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• A “400 W power supply” (PSU) is a switched-mode converter that can deliver up to ~400 W of continuous DC power.
• In a modern ATX PC it is sufficient for entry-level or office systems (integrated graphics or low-power GPU, ≤65 W CPU) provided the +12 V rail can supply ≈30 – 33 A and the unit is of good quality (80 PLUS Bronze or better).
• Suitability depends far more on rail current capability, efficiency, age and build quality than on the nominal wattage number.
Electrical fundamentals
• Total PSU rating:
\[ P{\text{out,max}} = V{12}\,I_{12} + V_5\,I5 + V{3.3}\,I_{3.3} + \dots \]
For today’s PCs the +12 V term dominates (CPU VRM, GPU, drives, fans).
• ATX tolerance: ±5 % on +12 V, +5 V and +3 .3 V rails; +5 VSB at ±5 %.
• Head-room rule of thumb: keep sustained system draw ≤70–80 % of PSU rating to maintain regulation and minimise fan noise.
Typical rail allocations in a 400 W ATX 2.3 unit
• +12 V : 30–33 A (360–396 W)
• +5 V : 15 A (75 W)
• +3.3 V : 15 A (49.5 W)
• +5 VSB : 2.5–3 A (12.5–15 W)
Compatibility with a PC using MSI 7728 v2.0 / H61 board
• Office/light gaming build (i3/i5 ≤77 W TDP, SSD, one HDD, GTX 650-class GPU) → 400 W good if the unit is healthy.
• Mid-range GPU (GTX 1660, RX 5500 XT) or >4 storage devices → practical minimum shifts to 500–550 W.
Ageing and quality factors
• Electrolytic capacitors lose capacitance ≈ 20 % per 3–5 years at 40 °C; ripple increases, rails sag under load.
• Lower-tier brands may print “400 W” peak, not continuous. Always read the label for combined +12 V rating and check for 80 PLUS or CE/UL markings.
Diagnostics for “1 beep then no start” symptom
• POST beep = firmware completed basic tests; failure later often points to PSU droop on +12 V when GPU or disk spins up.
• Recommended step: swap in a known-good 450–550 W 80 PLUS Bronze/Gold PSU; if the machine boots, the old 400 W unit is defective or overloaded.
• Instrumentation: use a PSU tester or multimeter on the ATX-24 and PCI-E plugs; observe +12 V during power-on. < 11.4 V is out of spec.
• ATX 3.0 & 12VHPWR: New PSUs ≥450 W now include the 12V-2×6/16-pin connector supporting instantaneous GPU transients up to 200 % of rated power; true 400 W ATX 3.0 units are rare—manufacturers start at 450 W.
• Efficiency: mainstream models moved from 80 PLUS White/Silver to Bronze/Gold; 90 + % efficiency at 20–50 % load reduces wasted heat and improves longevity.
• Topology: LLC + synchronous rectification is standard; some premium <500 W units already use GaN or SiC FETs for even higher efficiency.
• Form factors beyond PCs: fan-less 400 W industrial open-frame supplies (e.g., Mean Well LRS-400) or audio-grade SMPS (Hypex SMPS400A400) for Class-D amplifiers.
Example reputable 400 – 450 W PC PSUs
• Seasonic Core GX-450 (Gold, single-rail 37 A @ 12 V, 7-year warranty)
• be quiet! Pure Power 12 M 450 W (ATX 3.0, Gold, modular)
• Corsair RM450x (newest revision, Gold, 12VHPWR adapter included)
Voltage sag demonstration (simplified):
• Idle: 12.10 V
• When GPU engages: 11.25 V → below ATX spec → system resets / black screen.
• Safety: Choose CE-, UL-, TÜV-marked devices; never open a PSU unless trained—lethal voltages persist on bulk capacitors.
• Environmental: RoHS compliance eliminates Pb/Cd but units still contain e-waste; dispose through authorised recycling centres.
• Warranty tampering stickers—breaking them voids warranty in most jurisdictions.
Selection
• Target 80 PLUS Bronze/Gold, single +12 V rail ≥30 A, OVP/OCP/OTP/OPP/SCP, 5-year warranty minimum.
• Check connector set: 24-pin ATX, 4 + 4 pin EPS12V, at least one 6 + 2 pin PCI-E, 3× SATA.
Installation
• Ensure unobstructed airflow, fan facing case vent.
• Use the supplied modular cables only; mixing cables across brands can kill hardware.
Testing / troubleshooting
• Paper-clip jump-start (green to black) only verifies that standby and PS-ON logic work.
• Load testing: run OCCT or AIDA64 power test while monitoring with HWInfo; voltage deviation > 5 % and/or ripple > 120 mV pp indicates replacement time.
Replacement cycle
• Office use: 6–7 years or when capacity headroom drops below 25 %.
• 24/7 or high ambient: budget 4–5 years.
• Marketing “peak” wattage: Some budget units list 400 W but allow only 250 – 300 W continuous; always read the combined 12 V output.
• 400 W SFX or TFX units exist but have tighter thermal constraints; verify case airflow before using them at ≥80 % load.
• Examine Intel ATX12VO (single-rail 12 V, motherboard-side conversion) for future low-power desktops.
• Review PSU transient response per ATX 3.0 graphics-card excursion tests (2× & 3× rated power for 100 µs).
• Investigate GaN-based PFC/LLC stages for higher-frequency, smaller PSUs.
• For specialized applications (LED lighting, robotics), study constant-current or programmable 400 W SMPS modules.
A good-quality 400 W PSU remains viable for modest, energy-efficient systems but must supply a strong, stable +12 V rail, comply with ATX specs, and incorporate modern protections. When diagnosing boot failures after a single POST beep, the PSU is the first point to test—swap in a known-good 450 – 550 W 80 PLUS unit to rule it out. For new purchases, prefer current ATX 3.0 / 80 PLUS Gold models from reputable manufacturers to ensure efficiency, safety and future compatibility.
