BFG MX-680 Power Supply Review
By Gabriel Torres on July 15, 2009


Introduction

Hardware Secrets Bronze Award

LS and MX are sister power supply series from BFG, with models from MX series coming with a modular cabling system, a 120 mm fan and an auxiliary 80 mm fan on the rear (which only kicks in when the temperature on the secondary heatsink is above 50° C), while models from LS series use a 135 mm fan and no modular cabling system or second fan. Initially both series used the same internal design, however this is not entirely true anymore. All models from MX series are manufactured by Fore Point (one of the factories from Fortrex), but LS-450 and the new version of LS-550 (marked as “LS-550 (New)” on their website, being the model currently available at Newegg.com) are manufactured by Enhance. The old LS-550 and LS-680 are manufactured by Fore Point with the same design as models from MX series. According to BFG they will move all models from MX and LS to Enhance in the future, and when this happens units will be either released with new wattages (e.g., LS-700 instead of LS-680) or will have the name “New” added to their name.

BFG MX-680 power supply
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Figure 1: BFG MX-680 power supply.

BFG MX-680 power supply
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Figure 2: BFG MX-680 power supply.

BFG MX-680 is a relatively long unit, being 7” (180 mm) deep (counting the projection from the modular cabling connectors), using a 120 mm fan on its bottom and an 80 mm fan on its rear (which only turns on when the temperature on the secondary heatsink reaches 50° C) and featuring active PFC, of course.

The main motherboard cable (20/24-pin connector) and the ATX12V/EPS12V cable (two ATX12V connectors that together form an EPS12V connector) come from inside the power supply and have a nylon sleeving that also comes from inside the unit. They are long (23” or 58 cm each), which surely helps installing the unit inside a full tower case.

All other cables are available on the modular cabling system, which has eight connectors. MX-680 comes with nine cables, all measuring 21” (53 cm) between the end that goes on the modular cabling system and the first connector on the cable and 5 ½” (140 mm) between connectors. The cables included are:

All wires are 18 AWG, which is the correct gauge to be used. The number of cables is enough for a PC with up to two high-end video cards. The highlight here is the high number of SATA power plugs (eight).

BFG MX-680 power supply
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Figure 3: Cables.

Now let’s take an in-depth look inside this power supply.

A Look Inside The MX-680

We decided to disassemble this power supply to see what it looks like inside, how it is designed, and what components are used. Please read our Anatomy of Switching Power Supplies tutorial to understand how a power supply works and to compare this power supply to others.

BFG MX-680 power supply
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Figure 4: Overall look.

BFG MX-680 power supply
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Figure 5: Overall look.

BFG MX-680 power supply
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Figure 6: Overall look.

Transient Filtering Stage

As we have mentioned in other articles and reviews, the first place we look when opening a power supply for a hint about its quality, is its filtering stage. The recommended components for this stage are two ferrite coils, two ceramic capacitors (Y capacitors, usually blue), one metalized polyester capacitor (X capacitor), and one MOV (Metal-Oxide Varistor). Very low-end power supplies use fewer components, usually removing the MOV and the first coil.

This power supply is flawless on this stage, having two X capacitors and two Y capacitors more than required.

BFG MX-680 power supply
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Figure 7: Transient filtering stage (part 1).

BFG MX-680 power supply
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Figure 8: Transient filtering stage (part 2).

In the next page we will have a more detailed discussion about the components used in the BFG MX-680.

Primary Analysis

On this page we will take an in-depth look at the primary stage of BFG MX-680. For a better understanding, please read our Anatomy of Switching Power Supplies tutorial.

This power supply uses one GBU1506 rectifying bridge in its primary, which can deliver up to 15 A at 100° C. This component is clearly overspec’ed: at 115 V this unit would be able to pull up to 1,725 W from the power grid; assuming 80% efficiency, the bridge would allow this unit to deliver up to 1,380 W without burning this component. Of course we are only talking about this component and the real limit will depend on all other components from the power supply.

This is a different component from MX-550, which uses a 10 A bridge. Nice.

BFG MX-680 power supply
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Figure 9: Rectifying bridge.

BFG MX-680 uses two SPP24N60C3 power MOSFET transistors on its active PFC circuit, each one capable of delivering up to 24.3 A at 25° C or 15.4 A at 100° C in continuous mode (note the difference temperature makes) or up to 72.9 A at 25° C in pulse mode, presenting a resistance of 140 mΩ (typical) when turned on, a characteristic called RDS(on). This number indicates the amount of power that is wasted, so the lower this number the better, as less power will be wasted thus increasing efficiency. These transistors are more powerful than the ones used on MX-550 (16 A at 25° C or 10 A at 100° C). Very nice.

The electrolytic capacitor in charge of filtering the active PFC output is Japanese from Matsushita (Panasonic) and labeled at 85° C.

BFG MX-680 power supply
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Figure 10: Active PFC transistors and diode.

