EnergyKnight SS-350ET-T3: the cheapest Seasonic unit (updated)

Primary side

On the first look it is clear that these later units use different platform then the previous one, lasting for about a decade. It is based upon the current retail variant of S12II (SS-xx0GB). The whole unit has been reconfigured, on the primary side there is only one heatsink now and also the driving daughterboard is now elsewhere. As always, we begin with input bridge rectifier, the GBU406. It has its own small aluminium heatsink thanks to which it handles up to 4 A at 100 °C and 600 V continuously, or 150 A peak (for 8.3 ms). The voltage drop (which counts for losses) is rated as 1.0 V per diode at 2 A. The PFC coil appears to be wound using two strands of wire, soldered together somewhere on the way. It has some extra insulation using plastic film and a lot of silicone. Single Infineon IPP50R399CP (9/20 A at 560 V, RDS(On) 0.399 Ω at 4.9 A and 25 °C, 0.90 Ω at 150 °C) transistor in TO-220 package switches in the PFC circuit. The diode is an STMicroelectronics STTH8S06D (8/60 A, 600 V, drop of 3.4 V @8 A and 25 °C, 1.9 V at 125 °C) in TO-220AC package.

The bulk capacitor that gets charged from the PFC is a Nippon Chemi-Con SMQ 220 μF/400 V. I measured its true capacitance at 192 μF. This series has a minimum lifetime of 2000 hours at the maximum temperature (just 85 °C) and ripple. Well, it could be worse. The switching transistors it feeds are the same IPP50R399CP as the one in the boost PFC circuit, in the usual forward configuration. The main transformer is of VRL-35 size.

As for the driving circuitry, while Seasonic has been using Champion Micro silicon for years, they have now switched to Infineon, the ICE1CS02P PFC/PWM combo controller in particular. The PFC works at fixed 65 kHz while the PWM switching occurs at rather high 130 kHz with maximum duty cycle of 47 or 60 %. There is a Su’scon MF 4.7 μF/50 V capacitor to filter the controller’s power supply line.

+5 V stand-by rail

The stand-by rail in the SS-350ET-T3 uses a PWM microchip with integrated transistor, the Fairchild FSQ0165R, operating at 55.6 kHz. With the embedded transistor having RDS(On) of 10 Ω, it can deliver up to 13 W in open-frame units like this. The capacitor on the chip’s power supply input is a Su’Scon MF again, 68 μF/35 V.

For output rectification Seasonic opted for the good ol’ Diodes SBR10U45 (10/125 A, 45 V, 0.57 V @10 A/25 °C, 0.54 V @125 °C) in TO-252 SMD package. It is almost exactly under the large filter capacitor, Teapo SJ 3300/10. The other cap, after the Pi coil, is an SC 1000/10.

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