents from da...
dtorrese

dtorrese

@copilot I'm designing an ESC for a bldc motor, I want to prevent inrush currents from damaging my mosfets, the maximum current consumption I Expect would be 20A, which NTC termistor do you recommend to put in series in the positive terminal of the circuit where I directly connect my battery?
2y

copilot

For an Electronic Speed Controller (ESC) for a BLDC motor designed to handle a maximum current consumption of 20A and aiming to negate inrush current impact on MOSFETs, an NTC thermistor is indeed a wise choice. NTC thermistors are used to limit inrush currents by presenting a higher resistance at lower temperatures (when the electronics are first powered up) and then reducing their resistance as they warm up, due to the current flow, to a point where their impact on the circuit’s performance is minimal.
Given your maximum current requirement, here are the considerations for selecting an appropriate NTC thermistor:
NTC Thermistor Selection Considerations
  • Steady State Current: The NTC should comfortably handle the steady-state current of 20A once it has warmed up. This ensures minimal power loss and voltage drop across the NTC in normal operation.
  • Initial Resistance: Higher initial resistance values offer better inrush current limiting but also need more time to "warm up" to nominal operating conditions. This is a balancing act between protection and efficiency.
  • Dimension and Thermal Capacity: The NTC must physically handle the dissipated heat during both the inrush and steady-state phases without degrading.
  • Voltage Rating: Ensure the NTC's voltage rating matches or exceeds the system voltage to prevent breakdown.
  • Recovery Time: Since ESCs for motors often stop and start, ensure the NTC can cool and thus recover its high resistance quickly enough for your application's demands.
Recommended NTC Thermistor Specs (Assumptions)
  • Steady State Current Rating: ≥25A (to provide a comfortable margin above 20A)
  • Initial Resistance: Approx. 5-10 Ohms (sufficient to limit inrush current but not excessively delay power-up; exact value may need adjustment based on your circuit's startup characteristics)
  • Thermal Capacity: Look for a component designed specifically for inrush current limiting purposes, which will have suitable thermal characteristics.
Example Component
While I cannot give specific model numbers due to database access limitations, an example specification to look for could be:
  • NTC Thermistor, Inrush Current Limiting
  • Steady State Current: 25A
  • Initial Resistance: 5 ohms
  • Size/Package: Suitable for your PCB layout and thermal considerations
Installation Note
  • Connect the NTC in series with the positive battery terminal to the rest of your circuit. Given the importance of efficient thermal design, ensure your layout allows for adequate heat dissipation from the NTC thermistor.
Reminder
Please verify the exact specifications by consulting the datasheets of the components you consider and adapt the values according to your specific application's startup characteristics and thermal management capabilities. Testing with your actual hardware setup is invaluable for confirming the appropriate selection.
2y
dtorrese

dtorrese

is 2 ohm inital resistance good enough?
2y

copilot

Using a 2-ohm initial resistance NTC thermistor for limiting the inrush current in your ESC system for a BLDC motor might be adequate, depending on a few key parameters of your application. Let's analyze this decision based on the scenario you described—designing for a system that expects a maximum current consumption of 20A.
Considerations with a 2-ohm NTC:
  • Inrush Current Limitation: A lower initial resistance (2 ohms in this case) means the limitation on the initial inrush current will be less aggressive than with a higher resistance value. This may be sufficient for your system if the inrush current isn't drastically higher than your steady current and your MOSFETs can tolerate this level without damage.
  • Voltage Drop and Power Dissipation: Once the system is operating normally, and the NTC has heated up and reduced its resistance, a 2-ohm starting value should not significantly impede performance. However, immediately upon powering up, the voltage drop across the NTC thermistor will be (V = IR = 20A \times 2\Omega = 40V), which is likely incorrect considering your battery voltage would be lower. The actual initial current will be limited by the NTC and other circuit impedances. Ensuring this voltage drop and resultant power dissipation ((P = I^2R)) is within acceptable limits for your application and for the thermistor's capabilities is critical.
Given a typical battery voltage used in ESC applications (which could vary widely but commonly ranges between 7.4V for small, portable systems up to 48V for larger systems), your actual initial inrush current and corresponding voltage drop across the NTC would be different and should be calculated based on your specific battery voltage and total startup impedance.
  • Thermal Response and Recovery: A 2-ohm thermistor should still provide a decent balance between limiting inrush current and reaching operational state quickly. However, it's crucial to ensure that the selected NTC can handle the thermal cycling in your specific application, especially if starts and stops are frequent.
Recommendation
A 2-ohm initial resistance NTC can be a good choice if:
  • Your MOSFETs and the rest of the system can deal with the initial inrush current that will flow given this resistance.
  • The NTC can dissipate the initial inrush power without exceeding its maximum temperature rating.
  • The voltage drop across the NTC at startup does not adversely affect the operation of your circuit.
Still, it's highly advised to:
  • Simulate your circuit with the specific NTC model included to estimate the initial inrush current and assess the voltage drop and power dissipation accurately.
  • Prototype and Test with the actual component in your circuit to verify its performance under real conditions, ensuring it meets your requirements without causing undue delay in operation or excessive heat.
In summary, while a 2-ohm NTC thermistor might be adequate based on your described needs, final confirmation should come from detailed analysis and testing within your specific application context.
2y
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