Question

In LT-SPICE, develop a forward converter with voltage feedback control with the following specifications: Vd=20-50V, Vo=12V, R=1?, Peak output voltage overshoot not to exceed 10%, ?Vo<=10mV @ R=1?. Thoroughly test the converter output voltage stability throughout the input voltage range. 100ms after the converter have reached steady-state, switch-in a higher load. Show how output voltage is maintained and the change that the PWM signal experienced.

Answer 1

LT-SPICE is a free wave spice package for computer aided design of circuits and electronic system.

following are the steps to design a forward convertor with voltage feedback specification

1. click start - all programs - swCADIII
2. File - New schematic
3. now you are in the schematic editor. where you defines your circuit
4. Now define the transformer - in the circuit the turn ratio of the transformer is 2:1. The practical core has an A_L of 2.2H and primary inductance is 79 H and secondary inductance is 19.8H the coupling factor between the two inductor is ideal.
5. Press edit - L and place the inductor on the schematic sheet
6. change the value of L by right clicking on them, make L1 the primary winding and L2 the secondary winding
7. Add coupling factor between the two windings. click Edit > spice directive then in the text box type ' K L1 L2 I'
8. wires are added by pressing F3
9. Define the switch element by edit > component in the 'sym' library select component "sw" and place on schematic. for moving the object press F7, left click on the item to be moved, place in the desire location.
10. Add a GND symbol to the inverted input of the switch using Edit > place GND.
11. Add output diodes. click Edit > diode and place the output diode on the schematic.
12. Add output inductor , inductor symbol is in edit menu.
13. Add tertiory winding to the transformer
14. Add PWM generation circuit the RC filter is included to avoid the sharp edges. V2 is the voltage component which is a component in the 'sym' library. Place it on the schematic, right click on it and select advanced.
15. The voltage V2 is a triangular waveform which is compared with 'ctrl' voltage to generate PWM signal.
16. V2 is defined as a pulse waveform with ' V1 V2 T_delay T_rise T_fall T_ON period' values of ' 0 1 0 2.5 0 5'
17. Component B1is a bv symbol from the 'sym' library. It is a voltage source where the magnitude can be set to an arbitary function of other voltages in the circuit . This is a method for generating PWM waveform.
18. Place the bv on the schematic, right click on it, in the value attribute add V= 5*IF (v triangle )< v(ctrl) , 1,0) . the output is 1 if V(ctrl) is greater than v triangle , otherwise 0.
19. Node is labelled by pressing F4.
20. Now add control signal . It is also a bv component whose value attribute is " v = duty ratio".
21. Add the input voltage source ( a DC value of 28 V) input capacitor (4.7 F) load resistance (0.5 ? ) and GND symbol on the primary and secondary side.

The diagrams are as follow

finally the simulation parameters must be entered , right click on the schematic and chose edit simulation cmd