This article is meant to deliver some technically verified ideas of SMPS design. Readers can gather some ideas on the areas that need special care during designing their products, Read on…

The shortest distance between two conductors through air in between is called the Clearance. It depends on the factors mentioned below.

  • Peak Working voltage across insulation (Line-Neutral)
  • Transient Overvoltages that may occasionally appear on the line conductors.
  • Short term & long term Overvoltages that may occur between the line conductor and earth in electrical installations.
  • Altitude

Creepage: This is the shortest distance between two conductors  along the insulating surface in between. It depends on the above-mentioned factors of clearance as well as some other factors mentioned below.

  • Material group (l/11/llla/lllb)
  • Pollution  degree
  • R.M.S. working voltage

Note: Clearance and creepage distance shall be within the values mentioned in different Safety Standards like I EC 60950-1/1 EC 61010-1/1 EC 62368…etc)

Behaviour of Creepage & Clearance under High Stress

At high frequencies or in large clearances, the ions are trapped between  the electrodes, since the polarity changes too rapidly for them to be extracted at either  end of conductors on PCB.

At high frequencies or in large clearances, the ions are trapped  between  the electrodes, since the polarity changes too rapidly for them to be extracted at either  end.

Unlike clearances, creepage distances and solid insulation  are not replenishable because permanent damage, such as carbonized tracking and puncture, are likely.

Clearances are the leading cause of the phenomenon observed. Since charged ions travel along the electric field lines, any change in field direction could cause the ions to collide with, or be reflected away from, the dielectric surface. PD occurs in the clearances along the surface where the ionized gas is trapped, deteriorating carbon-based surface material, and making it more susceptible to carbonized tracking.

Determination of Clearances, Creepage Distances with High Frequency Voltage Stress

Insulation coordination is a discipline that seeks to achieve  the optimal technical and economical balance necessary to protect people and equipment against overvoltages, with the goal of enhancing system operability (e.g., continuity of the electrical supply)  and product safety.

It is implemented by matching the electric strength of equipment with respect to surrounding environmental conditions and the electrical stresses that are expected to occur throughout its anticipated lifetime, such as:

  • Repetitive  working voltages necessary for the intended functioning of the equipment
  • Transient overvoltages of atmospheric  origin or switching surges
  • Temporary overvoltages  due to earth fault in the power distribution system, from which the A.C. mains supply is derived
  • Transient overvoltages due to interconnection to other equipment, or connection to telecommunication networks  or cable distribution systems

Creepage Distances: Though the dimensioning of creepage distances is of little relevance to random transient overvoltages, it is commonly understood that the creepage distances must be equal to or greater than the associated clearances.

In summary, for frequencies not exceeding 30kHz, only the RMS-value of any periodic voltage is relevant. For frequencies greater than 30 kHz, the dielectric breakdown resulting from the effects of PD has priority over the phenomenon of carbonized tracking, due to the accumulation of charges along the insulating surface.

Dimensioning creepage distances can be ignored if the surface material is not likely to be deteriorated by thermal effects (e.g., ceramics). However, dimensioning clearances still applies in such cases.

Case Study: Investigation of a Switch Mode Power Supply

In this section, determining appropriate clearances, creepage distances and solid insulation will be illustrated through the investigation of a novel Switch Mode Power Supply (SMPS), designed for compliance with various energy saving standards.

In general, the choice of operating frequency is achieved by balancing the following factors:

  • Acoustic Noise: Ears can normally hear sounds in the range of 20 Hz to 20kHz; hence, the operating frequency  would  not fall into this range (usually above 20kHz).
  • Power Density: In theory, power density is proportional to the operating frequency; the higher the operating frequency, the smaller form factor.
  • Efficiency: Higher operating frequency  comes with higher switching loss and the associated conduction loss.
  • Components: The tradeoff  between high performance and cost-effectiveness.
  • EMC: Challenges associated with electromagnetic compatibility issues.

The operating frequency of a modern SMPS often ranges from 20kHz  to 1MHz. (For example, a high power  inverter  with  IGBTs, SMPS with Power MOSFETs, and DC-DC converters with MOSFETs would operate in a range of 20-30kHz, 50-200kHz and 100-400 kHz respectively.) The application of frequencies above 1MHz is rarely seen in practice, and a steep slope (i.e., a rapid change in behaviour)  is usually observed when the frequency  is above 1MHz.

