Introduction and Scope 

Electrical and electronic systems need low-voltage DC feeds to operate various sub-systems, controls, and displays, and provide safe human interface means. In addition to safe human access, proper isolation of circuits internal to power supplies and maintaining them in end systems is vital to proper functioning and safety compliance of the end system.

For both operational and safety reasons, isolating potential human contact areas from hazardous voltage and energy is fundamental to electrical systems engineering. To achieve this basic but critical objective, safety standards address methods of dealing with faults, fault energy, thermal excursions, material degradations, flammability, and so on. A good part of the above falls under the purview of power systems design and manufacturing, but isolation needs to be understood well and verified at various stages of end system design, qualification, and production.

There is abundant information and knowledge available in safety standards and handbooks like the National Electric Code (NEC) on acceptable methods of achieving isolation and test methods. The scope of this application note is to address the application considerations for systems engineers to incorporate into their development and qualification processes.

Isolation and insulation apply to a broad range of systems, but the emphasis of this application note is on switching power supplies, UPS, isolation transformers, and EMI filters. Although LED drivers and motor drives are not discussed specifically, isolation considerations concerning human contact are similar to other AC mains-powered systems.

The term “Power conversion” is used here for simplicity due to most systems commonly employing some form of AC to DC power conversion, which is normally the only stage that connects to the AC mains.

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General Architecture of Power Conversion Systems 

Before switching power supplies became commonplace, a line frequency transformer-rectifier-based power supply was commonly used. The transformer had all the isolation built in and there were no other blocks that needed to be isolated.

A general sectional view of the simple line frequency transformer showing where insulation/isolation is built in is shown in the figure below.

Transformer-Rectifier Power Systems 

Fig. Electrical schematic of single input-output transformer	Fig. Cross-sectional view of bobbin and windings

In the classical method, the enameled winding wires are wound in several layers with adequate insulating tapes between primary and one or more secondary windings. Primary winding is generally tied to the hazardous circuit while secondary winding is tied to the safe circuit. The whole winding stack is wound on a bobbin which sits over the magnetic core. In addition to insulation between windings, there’s also sufficient space or bobbin material between the windings and core, or the core may have a coating of insulation either wound or formed on it.

Quite often the bobbin is split into multiple sections and windings are placed spatially in each section achieving the needed insulation with bobbin material rather than insulation tapes.

A quick view of the two types of bobbins is below.

Fig. 3a. Windings, core, metal shell and insulation

To achieve the specified isolation, the windings should be isolated from each other, the core, and the overall metal shell, as shown in the simplified diagram below.  

Fig. 3b. Windings, core, metal shell, and insulation

Triple Insulation Wire

The triple insulation wire is a huge development in the quest to achieve the needed isolation without labor intensive insulation tapes and plastic structures, leading to ultra miniature magnetic components and consequently power supplies. The figures below show the general overview of the triple insulated wire structure and finished transformer.

It also eliminates the labor associated with winding insulation tapes which explains the popularity of this wire in all compact products.

Fig. 4 Triple insulated wire and winding example

Switching Power Supplies 

The advent of switching power supplies brought about more complexity, more building blocks, a feedback loop, and a compact layout in a compact enclosure. When laptops and portable electronics became common, compact switching power supplies technology evolved rapidly, with Class II power adapters becoming more commonly used. Likewise, home use medical systems evolved and needed power adapters with medical compliance.

A typical switching power supply power conversion method is shown in figure below.

Fig. 5 Basic switching power supply

Isolation specifications became more challenging to comply with as power supplies became more compact, to the point where creative solutions like triple-insulated wires and ultra-compact Y-capacitors were required to achieve high isolation and dielectric withstanding test voltage. In miniature wall warts and cell phone chargers need to incorporate the needed isolation with no exceptions as they involve human contact.

Referring to the block diagram from a safety perspective the overall power supply can be separated into three sections: hazardous input, low voltage safe output, control & system interface and protective earth. In safety parlance, they are called Live circuits, SELV circuits, and protective earth (PE). Other than the live circuit, everything else is considered a potential human contact area, hence needs to be isolated from the live circuit. The level of isolation, represented by an acceptable level of insulation, is shown by one or two lines to indicate Basic and reinforced insulation.

In medical grade products, it is common practice to refer to isolation from a patient safety perspective as a ‘Means of Patient protection’ (MOPP) as 1-MOPP or 2-MOPP as shown. The test values of 2-MOPP being double or higher than 1-MOPP. There’s also a level of protection slightly lower than patient safety, which is from an operator’s safety perspective, also called MOOP.

