Learn how to limit inrush current using NTC and PTC thermistors in this presentation by Ametherm. Read the full article here: http://www.ametherm.com/inrush-current/ptc-thermistors-for-inrush-current-limiting

1. IMPROVING INRUSH
CURRENT PROTECTION
BY: MEHDI SAMII, AMETHERM, INC.

2. PRESENTATION OVERVIEW
 Today we’re 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

3. WHAT IS INRUSH CURRENT?
a spike in current that occurs
when equipment is powered on.
Inrush current:

4. 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.

5. 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.
Welding equipment

6. 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.

7. 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.

8. WHAT IS NTC - BASED LIMITING?
 Various every-day items contain NTC thermistors:
Did You Know?
HVAC motors
Audio amplifiers
MRI machines
X-ray machines for airport security
Motor drives (found in treadmills)
PC power supply

9. 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.

10. 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.

11. 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?

12. 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.

13. WHEN SHOULD PTC – BASED LIMITING BE USED?
 A PTC thermistor should be used when:
 Ambient temperature is greater than 65°C.
 Ambient temperature is less than 0°C.
 Reset time needs to be near-zero second.
 To prevent a short circuit.

14. 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 IN
LOAD
Active Circuit
PTC
TIMER
COIL
DC
SOURCE
5V, 24V
DIODE
BRIDGE
FILTER
CAP
FILTER
CAP
300VDC
240V
120V

15. 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 Ω
1
10
100
10,000
1,000
100,000
Temperature ⁰C
-40⁰C 20⁰C 80⁰C 120⁰C
Tc
180⁰C40⁰C
NTC
PTC
Roo
m Temp.

16. 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.

17. SUMMARY
 NTC Inrush Current Limiter
 Commonly used in wide variety of equipment and applications.
 At high temperatures, its resistance is low.
 PTC Inrush Current Limiter
 Used in specific scenarios: extreme temperature conditions, near-
zero reset time, and short circuit.
 At high temperatures, its resistance is high.

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