This power supply uses two SPP20N60C3 power MOSFET transistors on the traditional two-transistor forward configuration on its switching section, presenting a maximum current of 20.7 A at 25° C or 13.1 A at 100° C in continuous mode (note the difference temperature makes) or 62.1 A in pulse mode at 25° C. These transistors present a typical RDS(on) of 160 mΩ. They are more powerful than the ones used on the 550 W version from this power supply (16 A at 25° C or 10 A at 100° C). Nice again!

BFG MX-680 power supply
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Figure 11: Switching transistors.

The primary is controlled by a CM6800 integrated circuit installed on a small printed circuit board. This component is the most popular PWM/PFC combo controller.

BFG MX-680 power supply
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Figure 12: PFC/PWM controller.

Secondary Analysis

BFG MX-680 has five Schottky rectifiers on its secondary. Here we could already see that the configuration was different from the 550 W model, which has four rectifiers.

The maximum theoretical current each line can deliver is given by the formula I / (1 - D), where D is the duty cycle used and I is the maximum current supported by the rectifying diode. Just as an exercise, we can assume a typical duty cycle of 30%.

The +12 V output is produced by two STPS41L60CT Schottky rectifiers connected in parallel, each one capable of handling up to 40 A (20 A per internal diode at 125° C, typical voltage drop of 0.67 V). So the maximum theoretical current the +12 V output from this power supply can deliver is of 57 A or 686 W. The 550 W model uses two 30 A rectifiers here.

The +5 V output is produced by two SBR30U30CT Schottky rectifiers, each one capable of handling up to 30 A at 140° C (15 A per internal diode, typical voltage drop of 0.50 V). So the maximum theoretical current the +5 V output from this power supply can deliver is of 43 A or 214 W. The 550 W model uses only one of these rectifiers here.

The +3.3 V output is produced by one SPR30L40CT Schottky rectifier, which is capable of handling up to 30 A at 110° C (15 A per internal diode, typical voltage drop of 0.41 V). So the maximum theoretical current the +3.3 V output from this power supply can deliver is 21 A or 71 W. This is exactly the same rectifier used on the 550 W model.

It is always good to remember that the real current/power limit for each output will depend on other factors, especially on the coils used.

BFG MX-680 power supply
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Figure 13: Rectifiers.

This power supply uses a PS223 monitoring integrated circuit, which features the following protections: over current (OCP), over temperature (OTP, although this power supply does not implement this protection), over voltage (OVP) and under voltage (UVP).

BFG MX-680 power supply
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Figure 14: Monitoring integrated circuit.

This power supply has two thermal sensors, one for each fan. We don’t understand why the manufacturer didn’t implement over temperature protection (OTP), as the monitoring integrated circuit has this protection, the only thing that would be needed was an additional thermal sensor.

All the electrolytic capacitors from the secondary are from JunFu and labeled at 105° C, as usual.

Power Distribution

In Figure 15, you can see the power supply label containing all the power specs.

BFG MX-680 power supply
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Figure 15: Power supply label.

This power supply features four +12 V virtual rails distributed like this:

We think this distribution is perfect.

Now let’s see if this power supply can really deliver 680 W.

Load Tests

We conducted several tests with this power supply, as described in the article Hardware Secrets Power Supply Test Methodology.

First we tested this power supply with five different load patterns, trying to pull around 20%, 40%, 60%, 80%, and 100% of its labeled maximum capacity (actual percentage used listed under “% Max Load”), watching how the reviewed unit behaved under each load. In the table below we list the load patterns we used and the results for each load.

If you add all the power listed for each test, you may find a different value than what is posted under “Total” below. Since each output can vary slightly (e.g., the +5 V output working at +5.10 V), the actual total amount of power being delivered is slightly different than the calculated value. On the “Total” row we are using the real amount of power being delivered, as measured by our load tester.

+12V1 and +12V2 are the two independent +12V inputs from our load tester and during out tests the +12V1 input was connected to the power supply +12V1 (main motherboard cable and peripheral power connectors) and +12V3 rails (video card auxiliary power connector), while the +12V2 input was connected to the power supply +12V2 rail (EPS12V connector).

Input

Test 1

Test 2

Test 3

Test 4

Test 5

+12V1

5 A (60 W)

10.5 A (126 W)

15.5 A (186 W)

20.5 A (246 W)

25.5 A (306 W)

+12V2

5 A (60 W)

10.5 A (126 W)

15.5 A (186 W)

20.5 A (246 W)

25.5 A (306 W)

+5V

1 A (5 W)

2 A (10 W)

4 A (20 W)

6 A (30 W)

8 A (40 W)

+3.3 V

1 A (3.3 W)

2 A (6.6 W)

4 A (13.2 W)

6 A (19.8 W)

8 A (26.4 W)

+5VSB

1 A (5 W)

1 A (5 W)

1.5 A (7.5 W)

2 A (10 W)

2.5 A (12.5 W)

-12 V

0.5 A (6 W)

0.5 A (6 W)

0.5 A (6 W)

0.5 A (6 W)

0.5 A (6 W)

Total

138.4 W

276.7 W

412.7 W

546.6 W

678.4 W

% Max Load

20.4%

40.7%

60.7%

80.4%

99.8%

Room Temp.

