Unregulated power supply PCB design problem

Thread Starter

nikolopikolo

Joined Sep 17, 2020
33
Hello, I'm in the process of designing a PCB for a simple dual polarity unregulated power supply and I have a question regarding zones and traces. The PSU is designed to supply 4A and 40V on each of the two rails. There are two 10mF capacitors implemented to avoid ripple. So through each of the caps there will be 4A of current and I chose the width of 2mm and did quite a thick zone around the cap.

My question is: do you think it's okay to use zone or will it affect the current too much, and is 2mm enough or should I go for more sience it's really huge capacitors? I'm using 3.9k resistors to discharge capacitors

Thank you for your help, I really appreciate it.

Nik
 

hrs

Joined Jun 13, 2014
321
Can you post a schematic of the circuit and a screenshot of the PCB layout? What is the load going to be?
 

Papabravo

Joined Feb 24, 2006
16,165
Why do you care about ripple in an unregulated supply? If you need low ripple, then you need a regulated supply. Maybe you were misinformed at some point in your education.
 

TechWise

Joined Aug 24, 2018
151
If there's space to do so, I would always use large copper pours for high current connections. If the space is there, why etch away good copper to turn it into a 2mm trace? When I've routed power supply sections, I don't think I've ever used a single copper track. I always use copper polygons/planes to offer the maximum possible copper area. You'll find that this is the approach taken in the recommended layout sections of most switch mode and linear regulators.

As above, a schematic of what you're trying to do and a screenshot of the proposed layout will be useful. The people here are here to help!
 

Thread Starter

nikolopikolo

Joined Sep 17, 2020
33
Transformer has dual secondaries rated at 40V 4A each. The rest I think is clear. Its a pretty simple design, and very oldschool, but it's what I need to power an amplifier. Any recommendations about the PCB are welcome, but I've 100% decided on this design.

Thank you for your help.
 

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TechWise

Joined Aug 24, 2018
151
Transformer has dual secondaries rated at 40V 4A each. The rest I think is clear. Its a pretty simple design, and very oldschool, but it's what I need to power an amplifier. Any recommendations about the PCB are welcome, but I've 100% decided on this design.

Thank you for your help.
If it were my design, I would remove the few remaining normal traces from the PCB and replace those with copper polygons as well like you have for most of the connections. The traces will probably be more than adequate for the currents you're working at, but as I said before, why etch away good copper to turn a large polygon into a small trace. I would aim to make use of all the copper available to you, proper clearances excepted of course.
 

hrs

Joined Jun 13, 2014
321
some random online PCB trace calculator says that 2 mm is marginal for 1 oz/ft^2 copper. The large ground pour makes no difference when connected with a 2 mm ground trace. I would make them as big as I have space for.

Your diagram says 55V so I assume the 40V you mentioned is RMS. With no load you will probably end up with +/- 60V. Then you get 60*60/3.3 = 1 Watt per discharge resistor. That's a bit much maybe. I don't see why you couldn't use 47k instead of 3.3k.
 

Thread Starter

nikolopikolo

Joined Sep 17, 2020
33
Alright i will fill the ground traces as large as I can.

I was gonna order two 3.9k discharge resistors ratee at 5 watts each. I chose this low resistance value because of the faster discharge time.
 

Papabravo

Joined Feb 24, 2006
16,165
I need it to power an audio amplifier so thats why I need low ripple. Can you just tell me about the question I asked
It would be pointless since you obviously do not understand what you need. You do not need an unregulated supply for an audio amplifier. What you need is a well regulated supply with good temperature characteristics and low impedance paths for high frequencies to ground.
 

Ian0

Joined Aug 7, 2020
2,260
A decent design of audio amplifier will have good enough PSRR to cope with the ripple. No commercial power amplifier has a regulated supply.
Unregulated supplies are ideal for audio amplifiers because the voltage sags under high load. So, for a normal audio signal with a high crest-factor they can deliver the occasional high power peak, such as a drum beat.
You do not need discharge resistors - the amplifier bias current will discharge the capacitors quite well enough, and 60V is not high enough to require discharge resistors to ensure safety.
As everyone else has said, use the thickest tracks you can - don't throw away good copper! But make sure that all your earths connect together at one star point - otherwise it will hum.
 
Don't put mains traces close to the secondary "40VDC" pads.:(
You must use single-point grounding (star) with the transformer center-tap going directly to the filter capacitors FIRST then the GND takeoff for the power amp. You would be surprised at how much ripple appears across the milliohms of traces with big 10,000uF caps.

Is is good practive to include reverse-diodes on the rails, as one +40V can blow and the -40V rail goes positive which damages more parts.
 

Thread Starter

nikolopikolo

Joined Sep 17, 2020
33
I've read about the star grounding technique, but didnt quite get it. As far as I understand I'm supposed to wire each of the ground points on my amplifier seperatly going to the filter caps as a main ground point right? Thats why I placed 3 pins labaled earth ground there.

Also, deleted the skinny trace and swaped the fuse and switch. Great idea btw I totally missed it.

Thanks for help everyone, I really appreciate it. :)
 

Ian0

Joined Aug 7, 2020
2,260
Star grounding - you have several points on your board labelled as "earth", but only one can possibly really be earth, because all the others are separated from the one that really is earth by a resistance, the resistance of the track, even though it is a really low resistance. Any current flowing through that resistance will produce a voltage, so only one point can be ZERO volts. All the others are not-quite-zero Volts.
So, choose one point that you want to be called 0V, and make sure all tracks that are zero-volts lead to THAT point, and not another point a little bit further along a power track where the voltage isn't 0V.
 
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