# Linear power supply using LM338

I'm trying to design a linear power supply to provide 32V and up to 10 amps from a 50V source. Is this the correct way of using multiple LM338 regulators to achieve this?

• In theory yes If you have water cooling for two high power CPU's dumping 180W . The correct way is to search for > 320W Buck regulator – Tony Stewart Sunnyskyguy EE75 Nov 27 '19 at 0:25
• A very, very good idea is to add 0.1 ohms to each regulator's output, then add an op amp to set the adj level. See the LM338 data sheet. Letting them fight directly is a great way to get high-frequency noise in the system. – WhatRoughBeast Nov 27 '19 at 0:32
• I'd suggest figure 44 from the datasheet ti.com/lit/ds/symlink/lm338.pdf but that op-amp dies at 36V – Jasen Nov 27 '19 at 0:38

Problem 1

The LM338 has an input-output voltage differential of 40V, and you've got those great big caps in the feedback path. That will cause the output to start low and ramp up slowly (8-10 volts/second) -- so unless that 50V supply ramps up slower than that, you have problems. You can alleviate this with zeners across the in/out pins, though.

Problem 2

The LM338 has 50 to 100$$\\mu\$$A of current coming out of its adjustment pin. That means that the output voltage, as you've designed it, is between 36V and 41V. You need to reduce the values of all the resistors so the standing current in the adjust resistors better swamps the current from the adjust pin.

Problem 3

They're not going to share current nicely. One will try to carry the whole load until it fries, and then the other will try until it fries. Generally you want to design one power supply; when things get this power-hungry, you want to use a controller chip (like the LM723, if you insist on being old school) and pass transistors.

Problem 4

They're going to get hot. You're dropping 18V at 5A -- that's 90 watts per device. The junction-to-case thermal resistance is 0.7 degrees/W, so you would need to keep the case at 62 degrees C to keep the junction temperature to 125 degrees C. That's going to limit the ambient temperature you can operate in, and even assuming a 25 degree C room temperature (which, trust me, is unrealistic), you'll need a big heat sink.

• Those are the facts. Here's the opinion: use a switching supply. If you don't have the chops to design one (and a 320W switching supply is a challenge), consider finding a module (Vicor is a name to trust, but there are others) and just using it. If electrical noise is a concern, then there are ways to quiet down a switching supply that don't involve burning up almost 60% of your input power across pass transistors. – TimWescott Nov 27 '19 at 0:52
• I'm not adverse to using a SMPS. I tried using Texas Instruments WEBENCH Power Designer for a starting point but couldn't quite tick all the boxes and I'm not confident in making a SMPS suitable for an audio amplifier (TPA3250 based) in terms of noise. Vicor look really nice but far too expensive for my DIY level project. My starting point is a toroidal transformer with 35V output so any ideas/starting points are welcome. – Joe Mann Nov 27 '19 at 10:52
• Since it's DIY, have you checked the surplus outlets? You may luck out and find a DC to DC module, but it's more likely that you'll find an AC to DC power supply with a voltage that's close, or one that can be adjusted; you'd probably have to follow that with additional filtering. Really old-school would be to use a choke-input filter off of your rectifier without regulation, but that'd only deliver about 30V. – TimWescott Nov 27 '19 at 15:29
• I did have another go with the Texas Instruments WEBENCH design tool and it came up with this - WEBENCH design. It would be supplied by the rectified and smoothed output of my transformer. I'm just not sure how suitable it would be for supplying an audio amplifier? – Joe Mann Nov 27 '19 at 21:51
• It's not letting me in. I suggest you make a screenshot of the design, and ask in a separate question whether it'll work. It may be wise to say what it's for (I assume it's for a stereo amp, but still). – TimWescott Nov 27 '19 at 22:37