Isolation in solar power
converters: Understanding theIEC62109-1 safety standard
Xiong Li
Systems Engineer

Anant Kamath
Systems Engineering Manager
Isolation Interface Group
Texas Instruments

Understanding the IEC 62109-1 safety standard
for solar power converters enables you to pickthe right isolation solutions for solar power
conversion applications.
There is a pressing need to accelerate the development of advanced energy technologies
in order to address the global challenges of clean energy, climate change and sustainable
development [1]. Solar energy is the cleanest, most abundant renewable energy source
available, and plays a key role in the development of sustainable energy systems.
The solar power generation capability in the United States (U.S.) has gone through an
exponential growth in the past decade. The U.S. has installed 1,393 megawatts (MW) of
solar photovoltaics (PV) in the second quarter of 2015 to reach 22.7 gigawatts (GW) of total
installed capacity. This is enough to power 4.6 million American homes [2]. Globally, by theend of 2015, the PV installation capability reached 233 GW.
Solar power conversion systems are used to

Isolated analog-to-digital converters (ADCs) andconvert solar energy to electrical energy, which isisolated amplifiers are utilized to sense and convert

either fed to utility grids or used by off-grid electricalanalog front-end signals like voltages and currents

networks. In a solar power conversion system,

for the purpose of close-loop controls. Isolated data

solar panels are operated to convert solar energy

links are applied to transfer information from circuits

to electrical energy, and power converters arereferenced to high voltage to ones referenced

employed to further process the harvested electricalto earth.

energy. In a solar power converter, high-voltage

The International Electrotechnical Commission (IEC)and low-voltage circuits co-exist. Isolations are

62109-1 [5] is a safety standard for solar power

required between the high-voltage and low-voltage

converters. This standard defines the minimum

circuits for both functional and safety purposes.

requirements for the design and manufacture of

Fundamental isolation concepts and terminology are

power conversion equipment (PCE) for protection

presented in references [3-4].

against electric shock, energy, fire, mechanical

Digital isolators can be used to address the isolationand other hazards. This standard provides general

requirements. In a solar power conversion system,

requirements applicable to all types of PV PCE.

different types of isolators are adopted to serve

This paper discusses the electrical aspects of thevarious functions. Isolated gate drivers are used to

IEC 62109-1 safety standard and analyzes how its

drive insulated gate bipolar transistors (IGBTs) or

stipulations on insulation requirements translate into

metal-oxide semiconductor field-effect transistors

specifications for isolators used in solar(MOSFETs) in the high-voltage power stage.

power converters.

Isolation in solar power converters

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January 2019

Figure 1. Typical system block diagram of a transformer-less solar power conversion system.

This discussion is limited to a few selectare applied to solar power conversion systems in

configurations and uses example cases to provide

order to achieve higher system efficiency, power

an understanding of the main principles of IEC

density and cost-effectiveness. Isolated gate drivers

62109-1 relevant to isolators. For instance, we

provide the required drive voltage to turn IGBTs on

will discuss only three-phase systems with a rated

and off. Typically, a gate-emitter voltage of 15 V issystem voltage of up to 1000 VRMS and rated DC

used to turn on IGBTs, and a negative gate-emitter

link voltage of 1500 V. For a more comprehensive

voltage of –8 V is used for IGBTs turn off. The gate

understanding of these requirements, see thedriver supply voltages for SiC MOSFETs are typically

IEC 62109-1 standard [5].

+20 V and –5 V. Isolated current and voltage
sensors are used to sense the currents and voltages

Isolation in solar power converters

in order to realize a closed-loop control system.

Figure 1 describes a simplified system block

A control module that contains a microprocessor or

diagram of a transformer-less grid-tied solar power

field-programmable gate array (FPGA) processes

conversion system. The solar power is harvested by

the feedback signals from the voltage and current

a PV panel and processed by post-stage

sensors, and provides the right sequence of

DC/DC and DC/AC converters. The DC/DC

pulse-width modulated (PWM) control signals toconverter is used to implement maximum power

the IGBT/SiC MOSFET gate drivers at the rightpoint tracking (MPPT) of the solar energy. The

frequency, which regulate the voltages and currents

DC/AC inverter is utilized to convert DC power toof the power converters. The voltage and current

AC power, which can be interfaced by a utility grid.

regulations are intended to realize MPPT and power-

Conventionally, IGBTs with a switching frequency

flow control to the grids.

of tens of kilohertz are used to form the DC/AC
converter. Nowadays, more and more SiC MOSFETs

