Flexible overheat detection at multiple locations using PTC thermistors
18 May 2009
As the distributed power architecture is becoming more popular, the use of multiple Point Of Load converters means multiple areas that are at risk from overheating.
As ICs have become more integrated and contain more functionality, requirements for supplying power to these chips have become more stringent. Demands on the power supply now include low voltages, high currents, high responsiveness for rapid load fluctuation, and high efficiency for low power consumption. Distributed power supplies such as Point Of Load (POL) converters have emerged to supply the correct amount of power as physically close to each of the loads as possible. Some of today’s laptop PCs contain more than ten POL converters.
Although the conversion efficiency of individual POLs is very high, some heat generation is inevitable since large currents flow through the supply’s FET, which results in Joule heating because of the FET’s ON resistance. Power FETs generating a lot of heat can be located at various places on the circuit board as part of POL converter circuits. The problem is further compounded when one considers that such power FETs are usually used in pairs, like in the typical setup shown in Figure 1. When one of the FETs is switched on, the other is switched off, and vice versa. The control circuit shown on the left-hand side of Figure 1 controls this switching. If something goes wrong with the control circuit, both FETs can be mistakenly switched on at the same time, causing a short circuit in which large currents flow through both the FETs. This causes the FETs to overheat beyond their maximum safe operating temperature. Depending on the circumstances, their surface temperature can be as high as 160 to 200ºC.
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 risk goes up when multiple units are used. Overheating can result in the enclosure melting, smoking or in the worst cases, igniting.
To deal with this, heat detection circuits using temperature sensors, such as Negative Temperature Coefficient (NTC) thermistors have been introduced by manufacturers. However, monitoring overheating from ten power devices would require ten of these 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.
Overheat detection using PTC thermistors
It is possible to circumvent these overheating problems by using PTC (Positive Temperature Coefficient) thermistors instead of NTC thermistors. PTC thermistors' resistance increases when the temperature increases, the opposite of what happens to an NTC thermistor.
When the temperature around a PTC thermistor reaches a certain level, its resistance increases rapidly. For example, with Murata's POSISTOR range of ceramic chip PTC thermistors, the resistance rapidly increases by a factor of over 1000.
Circuit A in Figure 2 is an overheat detection circuit built using a PTC chip thermistor. As an example, when Murata POSISTOR part number PRF18BC471Qx is used, the dividing resistance R is 10 kΩ and the voltage applied, Vcc, is 3.3 V, the relationship between the voltage output of the circuit Vout and the POSISTOR temperature is shown in the chart on the right-hand side of Figure 2. Vout at room temperature is 0.15 V, but it increases to 1.06 V when the temperature reaches 105ºC, at which point the thermistor has a resistance ten times its resistance at room temperature. Vout increases to 2.72 V when the sensing temperature reaches 120ºC, with thermistor resistance 100 times that of room temperature. If an appropriate dividing resistor is selected, the circuit can directly drive transistors or FETs to halt the power supply. This eliminates the necessity for hardware such as A-D converters, and control software in the overheat protection circuit.
It is possible to sense temperatures in multiple areas by connecting multiple PTC chip thermistors in series as shown in Circuit B in Figure 2, because of their large resistance change. If four PTC thermistor chips were placed at critical sensing points around a PCB and connected in series, when at least one of them reaches 120ºC, Vout rises to 2.72 V from 0.52 V at room temperature. If the output of the circuit were to be connected to a comparator with its threshold voltage set at around 2.7 V, we can connect more than ten PTC thermistors in series.
Low cost, flexible temperature sensing circuit
The application example in Figure 3 shows a notebook PC motherboard. Here, five thermistor chips can detect overheating in five locations totally independent of the sophisticated signal processing and control circuits which would be required by typical temperature sensing circuits employing NTC thermistors. The NTC version would also require additional sensors and sensing circuits whenever another location needed to be monitored. This also makes verification of the circuit more troublesome.
Using the method described in this article, only one detection circuit is needed as long as one PTC chip thermistor is placed at each location to be monitored. In addition, we can reduce not only the cost of the circuit but also the design and verification times, as shown in Figure 4.
This method is also advantageous for the flexibility of the design. For example, if a location is considered during the design phase to be potentially at risk from overheating, after prototyping it may be confirmed that there is no danger, and the corresponding thermistor can simply be short-circuited instead of having to make a design change in the detecting circuit. This is because the number of PTC thermistor chips connected has very little effect due to the parts’ large resistance changes.
Conversely, by placing lands at locations with low risk of overheating, the designer can decide later or after prototyping whether they need to mount a thermistor chip there. In some cases, the wiring pattern may not need to be changed. This type of superior flexibility can contribute greatly to shortening the design term.
An additional feature to consider is that the overheat temperature to be sensed can be chosen specifically for that part of the circuit by specifying a PTC thermistor with a different sensing temperature. For example, Murata's POSISTOR range of PTC thermistors includes parts that have their initial sensing temperature (the point at which the resistance begins to rise) between 65 and 145ºC at 10ºC intervals. Thus, sensing temperatures at various locations can be altered simply by exchanging the POSISTOR type used without making changes in the circuit design.
The method described here using multiple PTC thermistor 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. This realistic and inexpensive solution is already used in several notebook PCs which are equipped with more than ten POSISTOR chips each. Its applications are now expanding beyond distributed power systems to encompass other systems using multiple power devices such as plasma TVs and audio amplifiers.
Munenori Hikita is the European Product Manager for Sensor Products at Murata.
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