Achieving Regulatory Compliance For Leakage Currents With Class I Vs. Class Ii Smps

Defining Leakage Currents

Leakage currents refer to small uncontrolled currents that flow through insulation and over surfaces in electrical equipment. These currents occur due to capacitive and resistive paths in circuits and can pose safety hazards like electric shocks. Defining allowable limits for leakage currents is a key part of safety regulations and standards for electronic devices.

There are several types of leakage currents:

Earth leakage current – Current that flows from live parts to the earth ground.
Touch current – Current that flows when a human body comes in contact with exposed conductive parts.
Patient leakage current – Leakage current measured through a patient connected to a medical device.

Limits for earth leakage and touch currents help ensure user safety by preventing electric shock hazards. Compliance requires keeping these currents below specified thresholds over the device’s rated voltage, temperature, and lifetime ranges.

Class I vs Class II SMPS Designs

Switched-mode power supplies (SMPS) come in two safety classes that take different approaches to managing leakage currents:

Class I SMPS

Relies on earth ground connection to shunt leakage currents
Has earthed conductive chassis that helps drain leakage currents
Requires safety ground wiring in power cord
Higher earth leakage currents permitted

Class II SMPS

Prevents hazardous leakage currents instead of shunting
Reinforced or double insulation between circuits and chassis
No earth ground connection
Much lower allowable earth leakage currents

Class II designs are more complex but isolate user from hazards, while Class I depends on earth ground link being present to ensure safety. Class selection impacts techniques needed to meet leakage current regulations.

Key Standards and Regulations

Safety standards and regulations for leakage currents help define design requirements to protect users. Key references include:

IEC 62368-1

Applies to electrical equipment across wide range of voltages
Harmonizes various national safety standards
Defines tests for measuring earth, touch, and patient leakage currents
Specifies maximum acceptable current thresholds

IEC 60950-1

Legacy standard for Information Technology Equipment (ITE)
Being superseded by IEC 62368-1 but still used
Focuses on fire and electric shock hazards
Has more narrowly defined leakage current limits

UL 62368-1

North American safety standard aligned with IEC 62368-1
References European limits for leakage currents
Provides guidance on tests for certification

Designing SMPS for leakage current compliance requires cross-checking against applicable national/international safety standards early in the design process.

Measuring and Calculating Leakage Currents

Quantifying leakage currents to validate compliance involves both calculations during design and empirical measurements during testing:

Calculation

Model parasitic capacitance/resistance in circuit simulations
Compute expected earth leakage and touch currents
Verify compliance to maximums permitted in standards
Guide design changes to reduce leakage

Measurement

Use digital multimeter in current measurement mode
Utilize automated test equipment (ATE) for production testing
Attach probes between chassis, earth ground to quantify leakage
Compare readings versus allowable limits across voltage, temperature ranges

Accurately calculating and measuring leakage currents ensures the SMPS design meets safety margins defined in the standards prior to certification testing.

Design Techniques to Reduce Leakage

To avoid electric shock hazards, SMPS designs must keep leakage currents under specified limits across expected operating conditions. Key techniques to reduce leakage include:

Improved Printed Circuit Board Layout

Increase separation between high voltage and low voltage circuits
Optimize conductor spacing to reduce parasitic capacitance
Place ground planes to shunt unwanted leakage current
Model electric fields with simulation to guide layout

Adding Components Like Y Capacitors

Connect conductors to chassis ground via Y caps to bypass leakage
Use X capacitors for filtering rather than line-chassis paths
Properly rate capacitor voltage to avoid overstress failure

Selecting Components Like Fans

Avoid unprotected fan blades that generate additional leakage current
Specify low current fans with insulated power cables
Protect user from all accessible conductive parts

Leveraging such methods ensures target leakage current levels are achieved to satisfy applicable safety limits.

Testing and Validating Compliance

To receive certification for product safety, SMPS designs must undergo standardized testing to validate leakage current limits. Key aspects involve:

Working with accredited laboratories to perform tests
Measuring leakage over voltage, temperature ranges
Verifying operation under single fault condition
Testing to applicable regulatory safety standards (IEC, UL)
Submitting test reports to agencies for certification
Monitoring for changes to safety limits requiring re-validation

Early engagement with labs to align test plans with the appropriate methods and pass criteria accelerates the certification process for timely market introduction.

Example Circuits and Component Selections

For illustration, example SMPS circuits can demonstrate design and component selection to reduce leakage current and achieve regulatory compliance:

Medical Applications:

Reinforced insulation required between patient ports and internal electronics
Opto-couplers ensure electrical isolation
Shielded cables manage radiated emissions that impact readings
Y capacitors safely pass EMI noise to chassis ground

Consumer Applications:

Class II design with conformal board coating for pollution degree 2
Fuse resistors limit fault current to avoid RFI filters overheating
Fan grille connected to filter capacitors eliminates exposed blades
X capacitors reduce conducted emissions for EMC compliance

Such circuit implementations demonstrate design choices tailored for target use environments and safety class to achieve leakage current compliance.