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Guard bands minimise MOSFET failures in space (Part 1)

Posted: 23 Dec 2015 ?? ?Print Version ?Bookmark and Share

Keywords:testing requirements? MOSFETs? radiation? guard band? transistors?

The guard band gives us confidence that that the device and its associated circuitry in the aerospace module will function properly.

ICs used in space have testing requirements that go far beyond those for Earth-bound devices. I explained the process in my article Test power ICs to withstand radiation. Now, I'll look at what happens to power devices that drive motors in space when subjected to radiation.

Because ICs in space are subject to radiation levels that exceed those on Earth, we must test the devices with additional guard bands around datasheet specifications, which allows for drift. The guard band gives us confidence that that the device and its associated circuitry in the aerospace module will function properly.

Radiation plays a particular role in how MOSFETs that drive electric motors that function in space. In particular, a MOSFET's on resistance (RDSON) used in a DC-AC converter module value can change. The power converter delivers the electric power to the motors in an aerospace module. A MOSFET's on resistance depends on the overdrive voltage, that is, the difference in voltage between the MOSFET's gate and the source terminals (VGS) and the threshold voltage, VGSth according to the equation below. Figure 1 shows how RDSON is a function of that voltage difference.

Figure 1: The n-channel power MOSFET acts as a resistor whose conductance depends on the overdrive voltage VGS-VGSth. (source: University of Pisa, Informatic Department)

The variation of the threshold voltage comes from the variation of the charges trapped inside the silicon oxide and the recombination of electrons flowing inside the inducted channel between the source and drain terminals. That, in turn, causes a variation in RDSON. This variation can impact the functionality of the whole power module because a bridge of power switches act as a voltage divider whose voltage-conversion ratio is determined by mean of the resistance values of the ICs.

If the resistance value of the bridge changes, the DC voltage supplied to each branch of the electric motor will change accordingly. This problem must be solved because motors are widely used in aerospace modules. One example of power module made by MOSFET transistors for AC motor control by a battery source is the module in figure 2 by Mitsubishi electric.

Figure 2: A power module for electric motor control consists of n-channel MOSFETS (source: Mitsubishi Electric).

In Part 2, I'll explain an analytic methodology for choosing the correct guard-band to use during dice testing. Adding guard bands to automated tests lets us screen the outlayer parts and prevent a drifting of a MOSFET's electric parameter from exceeding published limits. Proper selection ensures a power module will withstand radiation in space. Part 2 also provides ATE test code that you can download and modify.

About the author
Paolo Scalisi is senior test engineer at STMicroelectronics Italy.

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