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Soft Start for PSU

Be nice to your power supply – and its load

Soft Start for PSU
Soft Start for PSU
Labs Project
Based on a Labs project | September 2017 | Find it here
The circuit presented here is a general-purpose soft-start accessory for both homemade and commercial power supplies.The simplest lab power supplies have an adjustable output voltage and (ideally) short-circuit protection. That’s fairly basic, but for a budding electronics enthusiast it’s an excellent way to start their career. The better models also have adjustable output current limiting, and precise indication of the output voltage and current is a feature that should be present in any respectable lab power supply.
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Materials

Gerber file

CAM/CAD data for the PCB referred to in this article is available as a Gerber file. Elektor GREEN and GOLD members can exclusively download these files for free as part of their membership. Gerber files allow a PCB to be produced on an appropriate device available locally, or through an online PCB manufacturing service.

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PCB

Component list

Component List
Resistors
R1 = 100Ohm, thick film, 5%, 0.1W, 150V
R2,R7 = 10kOhm, thick film, 5%, 0.1W, 150V
R3,R4 = 1kOhm, thick film, 5%, 0.1W, 150V
R5 = 3.6kOhm, 1%, 125mW, 150V, 0805
R6 = 240kOhm, 1%, 125mW, 150V, 0805
Capacitors
C1 = 100nF, 50V, X7R, 0805
C2 = 22µF, 10V, X5R, 20%, 1206
C3 = 1µF, 50V, 10%, X7R, 1206
C4 = 220nF, 50V, X7R, 0805
C5 = 1nF, 50V, 10%, X7R, 0603
Semiconductors
D1,D2 = 1N4148WS (100V, 200mA, 4ns)
LED1 = bicolour red/green, 3mm
T1,T2 = NVD6824NLT4G (n-channel MOSFET, 100V, 41A)
T3 = 2N7002 (60V, 250mA, 300mW, Vgs = 4.5V, RDS(on) = 1Ohm)
IC1 = MAX5024 (voltage regulator, 5V, 150mA, LDO)
IC2 = MAX16126TCA+ (voltage supervisor)
IC3 = MAX16054AZT+T (pushbutton debouncer)
K1 = 2-way PCB screw terminal block, 0.2" pitch, 630V
K2A = see text
K2B = 2-way PCB screw terminal block, 0.2" pitch, 630V
K3,K4 = 2-pin (1×2) pinheader
S1 = pushbutton NO (e.g. Multicomp R13-24A-05-BR)
Miscellaneous
PCB 160383-1 V1.1 with IC2 premounted (see text)

Discussion (0 comments)

elektor

marcosbuydid 2 years ago

I have a question regarding to what happens if at the output of this board occurs a short circuit in the load. Will this design tolerate this? having in mind that reverse polarity and overvoltage is also contemplated in the circuit. Thanks so much in advance.
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elektor

Content Director, Elektor 2 years ago

This is a project from 2018. We will notify the lab team to see if they have any feedback. If so, comments will be posted in this discussion section. Thanks! 

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elektor

Mathias_Claussen(Elektor) 2 years ago

Hello,

if you have a short in your load the FETs will still transport current. There is a thermal shutdown within the design but if this will kick in before the FETs get damaged is hard to say and depends on you power supply and the current you will draw with yout short.

Best Regards

Mathias Claußen
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2 Comment(s)
elektor

EN0196684ID 1 year ago

Hi!
There was mentioned that the maximum current is 5A.
Is it possible to increase it to 10A or a bit more and how?
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elektor

J.F. Simon, Elektor 1 year ago

Hello! Neither the publisher, nor Elektor, nor I can be held responsible if it doesn't work or if something goes wrong. Having said that, what limits the maximum current is the heat dissipation in T1 and T2 when they are fully conducting. That heat is P = R.I² with R being the RDSon of the transistor and I the current. The FETs used in the article have a RDSon of about 0.02 Ω. With a current of 5 A that gives P = 0.5 W (which will be dissipated in surrounding air and in the copper plane of the PCB). If T1 and T2 were fitted on big heat sinks, you could use them at much higher currents without problems. Here, cooling will be the limiting factor. To go to 10 A while keeping the power dissipation in the same ballpark, it's needed to choose other FETs with a much lower RDSon, around 0.005 Ω for example, while keeping current and voltage ratings similar or greater than previous ones. You can use the parametric search of your favorite supplier and see what's available in the same package (DPAK). As an example I found the IPD050N10N5, there are probably other possibilities.
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elektor

EN0196684ID 1 year ago

Hello!
Thanks for the detailed explanation!
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