Fig. 6 Isolation levels in power supplies

Isolation and Insulation 

Basic purpose of isolation is to unconditionally ‘separate’ the user from accidental contact or from destructive energy. Safety engineering involves a little more than just separation, but a separation that is consistent with the voltage, energy, time and temperature effects, and so on. To present us with an easy-to-interpret and follow method, the safety standards define the type of insulation that can be provided and validated. The means of validating the insulation happens to be the Dielectric strength test, in short HiPot. However, achieving the stated Hi Pot test compliance does not waive provision of the needed clearance and creepage.

There are innumerable examples of systems where a clear visible separation between hazardous circuits and human contact is not feasible. The most common example is an appliance power cord plugged into a wall socket and a human hand holding the wire. There is clearly no room to provide several mm of space, but the presence of an insulating layer of plastic of sufficient thickness and with a safety certified material is sufficient to achieve the stated insulation and deemed to isolate the hazardous circuit adequately. That’s the case with most commonly used home appliance cords that come with hair dryers, irons, vacuum cleaners, and lamps.

Typically, a 0.5mm thick insulation over copper wire is adequate to qualify as isolated, but must also survive a defined Hi Pot test. Safety agencies inspect production lines to ensure that certain minimum standards are maintained in the manufacturing of the plastic and the final cord to provide continued assurance of safety.

To provide clarity, safety standards provide specific values for clearance, creepage, and HiPot for the different isolation and insulation classifications, as shown in the example of a medical end-use system in the table below.

It should be noted that isolation levels are higher when patient contact is involved.

Note: MOOP= means of operator protection; MOPP= means of patient protection

Factors Affecting Isolation 

While isolation and insulation are built into the design and manufacturing process, it is imperative that safety is forever and should meet the target specifications. That means the products structurally don’t degrade during the course of the end product’s intended usage, age, or environmental effects. While standards have enormous details on materials, design, manufacturing processes, and inspection methods, a quick snapshot of factors that affect isolation will be helpful to buyers and end users. Defective insulation materials with pin holes, cracks, and incomplete filling materials

• Conductive debris lodged between parts accidentally during manufacture
• Sustained build-up of debris (pollution) over time due to local operating environment.
• Internal defects in bridging components like Y-capacitors, optoisolators, ceramic capacitors
• Errors in building with wrong parts, especially in bridging parts
• Defective PCB and/or contaminated surface finishes such as conformal coating
• Untrimmed component leads under PCBs poke holes into insulating sheets under PCB
• Defective AC inlets, EMI filters or feed-through terminals
• Local polluting effects build up a partially conductive layer over time
• Infestation and secretions from insects and micro-organisms degrading insulating materials

Much of the factors listed above are methodically addressed in the standard, some quick practical methods to verify the integrity of insulation and consequently isolation with Hi-Pot and leakage tests is a very established practice in component, sub-assembly, and final system testing.

Final system-level compliance statements in published documents require final Hi-Pot and leakage tests on a 100% basis.

Material classification and Comparative tracking index (CTI)

Safety standards recognize that all insulation materials are not the same. When two circuits are separated along the surface of an assembly board, the board’s material insulation ability has a lot to do with how resistant it is to break down along its surface, or tracking. Pollution tends to gradually deteriorate the board’s insulating property faster than any other effect. Conductive pollutants and the presence of moisture accelerate tracking and drastically reduce insulation, hence degrade designed in isolation. Common office equipment like printers and copiers release significant amounts of carbon which tend to deposit on circuit boards and insulating materials.

Safety standards define isolation requirements for different pollution degrees and material groups used in the design.

Higher pollution degrees require higher CTI material to qualify for stated isolation. Systems engineers should be critically aware of their intended operating conditions and state them in the procurement specifications of power converters.

• Degree 1 environments include clean rooms and sealed components 
• Degree 2 environments include offices and laboratories 
• Degree 3 environments include industrial manufacturing areas
• Degree 4 environments include outdoor environments with excessive humidity and/or dust

Where pollution cannot be avoided, as in most cases, care must be taken to prevent conductive debris from reaching the circuits or external filters used at air inlets.

The criteria for factoring in pollution picking the appropriate material types and incorporating isolation into the design apply to all components as well as the end system. This is because isolation barriers are all in parallel and a breakdown in one is enough to jeopardize the safety of the whole system.

Isolation Specifications of Low Power, High Power, and UPS Systems 

There are several types of switching power supplies and systems. From the simple single input-single output nonisolated designs to the complex multi-stage UPS systems, the basic methods of specifying isolation and testing remain the same. The degree or level of isolation however depends on the values of input and output voltages and the installation class. A quick summary of the types of power converters and isolation features for AC to DC power converters are shown in the table below.

