LM723 Basic PSU Feedback

[flindersit]

Hi am a newbie to electronics, i have designed a circuit as seen below, question is would this work to drive a bank of 2n3055 from the output of the pn2222a, or do i need to drive the output directly from pin 10 to all the 3055? i decided to use the 2222a as the 723 max output current of 25mA
TIA

https://prnt.sc/qa46w1TPRRRv

[Kleinstein]

The PN2222 is a big weak in driving already  1 x 2N3055.  It may be OK with the current, but would likely not get rid of the heat. If a high current and thus more than 1 power transistor is planed one would also need a larger transistor (e.g. TO220 case, like D44H...) for the base current. At high current the current gain of the 2N3055 is only at some 20, so expect 1/20 the current and power in the driver.

The output stage as shown has quite some drop out voltage. One can get a little less drop with a Sislaki like configuration with the IC internal NPN,  and external PNP and than NPN output transistors.
With the relatively low set voltage this may be an issue, but it depends on what voltage to start with.

The current limit at the base current is not very accurate, but the LM723 internal way is not accurate anyway. So it can be an OK compromise, saving on the shunt resistor power rating.

The feedback part of the circuit is not working right. That part is essentially all wrong: R1 should go to the output and the ref. signal has to go to the + input of the error amplifier.

[LarryAndrews]

Interesting that you are using an LM723. It's a part that's older than I am, and I'm old. I have seen schematics that use the 2N2222A for driving a 2N3055, but not in practice. Seems to me that driving a 2N3055, we used darlington pairs as in the TIP series of transistors. There are a million designs and books which describe designs using this type of linear power supplies from the 1970s. Book thrift stores should have them. I really enjoyed the programmable loads of that time era that used banks of 2N3055s. Still use them today. But, it seems that devices such as IRF MOSFETS took over the 2N3055 in power applications because of their durability and ease of use.

[dobsonr741]

Careful with the IRFs replacing the venerable 2N3055. Today’s power mosfets are optimized for switching and the DC safe operating area is minuscule. The smallest thermal overload on the chip surface will start a spreading pattern and eventually lead to failure. There are very few linear mosfets, usually called lateral or planar, that will be in par with power BJT at DC. The other indication of these parts is the higher Rds - ~0.5 ohm or up.

The key to if 2N2222 is appropriate is to calculate the currents for the pass element and its driver. Take the worst case beta from the data sheet, and check whether 2N2222 can supply that much within its limits, and at what power dissipation on the 2N2222. Good exercise it have not done before.

[adinsen]

I'm a year younger than the 723 so I'm nostalgic about it. Also, it's a good regulator. TIP3055 is a good alternative to 2N3055, actually it's the same, but in a modern package.

But your design will not work as you have the non inverting input on the 723 floating, so it will not regulate the output voltage. Also, current limiting will also not work as it will depend on the hFE of the 2N3055 which varies a lot and depends on Ic.

The 723 can supply about 150 mA so with 2N3055 having min gain 20 @ 4A, it should be fine driving it directly with a supply current of up to 3A.

The attached schematic is a basic 15V regulator from the 723 data sheet, I suggest you start from that 

There's also helpful information in this thread https://www.eevblog.com/forum/projects/lm723-die-pictures/

[LarryAndrews]

dobsonr741, That was interesting what you said about today's MOSFETs. I used IRF150 Power FETs when they were first released in the 1980s. We used them in switching power supplies, but I had always been taught of the Rds positive temperature coefficient of the devices and that as long as they were properly heat sinked, I would never have to worry about thermal destruction. The issue we had was with VDS. If we violated VDS, even for a short period of time during operation, we destroyed the device. So we would institute "snubber" circuitry to dampen voltage overshoots for protection. Perhaps what we were experiencing was this effect you speak of. Looking at devices in which a gain positive temperature coefficient is an issue at low gate voltage will be something I will look into. Thanks for bringing that to my attention.

[dobsonr741]

IXYS has good literature on the subject, as well nice linear capable products: https://www.ixys.com/documents/articles/article_linear_power_mosfets.pdf