Optocoupler vs transformer

APPLICATION NOTE 1002

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Isolink Optocouplers vs. Pulse Transformers

In isolation applications where one needs to pass signals in presence of transient or continuous high voltages, reject extreme noise, and break ground loops, optical (optocouplers) and magnetic (pulse transformers) coupling isolation are often used.

However pulse transformers are much more difficult to design with relative to optocouplers. When dealing with pulse transformers, the load becomes more critical than with any other type of transformer. Leakage and primary inductance values take on added significance because of the effect they have on the output waveshape.

If the loaddisturbs this wave shape, severe problems can be created. [1] In many applications, it is difficult to obtain a usable waveform with a pulse transformer (See Figure 1). This is essentially due to three factors: waveforin droop, effects of the turns ratio on risetime, and backswing. [2] A comparision between transfromer and optocoupler characteristics are presented in Table 2.

Optocouplers are being used to replace pulse transformers due to their ease of design, simplicity, and smaller size. The use of state-of-the-art AlGaAs emitters in Isolink’s optocouplers has greatly improved performanceand reliability. AlGaAs LED emitters are brighter, faster, more efficient and linear, and slower to degrade than GaAs and GaAsP type emitters used in older optocoupler designs. For hybrid assembly applications, Isolink offers the OLI optocoupler family in a small size,.100″ X.110″ X.045″ high, package.

Figure 1. Output Waveform Comparisions:
Optocoupler vs. Pulse Transformer

for “chip and wire” assembly. For applications requiring hermetic packaging, optocouplers are available in 6-pin leadless chip carrier (LCC) surface mount packages or TO-5 packages. Isohnk’s products range from simple phototransistor outputs to high speed, high common mode rejection logic gates. The typical data rates range from 5OKBaud to 10 MBaud. (See Table 1).
NOTES AND REFERENCES
1.
2.

”Pulse Transformers”, Eric Marchese,
Powertechnics Magazine, December 1989.
Hewlett Packard Tech Brief 103.

Table 1. Isolink Optocouplers

Product

Descriptions

Typical
Data Rate[1]

Recommended
Input Current

Typical Common Mode
Transient Immunity [2]

OLX
OLX
OLX
OLX

Transistor
Photodiodes Transistor
Photodiodes Darlington
High Speed logic gate

100 K bits/s
1 M bits/s
100 K bits/s
10 M bits/s

5 mA
10 mA
0.5 mA
5 mA

± 5,000 V/us
± 5,000 V/us
± 5,000 V/us
± 10,000 V/us

100
300
400
500

Notes: 1. At 25°C and recommended operating conditions for the particular product 2. Vcm = 50V p-p

Table 2. Advantages of Optocouplers Over Pulse Transformers

CHARACTERISTICS

PULSE TRANSFORMERS

OPTOCOUPLERS

Waveform droop is a linear function of the pulse width, for example, in a given
transformer of 10% droop at I us, droop will be 20% at 2 us. The magnitude of
the droop should not exceed the designed allowable threshold or fraction of the
initial amplitude. This constrains duty cycle and speed. Better magnetic flux
coupling will improve droop characteristics but usually results in higher primary
to secondary capacitance, which reduces common mode transient immunity and
dielectric withstand voltage.

No waveform droop

2. Waveform backswing

Backswing could damage adjoining sensitive circuitry. Clamping diode might be required to reduce backswing amplitude. Backswing also limits transformer duty cycle and performance.

No backswing

Primary to Secondary
Turns Ratio

Capacitance depends on primary to secondary turns ratio. Equivalent capacitance will increase with higher turns ratio increasing rise time. Unity turns ratio must be used for high speed applications consuming minimum
voltage and current at the primary to achieve a usable logic level signal at the output

No turns ratio effects

Data Format Requirements

Complex and expensive data encoding and decoding usually required due to constraints to duty cycle and to eliminate the signal’s DC components.

No need for data
Conversion

Common Mode Transient
Immunity

Common mode capacitance typically higher optocouplers. Fast common mode signals are more likely to be coupled through to the secondary. Improvements such as incorporating a Faraday shield between the primary and secondary or using two transformers in series will increase complexity, cost and board space.

Higher common mode transient immunity then most pulse transformers, without extra complexity.

Size

Usually larger and higher profile than optocouplers.

Small size and low profile. Optocouplers for hybrid assembly are about .100″
square and .045″ high.

Output Waveform Fidelity
1. Waveform droop

Isolink assumes no responsibility for the accuracy of data described herein and make no representations or warranties, express or implied, that such data is free from error.

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