Technical news

Overheat Detection at Multiple Locations with POSISTOR®

POSISTOR®, developed by Murata in 1959, is the world's first ceramic positive temperature coefficient (PTC ) thermistor. Its resistance rapidly increases to over 1,000 times at a specified temperature.

This unique characteristic enables a simple circuit to detect overheating of multiple power devices located at various locations, such as distributed power supplies, and helps avoid hazards, such as equipment enclosures melting, smoking and igniting.

1.Overheating risks increase as distributed power supplies become popular

Requirements for circuits supplying power to LSIs become more stringent as LSIs become larger-scaled and more multifunctional.

Such requirements include accommodation of low voltage and high current, high responsiveness for rapid load fluctuation, and high efficiency for low power consumption. Thus, distributed power supplies such as POL (Point-Of-Load: a high efficiency DC-DC converter), supply just enough power as close to each of the loads as possible. Some of the latest high-end notebook PCs are equipped with more than 10 POLs.

Fig 2 - Resistance/temperature characteristics of POSISTOR

Figure 1 - schematic diagram of example point-of-load converter

Although conversion efficiency of individual POLs is very high, some heat generation is inevitable.

That is because large currents flow through the power FET, resulting in Joule heating due to ON resistance. Today, power FETs generating high heat happen to be located at various spots on the circuit board.

With DC-DC converters, such as POL, power FETs are commonly used in pairs as shown in Fig.1. When one of them becomes ON the other one becomes OFF alternately. However when they are both ON simultaneously, it causes short- circuiting, and high current flows through both of them. In such case, power FETs may overheat beyond their guaranteed temperature. Depending on circumstances, their surface temperature can get as high as 160°C to 200°C.

Generally, measures are taken to prevent electronic equipment from overheating like this, however they cannot completely handle component defects or malfunctions caused by unexpected noise.

Although the possibility of overheating is small per individual unit, the chance of overheating increases when there are more units. Overheating can result in the equipment enclosure melting, smoking or igniting in the worst cases. To deal with this, abnormal heat detection circuits using temperature sensors, such as negative temperature coefficient (NTC) thermistors, are introduced by LSI manufacturers. However, monitoring overheating for 10 power devices would require 10 circuits. Even more are necessary if multi-phase type DC-DC converters are used. They also make the circuit design less flexible. Since the scale of detection circuits is relatively large, it is difficult to add or remove them after the prototype circuit board has been made.

2. Overheat detection using POSISTOR®

Fig 3 - Overheat detection circuit using POSISTOR® and its output voltage

We can circumvent the problems described above by using PTC thermistors "POSISTOR®" instead of NTC thermistor.

As shown in Fig. 2, when temperature reaches a certain level, resistance of POSISTOR rapidly increases to over 1,000 times. Circuit A in Fig. 3 is an overheat detecting circuit built using POSISTOR. When PRF18BC471Q is used for POSISTOR, dividing resistance R is at 10kW and applied voltage Vcc is at 3.3V, and the relationship between POSISTOR temperature and output voltage Vout is as shown on the right chart of Fig. 3.

Vout at room temperature is 0.15V, but it increases to 1.06V when the temperature reaches 105°C, the first sensing temperature (with resistance 10 times that of room temperature), and to 2.72V for the second sensing temperature (with resistance 100 times that of room temperature). If we select an appropriate dividing resistor, the circuit can directly drive transistors or FETs to halt power supply. This eliminates the necessity for hardware such as A-D converters, and control software.

It is possible to sense temperatures in multiple areas by connecting multiple POSISTOR chips in series as shown in Circuit B, because of their large resistance change. In the case where we have four POSISTOR chips, when at least one of them reaches 120°C, Vout rises to 2.72V from 0.52V at room temperature. If we set the comparator threshold voltage at around 2.7V, for example, we can connect over 10 POSISTOR chips. Figure 4 shows an application example of a notebook PC motherboard. Here, five POSISTOR chips can detect overheating in five locations totally independent of sophisticated signal processing and control circuits.

Fig 4 - Example of overheat detection circuit for multiple locations on a notebook PC

3. Low-cost, flexible temperature sensing circuit

The typical temperature sensing method using NTC thermistors requires additional sensors and sensing circuits whenever another location needs to be monitored. This also makes verification of the operation more troublesome. Using the method introduced in this article, we need only one detection circuit as long as we place one POSISTOR at each location to be monitored. In addition, as shown in Fig. 5, we can reduce not only the cost of the circuit but also the design and verification times. This method is also advantageous in terms of design flexibility.

For example, if a location is estimated as having a potential risk of overheating at the design phase, and confirmed that there is no danger after the prototyping phase or actual use, we can just short-circuit the corresponding POSISTOR instead of making a design change in the detecting circuit. This is because the number of the POSISTOR chips connected affects the threshold value very little due to the very large change in POSISTOR resistance. Conversely, by placing lands alone at locations with low risk of overheating, we can decide later after prototyping whether or not we need to mount POSISTOR chips there. In some cases, we may not need to change the wiring pattern at all. This type of superior flexibility can contribute greatly to shortening the design term. Also, the temperature to be sensed can be freely determined by selecting the POSISTOR type. For example, as indicated in Table 1 and Fig. 6, there are various POSISTOR types having the first sensing temperature between 65°C and 145°C at 10°C intervals.

Cost comparison for a system having many locations to monitor

Thus, sensing temperatures at various locations can be altered simply by exchanging the POSISTOR type used without making changes in the circuit design. Since the temperature difference between the monitored power device and POSISTOR changes due to the wiring pattern and heat capacity of mounted components, being able to determine the sensing temperature after evaluating the actual board is a great advantage.

While distributed power supply systems support advanced equipment, they tend to have more spots with a high risk of overheating. This method of using multiple POSISTOR chips to detect overheated power devices at multiple locations can prevent hazardous conditions (such as the equipment enclosure melting, smoking and igniting) at low cost and with high flexibility.

Already, there are several notebook PC models equipped with up to 10 or more POSISTOR chips. Its applications are now expanding beyond distributed power supply systems to encompass other systems using many power devices such as plasma TVs and audio amplifiers.

We highly recommend using multiple POSISTOR chips for overheat detection of multiple locations on circuit boards since they provide a realistic and inexpensive solution - and excellent security for all types of equipment using multiple power devices.

Combo image: Table 1 - Temperature characteristics for chip POSISTORS for overheat detection with Fig 6 - Variation in sensing temperature for POSISTOR

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