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Want to know the best location for replica and sense devices in power transistors?

Daniel Nenni

Staff member
Thermal Sense Device and Replica Placement in Power Devices

Who protects power protection chips?
Power protection chips are widely used these days to protect sensitive circuitry from over-voltage and over-current stress. However, these workhorse chips are often subjected to extraordinary thermal stress themselves and need to be protected from burning up – literally.

Power protection chips work like electronic fuses, switching off supply lines when voltage or current passing through them becomes too high. They have several advantages over other types of fuses though. First off they can be reset automatically or at the control of external logic. Additionally, they can limit current or voltage to allow a sensitive device to continue to operate normally even if there are spikes in the supply line.

ON Semiconductor’s electronic fuses are used heavily for hot swap devices like USB ports, and SAS and SATA disk drives. Upon insertion there is usually a power surge that can damage controller circuitry. The ON Semi devices provide soft start so devices see their power supply presented in a manageable way.


Looking at the block diagram for an electronic fuse, we see that there is circuitry for over-temperature and over-current protection. Both of these need sensors to provide the raw input for each of these operations. While it is obvious that the thermal sensor needs to collect accurate thermal information, it is also the case that the current sensor, which is a replica device, needs to have to same operating conditions as the large power transistor that controls the output.

The problem with large power transistors is that there are significant temperature gradients across the surface. The temperature is determined by the joule heating and the device’s ability to dissipate that heat. However, the junction temperature in turn determines the electrical operation of the segments of the transistor. In other words, there is an interdependency between localized temperature and operating voltage and current.

As a result, the highest temperature in a power transistor can be hard to locate and predict without the right tools. The penalty for placing either sensor in the wrong location can be incorrect over-current protection, or even worse, complete and dramatic device thermal failure. The highest power dissipation occurs at the highest current and voltage operation. This is where the risk of device failure is most extreme.

On Semi has chosen to rely on the “Power Transistor Modeler – Electro-Thermal” (PTM-ET™) tool from Magwel® to pinpoint the hottest location within their power transistors.

PTM-ET™ reads in the layout and also a model for the active area devices used in the power transistor. PTM-ET™ extracts the metal and poly structures to be able to fully model the current flow in the device. Then, user defined stimulus is applied over a time interval. PTM-ET™ uses advanced modeling methods to concurrently solve for transient electrical and thermal behavior. PTM-ET™ can also use surrounding thermal source and sink information to include an accurate view of the device including substrate, bond-wire, package and even board thermal behavior.

The end result is a 3-D visualization of the transient thermal and electrical performance of the entire device that can be graphically viewed or output in tabular report format. This can be used to determine the location and temperature of the device hotspot. With this information the designer can accurately place the over-temperature and replica devices in the optimal location.


There is a complicating twist however, once the device layout is altered to place the sense devices, the active area changes, resulting in movement of the hotspot. PTM-ET™ can be used again after sense device placement to ensure that they are placed as close as possible to the resultant hot spot(s).

PTM-ET™ is part of the suite of power transistor modeling tools from Magwel®. The base product, PTM®, is used to predict steady state Rdson and current density on the source/drain metal-poly network of large power devices. Also, PTM-TR™ can accurately predict non-uniform switching by fully modeling and simulating transient electrical behavior using the device the metal-poly gate network and the active area device model.

Companies like ON Semiconductor rely on Magwel® tools to solve challenging design problems so they can deliver high quality and high performance semiconductors.