How to Design Over Temperature Protection Circuit

Over temperature protection circuit design is not complicated as you may think. It could be done by using a thermistor and other discrete devices. If you have question on how to design over temperature protection circuit or planning to make one, this article is for you, so keep reading.

Key Definitions Used on How to Design Over Temperature Protection Circuit

I included definitions so that you could understand the terminology used in this article.

1. Over Temperature – it a scenario wherein the temperature of a system or devices is more than its recommended range. It is destructive and needs to be prevented.

2. Over Temperature Protection – from the term itself, it is providing protection to a system or devices against over temperature.

3. Over Temperature Protection Circuit – it is a circuit which primary purpose is to protect any system or devices against over temperature.

4. Over Temperature Protection Circuit Design – It refers to the design of a specific circuit to protect any system or device against over temperature.

5. OTP – abbreviation for over temperature

6. NTC Thermistor – this means negative temperature coefficient thermistor. As the temperature increases, the resistance decreases. This is popularly used thermistor for OTP circuit.

Generic Procedure on How to Design Over Temperature Protection Circuit

There are several ways on how to do the task. It will vary from technician to technician, engineer to engineer or person to person. Below, however, is a generic guide that I followed and teach to somebody that asks me.

1. Identify the hotspot area where you want to put the sensor/s. It is important that you are able to identify these hot spot locations so that the protection will serve its purpose.

2. Set the OTP target trip point. Put in my mind that when this trip point is reached, there must be no devices that will get damage.

3. Select the type of sensor you want to use. For boards, the economical solution is to use a NTC thermistor. Through-hole or surface mount could do (whichever preferable by the application). For system level, assembly type NTC could be used also.

4. Select the associated parts. Like selecting the comparator, the biasing resistors or you may add delay and bypass capacitors, or you may add a hysteresis. A hysteresis is a circuit that prevents two states to continuously keep changing. 5. Perform calculations and simulations. The only way to ensure the design will work before making an actual sample is to do calculations and simulations. There are freeware circuit simulators that you can do like LTSpice. If you want to learn, read LTSpice Simulation Tutorial.

6. Build actual circuit and perform testing. Building actual circuit could be expensive especially with multiple re-spins. To minimize the iterations, it is better to do your best in the calculations and simulations. In my case, I always perform detailed analysis and there is no more issue during actual circuit testing.

7. Optimize the design. Once you have the actual sample and can do the test, you can optimize the circuit values.

8. Implement the design for production or roll out.

Over Temperature Protection Circuit Design Sample

To better demonstrate and explain to you, let me give a sample on how to design over temperature protection circuit.

Steps #1 and #2

Let us assume that I already identified the hotspot of my system (step #1). My desired OTP trip point is 80’C (step #2).

Steps #3 to #5

I want to use a through-hole NTC thermistor as it is convenient for my application. I will also use a comparator with reference and biasing resistors and hysteresis circuits. I will also add a bypass (de-coupling capacitor). Then I will be showing my calculations and simulations as well. Below is the circuit that I came up with.

Circuit Operation

When the OTP set point is not reached, the positive input of the comparator (X1) is higher than the negative input. With this, the VOUT node will see the level of VCC due to the presence of the pull up resistor R2. By the way, X1 is an open drain type comparator. This means that its output pin will float when the positive input is higher than the negative input. This is the reason I added a pull-up resistor of 1k to VCC.

The thermistor used here is NTC. When the temperature in the hotspot location reaches the trip point, the RNTC value will be small such that the voltage drops across it is lower than the reference voltage (REF). This time, the VOUT will be zero. You can feed the VOUT to a circuit which could control the system turn-on and off. Basically, when VOUT is high, the system will not shut down and it will shut down when VOUT is low.

The purpose of D1 and R1 is to provide hysteresis. How this hysteresis works? When the output of X1 turns low, the R1 will form a parallel connection with RNTC. It is not a direct parallel due to the presence of D1 but the effect of D1 is not so significant that can be neglected during the analysis. The output will only turns low when the positive input of X1 is lower than the negative input. This will happen when the OTP circuit is working (there is an over temperature event).

With RNTC parallel with R1, the equivalent voltage will become much lower than the reference voltage. Because of this, there is a time delay before the system could turn-on again. This will prevent the scenario of rapid changing between turn on and turn off states.

Simulation Run

Let us examine how the circuit response looks like when run at simulation. In the simulation, I vary the resistance of the thermistor RNTC from 500 ohms to 10k. The x-axis shows the resistance while y-axis shows the voltage levels of VOUT, Votp and Vref. Based from the simulation result, the VOUT will change state from low to high when the value of RTNC is around 1.75k. This is the point where the voltage across RNTC exceeds the reference.