Peaktech 6225A review

Intro

I got a Peaktech 6225A power supply to power some things, as it seemed like a good deal, going beyond what one might find normally in these types of supplies: more display resolution and supposedly, lower noise. For this price, this supply is a good deal compared to other similar ones on the market. Let’s see how it performs.

You can even grab one on Amazon.de or

Amazon.fr through affiliate links which helps me support this website, while offering you the same low price.

Voltage and current set

For the first test, I have checked if the supply is really delivering on that 1mA / 10mV precision, using a calibrated multimeter. The results are below: for the voltage setting, the power supply seems to hold on pretty well from about 0.2V and higher, below which it is not that much control. On the current side, things are similar, current cannot be very accurately set below about 50mA. In both cases, the precision starts to be there from about 1% of the full scale and I think it is perfectly fine for this kind of product.

Burn test

First off, the power consumption test: the power supply burns 4.2W while on and with the output turned OFF. If the output is turned ON and set to 25V and no load, it consumes 5.2W, which I think is decent.

Of course, I had to grab my PSU burner to start stressing this guy out. I set the supply for 30V and 5A load and let it run for half an hour. The fan quickly started.

Mains power ON test

I checked what happens when the power supply is switched ON, from the mains switch at the back. With no load, there is a small 0.5V glitch that takes time to die down, due to the high filter capacitors. With a 10R resistor the glitch is surprisingly higher at 1V, but it extinguishes faster. Note: voltage is orange channel and current is blue channel.

With 10R load

Soft power ON test

Setting the power supply to 5V and turning ON results in this waveform. Note that the oscilloscope is suffering from external noise due to grounding.

And powering down, again no load shows quite a long time to discharge the capacitors

Transient

Here is the response when the supply is set to 5V and a 1A load pulse is applied. We can see the supply goes back on track within about 100µs. Note the voltage drop with load is not that high, but the Analog Discovery measures it after some wires. Disconnecting the load produces about 1V higher peak which is not that good.

Constant current mode

This is one of the interesting thing to check, as it shows how the supply will perform in case of a circuit fault and limit the current. First test, 5V output with 1oomA limit, checking what happens if the PSU burner tries to pull 1A from the supply. It takes about 6-8ms before the current is limited to the set value! This is quite slow.

Next up, same test, but setting the supply to a 1A limit and trying to pull 2A from it, we see the same response time.

With the same 1A limit trying to pull 5A shows some more details: the response time is now shorter, about 4ms, indicating this is closer to the loop response, and not the discharging of the output capacitors. The oscillations also show the loop stabilization time.

This is further seen as what happens when starting the power supply shorted, but with a 1A limit, which allows the load to pull almost 4A at start.

Conclusion: the supply is quite slow and will allow a pretty high current to pass through a shorted load before the loop reacts, not recommended as protection to delicate circuits.

Noise test

Finally, let’s look at noise test, this time I removed the whole PSU burner and left the Analog Discovery only. Setting the supply to 5V and loading with a 10R resistor, I checked the noise.

The noise is about 80-100 mVpp, which through a slightly incorrect way to judge this, comes down to about 15mVrms, quite more than the 3mVrms quoted in the specifications.

Actually, manufacturers don’t like to measure noise like this, but by having a 20uF capacitor at the end of the load. So doing the same thing, I get about 12Vpp of noise, much better and quite in line with the specifications, which is really pleasing.

Teardown

Of course the fun does not stop here, so let’s look at a what is inside. Surprise surprise, there’s a big TIP3055 transistor on the bottom, more on that later.

In general we see things that are expected: the input socket contains a fuse and the 110/220V switch, with the power then going through a double switch. Furthermore there is some common mode filtering and your typical NTC to protect from high power up rush in current.

We can also observe the thermostat of the fan is bolted on the middle heat sink which cools the main switching transistor. My non contat thermometer shows the fan turns on at about 35-40°C.

Comparing to other types of these supplies, like the Manson NSP 3630 (a lot of the designs are copies of this more or less) the main board, visible in the middle, appears much simpler.

Actually, the design is rather different. On top there is a regular 50Hz transformer, with 3 secondaries, compared with an auxiliary switching supply found on the Manson. On the bottom, the blue transformer is the main power supply transformer, carrying out the load.

The supply is controlled by an OB2269 switch mode controller, not the typical TL494. The OB2269 has quite a few advantages, it’s designed to drive a MOS which can be more efficient and it is optimised for “Extended Burst Mode Control For Improved Efficiency and Minimum Standby Power Design” which makes it more suitable for this type of application.

Unfortunately, as expected at this price range, the “Chong” filter capacitors are not the strongest point of this device.

The output filter of the main switching supply is a pair of probably low ESR 1000uF/50V 105C SME capacitors. Can you spot anything missing? Yup, no inductor filter on the output.

