Corsair RM850x (2021) PSU Review. The Best 850W PSU?

How you update something that is already on top of its category? Corsair knows this well, because its people already did it with the highly popular RM850x. The newest revision of this product was introduced on 2021 featuring a magnetic leviation fan and three EPS connectors for power hungry CPUs. 

I cannot hide it; I am a big fan of the Corsair RMx PSUs because they offer high performance and reliability at a price that won’t empty your wallet. Currently, the RMx members are among the best in the mid-price category, in every Wattage, from 550W up to 1000W. The OEM of this line is CWT, which also made the previous RMx models, and the exterior design allows you to distinguish the new RMx units from the older ones easily. The triangular perforations on the punched fan grill and the front exhaust are characteristic of Corsair’s fresh PSU designs.

The RM850x is rated Gold by both Cybenetics and 80 PLUS, and it also has a Cybenetics A- rating, meaning that its average noise output is within 25-30 dBA at normal operating temperatures (28-32°C). A more aggressive fan speed profile drives the magnetic bearing fan that the new model has, but still, the PSU’s noise won’t trouble you under normal conditions unless you are super sensitive.

 

Cybenetics Report

 

Technical Specifications:
  • Manufacturer (OEM): CWT
  • Max Power: 850W
  • Cybenetics Efficiency: [115V] Gold (87-89%)
  • 80 Plus Efficiency: Gold
  • Noise: Cybenetics A- (25 – 30 dBA)
  • Compliance: ATX12V v2.53, EPS 2.92
  • Alternative Low Power Mode support: Yes
  • Power 12V: 850W
  • Power 5V + 3.3v: 150W
  • Power 5VSB: 15W
  • Cooling: 140 mm Fluid Dynamic Bearing Fan (NR140P)
  • Semi-Passive Operation: Yes
  • Modular Design: Yes (Fully)
  • High Power Connectors: 3x EPS (3x cables), 4x PCIe 6+2 pin (2x cables)
  • Peripheral Connectors: 14x SATA (4x cables), 4x 4-pin Molex (single cable)
  • ATX/EPS Cable Length: 610/650mm
  • Distance between SATA connectors: 110mm
  • Distance between 4-pin Molex connectors: 100mm
  • In-cable capacitors: Yes (ATX, EPS, PCIe)
  • Dimensions (W x H x D): 150 mm x 85 mm x 160 mm
  • Weight: 1.71 kg (3.77 lb)
  • Warranty: 10 years

Box & Bundle

Thanks to the product photo on the front, you can quickly tell from the package’s design that this is the most recent RM850x version. The protection inside the box is adequate, and the bundle includes all the necessary stuff, along with several zip ties.

Product Photos

From the moment it has a semi-passive mode, I would like to see a switch on the PSU’s front, to deactivate should I want the fan to spin constantly. Besides that, I like the exterior design, and the fan grill is not restrictive.

Cables

Since this is an older unit, don’t expect to find 12VHPWR connectors, but I am sure Corsair will soon update its RMx line offering the new PCIe connectors. The three EPS connectors make this PSU suitable for potent mainboard-processor combinations, and the four PCIe connectors are enough for a powerful gaming system. Lastly, the amount of SATA connectors is insane!

Protection Features

Protection Features

OCP (Cold @ 25°C)

12V: 87.2A (123.16%), 11.879V
5V: 31.5A (157.5%), 4.946V
3.3V: 31.8A (159%), 3.276V
5VSB: 5.1A (170%), 4.977V

OCP (Hot @ 43°C)

12V: 87.4A (123.45%), 11.9V
5V: 31.5A (157.5%), 4.948V
3.3V: 31.7A (158.5%), 3.269V
5VSB: 5.1A (170%), 4.981V

OPP (Cold @ 28°C)

1049.07W (123.42%)

OPP (Hot @ 45°C)

1053.31W (123.92%)

OTP

✓ (140°C @ 12V Heat Sink)

SCP

12V to Earth: ✓
5V to Earth: ✓
3.3V to Earth: ✓
5VSB to Earth: ✓
-12V to Earth: ✓

PWR_OK

Proper operation

NLO

SIP

Surge: MOV
Inrush: NTC Thermistor & Bypass relay

OCP at 12V and OPP are correctly set. On the other hand, OCP on the minor rails is set sky-high. There is no need for such high amperage on the minor rails, especially the 3.3V one.

