Lesson 8/1747%

MODULE 08

Component Selection Criteria

A systematic selection methodology: derive the isolation requirement from the application, filter by speed and output type, size CTR with lifetime margin, respect temperature and package constraints, and optimize cost across volume scenarios — ending in a reusable selection worksheet.

Learning Objectives

After completing this module, you will be able to:

Derive isolation requirements from system architecture

Filter device families by speed and output needs

Apply CTR margin methodology for lifetime reliability

Trade off package, temperature, and certification constraints

Optimize selections for cost at volume

Where this fits. Modules 5–7 taught you the families, the parameters and the datasheet; Module 9 the ageing margins. This module puts them in order: a repeatable funnel that turns an application into a short list of parts that can be defended in a design review. The isolation numbers here are representative IEC 60664-1-style values; your end-product standard (Module 16) is the authority.
Illustrations and animations are original teaching drawings, not to scale. Values marked representative are typical of common parts; always confirm against the current datasheet or standard you design to.

Selection goes wrong when it starts from a favourite part or from price. The robust order is: hard safety constraints first, then function, then life, then manufacturing, and only then cost. Each stage removes candidates, so you optimise price among parts that already work.

Funnel diagram with six stages from all isolators on the market to two or three candidates: isolation requirement, speed and output type, CTR with lifetime margin, temperature range, package and certificates, cost and supply — **Figure 1.** The selection funnel. Stages 1–5 are pass/fail; stage 6 is optimisation.

Animation: twelve candidate optocouplers are filtered step by step for a 230 volt industrial PLC input, leaving two — **Animation 1.** Twelve plausible candidates, five filters, two survivors. Every rejected part fails a stated requirement, which is exactly what a reviewer wants to see.

The isolation requirement is derived from the system, not from the part. Five inputs decide it:

Five panels comparing functional, basic, supplementary, double and reinforced insulation between a hazardous voltage and a user-accessible part — **Figure 2.** Insulation classes. An optocoupler between mains and a touchable SELV circuit usually needs reinforced insulation (or double).

Input	Where it comes from	What it sets
Insulation class	Hazard analysis & end-product standard (what is touchable?)	Basic vs reinforced multiplier; DTI/PD requirements
Working voltage	Highest rms (for creepage) and peak (for V_IORM) across the barrier, incl. DC bus, ripple	Creepage table row; V_IORM
Overvoltage category	Where the equipment connects to the mains	Rated impulse → clearance, V_IOTM
Pollution degree	Environment inside the enclosure	Creepage table column
Altitude	Installation site (> 2000 m needs correction)	Clearance multiplier (e.g. ≈ 1.14 at 3000 m, 1.29 at 4000 m)

Four panels showing overvoltage categories IV, III, II and I with rated impulse voltages of 6, 4, 2.5 and 1.5 kilovolts for a 300 volt system — **Figure 3.** Overvoltage categories for a 300 V line-to-earth system (representative). Reinforced insulation uses the next impulse step up.

Calculator: Isolation Requirement

Representative values — confirm with the governing standard table. Creepage from IEC 60664-1-style tables (doubled for reinforced), clearance from the rated impulse (one step higher for reinforced) with altitude correction. End-product standards (IEC 62368-1, 61010-1, 60601-1, 61800-5-1) can require more.

Working voltage (rms or DC)

Insulation class

Pollution degree

Material group

Overvoltage category

Altitude

Waveform

Minimum V_IORM (peak working)–

Minimum creepage–

Rated impulse → minimum clearance (altitude-corrected)–

Transient rating to look for (V_IOTM)–

Solid insulation–

Package families that fit–

–

Speed decides the family before anything else in the datasheet matters. Use the data rate and the timing accuracy you need:

Requirement	Rule of thumb	Families that fit
≤ 1 kHz, status / relay signals	Any	Phototransistor, Darlington, AC-input
1–50 kHz	Phototransistor with R_L ≈ 1 kΩ and moderate overdrive (Module 4)	Phototransistor (PC817, 4N35 + R_BE)
50 kHz – 1 Mbit/s	Needs photodiode-based detection	6N135/6N136, logic parts
1–15 Mbit/s	PWD ≤ 10–20 % of bit time; skew budget	6N137, TLP2361, HCPL-26xx
> 15–25 Mbit/s or tight skew	Optocouplers run out of margin	Digital isolators (Module 15)
PWM for power stages	Delay matching between channels for dead time	Gate-driver couplers with specified skew

Worked example — PWM duty accuracy

20 kHz PWM (50 µs period) must keep duty error ≤ 0.5 %.