Looking at the board from behind, we can see some clear demarcation of the high and low voltage regions, which appear to be safely separated. Also here you see an element which is out of place: the wires of the fan are soldered directly on the back, even though there is a connector in place there. While the supply is quite low cost, there don’t seem to be any other elements inside aimed at saving such a tiny amount.

Next to the clandestine fan connection you can see the 3 rectifier bridges generating the 3 auxiliary voltages used for the supply control. The fan is supplied by 10VDC and there is another 30VDC supply and a -6VDC negative supply, all going to the front panel.

Moving on to the front panel, it connects with a ribbon cable carrying those 3 supplies plus another wire going to the base of the TIP3055 transistor on the bottom. There are 3 more thick wires connecting to the banana connectors on the front panel.

Here is an overview of the whole front panel, first the front side. We can see the two 4 digit, multiplexed displays, the 3 LEDs, 2 rotary encoders and the soft ON/OFF button. Across the outputs there is a 100nF capacitor, plus  2 more from each pole to the earth. There is a reverse protection diode as well and the bottom left shows the current shunt.

Moving towards the back now. Voltage seems to be regulated by a couple of TL431 (top, middle) and transistors. The big decoupling capacitors are SME 470uF/50V, 105C each and they are directly in parallel to the output, so the current measured in the shunt will be delayed by the discharge of the capacitors.

The design is much better than the Manson and other supplies, which relied on a lousy ATMEGA micro controller generating quite low frequency PWM signals which are then filtered to generate the voltage references. The readout was then carried out with dedicated voltmeter ICs.

Here we see a more modern approach: there is a micro controller handling everything, which of course has the markup removed.

Starting on the top, the display is controlled with two 74595 shift registers, one drives the segments and the other the digits, through the transistors on the right side. I have not poked around as to why there are only 6 of them and not 8, one for each digit. The buzzer and driving circuits are not populated.

On the bottom side we can see the micro controller and the control loop. Based on the capabilities of the supply, the microcontroller is probably a more modern device featuring 12 bit ADC and maybe a 12 bit DAC. Given the 3.4V supply I measured, I suspect filtered PWM is not involved here. Something like an XMEGA32E5 could do it, but the pins don’t match. The microcontroller is a 32 pin LQFP with 0.8mm pitch, power seems to come from pins 7 (GND) and 8 (VCC), and pin 1 might be reset. Any ideas?

The loop is controlled by 2 OP07C opamps, which feature very low offset (60µV typical) as a requirement to keep things under such high precision. Unfortunately the op-amps are also quite slow, with a unity gain bandwidth of only 0.4MHz, which can be seen as well in the loop reaction time, in the tests above. There is another 4558 opamp which I believe checks if the supply is in CC or CV mode.

Going back to the full view, the opamp near the connector is a good old 741, which I believe drives the transistor next to the connector which in turn drives the big TIP3055 transistor in the supply.

Speaking of which, what exactly is the big TIP3055 transistor for? Well, it is actually used as a linear regulator following the switch mode supply, which is a feature this supply has and not the older designs. I have measured and it keeps about 2.3 – 2.5V across collector-emitter, depending on load current, which means we are looking at about 12.5W of dissipation at max load. This secondary linear regulator allows the supply to provide such low output noise compared to others.

The way it works is that the opamps on the front panel drive this output transistor to the required output voltage. The switch mode supply follows suite and is designed to supply about 2.5V more than the output voltage, as seen by about 2.5V CE on the transistor. This design provides the best of both worlds, the switch-mode supply gives higher efficiency in a lower size and weight, while the following linear regulator can filter out most noise and provide a cleaner output.

I tried poking around the two un-populated headers with both the Analog Discovery digital IOs, a USB-serial adapter and a ST-LINK V2 and could not get any response out of the micro-controller in any way. I don’t intent to spend more time reverse engineering this, since it needs to go to work.

Conclusions

The power supply delivers great results for the price compared to other similar products, I believe it is a good deal to have one of these for general purpose, with the exception of very delicate things. Furthermore, the dual switching and linear regulator achieves a low weight and compact device with low noise, as promised.

However, the supply is not without it’s limits: the high output capacitance and rather slow loop means it is capable of delivering quite some punch before the limit sets in, so not everything is safe to be powered from it. Do note however than linear or not, only much more expensive lab power supplies have a fast acting loop with little output capacitance, as such a feature costs and is infrequently required. Quite a few other small improvements could make it better: screw type bananas are better for connecting wires directly and some sort of proportional to temperature fan control is quieter and welcome in places like a home lab.

Finally, I do recommend this for a well made and great performance for the price this product offers. You can even grab one on Amazon.de or Amazon.fr through affiliate links which helps me support this website, while offering you the same low price.

