High-current LDO + capacitance multiplier + outboard transistor booster - Page 1
Forum section: electronics / beginners

author: Nominal Animal
author_rank: Super Contributor
timestamp: July 19, 2021, 10:29:51 am
content: A complete newbie wrt. power supply circuits is asking for your help.Having a class II isolated 7.5V 8A power supply (adjustable up to 8V), I want to regulate it down to a nice, low-ripple 5 VDC –– say, adjustable between 4.5V and 5.5V, but at full 8A. MIC29752 comes close (7.5A), but for full 8A output, it seems I need an outboard transistor booster. Slow turn on is not a problem, although I do want to avoid any voltage overshoot. Combining the advice from Art of Electronics, here's the circuit I've come up with:Q4 is a Darlington transistor, used as a capacitance multiplier. R10, C1, R11, C2, R13, C3 form a three-stage low-pass RC filter, with total series resistance suitable for Q4 base current (say about 50-100mA for TIP142). L1 is a ferrite bead to absorb any leftover very high frequency noise.R7, RP1, R8 is the voltage divider for adjusting the regulator output. R7 and R8 limit the adjustable range to the designed safe and tested region (wrt. heatsinking).R9 is the minimum load, if the regulator needs one.Q1 is the high-current pass transistor. When the current through R1 is high enough, the voltage drop over it turns Q1 on; AoE uses 0.6 Ohm resistor here.It should be a PNP transistor with low emitter-collector drop, in its active (non-saturated) region. (I do believe an OnSemi FJA4213 or NJW0302G would be suitable here; recommendations?)R2-R6, Q2, and Q3 implement overload protection. When the voltage drop over R4 is large enough, Q2 turns Q1 off. While Q2 does not normally conduct, it has to be able to dissipate full load current during overloads. R5, R6 form a voltage divider that control how Q3 reacts to voltage drop over R4, for foldback current limiting.Does this make any sense, or have I missed something important here? How could I implement the same for a negative rail? The supplies I have are fully isolated (class II, no capacitors on output to protective ground).Since I kinda want four of these (±4.5-5.5V @ 8A, ±11-13V @ 6A), and I'll have accurate monitoring of the output (adjusted via a potentiometer), I'm thinking I might be better off doing discrete regulators based on a voltage reference (shunt, say TL431) suitably adjusted and filtered, with an opamp comparing the output to the voltage reference, and driving the base of a suitable pass Darlington transistor. With small changes (P↔N, opamp input range consideration), that should work for the negative rails too.Recommendations? Suggestions?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 19, 2021, 12:32:29 pm
content: For the step from 7.5 or 8 V to 5.5V there is no really much headroom for a capacitance multiplier. So one would have to use a single power stage and maybe the capacitance multiplier only for the reference / regulation circuit itself, not the main current. If the input part is a SMPS passive filtering with an inductor can be quite effective.Using a LDO (with internal power stage) together with an external transistor to boost the power level is usually not a good idea. These circuits often have a high drop out (e.g. 2 V) and stability / overshoot is really tricky. The parts are not that well defined to know upfront how bad the behavior would be before actually building it.There are special regulator chips to work with an external power transistor (e.g. LM723, LT175, some Micrel parts) However these are mainly available for the positive side. Instead of a LDO, even a simple TL431 may be a better starting point for a high current regulator.For the negative side one may have to go for a classical regulator made from OP(s), reference and power transistor. The LM723 is actually not that much more than a reference + an OP.An important factor for the circuit is the load capacitance: is it fixed, known , low ESR ?Another design point is if there is a current limit needed, either just for protection or as full lab supply with adjustable current ?For the positive side there are also a few high power LDOs that may be enough for 6 A.

author: Ian.M
author_rank: Super Contributor
timestamp: July 19, 2021, 12:54:31 pm
content: Analog Devices have an article:Paralleling Very Low Dropout Linear Regulators for Increased Output Current and Even Heat DistributionIts got a 12A 4xLT3033solution, using aMAT14matched quad NPN array as a current mirror + a handful of passives to enforce sharing. Delete a LT3033 and you'd have a 9A regulator. If you wanted to you could add current set resistors between ISET and GND of each LT3033 to enforce an 8A current limit.

author: Nominal Animal
author_rank: Super Contributor
timestamp: July 19, 2021, 01:11:53 pm
content: Quote from: Kleinstein on July 19, 2021, 12:32:29 pmTrue. The supplies I have have about 100mV of ripple, and looking at component selection, there's just half a volt or so headroom, which may not be enough in practice.Quote from: Kleinstein on July 19, 2021, 12:32:29 pmUnknown. (Which is quite annoying.)Quote from: Kleinstein on July 19, 2021, 12:32:29 pmOnly as protection. I would prefer it to be rather robust against stupidity, because I do stupid stuff when I'm still learning.This could be seen as a lab supply, but with a very limited voltage selection, and no constant current mode; only current limiting.Some background:I have a number of circuits, including Linux SBC:s, that take 5V or 12V at relatively high currents. (A friend has some audio circuits they'd like to play with, with similar voltage requirements.) I have microcontrollers and circuits that can measure both the voltage and the current at sufficiently high rates (~ 100 kHz sample rate) for me to detect stuff like current demand spikes, and the reaction of the power supply to that. Being completely isolated, I can connect the SBCs to my computer via USB3 and not worry about isolation (ground loops).I do have a proper power supply without overshoot and CC and CV modes for testing circuits. However, I'd love to have something dedicated to ±5V and ±12V, no CC mode other than overload/thermal cutoff; slightly adjustable (probably 4.5V-5.25V and 11.0V-12.25V would suffice), and completely isolated (since the supplies are, with minimal leakage too). If I could safely use them in series with a shared zero volt line, even better.Quote from: Kleinstein on July 19, 2021, 12:32:29 pmYes, I mentionedMIC29752, which can do 7.5A with less than 750mV dropout.In the same series, the MIC29502 is more affordable, and can do 5A with less than 600mV dropout.