Is it possible to make a buck converter with NPN transistors

[CasualKilla]

I am trying to test a simple buck converter on 123D.circuits, but for the life of me I can't figure out how to make this work properly with NPN transistors.

 

Lets say for the moment I want to convert 24V to 10V.

 

Basically I want a to drive the base of an NPN transistor with a 5V control pin. The reason for this is that the 5V controller voltage is more stable than the input voltage of the buck and I don't want massive variation in my base current when it is sinking into the controller.

 

Now the problem comes with the fact that there seems to be no good position to put a NPN transistor in the buck circuit.

 

 

I have been googling around and have seen the circuit below a few times now, but it doesn't seem to make any sense, since the inductor is not in series with load when the square wave is high, so it is different to a normal buck.

 

 

Anyway, the best I have been able to do is the following:

 

 

Which seems to have the problem of the output and input voltage not have the save reference due to the voltage drop across the NPN transistor. This may make it difficult to sample the output with the controller then the controller needs to be grounded to different grounds.

 

I remember having a similar problem when designing a NPN H-bridge a while ago.

 

Am I missing something, or is it the case that sometimes the requirement ie. NPN with base driven by 5V controller not possible and I just need to use a different transistor type or accept that the controller is going to be sinking highly variable current due to variable input voltage?

Edited December 17, 2015 by CasualKilla

[studiot]

The problem comes with the earthing arrangements.

 

The conventional arrangement is to use a collector follower not an emitter follower as shown in my Fig1.

This leads to a PNP drive drive transistor but allows through earthing as shown in the negative rail.

 

If you use an NPN drive transistor you have to place it in the negative rail so you have an earthing issue between the battery negative and the output negative and cannot use through earthing as shown in Fig2.

 

So long as you usage can accomodate this then ther is no reason for not using an NPN driver transistor.

 

MosFETS are used the days and the same thing P-channel (as Fig1) v N-channel (as Fig2) applies.

 

Note also that the collector follower is inherently short circuit proof, unlike the emitter follower.

 

 

 

Edit please note the mistake in fig1 as the transistor is the wrong way round.

 

I will correct this later.

Edited December 17, 2015 by studiot

[Danijel Gorupec]

CasualKilla, the circuit you found makes sense, but it is a step-up converter.

 

For what you are asking, I think the best would be to combine two transistors. Like on the first picture below. If you insist on NPN-only solution you might try the second picture, but I don't think it will be as good.

 

 

 

[studiot]

The following pages may help.

They are for the design of a self oscillating switching regulator.

 

Pulse driven ones are also possible.