Siemens 6ES5955-3NC4 Power Supply Module – SIMATIC S5

Siemens 6ES5955-3NC4 — Regulated DC Power Architecture for SIMATIC S5 Rack Systems

The Siemens 6ES5955-3NC4 is a rack-mount power supply module engineered for the SIMATIC S5 programmable controller platform. It occupies a dedicated PSU slot on the S5 central rack and delivers two independently regulated DC rails — 5 V at up to 9 A and 24 V at up to 1 A — to the S5 backplane bus. These rails power the CPU logic circuitry, memory subsystems, and the sensor/actuator interface circuits of installed I/O modules simultaneously. The module accepts a wide AC input range of 120 V or 230 V (manually switchable via an internal selector), making it deployable across North American and European grid standards without hardware substitution.

Within the S5 control loop, the PSU is the first point of energy conversion and the last line of defense against power-related faults. Its internal linear regulation topology maintains output ripple below 50 mV peak-to-peak on the 5 V rail under full load — a figure that directly determines the noise floor seen by the CPU’s address and data buses. Voltage deviation beyond ±5% on the 5 V rail is sufficient to cause indeterminate CPU states in the S5-115U and S5-135U architectures; the 6ES5955-3NC4 holds regulation within ±2% across the full operating temperature range of 0 °C to +60 °C.

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Technical Parameters

Hardware Logical Analysis

The 6ES5955-3NC4 employs a linear regulation topology rather than a switched-mode design. This architectural choice has measurable consequences for the S5 system environment. Linear regulators dissipate excess input-to-output voltage differential as heat across pass transistors, which imposes a thermal management requirement — the module’s heatsink must maintain adequate airflow within the rack enclosure. In exchange, the absence of a switching oscillator eliminates the primary source of conducted electromagnetic interference (EMI) that plagues SMPS-based supplies in the 50 kHz–500 kHz band.

The S5 backplane bus operates at TTL logic levels (5 V). Any high-frequency noise injected onto the 5 V supply rail couples directly into the bus transceivers of every module installed in the rack. The 6ES5955-3NC4’s linear output stage, combined with its LC output filter network, achieves a power supply rejection ratio (PSRR) sufficient to keep bus noise below the TTL noise margin threshold of 400 mV. This is particularly relevant in installations where the rack shares a distribution panel with variable-frequency drives (VFDs) or large contactor banks — noise sources that generate significant conducted disturbances on the AC supply line.

The 24 V DC rail serves a distinct function: it powers the sensor supply circuits of digital input modules and the coil drivers of relay output modules. This rail is isolated from the 5 V logic rail at the regulation stage, preventing load transients from actuator switching events from propagating into the CPU power domain. The isolation boundary is implemented via separate secondary windings on the internal transformer, with independent rectification and filtering for each rail.

The module incorporates an input EMC filter on the AC side — a common-mode choke in series with the line and neutral conductors, combined with X- and Y-class capacitors to ground. This filter attenuates both differential-mode and common-mode conducted disturbances entering from the AC supply, reducing susceptibility to grid-borne transients. The design meets the conducted emission limits of EN 55011 Class A, appropriate for industrial environments.

Fault detection logic monitors both output rails continuously. If either rail drops below its undervoltage threshold — approximately 4.75 V on the 5 V rail — the module asserts a fault signal on the backplane bus, which the S5 CPU interprets as a power fail interrupt. This gives the CPU a deterministic window to execute a controlled shutdown sequence and write process state to retentive memory before the supply collapses completely. The duration of this window is determined by the holdup capacitance on the 5 V rail, typically 20 ms at full load.

System Integration Benefits

• Direct backplane compatibility: The 6ES5955-3NC4 mates to the S5 rack PSU slot via the standard 48-pin backplane connector. No adapter, cable, or firmware configuration is required — the module is recognized by the rack bus arbitration logic immediately upon insertion.
• Dual-rail architecture eliminates external 24 V supply: Many S5 installations that use third-party PSUs require a separate 24 V DIN-rail supply for sensor circuits. The 6ES5955-3NC4 provides both rails from a single module slot, reducing panel wiring complexity and eliminating a potential single point of failure external to the rack.
• Deterministic power-fail signaling: The module’s undervoltage detection and backplane fault assertion give the S5 CPU a predictable, hardware-guaranteed notification window before power loss. This enables process-safe shutdown sequences in applications where uncontrolled CPU halt is unacceptable — chemical dosing systems, press brake controls, and conveyor sequencing are representative examples.
• Low conducted EMI preserves bus signal integrity: The linear regulation topology and AC-side EMC filter maintain the 5 V bus noise floor below levels that would cause marginal logic transitions on the backplane. This is measurable as a reduction in parity error events and watchdog resets in electrically noisy environments.
• Thermal behavior is predictable and monitorable: Unlike SMPS designs where efficiency varies with load, the linear regulator’s thermal dissipation scales linearly with output current. Maintenance engineers can estimate heatsink temperature from load current measurements, enabling condition-based maintenance scheduling without additional instrumentation.
• Wide AC input range supports multi-region deployment: The 120 V / 230 V switchable input allows the same module to be stocked and deployed across facilities in different countries without procurement of region-specific variants. This simplifies spare-parts inventory management for multinational manufacturers.
• 12-month warranty coverage reduces lifecycle cost: Each unit shipped from Xiamen carries a 12-month replacement warranty. For legacy S5 systems where downtime cost is high and replacement lead times from other sources can extend to weeks, this warranty provides a quantifiable risk reduction for maintenance budgets.
• Genuine Siemens traceability supports regulated-industry audits: Pharmaceutical, energy, and rail applications operating under IEC 61511, FDA 21 CFR Part 11, or EN 50126 require documented component provenance. Genuine Siemens part numbers with verifiable manufacturer markings satisfy audit requirements that third-party or remarked components cannot.

Quality Assurance & Global Logistics

Every Siemens 6ES5955-3NC4 unit dispatched from our Xiamen, China facility is a genuine Siemens-manufactured component. Units are sourced through verified supply channels and subjected to incoming inspection that includes label authentication, physical condition assessment, and powered functional verification of both DC output rails under resistive load. No remarked, counterfeit, or non-original parts are handled or sold.

Packaging follows anti-static handling protocols: each module is sealed in a conductive poly bag, cushioned with foam inserts, and boxed in a double-wall corrugated carton rated for international air freight handling. Shipments from Xiamen reach major industrial hubs in Southeast Asia within 3–5 business days via DHL Express, and European and North American destinations within 5–7 business days. Sea freight consolidation is available for bulk orders exceeding 20 units. A certificate of conformance is available upon request for regulated-industry procurement.

The 12-month warranty covers defects in materials and workmanship from the date of shipment. DOA (dead-on-arrival) units are replaced at no charge with expedited re-shipment. Warranty claims are processed within 48 hours of receipt of the returned unit at our Xiamen facility.

Contact Information

Email: [email protected]
WhatsApp: +86 18359268345
Web: siemensplc.com
Location: Xiamen, China
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