General SMPS question. - using a power transistor of some kind.

Thread Starter

swmitchell

Joined Nov 14, 2013
18
Gday' everyone,

I recently built my first SMPS (a simple lm2596 in a to220 package on a little prototype board). Inspired by the DC DC buck converter modules ya buy off ebay. So now I actually understand what's going on a bit more... I have a general question (3A is stacks for me so this on only a theory question...)

Could the maximum current be increased by putting some kind of power transistor on the switched output of the buck converter and running the 'output' of the transistor through a low pass filter and use the feedback of that?

Let's try a pickie to help me explain what I mean. I know there are bits missing off this schematic, please gloss over the obvious mistakes and missing/component choice, this is not a build schematic.

Thanks in advance everyone!
 

Attachments

CharlesWMcDonald

Joined May 16, 2019
233
No, that will not work. You need to choose an IC that supports an external switch. You also do not have the buck regulator topology correct, you omitted the diode. Here is a simple diagram illustrating the buck topology:

Red lines indicate the current flow with the switch in the on position and then in the off position. When the switch is on the current through the inductor establishes a magnetic field. When the switch turns off, the magnetic field collapses and generates a voltage across the inductor of opposite polarity.
Just adding an external switch to your IC will not work but you can use an external switch to increase the current. This would require a new design, including the inductor since it must be able to handle the higher current. Other parameters would also change depending on the requirements of the design. There are lots of resources online to learn about SMPS.
 

RPLaJeunesse

Joined Jul 29, 2018
62
Section 8.2 of the LM2596 datasheet shows that the switch current is monitored for overload protection. If you were to add an external boost transistor then you lose this protection. A shorted load becomes a true fire hazard, not safe!
 
Top