Allowed PWD = 0.005 × 50 µs = 250 ns.
A phototransistor (µs-class asymmetry) cannot meet it; a 6N137-class part (PWD ≤ ≈ 35 ns) passes easily.

For CTR-specified families, select the rank from the end-of-life requirement, not the new-part behaviour (Modules 6 and 9):

Required CTR_min (rank) = CTR_needed × margin / (k_IF · k_T · k_VCE · k_age) margin ≈ 2 for switching

Worked example — choosing the rank

Circuit needs CTR 25 % at end of life (Module 4 saturation checker). Factors: 0.9 × 0.8 × 0.9 × 0.5 = 0.324. Margin 2.

Required rank minimum = 25 % × 2 / 0.324 ≈ 154 %.
Rank B (130 %) is not enough; rank C (200 %) passes. Alternatively raise I_F (costs life) or halve the needed CTR with a larger pull-up (costs speed).

For logic and driver parts Select on the I_F threshold instead: design I_F ≥ 1.5–2 × the maximum threshold at end of life, and within the recommended I_F(on) range.

Module 5 covered each family in depth. In selection terms, the output requirement usually narrows the choice to one or two families:

Your output needs to…	Choose	Watch
Drive a logic input	Phototransistor (slow) or logic output (fast)	Pull-up, V_CE(sat) vs V_IL
Carry an analog value accurately	Linear (IL300/HCNR201) or ADC + digital isolator	K3 tolerance, op-amp supplies
Be part of a feedback loop	Phototransistor in linear region	CTR spread → loop gain spread; opto pole
Switch a small DC/AC load	PhotoMOS SSR	R_ON, leakage, surge
Switch a mains load	TRIAC driver (zero-cross or random-phase)	V_DRM, snubber
Drive a power switch gate	Gate-driver coupler	Peak current, UVLO, CMTI, DESAT
Detect AC presence / zero cross	AC-input coupler	Resistor power and voltage

Match the part’s rated range to the internal ambient of the enclosure, which is often 15–30 °C above the room:

Grade	Typical ambient range	Notes
Commercial	0 … +70 °C	Rare for optocouplers; most are industrial by default
Industrial	−40 … +85 / +100 / +110 °C	Check the upper limit and the derating curves
Extended / automotive	−40 … +125 °C	AEC-Q101 (discretes) or AEC-Q100 (IC-based couplers) qualification, PPAP, change notification
High-temperature	up to +125 °C and beyond	Gate drivers near power modules; ageing budget becomes dominant

The selection question is not “is 85 °C inside the rating” but “does the part still meet CTR, leakage and derating at the enclosure’s worst temperature for the full life” (Module 9). A part rated to 110 °C running at 85 °C has far more derating headroom than one rated to 100 °C.

With the isolation requirement known, the package is largely decided by creepage and assembly (Modules 3 and 11):

Constraint	Implication
Creepage ≤ 5 mm	SOP-4/SO6 compact packages are fine
Creepage 5–7 mm	Standard DIP or DIP gull-wing
Creepage ≥ 8 mm	Wide-lead DIP, stretched SO6/SO8, wide SO16
Reflow-only assembly	SMD options; check MSL and floor life
Wave/THT line or harsh vibration	Through-hole DIP
Board area critical	SO6/SOP-4; stretched SO for reinforced in small area

Collect the certifications your end-product standard needs before shortlisting:

Need	Typical evidence
Isolation withstand	UL 1577 file (V_ISO)
Working-voltage / reinforced claim	IEC 60747-5-5 / VDE 0884-5 certificate (V_IORM, PD test)
Regional acceptance	CQC (China), CSA (Canada) as required
End-product standard specifics	e.g. IEC 62368-1, IEC 61010-1, IEC 60601-1 acceptance of the component certificates
Automotive	AEC-Q101/Q100, PPAP, change notification

Design trap — certificate options. The certificate lists which ordering options it covers. The wide-lead, SMD and standard versions may have different entries, or none. Match the BOM part number to the certificate.