 

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26 Comments

  1. Hey 🙂
    I am currently trying to insert a small circuit in the PSU in order to handle reverse currents coming from motors that are acting as generators during braking.
    My issue is that I can’t really find a reference voltage that resembles the set output voltage (and stays the same during CC). Does anybody have an idea where to look for it? My first idea was that the uC outputs two voltages for the output voltage and current setting but it does not seem to be that way…
    Many thanks in case you have an idea! 🙂
    Great Blog btw!

    • I did not get much chance to poke around because I needed to use this supply… But it is possible that the reference voltage and current are generated using PWM signals which are probably filtered close to the opamps.
      If you have an oscilloscope and poke around, you might find the original PWM signal.

      Note, if you intend to use the PSU like that, better to use a diode in series with the output so you cannot inject current. And maybe a fuse too!

      • Hmmm then I will retry probing tomorrow 🙂
        I also thought about using a diode on the output but I was fearing for the motor drivers to be damaged if the voltage on the output after the diode spikes very high…

          • Hey 🙂
            I couldn’t resist myself and reverse engineered the most interesting part about the schematics of the Peaktech 6225 A control loop.
            (https://www.dropbox.com/s/wj405m435snhx5f/Peaktech%206225%20A.pdf?dl=0)
            It seems that I may be able to use Pin 17 of the µC or even directly the output of the LM714 to control my P-MOSFET that I would like to use for dumping currents flowing inside the PSU.

          • That is excellent work, thanks for the schematic
            The very complex circuit used to turn on has a double role: it makes sure there is no output glitch while the power supply starts. You need to wait for C17 to charge before you can turn off the clamp T4.
            Can you elaborate on “dumping currents flowing inside the PSU”?

          • Thanks! 🙂
            I have updated the schematic with some part about how I would like to dump the currents flowing into the ECU.
            In case some higher voltage is applied to the ECU contacts the output of the voltage control loop should change from positive to negative, switching on the PNP-Darlington Transistors and short circuiting the input voltage. I have added also an optional diode and capacitor to the base of the transistor in case I want the voltage clamping to stop after some time to prevent the PNP transistor from overheating.

          • Hm… does the IC3 have a symmetrical supply to be able to go negative?
            I am not very ok with this configuration, it is quire risky to clamp the output. Plus, there is some possibility that you will get oscillations.
            BTW, you will need some resistor across C19 and it should be non polarized.

          • I haven’t measured the negative voltage of those OPs but according to their datasheet they don’t work with a single supply so I guess that they use the -6V supply you found.
            Oszillations may indeed be an issue but at least there is some area of the OPs between -1V and 1V when neither of both transistors are switched. This should hopefully stabilize sufficiently.
            I have added a resistor in the schematic that should limit the continuous clamped current to ~1A while the capacitor allows higher peak currents. Only during clamping there will be a negative voltage of -3…-4V which should not be critical.

  2. Sorry for “spam”, but I just wanted to say that I have bought this PSU, and after first use thermal switch for the fan failed. -_- It is shorted all the time, and the fan is on all the time. And it is loud. Other than that, PSU is great. I have ordered a new thermal switches (40oC) from Aliexpress.

    • Indeed, mine too. The switch turns on at 35C and off at about 25C, which may never be reached if the ambient is hot enough.
      I will fix mine by putting a resistor across the thermostat which can keep the fan running at a low level. I expect it will never start unless I am really pushing it. Similar to what i did for my 3D printer power supply http://www.electrobob.com/quiet-3d-printer/

  3. Thanks, Bogdan.

    I am in Serbia. Actually, the store that is selling this PSU has the “A” version, but on their website it is advertised as non-A. I have ordered it. It is about 100 Euros.

    BTW, are you Serbian?

  4. Actually, only the non-A version of this PSU is available in my country. 🙁

    • Maybe give it some time, since the model is rather new? I don’t know where you are from, but there are some stores in europe that will ship to quite a few countries, like reichelt.de (have never used them)

  5. I am thinking of buying this PSU. Can the OP07C be changed to a better (faster) one? Will that decrease the reponse time? Is it even worth it?

    • The speed is not limited just by the amplifier, but by the external loop around it. It is very likely that the loop is slower than the amplifier to leave some margin. Even if you place a faster amplifier, it will not be faster. And designing another loop that will be stable is a whole other issue, since it needs to not only work by itself, but also work with the switching part as well.

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  7. or STM32F042K LQFP32

  8. Looks like STM8S to me

  9. Microcontroller looks like a standard ATMega328P. You can see the upper bar of the Atmel logo which really limits the number of devices that it could potentially be. Might try poking around to see if there is an active SPI or I2C bus on the board…

  10. Pingback: PSU Burner « Electro Bob

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