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AND8199 Datasheet(PDF) 1 Page - ON Semiconductor |
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AND8199 Datasheet(HTML) 1 Page - ON Semiconductor |
1 / 4 page Semiconductor Components Industries, LLC, 2005 January, 2005 − Rev. 0 1 Publication Order Number: AND8199/D AND8199 Thermal Stability of MOSFETs Prepared by: Alan Ball ON Semiconductor Application Engineering Manager A variety of applications use hot−swap controllers, often to increase the reliability of a system. However, a failure in the hot−swap circuit would defeat that purpose. When you use MOSFETs in their active region to control current, such as you would for a controller that operates in a constant−current mode of operation, they have an inherent failure mechanism. In this mode, the MOSFET can get hot spots and fail, long before the device exceeds its Safe Operating Area (SOA) ratings. Engineers have long understood that MOSFETs are positive temperature coefficient devices. Therefore, as the temperature of the device increases, the resistance increases. In other words, higher temperatures result in lower currents. This fact is important if you want to operate MOSFETs in parallel. With a good thermal path between devices, the positive temperature coefficient reduces the current in the hottest device and forces more of it to flow in the cooler device, thereby avoiding thermal runaway. Engineers often think of a MOSFET as a single power transistor, but it is a collection of thousands of tiny power FET cells connected in parallel. In terms of sharing current, the same application of the positive temperature coefficient applies. In this case, the thermal path between the cells is better than that of separate packaged devices, because the cells are all on the same die. As the current density of a small group of cells increases, those cells heat up, increasing the resistivity of those cells and forcing current to flow in neighboring cells, which minimizes the thermal gradient and avoids hot spots. This process is an essential physical tenet that allows the parallel array of cells to function reliably. If the MOSFET exhibits a negative thermal coefficient, today’s parallel cell structure would cause serious reliability issues. In fact, in some modes of operation, the thermal coefficient goes negative. You can easily understand this phenomenon by looking at the transconductance curves for a FET device (refer to Reference 1). A typical set of transconductance curves clearly demonstrates this effect as shown by Figure 1. Below are curves from three typical devices used in hot swap applications. 24 12 8 4 7 6 5 4 3 2 1 0 0 89 16 20 0 Figure 1. Transfer Characteristics for NTD12N10 VGS, GATE−TO−SOURCE VOLTAGE (V) TJ = 25°C TJ = −55°C TJ = 100°C VDS ≥ 10 V Figure 2. International Rectifier IRF530 100 10 4.0 5.0 6.0 7.0 8.0 VGS, GATE−TO−SOURCE VOLTAGE (V) TJ = 25°C TJ = 175°C VDS = 50 V 20 ms = Pulse Width http://onsemi.com |
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