Seventeam ST-420BKV 420 W Power Supply Review
By Gabriel Torres on March 6, 2008
Even though power supplies from Seventeam aren’t sold in the USA retail market, they are very popular in several other countries, due to its low cost. In fact, it seems that Seventeam is the OEM manufacturer for some other brands – we discovered that the external power supply series from XG/MGE, Magnum, is in fact Seventeam Fanless series. ST-420BKV is a 420 W power supply without PFC. Let’s take an in-depth look at this power supply to check whether this unit is good or not and test it to see if it can really deliver its announced 420 W.
Even though its design resembles a high-end product – using a big 120 mm fan on its bottom and using a mesh on the back –, ST-420BKV finishing isn’t so good. Even though on Seveteam’s website this power supply is pictured as having a black automotive painting job and an orange fan, the model we bought for this review used a plain zinc-coated steel housing, making it to look like a very low-end unit.
On the other hand this power supply has a feature we’ve never seen before: a 12 V jack for powering external devices such as speakers and external hard disk drives (see Figure 2).
In Figure 2 you can also see that this power supply has a 110/220 V switch, indicating that it doesn’t have PFC circuit (power supplies with active PFC don’t have a 110/220 V switch).
Even though this is a low-end power supply, the main motherboard cable (and only this cable) uses a plastic sleeving.
In Figure 3, you can see a big finishing detail missing: the cables come out from a big hole on the power supply housing and there is no finishing to cover the unused space, so you can see a big hole there.
This power supply has four peripheral power cables: one Serial ATA power cable containing two SATA power connectors; two peripheral power cables containing two standard peripheral power connectors and one floppy disk drive power connector each; and one peripheral power cable containing two standard peripheral power connectors. This power supply doesn’t have any auxiliary PCI Express power connector, so you will need to use an adapter in order to power your high-end PCI Express video card.
The main motherboard cable has a 20-pin connector, with a 4-pin extension in order to transform it into a 24-pin connector.
The gauge of the wires used on all cables is 18 AWG, but the floppy disk drive connectors use 22 AWG wires.
Even though this power supply has an UL label, the model we've got didn't have any UL registration number. However, Seventeam contacted us and sent us the complete UL documents, explaining that this model is really UL-certified and their registration number is E141400. We could confirm that online.
We decided to fully disassemble this power supply to take a look inside.
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 inside and to compare this power supply to others.
In this page, we will have an overall look, while in the next page we will discuss in details the quality and rating of the components used.
On Figures 5 and 6 you can have an overall look from inside this power supply.
As we mentioned on other articles, 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 than that, usually removing the MOV, which is essential for cutting spikes coming from the power grid, and the first coil.
In this stage this power supply is flawless, as it has more than the minimum recommended number of components for this stage: two extra metalized polyester capacitors (X capacitors), two extra ceramic disc capacitors (Y capacitors) and two extra ferrite coils, see Figures 7 and 8.
In our first analysis we posted that this power supply didn't have a MOV. However a friendly technician that works at one Seventeam distributor pointed out to us where the MOV’s are located: they are squeezed between the two electrolytic capacitors from the voltage doubler (the two big capacitors located on the primary) and connected after the rectifying bridge, and not before this component as usual – and that is why we didn’t find them at the first time. My sincere apologies to our readers for my lack of attention.
A very interesting feature from this power supply is that its fuse is inside a fireproof rubber protection. So this protection will prevent the spark produced on the minute the fuse is blown from setting the power supply on fire.
In the next page we will have a more detailed discussion about the components used in the Seventeam ST-420BKV.
Seventeam ST-420BKV uses one PBU1005 rectifying bridge, which can deliver up to 10 A (rated at 100° C). No heatsink was used to cool down this component. This component is clearly overspec'ed: at 115 V this unit would be able to pull up to 1,150 W from the power grid; assuming 80% efficiency, the bridge would allow this unit to deliver up to 920 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.
On the switching section two 2SC3320 NPN power transistors are used under the half-bridge configuration, which is a typical configuration used by power supplies without active PFC. Note that the transistors are regular BJT transistors and not MOSFET ones. Each transistor has a maximum rated current of 15A @ 25° C.
