Electronics > Projects, Designs, and Technical Stuff
LM338 Power supply update
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rpetitho:
So I built up my power supply from a previous post. (https://www.eevblog.com/forum/projects/my-first-power-supply-212774/msg2719512/#msg2719512) I had to make a few adjustments for performance and created a new schematic, pdf attached. Here are some performance curves, let me know your thoughts.

Transient Load Response: Set at about 9.7v, switch a ~10ohm resistor and drive about 1 amp. Drops by 600mv then recovers in about 3us. I think its good.


Overcurrent Cut-out. Set at about 9.7v and current limit at about 0.25 amps, instant connect ~10ohm load. Voltage drops most of the way in about 40us, and settles after about 150us. Decent?


Finally, a bit of a strange one here. A similar test to the first except this is the response when disconnecting a 10ohm load (about 1 amp). The response starts off as an inverse picture to the first curve, but then goes into a long saw tooth shape. Not sure why this is happening.  I found this on the internet reference a LM317 doing somthing similar. (https://e2e.ti.com/support/power-management/f/196/t/372417) I think the performance is still decent, but curios why it is happening.



-Bob
ezeisel:
The transconductance and so essentially  the speed of the NPN series pass transistor in the LM338 is proportional to its output current, hence the asymmetry: it can charge the output node up fast, but the node is only discharged by the 5 mA min load + quiescent current and output voltage setting divider current... so pulldown is slow.
That doesn't really explain why it's underdamped. But I think that is also related to the asymmetry: the output stage is very slow when the current is low, so the output pole is closer to the internal dominant pole, reducing the phase margin when voltage is dropping, leading to undershoot that is then corrected with another overshoot etc.

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