Semiconductor devices capable of handling higher levels of power are finding their way into increasingly higher power applications. To validate these new designs, researchers, fabricators and end-product designers need to be able to test experimental devices at higher power levels, which increases their risk of exposure to electric shock and fire.
When Keithley Instruments introduced a source measure unit (SMU) instrument capable of sourcing up to 3000 V, its designers had to figure out a way to protect both users and other instruments in the test setup. Although the Model 2657A High Power System SourceMeter SMU instrument is designed to handle this level of power safely, other instruments configured into the test system typically are not. If a device under test faults to another terminal, a high voltage output could easily destroy a lower-voltage SMU instrument.
The Model 2657A-PM-200 Protection Module is designed to protect lower-voltage source measure unit (SMU) instruments that are part of a testing configuration from damage by voltage sources that are greater than 220V. Keithley Instruments’ engineers designed this protection module for use in applications where a device breakdown or other potential failure could connect the high voltage output of an SMU instrument capable of sourcing up to 3000V to a lower voltage SMU instrument.
Keithley’s answer was the Model 2657A-PM-200 Protection Module (Figure 1), which protects against voltage outputs greater than 220 V and lets users safely connect multiple instruments to a probe. It addresses a number of challenges:
• The energy applied to a low voltage terminal must be limited to a level that is safe for equipment and operators in all conceivable fault conditions.
• All terminals must be safe for the rated voltage at that terminal.
• The system design must still allow for the full specified measurement performance.
A “look under the hood”:
1. Let’s start with the connectors. The connectors on the module’s high voltage side are rated for 3000 V. That is high enough for most power device testing, which typically is performed at 3x the rated voltage. The high-voltage triax connector has a special design that prevents a user from being exposed to high voltage. Additionally, unlike a typical through-hole or surface mount connector, the high voltage triax connector is mounted in a routed channel in the PC board. This type of in-board mounting is necessary to ensure sufficient high voltage spacing and low leakage. The high voltage connectors are spread as far apart as possible and employ a driven guard concept that reduces both leakage and settling time.
2. The connectors on the low voltage side are compatible with the high performance triax cables that the low voltage instruments require. By the way, the residual leakage current is less than 10pA, even at the full rated low voltage of 200V. Every terminal of the low voltage side is protected from overvoltage, including the HI force and sense lines, and the LO force and sense lines. Solid-state protection devices shunt current away from the load in response to a surge that exceeds the breakdown voltage.
3. Grounding is critical. Safety standards require providing a separate ground connection, because the shields of the triax cables are not considered a safe ground. The module’s case is constructed of metal and serves as chassis ground. The module comes with two grounding cables; both must be used. It must be able to sink more power to ground than the system could ever see from the instrumentation.
Thoughtful design of the connectors, good grounding and overvoltage protection devices make this module a handy way to protect instruments and people during high power testing. What’s more, the module design still allows for full measurement performance.