This power supply uses four power Schottky rectifiers on its secondary section: one SBL4060PT and three SBL4040PT, all of them capable of delivering up to 40 A @ 100° C (20 A per internal diode).
Here we can see that this power supply uses an old design. Current power supplies can deliver more current from the +12 V outputs, where the CPU and the video cards are connected to. On Seventeam ST-420BKV, however, the +5 V output is the one with the highest current limit, a typical scenario for six/seven years ago.
Since this power supply is based on the half-bridge design, calculating the maximum theoretical current for each output is easy: all we need to do is add the maximum current supported by each diode.
The +12 V output uses one SBL4040PT, so it can deliver up to 480 W. The maximum current this line can really deliver will depend on other components, especially the coil used.
The +5 V output uses two SBL4040PT in parallel, so it can deliver up to 80 A or 400 W. On more recent power supplies it is the +12 V output that has two rectifiers in parallel, not the +5 V output.
Usually the +3.3 V output on low-end power supplies based on half-bridge design is achieved by a 3.3 V voltage regulator connected to the +5 V output. This power supply from Seventeam, however, uses a better design, using an independent Schottky rectifier for producing its +3.3 V output.
The +3.3 V output is produced by the SBL4060PT, so it can deliver up to 132 W. Always keep in mind that the maximum current this line can really deliver will depend on other components, especially the coil.
Even though the +5 V line and the +3.3 V line have separated rectifiers, they share the same transformer output. So the maximum current both lines can deliver will depend a lot on the transformer.
On Figure 12 you can see a thermal sensor connected to the secondary heatsink, which controls the fan speed according to the power supply temperature.
The capacitors of the voltage doubler circuit are rated at 85° C and from Toshin Kogyo (TK), a Japanese vendor that sells rebranded Taiwanese caps from OST, and all other capacitors are Taiwanese, from CapXon (all rated 105° C).
In Figure 13, you can see ST-420BKV label stating all its power specs.
As you can see on the label, there are two +12 V virtual rails, each one able to deliver up to 16 A, but with a combined 26 A maximum. Inside the power supply these two rails are connected to the same place – the +12 V rail coming from the +12 V rectifiers. What happens is that each virtual rail is connected to its own over current protection (OCP) circuit, which shuts down the power supply if you pull more than 16 A on each rail or 26 A total, in the case of this power supply. Usually the OCP circuit is configured with a value a little bit above from what is printed on the label. During our tests we checked whether the OCP was really active, as you will see.
All +12 V (yellow) wires are connected to the first rail (+12V1) but the wires from the ATX12V connector, which are connected to the +12V2 rail, using yellow with black stripe wires. This distribution is correct for a power supply that doesn’t provide cables for video cards or lots of peripheral plugs.
Now let’s see whether this power supply could deliver its rated power or not.
We conducted several tests with this power supply, as described in the article Hardware Secrets Power Supply Test Methodology. All the tests described below were taken with a room temperature between 45° C and 49° C. During our tests the power supply temperature was between 48° C and 52° C.
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.
+12V2 is the second +12V input of our load tester and on this test it was connected to the power supply ATX12V connector. Since this connector was the only one connected to the power supply +12V2 virtual rail the +12V1 and +12V2 inputs from our load tester were really connected to the +12V1 and +12V2 virtual rails from the reviewed power supply.
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.
3 A (36 W)
6 A (72 W)
9.5 A (114 W)
12 A (144 W)
15 A (180 W)
2.5 A (30 W)
6 A (72 W)
8 A (96 W)
11.5 A (138 W)
14.5 A (174 W)
1 A (5 A)
2 A (10 W)
4 A (20 W)
5 A (25 W)
6 A (30 W)
1 A (3.3 A)
2 A (6.6 W)
4 A (13.2 W)
5 A (16.5 W)
6 A (19.8 W)
1 A (5 W)
1 A (5 W)
1 A (5 W)
1.5 A (7.5 W)
2 A (10 W)
0.5 A (6 W)
0.5 A (6 W)
0.5 A (6 W)
0.5 A (6 W)
0.8 A (9.6 W)
% Max Load
Ripple and Noise
We must say that ST-420BKV surpassed our expectations. We were expecting a low-end power supply with a lousy efficiency and not being able to deliver its rated power. Even though efficiency was below 80%, it wasn’t below 70% as we were expecting, so this unit isn’t that bad, especially when we think about the design that was used. And it could deliver its nominal power with a room temperature of 49.5° C, which is impressive.
