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January 24, 2012 / Aiman Kiwan pewa

On-resistance vs Output Capacitance for PhotoMOS Relays

In previous articles, we’ve shown how a PhotoMOS works, and how to optimize certain characteristics. Here we talk about the balance between  Output Capacitance  and On-resistance (C vs R).

Trade-off between On resistance and Output capacitance:

Implementing  a PhotoMOS relay is a simple task, especially in electronic circuits where the coil EMI in electromechanical relays can cause unwanted noise and false signals.   The PhotoMOS can be tuned to be suitable for different applications.  Since a MOSFET is being used to switch the output side, it is apparent that the MOSFET characteristics influence the behavior of the PhotoMOS relays.  By adding more photo-cells, the operating power can be reduced. In addition, the breakdown voltage, on-resistance and switching capacity can be changed by adjusting the design of the MOSFET.  For example, increasing the breakdown voltage requires slightly doped drain area.  However, this change will also decrease capacitance and increase on-resistance.  On the other hand, lowering the on-resistance by changing the drain area leads to higher capacity but lower breakdown voltage. So, getting the right formula is a lengthy and difficult task.

Low CxR  PhotoMOS Relays:

Low on resistance and low capacitance switches are needed to reduce signal loss and improve switching time and isolation characteristics, which are key requirements for high precision applications such as in measurement and data acquisition devices.  Since both the on-resistance and the capacitance are reduced, these devices are referred to as low CxR PhotoMOS.  These improvements in the CxR values are achieved by optimizing the layout of the MOSFET and improving the internal structure including the layout of the bonding pads, manner of wire bonding and new terminal leads.

By reducing the CxR value of the PhotoMOS relay, there is some trade off in other important values such as load voltage.  Table 1 shows a the characteristics of some typical low CxR PhotoMOS relays.


Table 1: List of low CxR  PhotoMOS relays

Applications for Low CxR PhotoMOS

Low CxR PhotoMOS are especially suited for measurement and data acquisition applications.  For example AQY221N3V has on resistance of 5.5W and output capacitance of 1pF, resulting in switching speed of 20ms and providing excellent isolation characteristics.  The reduced length of the internal bonding wires enable new smaller package.  The new SSOP (Shrink Small Outline Package) is 60% of the surface area of the conventional SOP (Small Outline Package).  This could translate to significant board space saving when the system requires several switches such as the case of Automatic Test Equipment (ATE).

In an ATE system, various AC and DC signals are applied to Device Under Test (DUT) where the switching speed plays a major role.  The input and output signals are monitored and the measurement is taken.  Because of the variety of signals in the test system, relays with different characteristics are needed.  Low CxR PhotoMOS with reduced on-resistance are needed for DC signals reducing signal loss while PhotoMOS with reduced capacitance are needed for AC signals providing optimized isolation.

Please describe yourtoughest design-in challenge with PhotoMOS relays in the Comments section.


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