Table 1: Typical isolation test voltages in common power conversion systems

Notes:

1. Board power is a generic term for low power low PCB mounted types
2. Medically certified products have higher values consistent with 1 and 2-MOPP; Also applicable to a higher power
3. Control and Interface signals are considered equivalent to the output circuit and isolated by the same level;
4. Depending on the manufacturer’s preference; higher output voltages are designed with higher Output to Earth isolation to enable series stacking.
5. DC Hipot test instead of AC is quite acceptable and common; 1500Vac= 2121Vdc; 3000Vac=4242Vdc; 2-MOPP is 4kVac=5656Vdc.
6. 3-Phase input power systems have higher values, subject to some variation among brands. In general, there is no human contact intended 

Benefits of Hipot Testing 

The primary objective of Hipot testing is to confirm the integrity of all insulation and isolations in the product in one quick test. This is one of the non-negotiable safety tests on the final product before it is packaged for shipment.

Similarly, it is also among the most critical tests for receiving and incoming inspection. A failure in Hipot is a definite reason for rejecting the failed unit and possibly inspecting the whole lot. A failure analysis and corrective action quickly follow to ensure there is no repeat failure. Quite often a Hipot test unravels one or more of the below defects that would otherwise be almost impossible to detect.

• Accidental foreign objects lodged
• Defective bridging components
• Wrong parts installed
• Defective PCB, magnetics, and overall insulation tapes
• Bent components due to handling
• Defects induced by improper packaging or damage induced by transportation

Bridging Components and System-Level Isolation Test (Hipot) 

In principle, providing the needed isolation between hazardous and safe circuits is straightforward, but the unfortunate reality is that all power conversion products need parts that ‘bridge’ the isolation. Examples are Y-capacitors used to mitigate EMI noise, Opto isolators for feedback and control, and insulators placed below and in-between PCBs and sheet metal enclosure (earth).

A common feature of safety-recognized bridging components like Y-capacitors and optoisolators is that the leads on either end are isolated galvanically or by qualified insulation that meets all the requirements of basic or reinforced insulation, Y2 capacitors can withstand basic insulation level or 1-MOPP equivalent Hipot test, Y1 capacitors withstand reinformed level and safety recognized Opto-isolators can withstand reinforced or 2-MOPP equivalent.

In addition, there are several components used within filter assemblies and other encapsulated components like board-mounted power converters that are invisible but appear in parallel with main isolation, creating a complex system of insulation. If the components, materials, and manufacturing are all in order, the collective insulation system performs like a single insulation. The only practical method of confirming the collective health of the isolation system is a Hipot test. In addition, leakage current will provide additional verification methods.

Verifying safety credentials (certificates) and safety logos on the components is good practice in QA and final inspection processes.

It should be noted that components like EMI filters have several Y-capacitors inside, all of them appearing in parallel due to AC terminals being shorted during the Hipot test. The combined value results in a significant inrush current when the Hipot test voltage is applied tripping the current limit detection. To prevent this problem, a soft ramp of test voltage is required. Safety test teams are well-versed in the problem and settings of the Hipot tester.

Bridging components like MOVs and gas discharge tubes used to mitigate lightning surges are commonly tied between the  HV AC circuit and Earth. They come with certain hi-pot test capabilities, but not equivalent to reinforced/2-MOPP level. When the final power supply or end system is tested for HiPot to reinforced/2-MOPP level, a substantial voltage appears across the front-end Y-capacitors and MOVs breaking them down. Safety agencies recognize this effect and allow for removing them during reinforced testing. More often than not, it is not possible to remove them after the final assembly. Due to this safety agencies permit only Basic level Hipot in final assembly, especially with a Class1 system. Class2 systems. A graphic depiction of all commonly used bridging components is shown in the figure below.

Fig. 8 Bridging components

Bridging components: practical examples

The example below is from a real flat panel TV power supply with isolation boundaries and bridging components identified. Although this is well-known to power supply designers, the concepts apply to their end systems as well. Where some portion of the system is tied to the primary hazardous circuit and others to the safe circuit, the system should maintain equivalent isolation capability if not higher.

As noted earlier, if the manufacturing process requires testing to reinforce/2-MOPP, the rating of all bridging parts should be taken into account and determined if the power supply is a known limitation. 

Fig. 9     Practical example of isolation design (TV power supply board)

Another commonly encountered front-end component is the EMI filter. It is generally integrated within the power supply as an assembly of discrete common mode inductors, X and Y capacitors all laid out in the hazardous area but with bridging Y capacitors tied to earth as shown in the figure below.

Fig. 10   Bridging components in EMI filter

Because the bridging components of the EMI filter do not connect to the secondary circuit, the isolation barriers are consistent with basic insulation. However, when the final assembly is tested for reinforced insulation, the bridging components see a voltage higher than the basic insulation level. When there is no clamping device like a MOV, there is nothing to trip the Hipot tester, which ends up damaging the Y-capacitors. To avoid this problem, the Hipot tester should be set to a lower current detection limit. It should be noted that all the Y-capacitors in the system appear in parallel during a Hipot test, which results in a combined leakage current much higher than estimated. The test voltage ramp should also be set low enough that it doesn’t drive current through the capacitors due to a fast ramp rate and trip the tester.