Voltage regulation was also one of the highlights during our tests. All outputs were within 3% of the nominal voltage during all tests, which is outstanding, as ATX spec states that regulation should be within 5%. Translation: the voltages were closer to their nominal values than what is stated by the ATX standard. The only exception was -12 V output during tests one and two, which was at -10.96 V and -11.40 V, respectively. Even though these values are still inside the ATX specification (-12 V has a 10% tolerance, while all other outputs have a 5% tolerance) we wanted to see values closer to -12 V, especially on the first test.
Electrical noise was also at a very low level, always below 29 mV on +12 V, below 26 mV on +5 V and below 22 mV on +3.3 V – ATX spec states a maximum noise level of 120 mV for +12 V and 50 mV for both +5 V and +3.3 V outputs. We wanted to see lower noise levels on +5 V and +3.3 V, even though they are within specs – very good power supplies have a noise level below 10 mV on these outputs.
Below we show the noise level we found on the power supply outputs while the unit was operating at its full load (test number five): +12V1 rail was at 28.6 mV, +12V2 rail was at 26.8 mV, +5 V rail was at 25.6 mV and +3.3 V rail was at 22 mV.
After these tests we tried to pull even more power from Seventeam ST-420BKV. Below you can see the maximum amount of power we could extract from this unit keeping it working with its voltages and electrical noise level within the proper working range. During this test room temperature was of 44° C and the power supply was working at 50° C.
15 A (180 W)
15 A (180 W)
9 A (45 W)
9 A (29.7 W)
2 A (10 W)
0.8 A (9.6 W)
% Max Load
We tried to pull even more power, but the power supply shut down, showing the over power protection (OPP) in action. We could also see the over current protection (OCP) in action in two ways. First we tried to determine if the OCP circuit was active and at which level. For this we configured our load tester to pull a lower current from +12V1 (5 A) and we increased the current on +12V2 until the power supply shut down. We found out that if we pulled more than 18 A from +12V2 the power supply would shut down, so OCP was set at 18 A, 2 amps above what was written on the power supply label, which is normal. Then we tried to pull 18 A from the two +12 V rails at the same time, but the power supply would shut down. Then we remembered that the manufacturer says on the label that the maximum combined current on +12 V is 24 A. The maximum we could pull was 15 A on each rail (30 A total), so the OCP circuit was again configured a little bit above from what was written on the label.
The bottom line is: both OPP and OCP circuits are active and working just fine. As we mentioned, when we tried to pull more power the unit would shut down or simply not turn on. ST-420BKV survived our tests.
Short-circuit protection is also working just fine, as we could test.
We could set the power supply to deliver more power, but it would shut itself down after a couple of minutes.
Under this condition noise level continued within specs, at 29.4 mV on +12V1, 25.6 mV on +12V2, 24.8 mV on +5 V and 19.6 mV on +3.3 V. This is really good.
The main problem of running this power supply at 450 W is its efficiency, below the 70% mark. As you can see, we were pulling 660 W from the wall to produce 450 W.
Another great feature about this power supply is its fan. When the power supply is cool, it runs very slowly, making almost no noise. But as soon as the power supply temperature reached 30° C the fan started spinning faster, making a lot of noise.
Seventeam ST-420BKV power supply specs include:
This power supply surprised us. Because of the design used in its primary – half-bridge, i.e., the same one used by old AT power supplies – we were expecting a power supply that would not be able to deliver its rated power and would have an efficiency below 70%.
Seventeam ST-420BKV was able to deliver up to 450 W at 45° C (420 W at 49.5° C), which is an outstanding result. As for its efficiency, it was between 70% and 80%, depending on the load.
On the down side we have the low number of SATA plugs – only two – and no power cable for video cards. So if you want to install more than two SATA devices and/or use a video card that requires an auxiliary PCI Express power connector, you will need to use adapters.
Of course we would prefer buying a power supply with at least 80% efficiency, but if you want a cheap power supply that can truly deliver its rated power and you are building a low-end or mainstream PC, Seventeam ST-420BKV is certainly an option. It survived to our "torture session" without burning!