47.6° C

46.6° C

49.1° C

49.8° C

48.6° C

PSU Temp.

47.4° C

48.0° C

49.8° C

55.3° C

53.5° C

Voltage Stability

Pass

Pass

Pass

Pass

Pass

Ripple and Noise

Pass

Failed on -12 V

Failed on -12 V

Pass

Pass

AC Power

168.5 W

330.8 W

500.7 W

680.0 W

877.0 W

Efficiency

82.1%

83.6%

82.4%

80.4%

77.4%

AC Voltage

113.1 V

113.2 V

111.7 V

108.8 V

105.4 V

Power Factor

0.986

0.996

0.998

0.998

0.999

Final Result

Pass

Pass

Pass

Pass

Pass

BFG MX-680 could really deliver 680 W at 48.6° C. If you pull up to 60% from its labeled capacity (i.e., up to 408 W) you will see a decent efficiency between 82.1% and 83.6%. When pulling 80% from its labeled power (i.e., 544 W) efficiency dropped to 80.4%, still above the 80% mark. But at full load (680 W) efficiency dropped below 80%, at 77.4%.

It is always good to remember that this is not necessarily a bad thing, as you probably won’t pull anywhere near its labeled capacity, as power supplies are designed for you to run them at 50% from their labeled capacity (click here to understand more).

The only problem we had with this power supply was with its -12 V output. During tests number one and five the ripple level from this output was low (18.4 mV and 38.4 mV, respectively) and during test number four it was high (93 mV), but still inside the 120 mV limit. But during tests number two and three the ripple on this output skyrocketed to 365 mV and 480 mV, respectively, far higher than the maximum allowed (120 mV).

BFG MX-680 power supply
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Figure 16: Noise level on -12 V during test number four (480 mV).

All other outputs presented very low noise levels, as you can see on the screenshots below. Just to remember, the maximum allowed for the +12 V outputs is 120 mV and the maximum allowed for the +5 V and +3.3 V outputs is 50 mV. All these values are peak-to-peak figures.  

BFG MX-680 power supply
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Figure 17: +12V1 input from load tester at 678.4 W (35.6 mV).

BFG MX-680 power supply
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Figure 18: +12V2 input from load tester at 678.4 W (35.2 mV).

BFG MX-680 power supply
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Figure 19: +5V rail with power supply delivering 678.4 W (18.2 mV).

BFG MX-680 power supply
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Figure 20: +3.3 V rail with power supply delivering 678.4 W (15.6 mV).

Now let’s see if we could pull even more power from MX-680.

Overload Tests

Before overloading power supplies we always test first if the over current protection (OCP) circuit is active and at what level it is configured.

OCP kicked in when we tried to pull more than 26 A from +12V2 input from our load tester (which was connected to the power supply +12V2 through the ATX12V/EPS12V cable).

We could pull up to 797.5 W from this power supply. If we tried to increase one amp the power supply would shut down, showing that a protection was in action, which is great. After all, the goal of our overloading tests is to see if the power supply protections are working and if the power supply burns when we overload it.

One thing that we noticed during our reviews was that the auxiliary 80 mm fan was not entering in action, even thought the temperature inside our “hot box” was at 50° C and the power supply housing was measuring 53° C. With the tip of a pencil we manually moved the blades of the fan and it started spinning. We had this exact same problem with the 550 W version from this power supply.

Input

Maximum

+12V1

32 A (384 W)

+12V2

32 A (384 W)

+5V

10 A (50 W)

+3.3 V

10 A (33 W)

+5VSB

2.5 A (30 W)

-12 V

0.5 A (6 W)

Total

797.5 W

% Max Load

117.3%

Main Specifications

BFG MX-680 power supply specs include:

* Researched at Newegg.com on the day we published this review.

Conclusions

Several manufacturers release higher capacity power supplies within the same series by simply replacing the secondary rectifiers with more powerful models. Fortunately this is not the case with BFG MX-680: the manufacturer replaces ALL components (PFC transistors, switching transistors and secondary rectifiers) with more powerful models compared to the 550 W product. This explains why we could pull up to 785 W from it.

If you are building a PC that is going to pull up to 544 W (80% of its labeled capacity), this power supply may be an option. At full load, however, efficiency drops below the 80% mark and we also had a huge ripple problem with the -12 V output when we pulled between 272 W and 408 W from it.

Another issue was that the auxiliary fan didn’t kick in automatically: we had to manually rotate it with the tip of a pencil in order to turn in on. This also happened with the 550 W model we reviewed.

A very important thing to keep in mind if you buy this unit: warranty. You have to register your power supply with BFG within 30 days of its purchase, otherwise you will only get a two-year warranty instead of the full five-year one. This is really tricky, as most users do not register their products with the manufacturer.

It comes with a somewhat attractive price tag – USD 130 –, but we think that there are better options on the market, like Seventeam ST-750Z-AF, which is cheaper, has a higher labeled wattage, also has a modular cabling system and provides a better performance.

Originally at http://www.hardwaresecrets.com/article/BFG-MX-680-Power-Supply-Review/760


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