AC-AC Double Conversion Power Conversion System (UPS) 

A simplified diagram of a double-conversion UPS is shown in the figure below. The building blocks are similar to conventional switching power conversion circuits each with inherent isolation attributes.

The main AC input to the AC Output power train involves multiple down and up conversations to be able to incorporate the battery, which is usually 12, 24, or 48V. Regardless of the battery voltage, its internal smart battery management system (BMS) is often considered low voltage and ties directly to human contact interfaces like on/off switches, fuel gauges, and status indicators. With both AC input and Output considered high voltage, it has to be isolated from potential human contact other than plugging in power cords, which are safety certified.

The overall UPS for both industrial, including consumer home use or medical system isolation specification and verification is not much different from a single input-output switching power supply.

The optional transfer switch adds a layer of insulation to the battery and interface circuits, as the AC output could be directly connected to the AC input when the UPS has an internal failure and the transfer switch is activated to continue the mission till a replacement system is available. Although direct human contact is not expected at the AC output port, its circuit is double insulated from any interface signals.

Fig.11    Isolation boundaries in Double conversion UPS

Multiple Power Supplies   

Quite often, control signals like Enable, Power Good, and Load share are referenced to Auxiliary DC output or isolated from everything else. They are treated like secondary main DC output, but isolated by a lower test level. Typically, a 500Vdc test applies. Regardless of the level of isolation, care must be taken to observe some precautions when power supplies are interfaced with the end system and potentially end up breaking the functional isolation. Likewise, when power supplies are series stacked, there is no easy way to tie the control signals together. An opto-isolated interface is needed for each PSU’s control signals as shown in the figure below.

Fig. 12   Series and parallel power supplies

Standards for Additional References: 

Several standards have addressed safety aspects of materials, components, systems design, and test methods to validate isolation and associated parameters in extraordinary detail. For details beyond the scope of this application note, the following standards serve as excellent references. Most of the standards are harmonized globally with additional exceptions by specific countries and complying with a specific standard is determined by the intended market for the end system.

• UL/IEC 62368-1: Safety standard-Consumer electronics, information technology, communication systems safety
• UL/IEC 60601-1: Medical electrical equipment safety
• IEC/EN 61000-x-y: Electromagnetic Compatibility (EMC)

Safety labels

Safety labels are designed and monitored rigorously by the safety agencies. From a user perspective, there is some critical information presented graphically with symbols defined by the standards. The symbols are governed to the finest detail to prevent fraud. Customs officials frequently inspect the labels for authenticity and validity. Consumers should get into the habit of checking the ratings, isolation, and disposal messages printed on the label.

An example of a product safety label on a desktop monitor is shown in the figure below.

Fig. 13   Product safety label

Summary and Conclusion

Insulation and isolation are fundamental to any electrical system. There may be several boundaries to cross as grid power is processed and converted from its natural form AC to different types of output voltages needed to run the various sub-systems. A clear isolation diagram of the overall system and its interface to the outside world should be undertaken before any real hardware development is undertaken. The technology of power conversion is highly dependent on the target isolation and several other aspects like leakage current, EMC, efficiency, density, size, cost, time to develop, and launch to market are highly dependent on the basic safety attributes.

Quite often, the configuration of the power conversion system can change lock, stock, and barrel due to a misunderstanding of the operating environment. The presence of conductive pollutants in the environment can drastically alter the selection of materials and design to meet the target isolation requirements at launch and over time. Hazardous locations with potential flammable gases add additional complexity to design and material selection.

The presence of a wide range of electrical and electronic systems all around us in our day-to-day life as well as going through an occasional medical procedure without worrying about shock is a testament to fine engineering and manufacturing practices that have been governed by standards perfected over decades of learning and practice.

Hopefully, this application note has served to highlight some of the basic principles of electrical systems engineering with emphasis on isolation for human safety.

Disclaimer: 

 The content in this application note is intended solely for general information purposes. It is provided with the understanding that the authors and publishers are not herein engaged in rendering engineering or other professional advice or services. The practice of engineering is driven by site-specific circumstances unique to each project. Consequently, any use of this information should be made only in consultation with a qualified and licensed professional who can consider all relevant factors and desired outcomes. The information in this application note was posted with reasonable care and attention. However, some information in this application note may be incomplete, incorrect, or inapplicable to particular circumstances or conditions.  

Astrodyne TDI does not accept liability for direct or indirect losses resulting from using, relying, or acting upon information in this application note.