Part Analysis

General Data
Manufacturer (OEM) CWT
PCB Type Double Sided
Primary Side
Transient Filter 4x Y caps, 2x X caps, 2x CM chokes, 1x MOV
Inrush Protection NTC Thermistor SCK-037 (3 Ohm) & Relay
Bridge Rectifier(s)
2x GBU1506 (600V, 15A @ 100°C)
APFC MOSFETs
2x Vishay SiHF30N60E (650V, 18A @ 100°C, Rds(on): 0.125Ohm)
APFC Boost Diode
1x On Semiconductor FFSP0865A (650V, 8A @ 155°C)
Bulk Cap(s)
2x Rubycon (400V, 470uF each or 940uF combined, 2,000h @ 105°C, MXH)
Main Switchers
2x
Infineon IPA60R190P6 (600V, 12.7A @ 100°C, Rds(on): 0.19Ohm)
APFC Controller
Champion CM6500UNX & Champion CM03X
Resonant Controller Champion CU6901VAC
Topology
Primary side: APFC, Half-Bridge & LLC converter
Secondary side: Synchronous Rectification & DC-DC converters
Secondary Side
+12V MOSFETs 6x On Semiconductor NTMFS5C430N (40V, 131A @ 100°C, Rds(on): 1.7mOhm)
5V & 3.3V DC-DC Converters: 2x UBIQ QM3054M6 (30V, 61A @ 100°C, Rds(on): 4.8mOhm) & 3x UBIQ QN3107M6N (30V, 70A @ 100°C, Rds(on): 2.6mOhm)
PWM Controllers: UPI Semi uP3861P
Filtering Capacitors Electrolytic: 7x Nippon Chemi-Con (1-5,000h @ 105°C, KZE), 5x Nippon Chemi-Con (4-10,000h @ 105°C, KY), 2x Rubycon (4-10,000h @ @ 105°C, YXJ), 1x Rubycon (6-10,000h @ 105°C, ZLH)
Polymer: 32x FPCAP, 5x United Chemi-Con
Change Over Switch 1x Sync Power SPN3006 MOSFET (30V, 57A @ 100°C, Rds(on): 5.5mOhm)
Supervisor IC Weltrend WT7502R (OVP, UVP, SCP, PG)
Fan Controller Microchip PIC16F1503
Fan Model Corsair NR140ML (140mm, 12V, 0.27A, Magnetic Levitation Bearing Fan)
5VSB Circuit
Rectifier
1x Galaxy Microcontroller D10PS45L SBR (45V, 10A) & IPS ISD04N65A
Standby PWM Controller On-Bright OB5282

Channel Well Technology (CWT) remains the OEM of all RMx units. Corsair didn’t use a standard platform from CWT’s portfolio but a custom one, which they co-developed with CWT’s engineers. You won’t find the RMx platform in any other non-Corsair unit. The ML bearing fan greatly enhances the PSU’s reliability since magnetic bearings don’t have a problem operating under harsh conditions for prolonged periods. The build quality is high overall, and the design is modern for the Gold (and Platinum) category’s standards. If you want interleaved PFC design and a full bridge topology, you should look at stronger units with Platinum, at least, efficiency.

Load Regulation

Load regulation is within 1% on all major rails, so I don’t have any complaints.

Ripple Suppression

Ripple suppression is outstanding!

Transient Response

I would like to see below 1% average transient deviation at 12V. The 3.3V rail doesn’t perform so well, either.

Hold Up Time

The hold-up time is long, and the power ok signal is accurate. So there is nothing more to ask here.

Timings

The PSU supports Alternative Low Power Modes.

Inrush Current

Inrush currents are low with both voltage inputs.

Efficiency Normal, Light & Super-Light Loads

Efficiency could be higher with normal loads. On the contrary, it is sky high with lights and super-light loads.

Average Efficiency 5VSB

The 5VSB rail is not so efficient.

Vampire Power

Vampire power is bottom low!

Average Efficiency

The average efficiency is not so high for the category’s standards.

Average PF

The APFC converter does a great job!

Average Noise

Compared to the previous RM850x, with the FDB fan, the new one is notably noisier. Still, its average noise output remains below 30 dBA so that it won’t bother you under normal operating conditions.

Fan Noise & Speed Maps @ 28-32 °C

Typically for a CWT-made unit, the load on the minor rails affects noise output. With close to 600W load, the unit passes the 30 dBA mark, and with more than 650W, it exceeds 35 dBA.

Overall Performance

The RM850x achieves high performance with 115V and a top place among Gold 850W PSUs, with 230V. The last time I reviewed the RM850x for Tom’s Hardware, the 850 G7 hadn’t been released, and as you can see on the 115V performance graph, it has a slight lead. I should also point out that I haven’t used the new scoring algorithm for the above graphs because I don’t have data for all PSUs, especially some older ones. The new algorithm considers more factors so that the final scores can change.

Epilogue

The RM850x is a definite improvement compared to its predecessor, offering ALPM support and a better fan. The new RM850x is noisier at high RPM, so overall noise output has notably increased. Still, you cannot call the new RM850x loud, but it lost the title of the quietest 850W Gold unit that its predecessor had. I prefer the ML fan over the FDB one since it is more reliable, especially at high operating temperatures.

I expect Corsair to update the RMx line soon, offering, among others, 12VHPWR connectors and ATX 3.0 compatibility. If you are not in a hurry to buy a new PSU, you could wait, but if you need a new unit now, you should include the RM850x in your buy list. Other suitable alternative options are the EVGA 850 G7 and the Seasonic GX-850. The Thermaltake GF1 850W is also a good choice.

 

Buy Corsair RM850x Buy EVGA 850 G7 Buy Seasonic GX-850 Buy Thermaltake GF1 850W

 

Pros:
  • Full power at 47°C
  • High overall performance
  • Top build quality
  • Properly set OCP at 12V and OPP
  • Quiet operation under normal conditions
  • Great ripple suppression
  • Highly efficient at light loads
  • Long hold-up time
  • Low inrush currents
  • High-performance APFC converter
  • ALPM support
  • ML fan
  • Lots of connectors, including 3x EPS
  • Fully modular
  • Dead low  vampire power
  • Quality, FDB fan
  • 10-year warranty
Cons:
  • High OCP on the minor rails
  • The transient response could be better
  • In-cable caps
  • The short distance between all peripheral connectors
  • No 12VHPWR connector (No PCIe 5.0 ready)

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