Electrical and Electronic Design

Electrical design begins before the schematic

A circuit can be technically elegant and still be unsuitable for the medical device. The engineer must first understand what the product is intended to do, who interacts with it, where it operates, what can cause harm and how electrical functions interact with software, mechanics, users and external systems.

See MTL-102 — Intended Purpose, Users and Use Environments and MTL-103 — User Needs and Design Inputs for establishing the product definition and testable requirements.

Build an electrical architecture that explains the design

The architecture should make power flow, signal flow, control, isolation, protection, diagnostics and external interfaces visible. It should also show how the design supports safety and essential performance.

Use explicit engineering budgets

Allocate system-level requirements to electrical elements before detailed design. Useful budgets include power consumption, battery capacity, accuracy, noise, latency, timing, thermal rise, communication loading, memory, diagnostic coverage and reliability. Record margins and assumptions so later changes can be assessed.

• Define normal, peak, start-up, charging, sleep and fault-condition power.
• Identify current return paths, reference potentials, shielding and chassis strategy.
• Allocate end-to-end accuracy and uncertainty across sensors, references, analogue stages, conversion, algorithms and calibration.
• Define start-up, reset, brownout, watchdog, power-loss and recovery behaviour.
• Specify interface voltage, timing, protocol, fault tolerance and compatibility.
• Separate safety-relevant, high-noise, high-energy and sensitive measurement domains where needed.
• Identify programmable devices and the boundary between hardware and software control.
• Show product variants and permitted combinations of boards, accessories and firmware.

See MTL-112 — Systems Engineering, Architecture and Interfaces for system allocation, states and interface control.

Design electrical safety into the product

Electrical safety is not achieved by passing a final dielectric-strength test. It depends on the equipment classification, applied parts, supply, environment, insulation system, accessible parts, energy sources and behaviour in normal and fault conditions.

The distinction between means of patient protection and means of operator protection affects construction and testing. It should be resolved with the complete use and contact model—not added to the schematic as an unexplained label.

External power supplies, isolation modules and certified components can support the strategy, but their certificates do not certify the finished medical device. Their ratings, conditions of acceptability, interfaces and use within the complete construction must be assessed.

Connect performance to risk

Electrical and electronic functions often implement clinical performance, protective measures or both. The design must show what happens when sensing, processing, power, communication or actuation is absent or degraded.

• Identify electrical functions that implement or support essential performance.
• Translate clinically meaningful limits into circuit and interface requirements.
• Analyse open circuit, short circuit, drift, saturation, stuck output, intermittent connection and out-of-sequence behaviour.
• Consider latent faults and whether diagnostics detect them before another failure occurs.
• Assess common causes such as shared power, references, clocks, processors, connectors and environmental stress.
• Define fault detection, annunciation, controlled continuation, recovery and shutdown.
• Verify that risk controls remain effective at component tolerances and environmental boundaries.
• Examine how cybersecurity compromise could affect electrical control, sensing or safety response.

Risk analysis should generate design requirements; design decisions should update the risk analysis. A failure-mode table completed after schematic release cannot substitute for this continuing engineering dialogue.

See MTL-113 — Essential Performance and Safety Concepts and MTL-105 — Medical-device Risk Management for the underlying safety reasoning.

Control sensing, measurement and actuation

Medical-device electronics frequently make decisions from small signals and then control functions with clinical consequences. The complete measurement or control chain must be understood, not merely each component’s nominal specification.

Accuracy requirements should include the complete chain and relevant uncertainty. A high-resolution converter does not create an accurate system when the sensor, reference, analogue front end, calibration, mechanics or environment dominate the error.

Boundary behaviour matters: start-up, warm-up, transition, overload, recovery and invalid data may be more safety-relevant than steady-state nominal operation.

Select components for the supported life

Component selection is a technical, manufacturing, supply and lifecycle decision. Nominal electrical fit is only the starting point.

• Confirm ratings across voltage, current, power, temperature, frequency, duty cycle and environmental stress.
• Apply justified derating and analyse tolerance, drift, ageing and worst-case combinations.
• Assess package, assembly process, cleanliness, moisture sensitivity, handling and inspectability.
• Review manufacturer quality, change notification, traceability, authenticity and supply continuity.
• Identify safety-critical and performance-critical components with controlled specifications.
• Record required approvals, certificates and conditions of acceptability.
• Evaluate single-source exposure, lifecycle status, obsolescence and authorised alternatives.
• Control programmable parts, factory configuration, calibration data and device identity.
• Ensure substitutions receive technical, risk, manufacturing and verification impact assessment.

The bill of materials should identify what is actually controlled. A generic value such as “10 kΩ resistor” may be sufficient in one location and inadequate in another where voltage rating, tolerance, stability, construction, flammability or safety approval matters.

Turn the circuit into a controlled physical design

The PCB and assembly are part of the circuit. Placement, routing, stack-up, materials, return paths, spacing, thermal behaviour and manufacturing processes determine whether the schematic’s intent survives in the product.

Use schematic, layout and manufacturing reviews with representatives from systems, mechanics, software, verification and production. The most damaging issues often occur at interfaces: connector orientation, shared references, mechanical stress, thermal paths, programming states or undocumented assembly options.

Engineer EMC rather than test for it

Electromagnetic compatibility requires both controlled emissions and adequate immunity. For medical devices, immunity acceptance criteria must be derived from safety and essential-performance behaviour, not limited to whether the device resets visibly.

• Define intended electromagnetic environments and nearby sources.
• Identify ports, cables, apertures, seams and power paths that can couple disturbances.
• Control loop area, return continuity, filtering, shielding, grounding and enclosure bonding.
• Place protection at the point of entry and provide a low-impedance path for disturbance energy.
• Manage switching-node area, edge rates, clock harmonics and common-mode currents.
• Protect high-impedance and low-level measurement circuits from conducted and radiated coupling.
• Define performance criteria for every relevant test and operating mode.
• Exercise representative loads, accessories, cables, communications and clinically significant functions.
• Plan pre-compliance investigation early enough to change architecture or mechanics.
• Record observations and recoveries even when the formal acceptance criterion is met.

Passing in one convenient configuration does not show that all marketed configurations are covered. Select and justify worst-case hardware, software, cables, accessories, power modes, radio activity, loads and operating states.

See MTL-304 — IEC 60601 Electrical Safety and EMC for the standards-led view.

Electrical engineering across the lifecycle

Electrical design continues from product definition through released-product support. The engineer’s evidence and decisions must mature with the device.

Verification, transfer and technical evidence

Verification should combine analysis and testing. Some conclusions—such as worst-case stress, tolerance, timing or thermal margin—cannot be demonstrated credibly by one nominal bench test. Others need physical evidence from representative assemblies.

Every important result should identify the requirement, method, equipment, configuration, environmental conditions, acceptance criteria, actual result, anomaly disposition and approval. Maintain traceability from system need and risk through electrical requirement, design output and evidence.

See MTL-106 — Verification and Validation for the wider evidence strategy.

Common misconceptions