Test and Measurement


How to use SSRs for reliable, fast- switching, low-loss semiconductor automated test equipment


By Rolf Horn, applications engineer at DigiKey I


ntegrated circuits (ICs) are more in demand than ever because they reduce hardware development costs, promote the miniaturization of electronic devices, and provide a wide range of functions. To ensure the quality of large production batches, semiconductor manufacturers require reliable and compact automated test equipment (ATE) that can quickly switch high-frequency AC and DC currents with low and high signal levels and minimal loss.


Solid-state relays (SSRs) based on photovoltaic MOSFETs are ideal for IC testers and ATE applications. Their miniature size and wear-free properties are particularly interesting.


This article briefly discusses ATE requirements. It then introduces different types of photovoltaic MOSFET relays from Panasonic’s PhotoMOS series SSRs and highlights their differences in component geometry and switching characteristics. Design tips for accelerated on/off switching and reducing PhotoMOS-specific leakage currents conclude the topic.


High packing density and short signal paths


An automated IC tester makes contact with the device under test (DUT) using densely packed needle adapters (probe cards) to perform functional testing. The modules in the test head generate and distribute high-speed test pulses, supply appropriate voltages, and switch measurement channels. Every test must occur in a confined space to minimize line losses, signal propagation times, interference, and channel crosstalk.


For this task, designers can use small-format switching elements such as Panasonic’s AQ series relays. For example, the voltage-controlled CC type AQY2C1R6PX PhotoMOS SSR comes in a TSON package that occupies 3.51 square millimetres (mm2) (1.95 × 1.80 mm) (Figure


22 October 2024


Figure 1: Shown are the housing dimensions for the AQ series small-signal PhotoMOS relays; dimensions are in millimetres. (Image source: Panasonic, modified by author)


test system and illustrates which PhotoMOS relay types are best suited to various signal paths in the measuring module of the test head. The AQY2C1R3PZ and AQY221N2TY PhotoMOS relays feature a low Cout of 1.2 and 1.1 picofarads (pF), respectively. This allows them to switch on and off at up to 10 and 20 microseconds (µs) (AQY2C1R3PZ), and 10 and 30 µs (AQY221N2TY). The tradeoff for both relays is increased Ron, 10.5 and 9.5 Ω, respectively, resulting in higher losses and component heating. These PhotoMOS relays are good for quickly switching measurement signals with low current flow, and they generate less reflection/phase shift with high- frequency signals.


Figure 2: The AQY2C1R6PX CC type PhotoMOS SSR (left) uses capacitive coupling and is voltage driven; the AQY221R6TW RF type (right) uses optical coupling and is current driven. (Image source: Panasonic, modified by author)


1). It uses capacitive coupling to provide 200 volt isolation protection and is voltage controlled, requiring only 1.2 milliwatts (mW) of control power.


The current-controlled RF type AQY221R6TW PhotoMOS relay has a small footprint of 3.8 mm², but its VSSOP housing is 3.6 times taller than the AQY2C1R6PX. It requires only 75 mW of control power and uses optical coupling to provide 200 volts of protective isolation. The leakage current (ILeak) of the CC and RF types is very low at 10 nanoamperes (nA). Figure 2 shows the circuit principle of CC type relays with capacitive coupling (left), and the RF type with optical coupling (right). The GE type AQV214EHAX also uses optical coupling and offers significantly higher protective insulation of up to 5 kilovolts (kV) between the control circuit (IN) and the load circuit (OUT). It comes in a larger 6-SMD


Components in Electronics


package that measures 8.8 mm x 6.4 mm with gull-wing leads. Requiring only 75 mW control power, SSRs from the GE series switch load currents of up to 150 mA at a maximum of 400 volts.


Optimizing contact resistance and output capacitance


As is typical for semiconductors, SSRs have an “on” resistance (Ron) and output capacitance (Cout) that cause heat loss and leakage current, respectively. Different relay types optimize for one or the other depending on the kind of signal to be switched.


SSR types with a particularly low Ron cause less attenuation when switching high- frequency AC test pulses. SSRs with a low Cout enable more accurate measurements for DC signals, while types with a high Cout are suitable for switching higher power levels. Figure 3 shows an automated semiconductor


The AQY2C1R6PX and AQY221R6TW discussed earlier are more suitable for slower- switching power signals and supply voltages with higher currents. While their lower Ron causes less component heating, their larger Cout has an integrator effect on the signals.


Minimizing signal distortion Semiconductor relays that only represent a simple on/off switch (1 form A) are examples of phototriacs for AC signals or optocouplers with bipolar transistors for pulsating DC signals. These devices cause distortions in the load signal due to threshold, ignition voltages, and switching delays. In addition, reverse recovery currents can generate harmonic overshoots (ringing) and leakage currents of several 10 to 100 milliamperes (mA). The FET half-bridge with driver circuit in Panasonic's PhotoMOS relays minimizes these signal distortions, hence their suitability for low-loss switching of AC and DC small signals such as high-speed test pulses, measurement signals, and supply voltages. When switched off, the leakage currents between the two OUT connections are below 1 microampere (µA). PhotoMOS relays are available in form A (single pole, single throw, normally open


continues on page 24 www.cieonline.co.uk


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