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LT8311

8311f

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APPLICATIONS INFORMATION

Picking the Opto-Coupler

The voltage feedback loop, explained earlier, uses an

opto-coupler to convey output voltage information from

the secondary side to the primary side (see Figure 17).

An opto-coupler is used because of its wide prevalence,

relatively low cost, and its ability to convey DC signal

information over an isolation boundary with potential

differences of up to 5000V.

The input of an opto-coupler typically consists of an

infrared light-emitting diode (LED), while the output

is typically a phototransistor. Current flowing into the

opto-coupler’s input LED, called IF, causes photons to

be emitted. These photons cross the opto-coupler’s

isolation barrier and get collected in the base of the output

phototransistor. This photo current, which essentially

forms the phototransistor’s base current, is gained up

by the phototransistor’s ß (current gain) before flowing

out of the opto-coupler, and is called IOPTO_OUT. The key

parameter of interest in an opto-coupler is the current

transfer ratio (CTR). CTR is typically expressed in units

of %, and is calculated as follows:

CTR(%)

IOPTO_OUT

Figure 17. Typical Opto-Coupler Configuration in a Voltage Feedback Loop

8311 F17

IOPTO_OUT

PRIMARY SIDE

OF VOLTAGE

FEEDBACK LOOP

VCC

OPTO

PRIMARY

SIDE

SECONDARY

SIDE

LT8311

OPTO-

COUPLER

ISOLATION

BOUNDARY

where IOPTO_OUT is the output current of the opto-coupler

and IF is the opto-coupler’s input LED current

Opto-couplers have historically been disliked, and jus-

tifiably so, for having CTRs that degrade with operating

lifetime, at higher operating temperatures, and at higher

input currents (IF). Much of this CTR degradation comes

from a reduction in the quantum efficiency of the input

LED, which is a function of the LED’s operating current

(IF), operating temperature and operating lifetime.

Fortunately, LED technology has matured over the last

coupleofdecades,andhasallowedimprovementsinopto-

coupler performance, a discussion of which is beyond the

scopeofthisdatasheet.AvagoTechnologieshaspublished

documentation showing 3-sigma CTR degradation of no

more than 10% over 30 field years of operation for their

opto-couplers manufactured with AlGaAs type LEDs run-

ning 5mA of input current (IF) at 100% duty cycle, and at

85°C ambient temperature.

Please refer to the application/design notes from opto-

coupler vendors such as Avago Technologies, CEL and

Vishay, to procure further information on opto-couplers.

Atypicallyrecommendedopto-coupleristhePS2801from

California Eastern Laboratories (CEL).

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