What is the use if we give a DC supply to the centre tapped primary coil of a transformer

Thread Starter

Ravi Teja 2

Joined Dec 5, 2016
17
Hi,
Recently i came across a circuit which has a transformer in which the primary coil is centre tapped and at the centre a DC supply of 5volts is being given .I have no idea what is the use of doing so. Please go through the image attached and help me out.

Click Here for the application note and go to Fig 13.
Please explain me the operation of the circuit.
 

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MaxHeadRoom

Joined Jul 18, 2013
19,063
The app note pretty much describes the circuit, it uses a Royer type self oscillating circuit.
Read up on DC to DC convertors and the Royer type.
The circuit produces AC.
Max.
 

Kermit2

Joined Feb 5, 2010
4,158
The oscillator depends on magnetic saturation of the transformer to initiate switching of the active components.
 

Papabravo

Joined Feb 24, 2006
12,407
Using a CT transformer, with the CT at +5 Volts and the ends driven by push-pull drivers, can provide an isolated digital data transmission method. Allen Bradley Data Highway and other protocols use this method.
 

Thread Starter

Ravi Teja 2

Joined Dec 5, 2016
17
The input and output is a sine wave which does not imply the transformer goes in to saturation.
Max.
I know i sound dumb but can u please give me one straight answer to "Why is a DC supply of 5V being given to pin 2 of transformer?"
Using a CT transformer, with the CT at +5 Volts and the ends driven by push-pull drivers, can provide an isolated digital data transmission method. Allen Bradley Data Highway and other protocols use this method.
what is the use of CT 5V Dc in a power supply? Here we dont transmit data isn't it?
 

MaxHeadRoom

Joined Jul 18, 2013
19,063
I know i sound dumb but can u please give me one straight answer to "Why is a DC supply of 5V being given to pin 2 of transformer?"
what is the use of CT 5V Dc in a power supply? Here we dont transmit data isn't it?
5v is the reference point for the C.T. of the primary, pins 1 & 3 are the outer ends of the primary and the voltage is 'Alternated' between these pins/windings with reference to the 5v in.
Max.
 

Papabravo

Joined Feb 24, 2006
12,407
I know i sound dumb but can u please give me one straight answer to "Why is a DC supply of 5V being given to pin 2 of transformer?"

what is the use of CT 5V Dc in a power supply? Here we dont transmit data isn't it?
Every time you pull one leg or the other low a pulse is created on the secondary which propagates down the twisted pair cable to a receiver at the other end. In both the data transmission case and the power supply case the purpose is to create an oscillator to produce an AC waveform.
 

Thread Starter

Ravi Teja 2

Joined Dec 5, 2016
17
So, If i remove the 5V DC supply at the CT will it effect my output . Apart from voltage alternating with 5v as reference what are the other uses of this CT 5V DC supply.
I am thinking that if i remove the 5V DC at the CT the voltage alternates with Zero volts as reference . Am i on the right track?
 

Papabravo

Joined Feb 24, 2006
12,407
So, If i remove the 5V DC supply at the CT will it effect my output . Apart from voltage alternating with 5v as reference what are the other uses of this CT 5V DC supply.
I am thinking that if i remove the 5V DC at the CT the voltage alternates with Zero volts as reference . Am i on the right track?
I don't think so. One reason to do it this way is that many devices are better at sinking current than sourcing current. The inductor however does not care. It will oppose a change in current in either direction and generate the corresponding voltage across it.
 

MaxHeadRoom

Joined Jul 18, 2013
19,063
So, If i remove the 5V DC supply at the CT will it effect my output . Apart from voltage alternating with 5v as reference what are the other uses of this CT 5V DC supply.
I am thinking that if i remove the 5V DC at the CT the voltage alternates with Zero volts as reference . Am i on the right track?
There will be no source for Q2 & Q3 with 5v removed.
Max.
 

Thread Starter

Ravi Teja 2

Joined Dec 5, 2016
17
There will be no source for Q2 & Q3 with 5v removed.
Max.
Thank you all very much for helping me understand that circuit.

Encountered a new problem now :
I modified the circuit to convert 12V DC to -325V DC . My requirement is that it has to source 5mA of current at -325V DC , but when i connect a load at the output i am getting a voltage drop and also current is decreasing. Is there anyway i can avoid it ?
 

Hypatia's Protege

Joined Mar 1, 2015
3,199
The oscillator depends on magnetic saturation of the transformer to initiate switching of the active components.
As a point of interest - the circuit depicted in the diagram is a resonant Royer oscillator - the 220nF capacitor forms a parallel resonant ('tank') circuit with the transformer primary. Properly engineered, the circuit will exhibit sinusoidal output over its design load range... Although saturation effects may play a role in rapid start-up under heavy load conditions - such is not essential as may be noted in the fact that the topology is likewise applicable to non-saturable (e.g. air core) transformers...

Best regards
HP:)
 

Hypatia's Protege

Joined Mar 1, 2015
3,199
I modified the circuit to convert 12V DC to -325V DC . My requirement is that it has to source 5mA of current at -325V DC , but when i connect a load at the output i am getting a voltage drop and also current is decreasing.
Sounds as if you're 'swamping' the oscillator -- that said, there's nothing unreasonable in what you're attempting (Indeed < 2W is well within 'range' of even very low power CCFL inverters)...

Is there anyway i can avoid it ?
Of course!:) Please post a schematic of your modified circuit:cool:

Very best regards
HP:)
 

Hypatia's Protege

Joined Mar 1, 2015
3,199
@Ravi Teja 2

Addendum to post #17

If your redesign incorporates a regulator scheme (as shown in your OP) -- Your difficulty may be mere misapplication/misadjustment of same -- Again, a schematic would be most helpful:)

Best regards
HP
 

Hypatia's Protege

Joined Mar 1, 2015
3,199
I modified the transformer values and left the entire circuit as it is.
Lp=20uH
Ls=3.33mH
Lf=1.25uH
While the implied ratios and (unloaded) resonant frequency (i.e. ≈ 76kHz) appear reasonable, more information is required am I to be of assistance...

1) What is the designed (i.e. 'on-label') application of the transformer? (Is it, for instance, a CCFL inverter transformer?) Did you rewind it or do the above values represent measurements taken from a 'stock' device?

2) If you indeed rewound the transformer, did you take care to correctly replace the reluctance spacers?

3) What is the resonant capacitor's dielectric material type/designation?

4) Is the circuit otherwise exactly as shown in the OP including the regulation loop?

Best regards
HP:)
 

Thread Starter

Ravi Teja 2

Joined Dec 5, 2016
17
While the implied ratios and (unloaded) resonant frequency (i.e. ≈ 76kHz) appear reasonable, more information is required am I to be of assistance...

1) What is the designed (i.e. 'on-label') application of the transformer? (Is it, for instance, a CCFL inverter transformer?) Did you rewind it or do the above values represent measurements taken from a 'stock' device?

2) If you indeed rewound the transformer, did you take care to correctly replace the reluctance spacers?

3) What is the resonant capacitor's dielectric material type/designation?

4) Is the circuit otherwise exactly as shown in the OP including the regulation loop?

Best regards
HP:)

Hi,
All my observations were mainly based on the simulation results only.
1/2/3. The values calculated for the transformers are purely based on trial and error method.
4.The entire circuit is left as it is.

But now i have rigged up the circuit and it is yet to be tested.
in that circuit the transformer used is the CCFL inverter transformer.
I'll keep you updated with my practical experimental results.Meanwhile can you give me some thoughts as to how to reduce the voltage drop.

Thanks.
 
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