Optocoupler is an electronic component that has a light source at its input side and a light detector or sensor in its output side. A light source is a LED while the detector or sensor is a phototransistor.

It is also called as optoisolator, photocoupler or optical isolator. This device is considered as a current controlled current source. This is because the input side is operated in terms of current while the output side is also in a form of current.

Figure below is the schematic symbol of an optocoupler. The input side has a diode symbol on it. On the other hand, the output side is like a bipolar junction transistor but with a floating base. There is no electrical connection between the diode and the baseless transistor but through the light coupling. Thus, optocoupler device is a very good choice for circuit isolation.

The same with a regular diode, the input terminals are labeled as anode and cathode. The output terminals are labeled collector and emitter the same with a BJT.

Optocoupler Electrical Parameters

The same with other electronic devices, there are parameters to consider in using optocoupler to ensure it will operate correctly and it will not get damage.

Input Side of the Optocoupler

Forward Current (I_F)

This is the current that flows from anode to cathode of the optocoupler. Optocoupler datasheet specifies the forward current limit. It is important to take note that the actual current flowing from anode to cathode must not exceed this limit. Otherwise, the optocoupler will get damage.

The forward current is controlled by a resistor place in series to the anode and cathode path. You must select the resistor value accordingly such that the forward current limit is not exceeded. To make the explanation more clear, consider below circuit.

There is a 1.5k resistor put in series with the anode. By doing KVL in the input side, the actual forward current is

I_F = (Vdd – V_F) / Rf

I_F = (5V – 0.7V) / 1.5k = 2.87 mA

Where I_F is the circuit forward current and V_F is the forward voltage drop of the LED. V_F is specified in the optocoupler datasheet. In this example, it is just a random value selected.

As a general consideration, the circuit forward current must be very much lower than the forward current limit to prolong the life of an optocoupler. A current stress of less than 75% is a good number.

Current Stress (LED) = (Circuit Forward Current / Forward Current Limit) x 100%

Current Stress (LED) <75% (to prolong optocoupler life)

Forward Surge Current (I_FSM)

The forward current rating is a steady state value. On the other hand, a forward surge current rating is a transient value. This means that an optocoupler can withstand a high forward current in a very short period of time. The same with the forward current, this rating must not be exceeded to prevent damaging the optocoupler.

Forward Voltage (V_F)

Forward voltage is also specified in the optocoupler datasheet. However, unlike forward current that could cause immediate damage to the optocoupler once exceeded, the forward voltage is just a characteristic value. This means that it is necessary for the circuit analysis but cannot cause a device failure due to exceeding its value.

Forward voltage is used in calculating the realistic value of the forward current. There is a relationship between forward current and forward voltage and this is specified in the datasheet in the form of a graph. Below is an example graph from Vishay Semiconductors with their optocoupler VO617A.

Another use of the forward voltage is to compute the power dissipation of the LED.

Reverse Voltage

Optocoupler input side is rated in terms of reverse voltage. This is the reverse voltage that the LED can still tolerate. Exceeding this value will damage the optocoupler. In most application wherein the positive supply is feed fixed to the anode side while the cathode side is in the ground (0V potential) level, reverse voltage is not an issue.

LED Power Dissipation

The LED power dissipation is the product of the forward voltage V_F and forward current I_F.

LED Power Dissipation =  V_F x I_F

This product must not exceed the LED power dissipation limit specified in the datasheet. Otherwise, the device will get damage. A good practice is to limit the LED power stress to below 75%.

LED Power Stress = (LED Power Dissipation / LED Power Dissipation Limit) x 100%

LED Power Stress < 75 (to prolong the life of the optocoupler)

Output Side of the Optocoupler

Collector Current (I_C)

Collector current is the current that flows to the optocoupler collector. The collector current limit is specified in the datasheet. Operating the optocoupler above the collector current limit will damage it.

The collector current is a function of the forward current and the CTR. CTR stands for current transfer ratio. This is analogous to beta or HFE in bipolar junction transistor.

CTR = (IC / IF) x 100%

Where IC is the collector current while IF is the forward current.

The collector current is dependent to two scenarios. First is linear operation and second is saturation operation.

At linear operation, the resistance in series to the collector or emitter path has nothing to do in controlling the collector current from exceeding the limit. The collector current during this scenario is given as

I_C linear = CTR x I_F

When operating at saturation, the collector current is just a function of the collector resistance and the voltage applied to the output side.

I_C saturation = Vcc / Rc

Where Vcc is the voltage applied to the collector and Rc is the resistance in series to the collector.

Collector Peak Current (I_CM)

The collector current is a steady state value. On the other hand, collector peak current is a transient value. This is means that an optocoupler can withstand this level of current which is much higher that the collector current in a short period of time.

Collector – Emitter Voltage (V_CEO)

This is an equally important rating of an optocoupler that when exceeded, it will damage the device.

This is the voltage measured across the collector to emitter junction when the base is not biased or there is no light detected from the input side.

To expound this further, refer to below circuit. When the diode side is not biased or not conducting, the voltage read at the Vout net must not exceed the V_CEO rating.

The same with the other parameters, the circuit collector-emitter voltage (V_CE) must not exceed the V_CEO limit to prevent damage. A good practice is to limit the V_CEO stress to less than 75%.

V_CEO Stress = (Circuit V_CE / V_CEO) x 100%

V_CEO Stress < 75% (to prolong the optocoupler life)

Collector-Emitter Saturation Voltage (V_CEsat)

This is not a stress related rating but a characteristic. This refers to the level of the voltage across the collector-emitter junction when the oprtocoupler is operating at saturation.

This voltage is necessary to compute the power dissipation of the output side at saturation.

