Trying Buck Converter on Pspice

[dhlee]

I wanted to make 5V output Buck converter, see the capture 2, there was a big change when I changed the BJT's Base Voltage. It just looks like Transistor-drop simulation.
Other Captures hold base voltage as 3V, and just changed inductor 22mH or 22uH(maybe)

or Is just a problem of BJT's library?

I think I'll use ST microelectronic's 2N2222. I knew only this NPN.

 

[dhlee]

Does smaller inductor making the Vout more stable?

 

[KrisBlueNZ]

What does the "TD = 0.5S" parameter for the base drive source mean? Does it mean you're switching the transistor at 2 Hz? That's FAR too slow! Try running it at 20 kHz, i.e. TD = 0.05 ms.

What does the "PW = 0.001s" parameter mean? If that's the width of the active part of the pulse, try making it about 1/2 of the total pulse period. For example, PW = 0.025 ms. That will give you an output voltage of about half the input voltage.

You cannot switch the transistor ON fully by applying a voltage of 3V on the base relative to the 0V rail. That is effectively operating the transistor in emitter follower or common collector mode. When the 3V is present at its base, the transistor will only pull its emitter up to about 2.3V (because V_BE is about 0.7V).

To fix this, you need to apply the drive between the transistor's base and emitter. In a simulator this is easily achieved - just disconnect the drive voltage's negative terminal from 0V and connect it to the emitter. You'll also have to add a current limiting resistor in series with the base. In a practical circuit there are various ways to do this.

The best way is to use an N-channel MOSFET instead of a transistor and generate a supply voltage for the drive circuit using a bootstrapping technique. Look at some integrated switching regulators with "VBOOT" pins and read the description of how they work.

 

[BobK]

Q1 is configured as a voltage follower. The most voltage you will get on it's emitter is 3V - 0.7V or 2.3V.

For a high side switch you need a PNP transistor and the pulse driving the base has to go from 12V (inactive) to 0V (active).

Bob

 

[dhlee]

TD = Delay time until first operation
TR/TF = rising/fall time
PW = pulse off time
PER = pulse period

Thank you, I'll change values and watch the output.

 

[BobK]

Did you read my post? Your circuit is not going to work.

Bob

 

[KrisBlueNZ]

Did you read my post? Your circuit is not going to work.

Ditto for my post #3 from the third paragraph onwards.

 

[dhlee]

Did you read my post? Your circuit is not going to work.

Bob

As I uploaded, output comes stable.
I wanted to know 2 things.

1.Voltage output : as you said, it depends on Vbase.
2.Other options : Inductor, Duty Cycle....etc(I think I can do it myself)

 

[electronicsbeliever.com]

With the circuit of yours, Q1 is functioning as an emitter follower. With base voltage less than 12V, you cannot see the expected output (as computation tell) because you are not getting the emitter voltage from 12Vdc but from the base of Q1. If you want to stick with an NPN, try making your base voltage higher than 12V (say 15V). The best device to use is MOSFET (either N or P). For NMOS, you need to supply the gate with voltage higher than 12V since this is high side driving. If you prefer PMOS, to turn on the PMOS you need to provide low potential, say ground level. However, to turn off the PMOS you need at least 12V applied at the gate.