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January 19, 2012 / Aiman Kiwan

How to Speed up the Switching Time for a PhotoMOS Relay

In the last PhotoMOS blog post, we discussed how to select the correct capacitor for the application. Now we’ll move on to switching time.

Minimizing Turn-on Time

There are some cases where turn on time is crucial and the circuit designer must find ways to reduce the amount of time it takes from applying the LED forward current until the MOSFET gate is closed.  Typically, higher LED current will lead to faster switching time.  On the other hand, it will increase power consumption and reduce the LED life.  Therefore, it is ideal to initially increase the LED forward current for switching operation and then reduce it afterward.  This operation can be done using the circuit in Figure 1

Capacitor C has no voltage drop across it when the circuit is in the off state.  Once a control signal is applied, the capacitor acts as a short circuit causing high inrush current limited only by RI through the LED.  After capacitor C is charged, it acts as an open circuit; thus RI and RF determine the LED current.  Based on the previous example, the maximum value for RI+RF=714Ω

For example to speed up the turn on time, we can increase the IF current from 5mA to 20mA.  The VF value at 20mA and 85oC from Figure 3 is 1.11V

Figure 1: PhotoMOS fast switching circuit

Thus, a standard resistor value of 150Ω should be selected for RI.  Now the value of RF can be determined as follow:

This yields a standard resistor of 560Ω.

Assuming same criteria as before, 5% tolerance and a temperature coefficient of 250ppm per oC, RF of 560Ω can no longer guarantee safe operation over the entire temperature range.

Ensuring Safe Operation

This value is higher than RFmax.  Therefore, RF must be reduced to the next lower standard value of 470Ω to insure safe operation over the entire temperature range.  The next step is to determine capacitor C value.  The time constant of an RC circuit is the amount of time it takes to charge the capacitor to a certain level.  After one time constant the capacitor will be charged to 63% of the applied voltage.  The following assumption can be made to calculate capacitor C value: time constant t = RI x C shall equal twice the maximum turn on time t =2 x Ton and the maximum Ton is 2ms

Thus a standard value C = 22mF should be used in this circuit to insure a maximum turn on time of less than 2ms.

If you have any questions on how to optizimize turn-on time of a PhotoMOS, please use the Comments section.

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