Improving Inrush Current Protection
IMPROVING INRUSH CURRENT PROTECTION BY: MEHDI SAMII, AMETHERM, INC.
PRESENTATION OVERVIEW Today were going to cover how to improve inrush current protection using NTC-based limiting and PTC-based limiting. This presentation will focus on the following topics:What is Inrush Current?Managing Inrush Current NTC-based Limiting PTC-based Limiting NTC vs. PTC Comparison
WHAT IS INRUSH CURRENT?
a spike in current that occurs when equipment is powered on.Inrush current:
MANAGING INRUSH CURRENT Inrush current can be properly managed through either NTC or PTC limiting. If inrush current is allowed to pass through the system, it can reduce the effective operating life or even damage equipment.
An uncontrolled flow of inrush current can damage the diode bridge and link capacitor, disabling the conversion of AC to DC current. This can lead to system failure.
MANAGING INRUSH CURRENT
Limiting inrush current becomes further complicated for systems that switch on and off quickly. This is because the limiter must reset instantaneously to protect the system.Later, we will cover how to address this using a PTC thermistor.
WHAT IS NTC - BASED LIMITING? NTC stands for negative temperature coefficient. The NTC thermistor provides variable resistance based on temperature. As temperature increases, the resistance drops from high to low and allows current to pass through.
WHAT IS NTC - BASED LIMITING? NTC thermistors are the most commonly used thermistor. They fit a wide range of applications including: automotive, military, industrial, and emissions controls.
WHAT IS NTC - BASED LIMITING? Various every-day items contain NTC thermistors:
Did You Know?
HVAC motorsAudio amplifiersMRI machinesX-ray machines for airport securityMotor drives (found in treadmills)PC power supply
HOW DOES NTC - BASED LIMITING WORK?
To limit inrush current, an NTC thermistor is placed between the power supply and system as shown in the figure to the left. Upon power on, the NTC thermistor provides high resistance to limit inrush current. As the inrush current drops, the NTC thermistor self-heats and its resistance drops to a low enough value to pass current through.
NTC - BASED LIMITING EXPLAINED Consider a system with 10 A continuous current, an inrush current of 100 A and an input voltage of 240 VAC. Upon power on, an NTC MS32 10015 thermistor has an initial resistance of 10 ohms. Now, consider the worst case scenario of turning on the system at peak voltage. This will result in a peak voltage value of 340 volts. Instead of passing 100 A, the NTC MS32 10015 only allows 34 A to pass through. As the NTC MS32 10015 self-heats, its resistance drops and the current lowers until the inrush current stops completely. The NTC MS32 10015 still continues to heat, dropping resistance to as low as 0.07 ohm, where it then reaches a steady state and passes current through with a minimum loss in efficiency.
NTC - BASED LIMITING ADVANTAGES Occupies only half the board space compared to a fixed resistor.Very simple selection criteria to design in the circuit. Requires no bypass circuit because resistance drops as it self-heats.Costs less compared to a limiting fixed resistor.
NTC-based limiting is the most cost effective way to limit inrush current. Did You Know?
WHAT IS PTC - BASED LIMITING? PTC stands for positive temperature coefficient. The PTC thermistor also provides variable resistance based on temperature. As temperature rises, resistance increases from low to high and blocks inrush current. Typically, NTC-based limiting is used for most applications. However, there are certain scenarios where PTC thermistors are the optimal choice.
WHEN SHOULD PTC BASED LIMITING BE USED?A PTC thermistor should be used when: Ambient temperature is greater than 65C.Ambient temperature is less than 0C.Reset time needs to be near-zero second.To prevent a short circuit.
HOW DOES PTC - BASED LIMITING WORK?A PTC-based limiting circuit requires a bypass circuit to send current back through the PTC thermistor to protect the system against shorts. By setting the bypass to 3X or 4X the amount of time it takes for the inrush current to settle, response time for the PTC-based limiter is extremely fast.
AC INLOADActive CircuitPTCTIMERCOILDC SOURCE 5V, 24VDIODE BRIDGEFILTER CAPFILTER CAP300VDC240V120V
PTC - BASED LIMITING ANALYSIS Resistance for an NTC MS32 10015 thermistor drops as it self-heats, while resistance increases for a PTC MCL20 500100 thermistor. At a specific threshold of 120 C for the PTC MCL20 500100, resistance increases sharply, enabling the PTC MCL20 500100 to respond quickly to inrush current. Also note how the PTC MCL20 500100 has a flat response at low temperatures, making it effective across the entire temperature spectrum.
Resistance 11010010,0001,000100,000Temperature C-40C20C80C120C Tc180C40C
PTC THERMISTOR TRADEOFFS
However, the increased responsiveness and advanced protection outweigh these tradeoffs.
Costs 1.5x more than an NTC thermistor.
Requires an active circuit to bypass PTC thermistor.
SUMMARY NTC Inrush Current Limiter Commonly used in wide variety of equipment and applications. At high temperatures, its resistance is low.
PTC Inrush Current LimiterUsed in specific scenarios: extreme temperature conditions, near-zero reset time, and short circuit. At high temperatures, its resistance is high.
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