Disadvantage of using Optocoupler with Low CTR for Switch Operation

In optocoupler, CTR stands for current transfer ratio. This is comparable to the current gain, beta or HFE in a BJT. This is the most important parameters of an optocoupler to decide its operation either a switch or an active or linear operation. In this article we are going to focus on what is the disadvantage of using Optocoupler with low CTR particularly in switch operation to give guidance to design engineers. There are several factors that affect the CTR, for more details, check here.

Mathematically, CTR is defined as

CTR = (Ic / If) x 100%

Where;

• Ic is the collector current, this is the current flowing from Vcc to R2, to the collector of the optocoupler then finally to the ground.

• If is the forward current, it is the current flowing from Vdd to R1, to the diode and finally to the ground.

Normally at linear operation, above equation is true. However when the optocoupler circuit saturates, it is not anymore valid to use the equation to get the exact collector current because at saturation, the collector current is only limited to the collector resistor. In order for the optocoupler circuit to saturate, the circuit CTR must be less than the device CTR.

Circuit CTR is the ratio of the collector current to forward current of the circuit. On the other hand, the device CTR is the current transfer ratio defined in the datasheet. In linear operation, these two are equal. The circuit CTR will always follow the device CTR as above equation.

Now, what is the disadvantage of using optocoupler with low CTR in switch operation? As mentioned in the above paragraph, at saturation the circuit CTR is less than the device CTR. Expressing it mathematically,

CTRcircuit<CTRdevice

Circuit CTR is the ratio of the collector to the forward current of the optocoupler circuit. So we can change the above equation as below

(Ic / If) < CTRdevice

Based on the CTR equation (first equation above), when the collector current increases, the circuit CTR will increase. In switch design, the collector current is mostly higher since the optocoupler is operating at saturation. If your device CTR is not high enough there is a tendency that the operation will go out from saturation. What will happen if the computed circuit CTR is equal to the device CTR? The opto-coupler will not anymore saturate. In actual design, the circuit CTR is purposely set very small compared to the device CTR (linear CTR). By doing this, you are erasing the gray areas in between.

Another big thing is that the device CTR will degrade with higher temperature and aging. Say the CTR of the device at nominal ambient temperature is 100%, this may become 60% considering the effect of temperature and aging (lifetime de-rating). So, if you are setting the circuit CTR just below the minimum device CTR, it is a problem over time. For complete discussion how CTR affected by temperature and aging read here.