How to Automate Low Loss Semiconductor Test Equipment with SSRs
Integrated circuits (ICs) are more in demand than ever as they reduce hardware development costs, facilitate the miniaturization of electronic devices, and provide a wide range of functionality. To ensure quality in high-volume production, semiconductor manufacturers need reliable, compact automated test equipment (ATE) that can quickly switch high-frequency AC and DC currents while keeping signals low and signals high with minimal loss.
Solid State Relays (SSRs) based on optoelectronic MOSFETs are ideal for IC testers and ATE applications. The miniature size and wear-free nature of these devices are of particular interest.
This article briefly discusses ATE requirements, introduces the different types of photovoltaic MOSFET relays in Panasonic's PhotoMOS family of solid state relays, and highlights the differences in component geometry and switching characteristics of these relays.
High Package Density, Short Signal Paths
Automated IC testers use dense pin adapters (probe cards) in contact with a device under test (DUT) to perform functional tests. Modules in the test head generate and distribute high-speed test pulses, provide the proper voltage and switch the measurement channel. Each test must be conducted in a confined space to minimize line losses, shorten signal propagation time and reduce interference and channel crosstalk.
To do this, designers can use small switching components such as Panasonic's AQ series of relays. For example, the voltage-controlled CC-type AQY2C1R6PX PhotoMOS solid-state relay is available in a TSON package measuring 3.51 mm2 (1.95 × 1.80 mm) (Figure 1). The device utilizes capacitive coupling to provide 200 V isolation protection and voltage control with only 1.2 mW of control power.
Figure 1: Shows the case dimensions (in mm) of the AQ series small signal PhotoMOS relays.
The current-controlled RF-type AQY221R6TW PhotoMOS relay comes in a small 3.8 mm² package, but its VSSOP case is 3.6 times taller than the AQY2C1R6PX. The device requires only 75 mW of control power and utilizes optocoupling to provide 200 V of protective isolation. the CC and RF types have a very low leakage current (ILeak) of 10 nano-amps (nA).
Figure 2 shows the circuit principle of the CC type relay with capacitive coupling (left) and the RF type relay with optical coupling (right).
Figure 2: PhotoMOS Solid State Relay Type AQY2C1R6PX CC (left) is capacitively coupled and driven by voltage; Type AQY221R6TW RF (right) is optically coupled and driven by current.
The GE Model AQV214EHAX also utilizes optical coupling to provide a high degree of protective insulation of up to 5 kV between the control (IN) and load (OUT) circuits. Available in a bulky 6-SMD package measuring 8.8 mm x 6.4 mm with gull-wing leads, the GE family of SSRs can switch load currents up to 150 mA at up to 400 V with only 75 mW of control power.
Optimized Contact Resistance and Output Capacitance
Like typical semiconductor devices, SSRs have an "on" resistance (Ron) and an output capacitance (Cout), which contribute to thermal losses and leakage current, respectively. Depending on the type of signal to be switched, different types of relays are optimized for one or the other of these two parameters.
Solid state relays with a particularly low Ron have less attenuation when switching high frequency AC test pulses. Solid state relays with low Cout allow for more accurate measurements of DC signals, while solid state relays with high Cout are suitable for switching higher powers. Figure 3 shows an automated semiconductor device test system and illustrates which PhotoMOS relay types are best suited for the various signal paths in the test head measurement module.
Figure 3: Each signal path in this automated semiconductor test system requires a specific type of PhotoMOS relay.
The AQY2C1R3PZ and AQY221N2TY PhotoMOS relays have a low Cout of 1.2 pF and 1.1 pF, respectively, which results in switching times of up to 10 µs and 20 µs (AQY2C1R3PZ), and 10 µs and 30 µs (AQY221N2TY) for these devices. The trade-off between these two relays is an increase in Ron to 10.5 Ω and 9.5 Ω, respectively, and an increase in losses and component heating. These PhotoMOS relays are suitable for fast switching of measurement signals at lower currents and produce less reflection/phase shift at high frequency signals.
Minimizing Signal Distortion
For example, opto-electronic triacs for AC signals or optocouplers with bipolar transistors for pulsating AC signals, semiconductor relays representing only simple switching (1 form A) are devices that cause distortion of the load signal due to thresholds, trigger voltages and switching delays. In addition, the reverse recovery current generates harmonic overshoot (transient oscillations) and leakage currents of 10 mmA to 100 mA.
In Panasonic's PhotoMOS relay, the FET half-bridge with driver circuitry minimizes these signal distortions, making it suitable for low-loss switching of small AC and DC signals such as high-speed test pulses, measurement signals, and supply voltages. Leakage current between the two output connections is less than 1 microampere (µA) when switched off.
PhotoMOS relays are available as form A (single-pole, single-throw, normally-open contacts (SPST-NO)) or form B (normally-closed contacts, SPST-NC), as well as multiplexed products. Designers can combine form A and form B devices to create form C switches such as single-pole, double-throw (SPDT), single-pole transfer switches, and double-pole, double-throw (DPDT) switches.
For example, the AQS225R2S is a 4-way PhotoMOS relay (4SPST-NO) in a SOP16 package that can handle up to 70 mA at switching voltages up to 80 V. The AQS225R2S is a 4-way PhotoMOS relay (4SPST-NO) in a SOP16 package that is capable of handling up to 70 mA with switching voltages up to 80 V. In addition, the AQW214SX is a dual PhotoMOS relay (2SPST-NO) in a SOP8 package that can handle load currents up to 80 mA at switching voltages up to 400 V. The AQW214SX is also available in a SOP16 package.
Figure 4: SSRs and optocouplers cause distortion in the output signal due to threshold and trigger voltages; PhotoMOS relays switch AC and DC signals without distortion.
In order to protect the PhotoMOS output stage by attenuating the feedback effects of inductive and capacitive switching loads, the designer must add clamp and current-supply diodes, RC and LC filters, or varistors on the output side. In the CC series, clamp diodes protect the input oscillator from overvoltage spikes and limit the control signal to 3 V to 5.5 V, while RC filters ensure residual ripple is less than ±0.5 volts.
In conclusion, by using small, non-wearing PhotoMOS relays, designers can increase signal density and measurement speed in ATE applications while reducing maintenance requirements.