Isolation in solar power converters

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January 2019

The control module interfaces with the rest of the

The control module, input side of the isolated gate

control network (for example, to a computer ordrivers, and input side of the isolated amplifiers are

human-machine interface ([HMI]) through standard

referenced to the DC– bus in Figure 2.

communication interfaces such as RS-485, CAN or

In this case, additional digital isolation provides

industrial Ethernet.

safety isolation between this control module (which

The control module has parts that are accessible

is connected to high voltage) and a second control

to humans, for example, the connectors of theor communications module that is earth-referenced.

communication interface. Sufficient safety isolation

The human-accessible parts or interfaces lie on

is required between these exposed parts and thethe second control module. In this architecture,high-voltage circuits (circuitry connected to theisolation is still required in both the gate drivers andDC buses and utility grids). This isolation can be

the isolated current and voltage-sense modules

achieved through isolated gate drivers and isolated– not from an electrical safety perspective but

voltage and current-sense amplifiers. In Figure 1,

for functional purposes. Here, isolation allows

the human accessible control module, input side

the first control module (referenced to DC–) toof the isolated gate drivers, and input side of thecommunicate with the IGBTs and sensors, whose

isolated amplifiers are referenced to the earth, which

ground references are the inverter outputs that are

is safety-isolated from the high-voltage systems.

switching high voltage with respect to the DC– bus.

Additional isolation can be introduced between thecontrol module and communication interface, as
shown in Figure 2.

PV

DC
DC

Figure 2. Alternative system block diagram of a transformer-less solar power conversion system.

Isolation in solar power converters

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January 2019

Shown in Figure 3 is a system diagram of a

3. Grid-tied circuits – The system voltage depends

transformer-based, grid-tied solar converter. In thison the ground-earthing patterns [6].

architecture, a high-frequency transformer is used

• TN and TT systems – The root-mean-square (RMS) value

to implement high-voltage isolation between the PVof the rated voltage between a phase and earth.

circuits and grid-tied circuits, which adds additional

In TN and TT systems, both line frequency transformer

safety margins. However, inclusion of the isolation(or generator) neutral and equipment frames are earthed.

transformer brings extra power loss and accounts

In a TT system, equipment frames are earthed locally,for further board space, which means more cost.

and are not connected to the line neutral. In a TN system,

The isolation requirements of the PV circuits andthe equipment frames are earthed by connecting to thegrid-tied circuits need to be considered separatelyline neutral.

for this case. More details will be presented later in• A TN system is called corner-earthed system if one phase

this paper.

earthed. The system voltage is the RMS value ofphase-to-phase voltage

IEC 62109-1 definitions

4. IT systems.

1. System voltage – In a solar power system, there• The RMS value of the rated voltage between a phase andare two sub-circuits, which are PV circuits and grid-an artificial neutral is used to determine thetied circuits. The system voltages of the PV circuits

impulse voltage.

and grid-tied circuits are determined separately.

• The RMS value of the rated voltage between phases is

2. PV circuits – The system voltage is the open

used to determine the temporary overvoltage.

circuit voltage of the PV panels.

5. Single-phase IT systems – This is the RMS valueof the rated voltage between phase conductors.

Figure 3. System block diagram of a transformer-based solar power conversion system.

Isolation in solar power converters

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January 2019

L1
L1
L2
L3
L2
L3

Figure 4. (a) A star-earthed TN system that is neutral-earthed; and (b) a corner-earthed TN system where one phase is earthed.

Applying the IEC62109-1 standard toa solar power converter design

6. Overvoltage category (OVC) – addresses thetransient voltages. According to the IEC 62109-1
standard, equipment falls into one of these

Now that you have an understanding of isolation

four categories:

with earthing concepts and terms, you can apply• Category I: Applies to equipment connected to a circuit

the IEC 62109-1 requirements to a solar power

where measures are taken to reduce surges andconversion system design step-by-step. Unlesstransient overvoltage.

specified otherwise, we use OVC III for grid-tied• Category II: Applies to portable tools and plug-connectedcircuits and OVC II for PV circuits, and pollution

equipment not permanently connected to the mains.

degree 2 in this discussion. Also, this document• Category III: Applies to equipment connected permanently

only covers requirements for isolators versus otherto supply mains downstream of the distribution board.

components in a solar inverter system, such as• Category IV: Applies to equipment permanently connected

power modules and heat sinks.

at the origin of the installation upstream of the main

Step 1: Identify the isolators present in the

distribution board.

system and determine if each needs functional,

Equipment in a higher OVC category sees higher

basic or reinforced isolation.

voltage disturbances with higher possibilities, so the

Sufficient safety isolation (or protective separationequipment needs to address a higher requirementin IEC 62109-1) is required between circuitries

on the surge voltage and temporary overvoltage. In

connected to high voltage and parts that area solar power conversion system circuits connected

accessible to humans. A designer can achieve

to the PV panels belong to OVC II, while the circuits

protective separation either by two basic isolators inconnected to the grid fall under OVC III.

series or through one reinforced isolator.

