High current, Low ripple Fixed voltage Linear Power Supply - Page 1
Forum section: electronics / projects

author: msngupta
author_rank: Contributor
timestamp: November 06, 2016, 08:06:58 pm
content: Hi,I am trying to design a high current (5A/10A) low ripple (as low as possible) fixed voltage (15V) linear power supply. I started with LM7815 with the standard circuit (with a step-down transformer,rectifier, filter,...) But 7815 can only give 1-1.5A, how would I increase the current rating of the circuit with keeping the ripple voltage as low as possible. Do I need to use some other voltage regulator to achieve this ?Can someone refer me to a circuit which would increase the current rating, and how much ripple would one expect?I see that commercial power supplies claim to have 0.25 mV RMS ripple, is it possible to achieve such low ripple?http://www.acopian.com/store/9-(7).aspx?min=12&max=16What all factors lead to a low ripple supply?Any hint to achieve will be really helpful.Thanks a lot,Sainath.

author: Cliff Matthews
author_rank: Unknown
timestamp: November 06, 2016, 09:03:46 pm
content: For the 5A you could look into the cheap and easy LD1084..http://www.mouser.com/ds/2/389/ld1084-974129.pdfbut I don't know if they can be paralleled easily.. and you can improve output ripple with tantalum caps.

author: danadak
author_rank: Super Contributor
timestamp: November 07, 2016, 12:29:10 am
content: There are designs in these handbooks that can show you approaches and actual designs -http://www.soloelectronica.net/PDF/Linear%20&%20Switching%20Voltage%20Regulator%20Handbook.pdfhttp://www.smcelectronics.com/DOWNLOADS/1980-VOLTREG.PDFhttps://archive.org/details/NationalSemiconductorLinearApplicationsHandbook1994Regards, Dana.

author: David Hess
author_rank: Super Contributor
timestamp: November 07, 2016, 02:57:25 am
content: The 1980 National Voltage Regulator Handbook linked by danadak covers how it can be done.Combine the High Current Regulator circuit shown in figure 7.3 on page 7-1 and the High Stability Regulator circuit shown on page 10-4. For lowest noise, use an LM317 (lower noise) instead of an LM7815 since the feedback resistors and reference in the High Stability Regulator circuit control the output voltage. With an LM317, the operational amplifier can drive the regulator's adjustment pin directly through a bypassed zener diode acting as a level shifter or I guess a small signal p-channel MOSFET could be used because of its relatively high Vgs; otherwise a rail-to-rail output operational amplifier may be needed. Or use an LM7805 with the buffer transistor shown although I use a PNP instead of a p-channel JFET.The output stability now just depends on the operational amplifier and reference circuit with the integrated regulator acting to protect the pass transistors. I have used this configuration before to get better than 10 microvolt stability (I did not have a better voltmeter available at the time) from zero to full load and no measurable output ripple. Almost any operational amplifier will work but a precision type will yield better performance.At 10 amps with a 20 volt input and output short circuit, the power dissipation for one transistor may be excessive in which case two transistors can be used in parallel by duplicating that part of the circuit. Mount the integrated regulator to the transistor(s) heat sink so it can also protect them thermally.With some design care, this circuit can be essentially blow out proof.

author: Someone
author_rank: Super Contributor
timestamp: November 07, 2016, 07:50:44 am
content: Quote from: blueskull on November 06, 2016, 09:26:16 pmNoise can incorporate errors down to DC, and at 10A you have would drop that quite quickly. You can throw your **** around and say how something else is much lower noise but anything below mV of rms noise is getting into specialist territory even the Agilent/Keysight B296xA "low noise power source" is only specified to 3mV rms noise at a 20MHz bandwidth unless you add their filter or "ultra low noise filter" which reduces the available current and gets you down to 10uV rms across 20MHz. At 10A you've already got 8uA of shot noise that cannot be eliminated (again into the standard 20MHz bandwidth), you're not special or adding anything to the discussion.

author: mariush
author_rank: Super Contributor
timestamp: November 07, 2016, 02:16:12 pm
content: LT1083 can do 7.5A but it's extremely rare. LT1084 can do 5A and it's basically same as LD1084 (but I'd say the ones made by Linear would be more rugged and more expensive).The datasheet for LT108* from linear even shows example circuit saying how to parallel two linear regulators for more current:http://cds.linear.com/docs/en/datasheet/108345fh.pdfThe current limits for lt1084 is at least 5.5A but typical is 6.5A ..

author: Someone
author_rank: Super Contributor
timestamp: November 08, 2016, 01:03:30 am
content: Quote from: blueskull on November 07, 2016, 02:25:28 pmAgain you're just throwing out headline specs as if they mean anything in the final application, at a 15V output @ 500mA TI quotes the noise for TPS7A4700 as 12uV rms. So you could try paralleling many of them and end up with theoretically something uV of noise, or add a pass element which will add more noise of its own or it they might just go completely unstable unless you compensate the loop. Its a pretty risky approach for someone who came in asking for ways to get a mV or so of noise for a 10A supply.Quote from: msngupta on November 06, 2016, 08:06:58 pmHello, TI provide a well known example of a 5A 15V supply using the LM7815/LM340T in their application note:http://www.ti.com/lit/an/snoa660a/snoa660a.pdfThe output noise can be minimised by measuring at different current outputs and selecting the one that produces the best result for your data sheet! Also having simple resistive loads and minimising the measurement bandwidth will reduce the number, so you need to start with an idea of what parameters are important and then measure the noise under those conditions.

author: BravoV
author_rank: Super Contributor
timestamp: November 08, 2016, 02:22:33 am
content: Well, if want really true low ripple and noise, and you're rich, parallel 50 of this LT3042s, at your 15V output which already at max, you will get this :RMS Noise : 0.8 µVRMS (10Hz to 100kHz)Noise Spectral Density : 2 nV/?Hz at 10kHzPSRR: 117dB@120Hz , 79dB@1MHz , 50dB@10Mhz

author: David Hess
author_rank: Super Contributor
timestamp: November 08, 2016, 01:02:50 pm
content: Quote from: BravoV on November 08, 2016, 02:22:33 amAnd if you are not so rich, use a discrete design with some modern fast triple diffused power bipolar emitter followers like the D44/D45 series, a low noise reference, and low noise operational amplifier.