Advantages of Negative Power Supplies

Thread Starter

JC13

Joined Jul 5, 2015
16
Hi everybody,

can someone explain what are the advantages of Negative Power Supplies against Positive Power Supplies.

Thanks in advance

Best regards
 

#12

Joined Nov 30, 2010
18,076
Negative power supplies are not "better" than positive power supplies. They are merely tools, necessary to achieve certain goals. Their, "advantage" is that, if you need a negative voltage, a negative power supply can do that better than a positive power supply.
 

MikeML

Joined Oct 2, 2009
5,444
Another view is that there is no such thing as a negative supply vs a positive supply. There are just power supplies. You get to determine if you call the more positive terminal or the more negative terminal the reference against which the other terminal is measured.

Look at this: V1 is identical to V2

ps.gif
 
Last edited:

ian field

Joined Oct 27, 2012
6,543
Negative power supplies are not "better" than positive power supplies. They are merely tools, necessary to achieve certain goals. Their, "advantage" is that, if you need a negative voltage, a negative power supply can do that better than a positive power supply.
Negative supplies are better for monster power valves (tubes) who's anode (plate) is a dirty great block of copper that has to be bolted to something and/or water cooled.

Most households have one - called a magnetron, in the microwave oven.
 

#12

Joined Nov 30, 2010
18,076
That sounds like a good use for a negative supply. In that case, it's the right tool for the job. Or you could make a positive supply and connect the tube the other way around. Maybe it's convenient to heat sink the positive end of a magnetron and call that, "common" but it just a different frame of reference to me.
 

MikeML

Joined Oct 2, 2009
5,444
You might characterize power supplies as to if the regulation is done along the positive rail or the negative rail.
 

crutschow

Joined Mar 14, 2008
23,352
Generally the most common use for supplies with a negative output with respect to ground are for circuits that require both a plus and minus supply, such as some op amp circuits and audio amplifiers.
 

crutschow

Joined Mar 14, 2008
23,352
You might characterize power supplies as to if the regulation is done along the positive rail or the negative rail.
But that doesn't necessarily characterize a supply as positive or negative.
This is perhaps a little picky, but if the input power to a regulator is isolated from ground, then either output of the regulator can be grounded to give either a positive or negative supply, independent of which leg has the regulator.
The regulator operates equally well with either connection.
 

#12

Joined Nov 30, 2010
18,076
There's also a question of corrosion. Why do all the Telephone Exchanges have a -48 volt bus?
My 1983 book about telephones says the, "wait" voltage is negative compared to the planet, but the telephone wires reverse polarity when the call is connected. There is a FWB inside the handset, so it doesn't seem to matter (to the telephone) which polarity arrives. This seems to lend credence to your idea.
 

mcgyvr

Joined Oct 15, 2009
5,394
From what I've heard.. The telecom system chose -48VDC because they believe it reduced potential corrosion and buildup on battery plates thus extending their lifespan compared to positive voltage systems.
Virtually all telecom equipment runs on -48VDC.. (for now) with some +24VDC systems for microwave,etc... Utilities may utilize +130VDC
 

Ramussons

Joined May 3, 2013
797
What I've learnt ago is that back in the days of Telegraphy, the "Posts" that carried the wires were -ve to the "ground" to avoid or reduce "electrolytic" corrosion" of the posts. Same with the wires and cables. The "Earth" return rods were also protected the same way. And it caught on and continues.
 

mcgyvr

Joined Oct 15, 2009
5,394
What I've learnt ago is that back in the days of Telegraphy, the "Posts" that carried the wires were -ve to the "ground" to avoid or reduce "electrolytic" corrosion" of the posts. Same with the wires and cables. The "Earth" return rods were also protected the same way. And it caught on and continues.
Yes that is part of what I was referring to.. in the first part and remembered the discussion about batteries too being involved in that due to the lead content.

