FEATURE
ELECTRICAL & ELECTRONIC COMPONENTS DESIGNING POWER SUPPLIES FOR
Bryan Angelo Bores, senior product applications engineer, and Noel Tenorio, product applications manager, Analog Devices, look into the design of power supplies in relation to IEC 61508
W
ith its operation affecting the entire functionality of an electronics system, the power supply unit is one of its
most crucial components. In the context of industrial functional safety –
international standard IEC 61508 – power supplies are considered elements and supporting services to electrical/electronic/programmable electronic (E/E/PE) safety-related systems (SRS) as well as other subsystems. However, due to the IEC 61508’s three
key requirements for functional safety (FS) compliance alongside recommended diagnostic measures, developing power supplies for industrial FS can be tiresome. So let’s take a look at what the basic functional safety standard states about power supplies.
REQUIREMENTS Let us start off by focusing on what the basic functional safety standard requires from power supplies.
The IEC 61508-4 defines E/E/PE systems as those used for control, protection or monitoring based on one or more E/E/PE device. This includes all elements of the system such as power supplies, sensors and other input devices, data highways and other communication paths, and actuators and other output devices. Meanwhile, a safety-related system is defined as a designated system that both implements the required safety functions necessary to achieve or maintain a safe state for the equipment under control (EUC). This is intended to achieve – on its own or with other E/E/PE SRS and other risk reduction measures – the necessary safety integrity for the required safety functions. This is shown in Figure 1, where power supplies also serve as an example of supporting services to an E/E/PE SRS aside from the hardware and software required to carry out the specified safety function.
POWER SUPPLIES AND COMMON CAUSE FAILURES The basic functional safety standard defines common cause failure (CCF) as a failure that is
the result of one or more events, causing concurrent failures of two or more separate channels in a multiple channel system, leading to system failure. One example is a power supply failure that can result in multiple dangerous failures of the SRS. This is shown in Figure 2 where a failure in the 24V supply, assuming the 24V input becomes shorted
to its outputs 12VCC and 5VCC, will result in a dangerous failure of the succeeding circuits. CCFs are important to consider when
complying with functional safety as it affects compliance to the IEC 61508’s three key requirements: systematic safety integrity, hardware safety integrity, and architectural constraints. The standard cited requirements
regarding CCF and power supplies in certain circumstances are:
• IEC 61508-1 Section 7.6.2.7 takes the possibility of CCF into account when allocating overall safety requirements. This section also requires that the EUC control system, E/E/PE SRS, and other risk reduction measures when treated as independent for the allocation, shall not share common power supplies whose failure could result in a dangerous mode of failure of all systems. • Similarly, under synthesis of elements to achieve the required systematic capability (SC), IEC 61508-2 Section 7.4.3.4 Note 1 cites ensuring that there’s no common power supply failure that will cause a dangerous mode of failure of all systems is a possible approach to achieve sufficient independence.
• For integrated circuits with on- chip redundancy, IEC 61508-2 Annex E also cites several normative requirements including
Technique/Measure
Measures against voltage breakdowns, voltage variations, overvoltage, low voltage, and other phenomena such as AC power supply frequency variation that can lead to dangerous failure
Figure 2. Example of a power supply CCF scenario
separation of input and outputs such as power supply among others, and use of measures to avoid dangerous failures caused by power supply faults. While these clauses prohibit sharing common
power supplies whose failure could cause a dangerous mode of failure of all systems, implementing such practice when designing a system will result in an increased footprint with greater board size and cost. One way to still use common power supplies is
by employing sufficient power supply monitoring. By doing this, dangerous failures brought by the power supply to an E/E/PE SRS can be reduced to a tolerable level, if not eliminated, in accordance with the safety requirements.
POWER SUPPLY FAILURES AND DIAGNOSTICS With the need to detect failures in the power supply, the basic functional safety standard specifies requirements and recommendations to address both systematic and random hardware failures. In terms of the requirements for control of
systematic faults, IEC 61508-2 Section 7.4.7.1 requires the design of E/E/PE SRS to be tolerant against environmental stresses including electromagnetic disturbances. This clause is
SIL 1 SIL 2 SIL 3 SIL 4
M low
M medium
M medium M high Table 1. Power supply monitoring requirement from IEC 61508-2 Table A.16 Component Power supply
Low (60%) Stuck-at
Medium (90%) DC fault model Drift and oscillation
High (99%)
DC fault model Drift and oscillation
Figure 1. E/E/PE system - structure and terminology 54 DESIGN SOLUTIONS JULY/AUGUST 2025
Table 2. Power supply faults and failures to be assumed according to IEC 61508-2 Table A.1
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