Pdiss_output_saturation = V_CEsat x I_Csat

Where V_CEsat is the saturation voltage specified in the datasheet and I_Csat is the collector current level when the optocoupler is operating in saturation.

Output Power Dissipation

This is the power dissipation in the transistor. It is the product of the collector-emitter voltage (VCE) and the collector current (IC).

Output Power Dissipation = V_CE x I_C

The circuit output power dissipation must not exceed the power dissipation specified in the datasheet. In failing to do so, the device will get damage. Limiting the output power stress to less than 75% is a good practice to prolong the device life.

Output Power Stress = (Output Power Dissipation / Output Power Dissipation Rating) x 100%

Output Power Stress < 75% (to prolong the life of optocoupler)

Entire Optocoupler Device Ratings and Characteristics

Current Transfer Ratio (CTR)

This is the most important parameter of an optocoupler in order to know the collector current or the circuit operation (linear or saturation). CTR is the ratio of the collector current (I_C) to the forward current (I_F) in percent.

CTR = (I_C / I_F) x 100%

The datasheet specify CTR in terms of range (minimum and maximum value). This means that the CTR will vary in between the range. Thus, it is important to consider it during the design stage.

Total Power Dissipation

This is the package power dissipation. This must be the sum of the LED and output power dissipation.

Total Power Dissipation = Pdiss_input + Pdiss_output

Total Power Dissipation = (V_F x I_F) + (V_CE x I_C)

The circuit total power dissipation should not exceed the device rating as specified in the datasheet. Otherwise, the optocoupler will damage. Limit the power stress to less than 75% to increase the longevity of the optocoupler.

Operating and Storage Temperature

Operating temperature means the temperature when the optocoupler is functioning. On the other hand, storage temperature is when the device is not working

The datasheet provides limit for both temperature. It is very important not to exceed these limits to avoid damaging the part.

Factor that Affects Optocoupler CTR

Forward Current

There is a relation between CTR and forward current. Using the graph below which is taken from Vishay Semiconductor’s VO617A optocoupler datasheet, it shows that CTR is higher when the forward current is higher also. However, this is not a linear relation. As you can see, the CTR will start to drop at a certain level of forward current.

When you design the circuit, it is advisable to set the forward current before the CTR value is starting to drop.

Operating Temperature

Below curves are still taken from VO617A optocoupler datasheet. The y-axis is the normalized CTR value for non-saturated and saturated condition. The x-axis is the ambient temperature. For instance, the maximum ambient temperature of operation is 60’C, the normalized CTR is around 0.8 using the saturated graph. This means that whatever the typical CTR value must be multiplied by 0.8.

Aging

An optocoupler is a device that uses LED as light source. LED intensity weakens over time. Thus, CTR will be affected.

Power De-rating

The given power dissipation in the datasheet either for the LED, output or package is based on the typical ambient temperature that most of the times at 25’C.

If the actual operation is beyond this temperature, the power dissipation capability or rating of the device will decrease. This is called power de-rating.

Pdiss_derating = (Tjmax – Tamb_max) / Rthja

Where Tjmax is the maximum junction temperature of the device as specified in the datasheet, Tamb_max is the maximum ambient temperature of operation and Rthja is the thermal resistance from junction to ambient which is given in the datasheet also.

Optocoupler Important Characteristic Curves

When you dig into an Optocoupler datasheet, you can see the following characteristic curves.

• V_CEsat vs Ambient Temperature
• CTR vs Forward Current (saturated and non-saturated)
• CTR vs Ambient Temperature
• I_C vs V_CE

All of them are important to help you further understand the behavior of an optocoupler device you are selected.

V_CEsat vs Ambient Temperature

This is a characteristic curve which describes how the collector-emitter saturation voltage varies with the ambient temperature. The VCEsat at 25’C is not anymore the same to that at 100’C. Below is an example curve from Vishay Semiconductors Opto-coupler. Thus, you must be mindful in your design.

CTR vs. Forward Current

There is a CTR provided in the early page (if not right at the front page) in the datasheet. This is not showing any relationship with respect to the forward current. If you want to know the possible CTR at a given forward current, you need the CTR vs. forward current curve. There are two versions of the curve. One is for non-saturation and the second one is for saturation.

CTR vs. Ambient Temperature

CTR is not constant value but vary with respect to ambient temperature also. In most cases, datasheet specified normalized CTR vs. ambient temperature. Normalize means whatever the number specified will be the multiplier to the nominal CTR. For instance in below curve from Vishay Semiconductor optocoupler, if the normalized CTR value is 0.8, this means that whatever the nominal CTR specified is to be multiplied by 0.8.

I_C vs V_CE

If you want to know how the collector current going to affect the collector-emitter voltage, you need to see the I_C vs. V_CE curve. This is not a big concern though when an optocoupler will operate at hard saturation.

Circuit Design Using Optocoupler

Here is a simple optocoupler circuit. The objective of this example is to calculate the collector-emitter voltage. Let us assumed the diode forward voltage (V_F) and CTR to be 1.2V and 100% respectively. You can derive these parameters from the datasheet.

Step 1: Calculate the forward current, I_F

IF = (V1 –  VF) / R1 = (10V – 1.2V) / 1k = 8.8 mA

Step 2: Calculate the collector current, I_C

IC = CTR x IF = 100% x 8.8 mA = 8.8 mA

Step 3: Calculate the collector-emitter voltage, V_CE

V_CE = V2 – I_C x R2 = 10V – (8.8 mA x 10k) = -78 V

You noticed that the computed V_CE is negative. There is no negative value is real application. This actually means that the optocoupler is at hard saturation. At hard saturation, the ideal value of V_CE is zero but in reality, it will be clamped to the V_CEsat voltage declared in the datasheet.

If you like to learn more about circuit design involving optocoupler, read the article:

• Optocoupler Circuit Design and Detailed Analysis