Electronics > Projects, Designs, and Technical Stuff
TO220 rail splitter?
Zero999:
Please do some measurements. I'd be interested as to how efficient it is and the accuracy of the voltage splitter.
BravoV:
Subbed. :-+
0xdeadbeef:
Hm. What kind of measurements do you suggest exactly? Obviously the DC/DC board will have the biggest impact on ripple/noise and efficiency. Also efficiency will vary over input/output voltage, current draw etc.
Anyway, I did a schematic of what I think should represent the virtual ground circuit. The DC/DC and the input circuit to the DC/DC is missing from this schematic. So the Micro-USB connector and the two 220µF caps (in parallel) for the input voltage are not included. The main circuit looks a lot like the one from the LM675 application circuit.
It's actually quite surprising that two of the 220µF caps were used to stabilize the input of the DC/DC but none (directly) at the output of DC/DC (the DC/DC board itself has a 220F on its input and a 100µF on its output).
There's a pair of two 220µF in parallel between +VCC and ground and another pair between ground and -VCC. The last cap is used in the feedback loop. Which again is surprising as this would mean a corner frequency of 0.07Hz.
All the caps on the main PCB (not the DC/DC) are all Elna 0522 SMD types rated 25V and 125°C. Could be RVK-25V221MH10U but I'm unsure as there is no specific type marking on them. Could be also low-ESR types or something, there are quite a few Elna product families with the same (printed) temperature/voltage ratings.
The layout is a bit weird. While the outputs of the DC/DC are connected directly to the output pins (through short wires soldered over the cutout) and the trace for virtual ground is very wide, the traces to the supply pins 3 (negative supply) and 5 (positive supply) of the TDA2030 are rather thin in comparison. It seems weird to use a trace width about 3 times as wide for the output of an OpAmp compared to its supplies.
0xdeadbeef:
OK, I did a first measurement regarding noise and symmetry. As expected, the noise on the output of the DC/DC (and thus on +Vcc/-Vcc) is quite substantial. The attached screenshot was taken with the output set to ~35V and the input at 5V (linear bench supply). There are two independent disturbances. One is triangular in shape and has a period of a bit more than 5µs (~190kHz) with a peak of around 150mV. The other one is a (somewhat) symmetrical glitch with a period of rougly 10µs (100kHz). In sum, the RMS noise is about 27mV.
[EDIT]
The 2nd (symmetrical) glitch has its origin somewhere else, not in the module. I currently blame my UPS but have to investigate further.
Still, the noise on the DC/DC module is far worse than on others I have with a (fake?) LM2577. Guess the problem is that this module doesn't have any LC filter at its output.
I then measured the voltage between the outputs to check for symmetry. With the DC/DC set to 34V (i.e. 34V measured between VCC+ and VCC-), the virtual ground was slightly off. VCC+ to VGND measured 17.03V while VGND to -VCC measured -16.97V. So there is a +/-30mV deviation at 34V. Still, 0.03V/17V*100% equals 0.176%. So the symmetry is pretty good.
Note that all measurements were done without load other than the onboard LED and the TD2030A. With 34V on the DC/DC output and a 10k resistor, the current through the LED should be 3.4mA. Additionally, the quiescent drain current of the TD2030A is supposed to be around 40mA. On the input, there were 348mA going into the circuit at the time. So the efficiency of the DC/DC for very low output currents should be something like
(43.3mA*34V)/(348mA*5V)*100% = 84.8%.
There's a bit of guesswork involved since I can't really measure the quiescent current drain of the TD2030A but I guess an efficiency of 80-90% seems realistic.
Besides, I guess we can assume this as a worst case scenario (low input voltage, high output voltage, low output current). So I did another test with the input voltage at 12V. The DC/DC output was 34.08V at this point and the current flowing into the circuit was 144.4mA.
(43.38mA*34.08V)/(144.4mA*12V)*100% = 85.32%.
Again, note that this is the efficiency of the DC/DC board only, since I assumed the quiescent current draw of the TD2030A and the LED as loads. Obviously the efficiency of the whole rail splitter circuit including the DC/DC will be catastrophic for small (external) load currents. Only if they are significantly higher than the internal load current (LED + TD2030 quiescent current), it will be possible to get into the ~80% area again.
Side note: when I first played around with the board today, I observed significant flickering on the LED. I first used USB connectors, then moved to the bench supply. But the flicker didn't go away.Only after some time, it disappeared and didn't appear (yet) again. I didn't observe and voltage fluctuation on the outputs though (other than the noise described above).
0xdeadbeef:
BTW: the thing gets pretty hot even when idling (i.e. no load). The DC/DC is only warm (about 46°C). I'm a bit puzzled though that the trimmer seems to be the warmest part there...
But the TO220 gets up to 54°C or so. If I got the numbers right, this should be only like 1.36W (40mA*34V).
Makes you wonder about temperatures at 3-4A. I actually have some doubts that this will be even usable for 1A for longer times unless the thermal design is improved.
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