The investigation discussed here was conducted  with an SMPS employing a variety of devices, including a PFC controller with standby mode programming, a half-bridge LLC resonant converter with  adjustable operating frequency  up to 500kHz  tailored for a broad range of load conditions, a Synchronous  Rectifier (SR) controller, and a synchronous step-down switching regulator.

General technical information regarding the SMPS is as follows

  • Rated voltage range: 100-240 V a.c., 50/60 Hz
  • Overvoltage category II: Pluggable equipment supplied by fixed electrical  installation of buildings
  • Pollution degree 2
  • Peak working voltage: 514 V peak
  • RMS working voltage: 270 V RMS
  • Short-term temporary overvoltages: 1,440 V RMS, if it is intended to be supplied by IT or TI power system
  • Operating frequency:  500kHz

Comparative tracking index (CTI): Material Group Ill


Clearances to withstand transient overvoltages

Rated Impulse Voltage referenced from IEC 60664-1: Nominal system voltage<::; 300 V A.C.

Overvoltage category: II

Rated impulse voltage for equipment: 2,500 V

Clearances for Case A (inhomogeneous field) from Table F.2 of IEC 60664-1: Required impulse withstand voltage: II

Pollution  degree: 2

Clearances for basic insulation= 1.5 mm

Clearances for reinforced  insulation =3.0 mm

Clearances to withstand periodic voltages (taken into account for high frequency voltage stress)

Clearances for Case A (inhomogeneous field) from Table 1 of IEC 60664-4, upon the condition that the operating frequency is above f (critical):

Peak working voltage<::; 600 V Peak Clearances for basic insulation= 0.065 mm Clearances for reinforced  insulation = 0.50 mm

Creepage Distances

Creepage distances to avoid carbonized tracking Creepage distances referenced from IEC 60664-1: RMS working voltage: 270 V r.m.s.

Pollution  degree: 2

Comparative  tracking index (CTI): Material Group Ill Creepage distances for basic insulation= 2.7 mm Creepage distances for reinforced insulation= 5.4 mm

  1. Creepage distances to avoid deterioration due to high frequency voltage stresses

Creepage distances from IEC 60664-4: Peak working voltage: 514 V peak

Pollution  degree: 2

Operating frequency::<; 500kHz

Creepage distances for basic insulation = 0.6 mm

Creepage distances for reinforced  insulation = 1.2 mm

For creepage distances, the effect of carbonized tracking takes priority over the effect of deterioration due to high frequency voltage stresses. Therefore, the minimum creepage distance for basic insulation is 2.7 mm, and that of reinforced insulation  is 5.4 mm. No alignment is needed as the minimum creepage distances are greater than the minimum clearances (Creepage of Reinforced insulation= 2* Basic Insulation)

A comparison between different product safety standards

Instead of a comprehensive comparison of the dimensioning of clearances, creepage distances and solid insulation found in IEC 62368-1and the legacy standards (i.e., IEC 60950-1and IEC 60065), this section offers a brief overview through  a series of simplified charts.

In brief, the clearances of the legacy standards are obtained  by assuming the A.C. mains supply does not exceed 300 V RMS (i.e., A.C. mains transient  of 2,500 V peak, representing equipment designed for a universal input  voltage range of 100-240 V A.C.), pollution degree 2, overvoltage  category II. Hence, the traces of the legacy standards depicted result in an offset of 2 mm for basic or supplementary insulation, and 4 mm for reinforced  insulation, and associated additional values in Table 2L of IEC 60950-1.

The first crossover of legacy standards and creepage distances for fundamental frequencies exceeding 700kHz  but equal to or less than 1,000 kHz (Derived from IEC 60664-1) occurs at 600 V peak. This means that, if peak working voltages exceeding 600 V peak and operating frequencies  exceeding 700kHz  are likely, a rapid demand growth for creepage distances is inevitable.


Emerging energy saving standards have Ied to a marked preference for switch mode power  supply topologies. However, the use of SMPSs has raised concerns over the sharp change in dielectric properties under high frequency voltage stress, and the attendant safety risks. This article has provided some basic information about the partial discharge phenomena, and the rationale underlying the applicable product safety standards, in an effort  to assist compliance  engineers to better understand applicable testing requirements.

Nirod Ranjan Samantaray is a Senior ReguIatory Compiiance Specialist.

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