+/-40V SMPS virtual ground

[j0no89]

Hey, I am currently designing a switchmode power supply with +/-40V @ 2A based on the TOP258YN. So, getting on with my question, in the reference designs for a split rail supply they use a transformer with a centre-tapped secondary winding, but the only transformer i can find on digi-key (where im ordering all my other parts) that is capable of putting out this much power and has an appropriate turns ratio has no centre tap on the secondary (Wurth Electronics 750311269).



To overcome this problem i found a 'virtual ground' circuit on http://tangentsoft.net/elec/vgrounds.html. So, my question is, what is the actual potential of the common rail in this virtual ground circuit w.r.t the mains input earth. I dont really understand how the virtual ground works so im not sure if it is a true split rail supply or is it just creating a reference halfway between the 2 rails and saying that the 2 rails are +/-40V w.r.t that reference (which would be 40V w.r.t earth). Mainly i just want to know if this common rail is safe to connect to the mains earth reference.



Thanks in advance for any help, if its unclear what im asking just let me know and ill try and be more clear!

[c4757p]

EDIT: You might not want to do this. This linear virtual ground must be able to handle at very least 80W (40V times 2A). That is a lot. I'd recommend doing a bit more research into a full, true switchmode. If they're isolated you can use two identical +40V ones joined at one point.

That virtual ground is just a voltage follower set halfway between V+ and V-. It's not defined as anything w.r.t. mains earth unless you already have mains earth connected somewhere else. Feel free to connect it wherever you like.

Additionally, if you need a more precise virtual ground (that one is not), you can fix it up a bit with an op amp. Run those 10R resistors directly between the emitters (not connected to the base diodes). Ground is still between them. Then ground the inverting (-) input of the amp (to the virtual ground), connect the output between the two diodes, and run two equal (4700 to 47K or so) resistors from the noninverting (+) input to each of V+ and V-.

[Smokey]

Center-tap transformers are pretty cheap elsewhere.  Digikey isn't great for everything.
While I'm sometimes guilty of it myself, designing a circuit around what is in stock at digikey isn't always the best way to go.   

[ptricks]

I don't recommend virtual grounds for anything but low power circuits, designing a proper one for high power really isn't fun.  The virtual ground is also called a floating ground and they do not behave well with mains ground depending on the circuit they can even be dangerous.

Search some of the surplus sites for transformers, you can get them a lot cheaper than digikey.
Another source is old audio amps from the 1980's, you can tell the ones that have large transformers by the weight.

[JoeyP]

One other point to consider about virtual grounds: While they may look fine on paper, and work OK at DC or low frequencies, their frequency response is finite, so they can be very unpredictable (or down right dangerous) with transients.

[amyk]

How about this.

[Zero999]

How about this.

You beat me to it.

Just make sure the capacitors can handle the current. You may need to put several in parallel.

[muvideo]

How about this.

That will not be a flyback topology anymore, I've never worked with a top258,
but i imagine that probably it will not be happy

To the OP, if it's one-off or few units just wind the transformer yourself,
powerint has some tools that basically makes the design for you.

[peter.mitchell]

if its just a one off, just use two transformers...

[SeanB]

Using that on a flyback converter will give a big surprise in one rail being +10V and the other being -200V, depending on which way that you wire the diode so it rectifies either the forward or the flyback pulse. That is why on some you see a diode with a 400V reverse rating used on a 10V or so rail.

[muvideo]

That is why on some you see a diode with a 400V reverse rating used on a 10V or so rail.

Yes, for example on high primary voltages if you try to use relatively low Vds mosfets.
Calculate the turns for low peak drain voltage on the primary and probably you will have
huge reverse voltages on secondary rectifier. Reflected voltages are nasty on flybacks.
This is why I suggest the op to calculate the  "transformer" (that in reality is a coupled
inductor), it's a useful learning excercise and there are many docs around that explain
how to do.

Fabio.

[Mechatrommer]

Using that on a flyback converter will give a big surprise in one rail being +10V and the other being -200V, depending on which way that you wire the diode so it rectifies either the forward or the flyback pulse. That is why on some you see a diode with a 400V reverse rating used on a 10V or so rail.

are you talking about circuit in OP or voltage doubler proposed by amyk?

[SeanB]

Amyk, OP used separate windings for each rail. Seen often on TV sets where you had a 10v rail from the LOPT and a 150V rail from the same winding, the 10V rail being loaded heavily by some digital logic fed via a regulator. The 150v rail being lightly loaded.

OT but Happy Eid.