Variable Transformer – A Building Block
About the writer: Harvey Morehouse is a contractor/consultant with many years of experience using circuit analysis programs. His primary activities are in Reliability, Safety, Testability and Circuit Analysis. He may be reached at harvey.annie@verizon.net. Simple questions for which I know the answer are free. Complex questions, especially where I am ignorant of the answers, are costly!!!
Summary:
In a previous article I showed how to create an ideal transformer.
This element by itself is useful, with or without the addition of
a core element. The ideal transformer itself, if it had a means of
adjusting the turns ratio dynamically, is useful both for investigating
and simulating designs where such a device may be used.
Ideal Transformer:
Ideal Transformer Model: The ideal transformer model was described previously described as having a symbol as follows.
Figure 1
Ideal Transformer Primitive Symbol
The ideal transformer model, slightly modified, was shown as:
Figure 2
Ideal Transformer Primitive Model
The netlist for this device, with a turns ratio of n, from primary to secondary, is:
| Circuit1 ************************ * B2 Spice ************************ * B2 Spice default format (same as Berkeley Spice 3F format) ***** main circuit F1 1 2 VAm1 1/n E1 6 4 N1 N2 1/n VAm1 5 6 0 RP 1 2 1e10 RS 5 3 1e-9 R1 2 4 1e10 .END |
The circuit primitive was modified slightly to make it controllable in an analysis, in Figure 3 following:
Figure 3
Modified Variable Transformer Primitive
The output was left isolated to allow the device to be 'rotated', and for generality. Devices E1 and F1 in the original circuit are replaced by non-linear sources. What is needed most generally is not a parameter to be passed but a direct input which, continuously or cycle by cycle, varies D. In that event the output would be a voltage which would, continuously or cycle by cycle, vary the transformer. Thus another pair of input terminals was provided where the 'D' level could be used as an input. It is presumed that the range of this input was somewhere between 0 and less than unity. The corresponding netlist becomes:
| ************************ * B2 Spice ************************ * B2 Spice default format (same as Berkeley Spice 3F format) ***** main circuit RP N1 N2 1e10 RS 6 N3 1e-9 R1 N5 N6 1Meg VAm1 10 6 0 B2 N1 N2 i=i(vam1)*v(5,6) B1 10 N4 v=v(1,2)*V(5,6) .END |
Note that the node numbers in the above will not be correct if and when grounds are used in the circuit. A suitable symbol is required for this device. One like the following may be used.
A test circuit was provided for this primitive. This is shown in Figure 5 following:
Figure 5
Variable Transformer test circuit
A netlist for the test circuit is as shown as follows:
|
*
B2 Spice |
This circuit was analyzed with a parameter sweep of voltage source V2. V2 was varied between 0 and 1 in steps of 0.25V. A plot of this simulation is shown in Figure 6.
Figure 6
Transient Parameter Sweep
Conclusions:
The preceding plot shows that a variable transformer can be created, which can be dynamically modified by a control voltage between zero and one volts. Later, time permitting, other articles will show how this may be used in the simulation of Switching Mode Power Supplies.
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