Working voltage – Working voltage is the continuous

In the solar power conversion system (Figure 1),

voltage presenting across the insulation barrier ofthe isolated gate drivers and isolated voltage andan isolator whenever the equipment is in operation.

current-feedback circuits both need to support

This value is not directly related to the systemreinforced isolation. Basic isolation is sufficient

voltage. Instead, it depends on system architecture

if another basic isolation is inserted through theand operation conditions.

isolated data links. In the solar power conversion
system (Figure 2), the digital isolator needs tosupport reinforced isolation because the isolated
gate drivers and amplifiers are referenced to DC–,and only functional isolations are implemented.

Isolation in solar power converters

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Step 2: Determine system voltage- The impulse voltage from the PV circuits on grid-tied
circuits is determined based on PV system voltage andAs discussed, system voltages for PV circuitswith an OVC that is one level lower than theand grid-tied circuits are defined separately. For

PV circuits.

PV circuits, the system voltage is the open circuit
voltage of the PV panels. For grid-tied circuits, the- The impulse voltage rating of the grid-tied circuits is

system voltage depends on the earthing scheme.

the higher one of (a) and the value calculated.
d. For solar power conversion systems

A three-phase 400 VRMS TN grid voltage that is(Figures 1-2) not providing galvanic isolation between

neutral-earthed has a system voltage of 230 VRMS. A

the grid and PV circuits, the impulse withstand voltage

three-phase 480-VRMS corner-earthed system has aratings of the grid-tied circuits and PV circuits are

system voltage of 480 VRMS.

determined as in (a) and (b), and the higher value is used

Step 3: Determine the requirement for temporary

for the combined circuits. The impulse voltage rating ofovervoltage and impulse/surge voltage for eachthe combined circuits also applies to the circuits whichisolator from the IEC62109-1 standard.

are connected to the combined PV circuits withouta. For the grid-tied circuits, the impulse and temporary

galvanic isolation.

overvoltage can be determined according to the system

IEC62109-1 lists the requirements for basic

voltage and overvoltage category (OVC III is assumed).

isolation. For reinforced isolation, use the next-

Interpolation of the system voltage is not allowed for thehigher impulse voltage and a temporary overvoltage

grid-tied circuits, thus the next higher system voltageof 2 times the basic requirement. To determine

needs to be used. For example, a system voltage of 230

clearances (Step 4) for reinforced isolation, use a

VRMS is treated as a 300 VRMS based on IEC62109-1.

temporary overvoltage of 1.6 times (not double) theb. For PV circuits, OVC II is assumed. The minimum

basic requirement and the next-higher

requirement of the impulse withstand voltage isimpulse voltage.

2500 V. Different from the grid-tied circuits, system

Step 4: Determine the clearance required fromvoltage interpolation is allowed for PV circuits.

every isolator used in the designc. For solar power conversion systems with galvanic
isolation between the grid-tied circuits and PV circuits

IEC 62109-1 lists the clearance requirements for(Figure 3), the impulse voltage rating of the grid-tied

a given temporary overvoltage and surge/impulse

circuits and PV circuits are determined in the previous

voltage. Using this information, you can obtain thetwo bullets. The isolation transformer between the PV

clearance requirement based on the temporary

circuits and grid-tied circuits can reduce the OVC. The

overvoltage and surge requirements determined at

OVC reduction needs to follow the approach below:

Step 3.

- The impulse voltage from the grid-tied circuits on theIEC 62109-1 presents isolation requirements for

PV circuits is derived based on the system voltage

altitudes of up to 2000 m. To operate at higherof the grid-tied circuits with an OVC that is one level

altitudes, clearance needs to be increased by alower than the grid-tied circuits.

certain degree to account for the fact that air breaks- The impulse voltage rating of the PV circuits is thedown more easily at higher altitudes. This correction

higher one of (b) and the value calculated.

