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January 10, 2012 / Aiman Kiwan pewa

Circuit Design Criteria for PhotoMOS Solid State Relays

In an earlier blog post (here) , we described the principle of operation of a PhotoMOS solid state relay. Now we’ll look at some considerations when designing the circuit around the PhotoMOS.

Choosing An Input Resistor:

The most basic method to drive a PhotoMOS relay is to apply a switchable voltage directly to the input pin of the PhotoMOS through a resistor to limit the current through the LED (Figure 1). It is very critical to choose the correct RF value to insure that the LED turn on to full intensity, but will not be overdriven by the input voltage.   The Rvalue can be calculated using the following formula:

Figure 1: PhotoMOS relay basic input circuit

Temperature must be taken in consideration when designing any electronic component.  Since the LED operating current increases as the temperature rises, we must use the typical IF value (typical recommended value is 5mA) at the maximum operating temperature of 85oC to insure safe operation.   The LED forward voltage (VF) depends on the forward current (IF) and the temperature.

Figure 2: LED forward voltage vs. ambient temperature

Let’s for example calculate the RF value for AQV210 PhotoMOS.  Figure 2 shows the LED forward voltage vs. ambient temperature graph for the AQV210 PhotoMOS.  The LED forward voltage with IF of 5mA at 85oC is 1.03V. The maximum RF value can be calculated as follow:

Assuming a 5% tolerance and a temperature coefficient of 250ppm (Parts Per Million) per oC, the appropriate RF value will be the next lower value from the standard resistors: RF=680W. This will insure safe operation over the entire temperature range.  If the supply voltage (Vcc) contains a ripple, the lowest possible Vcc value should be used for the calculations.

Figure 3: PhotoMOS relay transistor input circuit

Although power consumption and drive current for PhotoMOS relays are significantly lower than electromechanical relays, some logic circuits can not drive the PhotoMOS directly and require some additional components.  Using a transistor as a control mechanism to switch an external power supply is one method that is typically used by circuit designers.  Figure 3 shows the PhotoMOS input circuit with external power supply controlled by a transistor.  In this scenario the transistor is controlled by the output of the logic circuit.  When the transistor is turned on, it will create a path to ground for the power supply Vcc thus turning on the LED.  When calculating the RF in this circuit, we must account for the voltage (Von, typically 0.4 to 0.7V) drop between the collector and the emitter of the transistor.  Using the same example of the AQV210 PhotoMOS, RF can be calculated as follow:

Assuming a 5% tolerance and a temperature coefficient of 250ppm per oC, RF of 680W can no longer guarantee safe operation over the entire temperature range.  In this case it is recommended to use the next lower standard resistor to insure that RF is lower than the maximum allowed value of 714W: RF=560W.

In the next PhotoMOS post, we’ll discuss how to optimize the switching time .

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