Seasonic Focus Plus Gold 850 PSU Review

Teardown & Component Analysis

Parts Description

Before proceeding with this page we strongly encourage you to a look at our PSUs 101 article, which provides valuable information about PSUs and their operation, allowing you to better understand the components we're about to discuss. Our main tools for disassembling PSUs are a Thermaltronics soldering and rework station and a Hakko FR-300 desoldering gun. Finally, for the identification of tiny parts we use an Andonstar HDMI digital microscope.

General Data
Manufacturer (OEM)Seasonic
Platform ModelFX
Primary Side
Transient Filter4x Y caps, 2x X caps, 2x CM chokes, 1x MOV, 1x CM02X
Inrush ProtectionNTC Thermistor & Relay
Bridge Rectifier(s)2x?GBU1506 (600V, 15A)
APFC MOSFETs2x Infineon?IPW50R190CE (550V, 15.7A @ 100°C, 0.19Ω)
APFC Boost Diode1x STMicroelectronics STTH8S06D (600V, 8A @ 125°C)
Hold-up Cap(s)1x Nippon Chemi-Con (400V, 650uF, 2000h @ 105°C, CE)
Main Switchers4x UTC?GPT13N50DG (500V, 13A @ 100°C, 0.49Ω)
APFC ControllerChampion CM6500UNX
Resonant ControllerChampion CM6901T6X
TopologyPrimary side: Full-Bridge & LLC Resonant Controller
Secondary side: Synchronous Rectification & DC-DC converters
Secondary Side
+12V MOSFETs4x Nexperia?PSMN2R6-40YS (40V, 100A @ 25°C, 2.8mΩ)
5V & 3.3VDC-DC Converters: 6x Infineon?BSC0906NS (30V, 40A @ 100°C, 4.5mΩ)
PWM Controller:?APW7159
Filtering CapacitorsElectrolytics: Chemi-Con (1-5000 @ 105°C, KZE), Chemi-Con (4-10,000 @ 105°C, KY), W
Polymers: Chemi-Con
Supervisor ICWeltrend WT7527V (OVP, UVP, OCP, SCP, PG)
Fan ModelHong Hua HA1225H12F-Z (120mm, 12V, 0.58A, 2200 RPM, Fluid Dynamic Bearing)
5VSB Circuit
Standby PWM ControllerExcelliance EM8569

This platform is exactly the same as the one we saw in Seasonic's Focus Plus Gold 750. It's a new design featuring a full-bridge topology on the primary side, along with an LLC resonant converter. We find typical stuff on the secondary side: a synchronous design for the +12V rail and a couple of DC-DC converters for the minor rails. Unfortunately, the -12V rail is regulated through a plain diode and not a proper regulator, which would provide increased reliability and safety against overloading. The filtering caps come from Chemi-Con; some of them are special-order from Seasonic, since we can't find any information about them. The cooling fan only measures 120mm across, since a larger one wouldn't fit in this compact chassis.

The first part of the transient filter is on a small PCB located right behind the AC receptacle. A couple of Y caps and a single X cap are there, along with a CM02X that blocks current through the X cap discharge resistor when AC voltage is connected. The same IC also automatically discharges the X cap through the discharge resistor when AC is disconnected. Seasonic's EMI filter continues on the main PCB with two more Y caps and a single X one, two CM chokes, and an MOV. It's an effective filter, given the low conducted EMI transmissions we measured.

An NTC thermistor protects against large inrush currents; it is supported by a bypass relay.

A couple of bridge rectifiers (GBU1506) are bolted onto a dedicated heat sink.

The APFC converter uses two Infineon?IPW50R190CE FETs and a STMicroelectronics STTH8S06D boost diode. The bulk cap is provided by Chemi-Con and it has a capacity of 650uF. We measured a longer than 17ms hold-up time (the ATX spec's minimum), so the bulk cap clearly has enough capacity for this platform.

The APFC controller is installed on the main PCB's solder side, along with the LLC resonant controller. The first is a Champion?CM6500UNX, while the second is the omnipresent CM6901.

The primary FETs are four UTC?GBT10N50ADGs (the same ones in Seasonic's SSR-750FX) configured in a full bridge topology. An LLC resonant converter is also used for boosting the unit's efficiency.

On the secondary side, the +12V rail is rectified by four Nexperia?PSMN2R6-40YSes. Exactly the same configuration is used in Seasonic's SSR-750FX. So far, the main difference between its 850W and 750W Focus Plus Gold units are the APFC FETs.

Chemi-Con filtering caps on the secondary side belong to the company's KZE, KY, and W lines. Besides electrolytic caps, a few polymer Chemi-Con caps are also used for filtering purposes.


A couple of DC-DC converters are located on small vertical PCB, employing six Infineon?BSC0906NS?FETs and a single ANPEC?APW7159?PWM controller.

The supervisor IC is a Weltrend?WT7527V. This IC supports OCP at +12V for up to two channels. However, this PSU sports a single +12V rail. The same IC supports all important protection features except for OTP, which is implemented through another circuit.


The standby PWM controller is a Excelliance EM8569 IC.

On the front of the modular board, four polymer caps and four electrolytic caps, all provided by Chemi-Con, further suppress ripple.

Seasonic's soldering quality is pretty good, although some of the component leads could be shorter.

The fan is made by Hong Hua and its model number is HA1225H12F-Z. It features an FDB bearing and can rotate at high speeds if needed. This power supply does employ a relaxed fan profile, so you'll have to push it pretty hard under high ambient temperatures to make the fan run at full-speed. At that point, it generates about 45 dB(A) from one meter away.

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