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0-30V Stabilized Power Supply


redwire
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If three 2N3055 output transistors share the 5A of current then each has 1.7A and its minimum current gain is 40. Then the current in the BD139 is a max of 125mA. Its max voltage is 36.2V so its max power dissipation is 125mA x 36.2V= 4.5W.

The max allowed chip temperature of a BD139 is 150 degrees C but 130 degrees is safer. Your ambient might be 30 degrees. The thermal resistance from chip to case is 10 degrees/W and for thermal grease is about 0.3 degrees/W. Then with a perfect heatsink the case will be at 30 degrees C and the chip will be (4.5W x 10.3) + 30= 76.4 degrees C.

Your heatsink must have a thermal resistance of (100 degrees - 76.4 degrees)/4.5W= 5.2 degrees C/W or less.   

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I usually use this way to calculate the heatsink, it is wrong that way:
(130°C-30°C)/4.5W - (10C/W+0.3C/W) = 11,9°C/W



What is wrong with my heatsink calculation in 5 post ago??


Nothing wrong really.

Audioguru either made a small typo and actually wanted to type

(130°C - 76.4°C)/4.5W = 11.91K/W

or he substracted another 30°C of safety margin from the maximum junction temperature value of 130°C.

(130°C - 30°C - 76.4°C)/4.5W = 5.24K/W

Florian
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Very nice. Is this the latest REV2 version of the PSU or the original version? The shematic seems to be different, and you're only using one output power transistor.

It is a combination of the original schematic and the latest schematic:
1) It uses Q2 to solve the "Opamp Phase Inversion" problem with the TL081 opamps used in the original project but the MC34071 opamps do not have that probem.
2) The value of R15 is much too high.
3) Its single 2N3055 output transistor will get extremely hot (maybe hot enough to fail) when the output is set for 3A and it is shorted or has a low voltage. Its max dissipation is about 90W.
It has a huge heatsink and a fan so maybe it will survive.

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It is a combination of the original schematic and the latest schematic:
1) It uses Q2 to solve the "Opamp Phase Inversion" problem with the TL081 opamps used in the original project but the MC34071 opamps do not have that probem.
2) The value of R15 is much too high.
3) Its single 2N3055 output transistor will get extremely hot (maybe hot enough to fail) when the output is set for 3A and it is shorted or has a low voltage. Its max dissipation is about 90W.
It has a huge heatsink and a fan so maybe it will survive.


1) I deliberately kept Q2 because with it output voltage drop more quickly after power off.
2) R15 is 1k in original schematic and I don't know what will change if I replace it with lower value resistor since everything is working OK.
3) I decided to use only one output transistor, because the PS would rarely be used at low voltage and high amperage. With this heatsink and the fan running it manage to dissipate 60-70 W for a long periods without problem. For a couple of minutes it survived shorting at 3A.

BTW, for the second unit I put 2SC5589 instead of 2N3055, because it was ease to mount :)

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I've got four TIP3055 (TO-218 / SOT 93) here, and two 2N3055 (TO-3). I've decided to use silent CPU coolers. I am uncertain if a single BD139 will be enough to drive four TIP3055s. I am only aiming for 30V / 3A output, so the additional TIP3055s would just be there to compensate for the lower power rating and to dissipate the heat more easily.

I have two options now ... (see attached pics):

post-46370-14279144450682_thumb.jpg

post-46370-14279144450825_thumb.jpg

post-46370-14279144450963_thumb.jpg

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R15 wastes supply voltage. 1k is too high but 100 ohms is fine.

When output transistors are connected in parallel so they share the heat then each one needs a 0.33 ohm emitter resistor so that their base-emitter voltage differences do not cause the one with the lowest Vbe to hog all the current.

The minimum current gain of one 2N3055 with a current of 3A is about 27. Then the max current in the BD139 is 3A/27= 111mA.
Two 2N3055 transistors each have a max current of 1.5A then their minimum current gain is about 42. Then the max current in the BD139 is 3A/42= 71mA.
You can calculate the current in the BD139 if there are 3 and 4 output transistors.

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  • 2 weeks later...

As we know power dissipation of one of three 2N3055 output transistor is aprox. 65W and recommended thermal resistance for heatsink for one transistor is (200-35)/65 - (1.5 + 0.2) = 0,84 °C/W, but how can i calculate the required thermal resistance of heatsnik for three same transistor with the same power dissipation?
Is ther any equation for this or is eought that i get the heatsnik with thermal resistance three times smaller than thermal resistance for one of each used transistor??...sorry for my english!!
TNX

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The latest circuit diagram can be found here: http://www.electronics-lab.com/forum/index.php?topic=19066.msg1002666#msg1002666

According to this application note from Fischer Elekronik (heat sink manufacturerer)
http://www.fischerelektronik.de/pim/upload/fischerData/datasheet/base/technischeerlaeuterungen_d.pdf (sorry it's in German, there used to be an english version of this but it's gone) you calculate it like this.

irWHa.jpg

Florian

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One more question. Is there a way I can get +/- outputs on this supply. I am going to build it anyway but would be even more handy for me if I had both.

Make two completely separate projects (with separate transformers or separate transformer windings).
One can be a positive supply when its negative output is common and the other can be a negative supply when its positive output is common. Connect the commons together and to earth if you want.
Then you will have positive 0V to +30VDC at up to 3A and have negative 0V to -30VDC at up to 3A.
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Thank you audioguru for the info and please don't think I question your answer because I know next to nothing about electronics. Here's the rub, I just put a small voltage bench supply together, two 14v outlets, one plus, one neg, using center tapped trans. I was curious as to whether this one could be modified so as to have more output v and amps than the first one and have +/-. I have the parts for this 30v supply on order and thought maybe with some changes I could do what I asked about. Probably to late for that now?

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