Only now does price matter. Four cost levers, in order of impact:

Family

A phototransistor is typically an order of magnitude cheaper than a logic coupler or isolator channel; do not buy speed you do not need.

Volume tier

Price per part falls steeply with volume; quote at your real annual quantity.

Channel count

Multi-channel parts and digital isolators amortise cost per channel.

Hidden BOM

Supplies, bypass caps, pull-ups, TVS, board area and test time. An isolated DC-DC can cost more than all the couplers together.

Log-log chart of illustrative cost per channel versus annual volume for four options: PC817-class phototransistor lowest, reinforced stretched-SO phototransistor, 6N137-class logic coupler and 2-channel digital isolator per channel highest — **Figure 4.** Illustrative price model (not quotes). Crossovers depend on channel count and on whether an isolated supply already exists.

Interactive: Cost-at-Volume Comparator

Volume slider is logarithmic (10²–10⁶ units/year). Illustrative prices; replace with your quotes. Includes support parts and, if needed, a share of an isolated DC-DC supply.

Annual volume

Channels per unit

Isolated supply already present on the far side?

–

Second sourcing Pin-compatible alternates are common for PC817- and 6N137-class parts. Approve alternates only after comparing CTR ranks, timing and insulation tables (Module 7). Digital isolators are often pin-compatible across vendors but not with optocouplers (Module 15).

Application A — 24 V PLC digital input (industrial cabinet)

Step	Decision
Isolation	Field side and logic side are both SELV: functional isolation for noise/ground loops (verify with the PLC standard, IEC 61131-2). Creepage per functional needs; standard packages suffice.
Speed / output	< 1 kHz, logic input → phototransistor.
CTR	I_F ≈ 3 mA from the IEC 61131-2 input current window; needed CTR 25 % EOL → rank C (Section 4).
Temperature / package	−25…+70 °C cabinet (internal up to 85 °C); SMD SOP-4 or DIP SMD on a reflow line.
Result	PC817-class (rank C) SMD or TLP185-class; about 16 channels per module → consider 4-channel packages for area.

Application B — 230 V flyback feedback (IEC 62368-1, reinforced)

Step	Decision
Isolation	Mains primary to SELV secondary: reinforced; working 250 V_rms, OVC II, PD2 → creepage ≈ 5.0 mm, clearance ≈ 3.0 mm (representative), V_IORM ≥ 354 V_pk plus the flyback reflected-voltage peak across the barrier.
Speed / output	Linear feedback loop → phototransistor in the active region.
CTR	Narrow rank (e.g. B or C) to limit loop-gain spread (Module 4); EOL margin for regulation at CTR_min.
Temperature / package	Inside the supply up to 100–105 °C; standard DIP (≈ 7 mm) or wide-lead for margin; VDE-certified option.
Result	PC817-class VDE option, rank B, 4-pin DIP/SMD; confirm with the supply’s safety file.

Application C — isolated RS-485 at 1 Mbit/s in a 400 V drive

Step	Decision
Isolation	Touchable communication port vs 400 V (up to 300 V line-to-earth), OVC III, PD2 inside the enclosure: reinforced → creepage ≈ 6.3–8 mm, clearance ≈ 5.5 mm (6 kV impulse), V_IORM ≥ the DC-bus-referenced peak.
Speed / output	1 Mbit/s: PWD ≤ 100 ns and CMTI ≥ 2× drive dV/dt at the port (e.g. ≥ 25–50 kV/µs) → logic coupler or digital isolator.
Channels	DI, RO and DE: three channels plus isolated 5 V supply.
Result	Three 10–15 MBd logic couplers in stretched SO6 (8 mm) or a 3-channel digital isolator + isolated transceiver; cost comparator usually favours the integrated isolated transceiver at volume (Module 15).