Here is a quote from the book "DC Power System Design for Telecommunication" by Whitham D. Reeve
The positive terminal on 48VDC power systems originally was connected to earth to reduce galvanic corrosion due to leakage currents on buried lead-sheathed outside plant cables and other metallic pipes (water,gas) and components buried in the ground. Although lead is no longer used to sheath outside plant cables, the practice continues to minimize corrosion on existing lead-sheathed cables and other buried metallic components.

But outside of that negative supplies have other benefits as stated above.. rail-rail, opamp, etc...
 

Thread Starter

JC13

Joined Jul 5, 2015
16
Thank you all for your answers.

Sorry I didn't set my question corectly.
I wanted to ask which are benefits of Negative voltage.

I think that systems which use negative voltage are working with a lower current, and it is easier to achieve faster switching speed of power MOSFETs used in SMPS systems.

I don't know have I right about that, and I wanted know are there more advantages of using Negative voltage.

I posted before one question about connection of MOSFETs in power supply.
For example, when we need to produce negative voltage through a transformer (power supply with PWM), how we can do that with a MOSFET.
Transformer has 21 pins and always 3 pins are connected together (3 pins for primary, 3 pins for fly back voltage for comparison with Vref, and other pins are secondary part of transformer, power supply outputs). I think primary of transformer is connected to pins 1,2,3, on pin 2 is connected source of MOSFET, and drain is connected on +VDC (what is function of this MOSFET).
On pins 1 and 3 are connected power MOSFETs regulated with PWM IC and they produce a frequency for transformer.

I don't understand connections of these MOSFETs, what is their function.
 

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mcgyvr

Joined Oct 15, 2009
5,394
I wanted to ask which are benefits of Negative voltage.

I think that systems which use negative voltage are working with a lower current, and it is easier to achieve faster switching speed of power MOSFETs used in SMPS systems.

I don't know have I right about that, and I wanted know are there more advantages of using Negative voltage.
I routinely work with -48VDC and 800+ Amps... thats far from "lower current"

Are you really asking for recommendations/guidance on how to use Mosfets correctly to switch negative voltages?
 

absf

Joined Dec 29, 2010
1,942
From what I've heard.. The telecom system chose -48VDC because they believe it reduced potential corrosion and buildup on battery plates thus extending their lifespan compared to positive voltage systems.
Virtually all telecom equipment runs on -48VDC.. (for now) with some +24VDC systems for microwave,etc... Utilities may utilize +130VDC
Luckily, The DC-DC step down power supply for the logic chips have their negative side connected to GND ( I mean in an Ericsson AXE Exchange). Just cannot imagine how to trouble-shoot the circuit boards, if all the Vcc and Vdd are connected to GND.

Allen
 

Thread Starter

JC13

Joined Jul 5, 2015
16
Sorry guys for wrong schematic....and thanks for the answers....

now I think that I have made a good circuits. The main problem was that I didn't know is there positive or negative voltage in circuit.

MOSFET 1....it is connected like this on the picture below. He is connected on the input of the board.

MOSFET 2....pins 1,2,3 on transformer make me confuse and also connections to the voltage regulator (IC6). Diodes D9 and D10 are connected to the ground and then to Vin of the voltage regulator.
Also the pins 10,11,12....only I know it can be is that negative voltage goes through D6 to
Emitter of T5 and on emitter is negative voltage. T5 is giving 15V to ICs on the board.

And connection of T5 MOSFET, is it really like this, is it good connection.

Can someone explain me how those two circuits are working from the pictures below.

I will really appreciate that.

Thanks in advance and best regards

JovanMOSFET 1.jpg MOSFET 2.jpg
 

Thread Starter

JC13

Joined Jul 5, 2015
16
And on the picture MOSFET 1....how can he switch ground between Drain and Source....thats why I think it is negative voltage....
 

cmartinez

Joined Jan 17, 2007
6,467
Sorry guys for wrong schematic....and thanks for the answers....
now I think that I have made a good circuits. The main problem was that I didn't know is there positive or negative voltage in circuit.
I completely fail to see the purpose of this circuit:

Capture.JPG

How can switching no load between two ground points be useful?
 
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