What Is Wireless Charger Over Temperature Protection & How Does It Works ?

What Is Wireless Charger Over Temperature Protection ?

Over Temperature Protection (OTP) or say Over Temperature Control is one of the major safety measurement of Wireless Charger system.

As it indicates, OTP will protect the system by automatically shut down Wireless Charging or reduce the charging output to some lower degree once it detect system temperature is over certain standard.

Over Temperature Protection

It’s important to have this safety measure to avoid Wireless Charger over heating, which may result in melt of the plastic housing,or in sever case, may cause fire.

So OTP is so important, how does it really works ?

How Does Wrieless Charger Over Temperature Protection Works ?

The key functional component of Over Temperature Control system is called NTC, short for Negative Temperature Coefficient.

NTC refers to NTC thermistor. It’s type of resistor with changeable resistance based on different temperature.

In over-temperature protection applications, the NTC thermistor acts as a safety device, shutting down a system or a device if the temperature exceeds a certain limit.

Wireless Charger Structure

So once NTC switch on working, OTP will be in position, protect wireless charging from over heating. Either wireless charger will stop working to cool down, or it will reduce power output to a lower speed.

Once wireless charger cool down, OTP will switch off, and wireless charger will resume working when it detects there are still Qi device on top of the transmitter coil.

All in one, let’s say OTP is similar to a switch, it turns on or turns off based on temperature changes.

Two Kinds Of NTC Within Wireless Charger: Chip Fixed NTC Resistor VS Soldering NTC Resistor

There are two kind of NTC thermistor: Chip Fixed NTC Resistor & Soldering NTC Resistor

By its name, you can see, Chip Fixed Resistor is like a Chip Set which is pasted by SMT ( Surface Mounting Technology ) Machine on the PCB board. Below is a typical Chip NTC Resistor.

Wireless Charger NTC Resistor

SMT is some kind of high speed machine which can paste each component on the designed circuit board position with very high efficiency;

While Soldering Resistor is kind of Resistor with long tail, which can be soldered by human hands to the PCBA.

NTC With Sensor

Thanks to the long tail of the NTC, it can easily project out to some lengthy away position, to detect the temperature of some specific position, not only nearby.

As for wireless charger, this specific position is normally on top of Transmitter Coil.

Charging Coil With NTC Sensor

During wireless charging progress, PCBA – the major circuit board will have lots of heat arised, in the meanwhile, transmitter coil will also getting hot, during magnetic transfer.

As you may know, wireless charging have some roughly 70% transmit efficiency.

It means 70% of the power can be received by Qi smart phone, but 30% will be wasted, through a way transforming into engergy, which causes charging coil getting hot gradually.

When we put smart phone on the wireless charger pad, the close part to the smart phone is transmitter coil. So it’s very helpful to have NTC sensor positioned directly in the middle of charging coil.

PCBA Wth Soldering NTC

Once the NTC sensor detect high temperature on top of charging coil, it will simply shut down the sytem, make wireless charger and smart phone stay safe.

The Chip Fixed NTC sensor, on the other hand, will only detect the temperature of nearby position, because it is pasted on the PCBA board. So only wireless charging PCBA board getting hot, NTC will be working. Shut down the sytem or reduce working rate.

Wireless Charger With NTC

By comparision, Soldering NTC Sensor are more practical bacause it detect charging coil directly, it provide most safety to the smart phone. But because it needs human labor for soldering, there are extra cost, compared with Fixed Chip NTC.

Fixed Chip NTC is functional as well, it hides from extra labor cost while it still works great for PCBA temperature detection.

Does Every Wireless Charger Have NTC For Protection ?

The answer is NO ! Due to budget control, some of the cheap wireless charger PCBA may not have NTC included. There are variety of solution suppliers, not all of them will use NTC on the charging PCBA. But as safety becomes more and more concerned, NTC will be the basic component for wireless charger system.

Why Some Wireless Charger Does Not Have NTC ? Is It Dangerous To Use ?

As talked above, some wireless chargers do not have NTC. So why this happens ?

One reason is to control the budget, the other reason is, from Smart Phone side, there are interior Power Management System. For example for iPhone, the smart phone will automatically slow down the charging speed once it detect iPhone body growing hot, over 38 degree celcus usually.

There are similar management within Sumsung smart phones. It automatically slow down or even close the charging process if smart phone detects over temperature.

So even wireless charger does not have NTC in the system, its output power will still be limited because smart phone will manage the input power on its own side. This is also the reason why some suppliers take the option to use PCBA without NTC.


NTC is cheap component, either by Chip Fixed NTC or Soldering NTC, it both works fine with needed purpose.

As the cost is really not much, we highly recommend you to ask your Wireless Charger Manufacturer to include this key function with your wireless charger products.

Still have any query ? Or need any assistance for your OEM / ODM wireless charger manufacture ? Pls feel free contact us, TopWirelessCharger are here to help !

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