Reduce Switching Costs in Automated Test Systems with FETs and SSRs
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Found in products from automotive engines to thermocouples, electromechanical relays are readily available, low-cost devices you can use to route signals that originate from almost anywhere. It is no wonder that these devices have become ubiquitous in the automated test market. Despite their industry adoption, electromechanical relays are relatively slow (hundreds of channels per second) and have a limited lifetime, which makes them unsuitable for applications such as semiconductor validation test.
Devices such as solid-state relays (SSRs) and field-effect transistor (FET) switches are better suited for applications such as semiconductor validation. With switching speeds as high as 50,000 channels per second and unlimited mechanical lifetime, FETs and SSRs reduce overhead in many applications by decreasing test times and eliminating the need for frequent switching component replacement. Additionally, their small size helps minimize up-front switch system costs.
What Is an SSR?
An SSR is constructed using a photo-sensitive metal-oxide semiconductor field-effect transistor (MOSFET) that is controlled with an LED. Light from the encapsulated LED actuates the photo-sensitive MOSFET so current can flow through it.
Figure 1. An SSR is constructed using a photo-sensitive MOSFET that is controlled with an LED.
SSRs are useful for high-voltage applications because the LED actuation provides a galvanic isolation barrier between the control circuitry and the MOSFET. Because the MOSFET is performing the switching, however, there is no galvanic barrier between its contacts. When there is no gate drive on the MOSFET, its drain-source channel has a very high resistance, which provides the disconnection between the contacts. Because SSRs use MOSFETs, which are solid-state devices, to switch states instead of a mechanical arm, they have infinite mechanical lifetime. SSRs also offer faster switching speeds than electromechanical relays. Switching time for SSRs is dependent on the time required to power the LED on and off, approximately between 0.5 ms and 1.0 ms. This is faster than the time required for the mechanical arm to move between states in electromechanical relays.
The NI PXI-2533 and PXI-2534 256-crosspoint matrix modules are examples of high-density switch products that incorporate SSR technology. Both PXI modules offer unlimited mechanical lifetime and unlimited simultaneous connections while providing you with the ability to switch up to 55 V at 1 A on all channels.
Figure 2. PXI-2533 256-crosspoint SSR matrix modules are high-density switch products that incorporate SSR technology.
What Is an FET Switch?
Like SSRs, FET switches are not mechanical devices and therefore have an unlimited mechanical lifetime. FET switches use a series of CMOS transistors to connect and disconnect circuits. Unlike SSRs, the control circuitry directly drives the transistor gates instead of driving an LED. Therefore, you obtain much faster switching speeds because the power-on and power-off time of the LED is not an issue. Also, because there are no mechanical parts or LEDs in the packaging, FET switches can be very small. One major limitation of the FET switch, however, is that it lacks a physical isolation barrier, so you can use it only with low-voltage signals.
FET switches are most often used in multiplexer and matrix configurations for higher-speed, low-voltage applications. The NI PXI-2535 and PXI-2536 544-crosspoint matrix modules are examples of FET-based switch modules. These PXI modules offer switching speeds up to 50,000 crosspoints per second.
Figure 3. The PXI-2536 is a 544-crosspoint FET switch module and offers switching speeds up to 50,000 crosspoints per second.
Advances in FET and SSR Technology
Until recently, FET and SSR devices have had a reputation for introducing measurement errors in test systems due to their high path resistance, which can sometimes exceed 1 kΩ. This characteristic has impeded these devices’ entry into the automated test market. However, recent advancements in transistor technology have made FET and SSR path resistance comparable to that of electromechanical relays. For example, the PXI-2533 256-crosspoint SSR relay has a path resistance of 1 Ω, which is less than or equal to the path resistance of most electromechanical relay modules. This substantial technological breakthrough has made it possible to leverage the inherent advantages of FET and SSR technology, which include unlimited mechanical lifetime and faster switching speeds, without suffering the consequences of higher path resistance.
PXI FET and SSR Switch Modules Reduce Test Costs
FET and SSR devices help reduce automated test system costs by lowering up-front spending, increasing switch system lifetime, and minimizing test times. The compact size of FETs and SSRs helps reduce the up-front cost of PXI switch systems. The cost of a PXI switch module is based on the cost of relay components, back-end circuitry, and materials such as the printed circuit board (PCB) that are used to build the module. The compact size of FETs and SSRs makes it easy to build very high-density single-slot PXI switch modules. This helps reduce the number of PXI modules and, consequently, PXI slots used in a chassis when building high-density switch systems such as those used in semiconductor validation testers. By using fewer modules, you spend less on raw materials and the back-end architecture. The PXI-2535 544-crosspoint matrix is an example of a very high-density PXI switch module built using FET technology.
The unlimited mechanical lifetime and faster switching speeds of FET switches also help minimize costs in test systems. For example, consider a system used to conduct 10 tests on a chip with 500 I/O points. You use the chip in numerous devices and its cumulative sales are estimated at 1 million chips per month. The test system, built with a single NI PXI-4130 source measure unit (SMU) and a switching front end used for routing all 500 points to the SMU, is required to run continuously without interruption. The cost comparison of using an FET-based switch product versus an electromechanical relay-based product is outlined below.
Using the 50,000 channels per second scanning speed of the PXI-2535 FET switch, you can test all 1 million chips in less than 12 days. Because FETs have unlimited mechanical lifetime, you incur no switch module replacement costs during the process.
Figure 4. You can test chips with the SMU and 544-crosspoint FET switch.
If you use an electromechanical relay-based switch module with the same density, expenses are much higher. Electromechanical relays have a typical lifetime of 1 million closures and a speed of 250 channels per second. Because each relay closes 10 million times during the process of testing all 1 million chips, you need to replace the relay module 10 times. This increases the total expenses incurred to maintain the system. The slower electromechanical relay speed also adds costs in comparison to the FET-based solution. It takes 231 days to test 1 million chips using electromechanical relays. Thus, using electromechanical relays adds the cost of maintaining and running a production floor for 219 additional days in comparison to the FET-based module. Longer test times also introduce challenges when managing inventory and shipping products to customers.
Although many factors, such as instrument speed and desired resolution, cumulatively determine test time, this example shows some of the benefits of using FET and SSR technology.
FETs and SSRs – A Viable Solution for Switching
There is no single solution for routing signals in automated test systems. In fact, the number of available solutions is continually increasing. FETs and SSRs are examples of switching solutions that have always been available to the market but have become viable options only recently due to advances in transistor technology. With these advances, you now can reap the benefits of solid-state switching, which include faster switching speeds and unlimited mechanical life, to build better, faster, and more economical test systems.
Jaideep Jhangiani is a product manager for NI switching products. He holds a bachelor’s degree in computer engineering from Texas A&M University.
View demonstrations of solid-state switches and their applications in semiconductor validation.
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