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.
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.
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
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.
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.
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.
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 through affiliate links which helps me support this website, while offering you the same low price (Disclaimer!).