Isolation in solar power converters

factor for higher altitudes is defined in IEC 62109-1.

7

January 2019For example, to operate at 5000 m, increase the

However, some margin provides a buffer for theclearance obtained from this standard by a factor

effects for various working-voltage-transient profilesof 1.48.

to increase design robustness and reliability.

Step 5: Determine working voltage (both PK and

Step 6: Determine creepage based on RMS

RMS) based on the actual operating condition ofvalues of working voltage expected according

an isolator

to IEC62109-1

The working voltage does not directly follow from

IEC62109-1 presents the results for basic isolation.

the system voltage, but depends on the system

For reinforced isolation, double the creepage

structure and operations of the solar power

requirement. The creepage requirement depends on

conversion system. A sinusoidal PWM-modulated

the pollution degree and comparative tracking index

bipolar voltage having a swing between DC+(CTI) of the isolator.

and DC– is applied to the barrier of the high-side

Conformal coating or potting can be used toisolated gate drivers in Figure 1. However, thereduce the creepage and clearance requirement

high-side isolated gate driver sees a unipolar voltage

by reducing the pollution degree under the coating

swing between 0 to 2 VDC+ in Figure 2.

and blocking the path for arcing between pins.

The earthing scheme also plays a role in the working

However, these methods add cost, need additional

voltage. For the case in Figure 2, the DC– businspection steps to check the quality of the coating,is at a pretty steady voltage with respect to earth

and have limitations on the maximum voltage levels

potential when the grid is star-earthed while thesupported. Choosing an isolator with a higher CTI

DC- sees a high voltage swing if the gridand higher values of creepage and clearance isis corner-earthed.

often the cheaper and more reliable alternative.

The operations of the converter affect the working

For cases where the creepage requirement isvoltage in another way. For example, a higher

lower than the clearance requirement, you needmodulation depth of the inverter forces a higher

to increase creepage to meet the clearance. This

working voltage across the barriers of the high-side

adjustment is necessary because the creepage

isolated gate drivers in Figure 1. All of these factors

path along the package surface is also a path along

complicate accurately determining working voltage.

which air breakdown can occur.

Generally, a working voltage equal to the DC link
voltage covers most cases for solar converters.
AC phase
DC link- phasevoltagevoltage(Vdc)(VRMS)

Earthing
schemeSystemvoltage(VRMS)

Systemvoltage per
IEC62109-1

Basic/
reinforced

Temporary
overvoltage(VRMS/VPK)

Impulse/Minimum Working
surge
clearance voltage
voltage
(mm)
(VPK)
(VPK)

Minimum
Material group
I

II

III

480

1500

Neutral-earth

277

300

Basic

1500/2120

4000

3.0

1500

5

7.1

10

480

1500

Neutral-earth

277

300

Reinforced

3000/4240

6000

5.5

1500

10

14.2

20

480

1500

Comer-earth

480

600

Basic

1800/2550

6000

5.5

1500

5

7.1

10

480

1500

Comer-earth

480

600

Reinforced

3600/5100

8000

8.0

1500

10

14.2

20

Table 1. Summary of requirements per the IEC 62109-1 for our example system.
Isolation in solar power converters

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Step 7: Choose isolator that meets VISO, VSURGE,

drivers, digital isolators, isolated delta-sigma ADCs

working voltage, creepage and clearance

and amplifiers, and isolated communication links

requirements obtained above

such as isolated RS-485 and isolated CAN. Recent

Table 1 summarizes these requirements for anofferings include the ISO5851, ISO5852S, ISO5451

example transformer-less solar power conversion

and ISO5452 reinforced isolated gate drivers, digitalsystems under different system configurations,

isolators ISO77xx, and isolated delta-sigma ADCs

with OVC III for grid-tied circuits and OVC II for PV

AMC1304x and AMC1305x in a 16-pin small-circuits, pollution degree 2, and altitude <2000 m.

outline integrated circuit (SOIC) package, with 8-mm

For the example solar power conversion system,

creepage and clearance.

the AC phase-to-phase voltage is 480 VRMS, the DC

Included are isolated gate drivers UCC53xx withlink voltage is 1500 VDC, and the open circuit voltage

basic isolation in 8D package and reinforced isolationof the PV panel is 848VDC. With the open circuitin 8-DWV package. The isolated gate driver family

voltage of 848 VDC, the surge voltage requirement

has three different pin configurations featuring active

is determined to be 4000 VPK. The working voltage

miller clamp, split outputs, and bipolar power supply

is assumed the same as the DC link voltage, which

separately. Each pin configuration offers different

covers most architectures. However, the actual

options of drive strength. Digital isolators ISO78xx

working voltage required from the isolator depends

are available with an extra-wide 14.5-mm creepage

on where the isolator fits in the system architecture.

