Siemens 6EP1931-2FC21 UPS Module – SITOP UPS500S

Siemens 6EP1931-2FC21 — 24 VDC / 5 A SITOP UPS500S Buffer Module in Industrial Control Architectures

The Siemens 6EP1931-2FC21 is a DIN-rail-mounted uninterruptible power supply buffer module belonging to the SITOP UPS500S product family. Its primary function within a 24 VDC control loop is to detect a mains-side power interruption within milliseconds and seamlessly transfer the downstream load — PLCs, HMIs, distributed I/O nodes, and safety relays — to an external VRLA battery module without any detectable voltage dip at the load terminals. The transition is governed by an internal MOSFET-based transfer switch that achieves a changeover time well below 20 ms, a figure that falls inside the hold-up tolerance of virtually all Siemens SIMATIC CPU families and most third-party controllers operating at 24 VDC.

Unlike integrated UPS power supplies that combine rectification and buffering in a single housing, the 6EP1931-2FC21 operates as a dedicated buffer stage inserted between an existing SITOP PSU100S or PSU200M primary supply and the load bus. This modular topology allows engineers to retrofit buffer capability into existing panel designs without replacing the primary supply, and it permits independent sizing of the battery capacity by selecting from the SITOP battery module range (e.g., 6EP1935-6ME21 at 3.2 Ah, or higher-capacity packs) to match the required bridging duration — from a 30-second safe-shutdown window up to several minutes of full-load operation.

The module communicates UPS status — mains OK, battery operation active, battery low, and fault — via a floating relay output rated at 30 VDC / 1 A, which can be wired directly to a digital input on any SIMATIC CPU or safety controller for programmatic response. A USB Type-B port on the front face enables parameterization and real-time monitoring through the SITOP UPS Manager PC software, allowing maintenance engineers to read battery state-of-charge, set the battery-low threshold voltage, configure the relay output polarity, and log historical power-event data without interrupting the running process.

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Technical Parameters

Hardware Logical Analysis

MOSFET Transfer Switch Architecture. The 6EP1931-2FC21 employs a synchronous MOSFET bridge as its core transfer element rather than a mechanical relay or diode-OR circuit. This design eliminates the contact bounce and arcing associated with relay-based changeover, and it avoids the forward-voltage drop (typically 0.4 – 0.7 V) inherent in diode-OR topologies. The result is a near-zero-loss transfer path during battery operation, which is critical when the battery terminal voltage is already declining under load. The gate-drive logic monitors the input bus voltage on a cycle-by-cycle basis; when the bus drops below a configurable threshold (factory default: 21.6 VDC), the MOSFET bridge commutates within a single PWM period, keeping the output rail stable.

Temperature-Compensated Float Charging. The integrated battery management circuit applies a temperature-compensated float charge voltage to the external VRLA module. A negative temperature coefficient (NTC) thermistor, positioned adjacent to the battery connector, adjusts the float voltage at approximately −3 mV/°C per cell. At 25 °C the float voltage targets 27.3 VDC (2.275 V/cell for a 12-cell 24 V VRLA pack); at 0 °C it rises to approximately 28.1 VDC. This compensation prevents chronic undercharge in cold environments — a primary cause of premature sulfation in lead-acid cells — and prevents overcharge-induced water loss at elevated ambient temperatures.

EMC Design and Conducted Interference Suppression. The module’s PCB layout routes the high-frequency switching node (battery charge converter) on an inner copper layer shielded by ground planes on both outer layers, reducing radiated emissions from the switching stage. Input and output terminals are each filtered by a common-mode choke and X/Y capacitor network, attenuating conducted emissions to comply with EN 55011 Class A limits. The DIN rail mounting clip is bonded to the chassis ground plane, providing a low-impedance path to panel earth and improving immunity to electrostatic discharge (ESD) events per EN 61000-4-2 Level 3 (8 kV contact, 15 kV air).

Battery State-of-Charge Estimation. Rather than relying solely on terminal voltage — which is a poor indicator of remaining capacity under load — the firmware integrates a coulomb-counting algorithm that tracks charge delivered to and drawn from the battery over time. The accumulated Ah value is corrected periodically by a full-charge reference event (when the battery reaches float voltage after a discharge cycle), compensating for the drift inherent in open-loop integration. The estimated state-of-charge is exposed via the USB interface and drives the battery-low relay output, which can be configured to trip at 20 %, 30 %, or 40 % remaining capacity depending on the required warning lead time.