Work through the funnel for your own interface. The worksheet generates a requirement specification and candidate families, saves your entries in this browser, and prints cleanly for a design review.

Interactive: Selection Worksheet

Representative isolation values; confirm with your end-product standard.

Application / interface name

Working voltage across barrier (V rms / DC)

Insulation class

Pollution degree

Overvoltage category

Output function

Max. signal rate (kbit/s or kHz)

Max. internal ambient (°C)

Required life (years)

Assembly

Automotive?

Requirement specification

Safety first

Derive the isolation requirement from the system before looking at parts.

Five inputs

Insulation class, working voltage, OVC, pollution degree, altitude.

Speed picks the family

Data rate and timing accuracy narrow the choice immediately.

Rank from EOL

Required rank = needed CTR × margin / Πk-factors.

Internal ambient

Select the temperature grade for the hottest point in the enclosure.

Package from creepage

Wide or stretched packages for ≥ 8 mm.

Certificates per option

Match BOM ordering codes to certificate coverage.

Cost last

Compare total cost (supplies, caps, area) at real volume.

Design checklist

Tick items as you review a design. Your ticks are saved in this browser only; use your browser’s print function for a sign-off copy.

Four parts: multiple choice, true/false, a matching exercise and engineering scenarios. Score 80 % on the multiple choice to pass the module.

Part A — Multiple Choice

Question 1 of 10 • Score: 0/10

Part B — True or False

Score: 0/8

Part C — Match the Term to Its Meaning

Drag each term onto its definition, or click a term and then click a definition.

Placed: 0/6

Part D — Engineering Scenarios

Scenario 1 — The cheapest part that failed certification

A team picks a SOP-4 phototransistor (5 mm creepage) for a 230 V product to be sold for installation in industrial cabinets (PD3, reinforced). It is the cheapest option. What should the funnel have caught?

Analysis. Step 1: at 250 V, PD3, group III, representative basic creepage is 4.0 mm, so reinforced needs 8.0 mm. The SOP-4 is out before price is considered. Fix: a stretched SO6 or wide-lead DIP with a VDE certificate covering that option; then compare prices among the survivors.

Scenario 2 — Rank or current?

A circuit needs 30 % CTR at end of life. Factors multiply to 0.36; margin 2. Rank B is 130 % min, rank C 200 % min. Which rank?

Analysis. Required minimum = 30 % × 2 / 0.36 ≈ 167 %. Rank B fails, rank C passes. Raising I_F to rescue rank B would shorten LED life (Module 9), so order rank C.

Scenario 3 — Optocoupler or digital isolator at volume?

A 4-channel 1 Mbit/s interface ships 200 000 units/year. The far side has no isolated supply yet. Option 1: four 6N137-class couplers + isolated 5 V DC-DC. Option 2: a 4-channel digital isolator with integrated isolated power. How do you decide?

Analysis. Both need far-side power, which dominates cost. Compare total BOM: four couplers + bypass caps + discrete DC-DC vs one integrated part (plus its caps). Also compare board area, CMTI, PWD and qualification. At high volume and 4 channels the integrated isolator is often cheaper and smaller; the couplers win on simplicity, availability and where analog or fail-safe behaviour matters (Module 15). Document the comparison with real quotes.

Additional Resources

Standards

IEC 60664-1 (insulation coordination) • your end-product standard (IEC 62368-1, 61010-1, 60601-1, 61800-5-1, 61131-2) • AEC-Q101/Q100 for automotive.

Vendor tools

Parametric search and cross-reference tools from optocoupler vendors; certificate databases (UL Product iQ, VDE certificate search) to confirm option coverage.

Keep learning

Module 10 turns selections into complete circuits; Module 11 carries the creepage onto the PCB; Module 15 compares with digital isolators; Module 16 covers the standards.

← Reading the Datasheet Next: Optocoupler Derating →