and clearance package. These extra-wide package
options are targeted to support high-voltage

Isolators for solar power
conversion applications(1500 VDC) solar power conversion applications.
These families of isolators have a 60-second

In response to these needs, Texas Instruments offers

temporary overvoltage of 5.7 kVRMS; bipolar, unipolar

several isolation offerings for solar power conversion

and DC working voltages of 1.5 kVRMS and 2121 VPK;

applications. These include isolated IGBT gate

and a surge voltage of 12.8 kVPK.

DeviceFunction

Creepage/Clearance (mm)Basic/Reinforced

System voltage
supported up to VRMS

Working voltage
supported up to
VRMS/VPKUCC53xxD

Gate Driver

4

UCC53xxDWVGate Driver

8

ISO585xDW

Gate Driver

8

Basic
Basic
Reinforced
Basic
Reinforced

300
600
1000
600
1000

800/1130
800/11301000/1414
800/1130
1500/2121

ISO77xxDWDigital Isolator

8

Basic

300

450/637

ISO78xxDWDigital Isolator

8

ISO78xxDWWDigital Isolator

14.5

Reinforced
Basic
Reinforced
Basic
Reinforced

1000
600
1000
1000
> 10001500/2121
800/1130
1500/2121
1450/2050
1500/2121

Table 2. TI isolation device capabilities per IEC 62109-1 (OVC III, pollution degree 2, altitude < 2000m).
Note 1: Occasionally package creepage and clearance limits the system and working voltage supported. For the intrinsic capability of the isolators,
see the corresponding product data sheets and reference [7]. For example, the intrinsic capabilities of the isolator can be realized at the systemlevel through conformal coating or potting.
Isolation in solar power converters

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January 2019

Additionally, these devices use a CTI > 600 (MaterialGroup I) package-mold compound, which enables
them to operate at higher working voltages at thesystem level, compared to competing devices
with the same creepage. The ISO78xx, ISO77xx,
ISO58xx, and UCC53xx devices also featurehigh common-mode transient immunity (CMTI),
low propagation delay, and split sink and source
strength, which can support high-performance
operations of SiC MOSFETs.
Table 2 summarizes the capabilities of these devices
when applied to various requirements of the IEC
62109-1 standard. By combining intrinsic isolation
strength with a Material Group I mold compound inwide packages, TI devices can address productswith a rated system voltage of up to 1000 VRMS and a
rated DC link voltage of up to 1500 VDC, with margin
to spare.

References
1. Technology roadmap: Solar photovoltaic energy-2014
edition, International Energy Agency
2. U.S. Solar Market Insight. Solar Energy Industries
Association (SEIA), June 9, 2016
3. Anant S Kamath, Kannan Soundarapandian. High-voltage
reinforced isolation: Definitions and test methodologies.
Texas Instruments White Paper, November 2014.
4. Isolation glossary (SLLA353). Texas Instruments,
October 2014.
5. IEC 62109-1 Edition 1.0, Safety of power converters
for use in photovoltaic power systems – Part 1: General
requirements. Apr. 2010.
6. IEC 60364-1: Electrical installations of buildings –Part 1: Fundamental principles, assessment of generalcharacteristics, definitions. International ElectrotechnicalCommission, Geneva.
7. Download these data sheets: ISO5851, ISO5852S,

Conclusion

ISO5451, ISO5452, ISO7720, ISO7721, ISO7740,

Isolators are essential components of modern

ISO7741, ISO7760, ISO7761 ISO7842, ISO7841,

solar power conversion systems and must be

ISO7840, ISO7821, ISO7831, AMC1304M25,

chosen carefully to ensure optimal insulation

AMC1305M05.

coordination. In this paper, we examined several
aspects of isolator selection for solar power
conversion systems to comply with IEC 62109-1.
This includes requirements on transient overvoltage,
impulse voltage, working voltage, and creepage
and clearance. A combination of intrinsic isolation
strength, superior mold compound and availabilityof wide-package options enables TI devices toaddress the requirements of solar inverter designs
with rated system voltage up to 1000 VRMS and a
rated DC link voltage of up to 1500 VDC.

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