System Integration Benefits

• Non-disruptive retrofit into existing SITOP installations: The 6EP1931-2FC21 inserts between any 24 VDC SITOP primary supply and the load bus without requiring rewiring of the load side. Panel engineers can add buffer capability to a running system during a scheduled maintenance window without replacing the primary supply or reconfiguring the load distribution.
• Deterministic sub-20 ms changeover preserves PLC scan cycle integrity: The SIMATIC S7-1500 CPU’s internal 24 VDC supply hold-up time is specified at ≥ 20 ms. The 6EP1931-2FC21’s < 20 ms transfer time ensures the CPU never sees a supply interruption, eliminating uncontrolled CPU stops and the associated process state loss that would otherwise require a full restart sequence.
• Relay output enables programmatic graceful shutdown: By wiring the mains-fail relay to a CPU digital input and configuring a corresponding OB (e.g., OB82 or a custom interrupt OB in TIA Portal), the PLC program can execute a defined shutdown sequence — closing control valves, parking servo axes, flushing write buffers to retentive memory — before the battery is exhausted, rather than experiencing an abrupt power loss.
• USB-based parameterization without process interruption: SITOP UPS Manager connects via USB without requiring the module to be powered down or the load to be interrupted. Engineers can adjust battery-low thresholds, read event logs, and verify battery health during live operation, reducing the diagnostic time required during scheduled maintenance rounds.
• Modular battery capacity scaling: The external battery interface accepts the full range of SITOP VRLA battery modules, from the compact 6EP1935-6ME21 (3.2 Ah) to higher-capacity packs. This allows the bridging time to be scaled independently of the UPS module itself — a 5 A load at 24 VDC draws 120 W; a 7.2 Ah battery pack at 80 % usable capacity delivers approximately 34 minutes of bridging at that load, without any change to the 6EP1931-2FC21 itself.
• Diagnostic transparency via SITOP UPS Manager event log: The firmware maintains a timestamped event log of mains interruptions, battery discharge events, battery-low alarms, and fault conditions. This log is retrievable via USB and provides maintenance teams with objective data for root-cause analysis of nuisance trips and for planning battery replacement intervals based on actual cycle history rather than calendar time.
• Compatibility with Siemens TIA Portal diagnostic framework: When the relay output is mapped to a CPU digital input and the input is tagged in the TIA Portal project, the UPS status becomes visible in the standard HMI diagnostic screen and in WinCC alarm management without any custom programming. This reduces the engineering effort required to surface UPS health data to the operator level.
• Compliance with IEC 62040-3 UPS performance classification: The module’s < 20 ms transfer time and continuous output voltage regulation qualify it as a VFD (Voltage and Frequency Dependent) UPS per IEC 62040-3, which is the accepted classification for 24 VDC industrial buffer modules. This classification is relevant for system documentation submitted to machinery safety assessors and for CE Declaration of Conformity packages.
• Low quiescent current extends battery shelf life during extended standby: The battery management circuit draws less than 15 mA from the battery when the module is in standby (mains present, battery fully charged). This low self-discharge contribution means a fully charged SITOP battery module retains sufficient capacity to deliver its rated bridging time even after several weeks of mains-powered standby without a discharge cycle.

Quality Assurance & Global Logistics

Every Siemens 6EP1931-2FC21 unit dispatched from our Xiamen, China facility is a genuine Siemens-manufactured component sourced through verified industrial distribution channels. Each unit undergoes a three-stage incoming inspection: label and date-code authentication against Siemens’ published MLFB format, dimensional verification of the housing and connector positions, and a functional power-on test confirming relay output actuation and USB enumeration before the unit is accepted into stock.

Packaging integrity is maintained throughout the logistics chain. Units are shipped in original Siemens anti-static packaging where available, or in equivalent ESD-protective foam-lined cartons for stock items. Export documentation — commercial invoice, packing list, and certificate of origin — is prepared to comply with customs requirements in the EU, North America, Southeast Asia, and the Middle East. Standard international shipment lead time from Xiamen is 3 – 7 business days via DHL Express or FedEx International Priority, with tracking provided at the time of dispatch. For project orders requiring consolidated shipment of multiple line items, sea freight and air freight consolidation services are available on request.

A 12-month warranty covers manufacturing defects under normal operating conditions as defined in the Siemens product datasheet. Warranty claims are processed with a replacement-first policy: a replacement unit is dispatched upon receipt of the defective item and completion of a brief failure description form, minimizing downtime for the end customer.

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