Help! PWM DC Chopper (DC-DC Converter)

kadzuki2

Hi guys, i need help on my assignment. The problem i have is reading the output of this IC chip (UC3524). Apparently you can't just connect an oscilloscope to the output and get a reading. My teacher said something about using transistor?

Here is the IC chip's schematic UC3524

All i know is that you have to connect them in parallel. Pin 12 with pin 13 and pin 11 with pin 14.

Another thing what is a MOSFET Chopper Switch?

MATLABdude

Shouldn't this be in the homework help section? Ah well...

First off, try to link to the manufacturer's website, not these second-source / last resort datasheet vendors:
http://focus.ti.com/lit/ds/symlink/uc3524.pdf

The ability to parse a datasheet is one of the most important skills an EE acquires. The Features section tells you what the IC does, the Description gives you more details, and the Principles of Operation tells you how it does it. The Reference Circuits section (rolled into the Application Information tells you how to hook up the IC so that it does various things.

I'm assuming that your teacher is asking you to hook the circuit up in the open loop test configuration (Fig. 8 note that there's a typo in the pin numbering) so based on the above information, what should you see at the output?

As to the other question of what a MOSFET chopper does... I'll break with the homework template and say that all it does is chops up a DC input into a square wave (of some desired frequency and pulse-width). This is used for analog-like power control (e.g. controlling motor speed beyond on-off, and without having to do analog voltage output), as the basis of switching regulators (buck, boost, inverting, etc.) In short, it can be used for a lot of things.

kadzuki2

Thanks! I get it now. (For the MOSFET part)

I still don't understand the output side. In Fig. 8 the output seems to be different from each other, the Pins 12,13 and Pins 11,14. They are connected differently. I do see that they are connected in parallel. But if i recalled my teacher said something about connecting up to a transistor.

Did he mean biasing?

MATLABdude

No, they just don't have them connected together. Note that the emitters (11 and 14) are just grounded (so the outputs are relative to ground) Regardless, it really depends on what configuration you've got your UC3524 in (and presumably, that it's wired up properly). The output stage is already a transistor (see the Block Diagram), and you'd probably be able to measure a potential difference between the collector and emitter, whether with an oscilloscope or multimeter.

Actually, I just noticed that these are open collector... You'll note in diagram 8 that they have pull up resistors (2k) to bias the built-in BJT. That's probably what they meant.

That's about all I have, so good luck!

kadzuki2

OK thanks! I wouldn't know until i tried it practically

berkeman

Since this is more project-oriented, I'll let the thread stay here for now. Good help by MATLAB.