150W Boost Converter Schematic

In a recent project, I needed a boost converter to step up 5V to about 8V at a few amps.
 A few different Chinese-made boost converter modules are available from various sources:
I’ve seen them on eBay and Amazon. One very common one is known as the ‘150W Boost
Converter’. I believe it’s intended for charging laptops from car batteries. It’s specified to
take an input of 10-32V and output 12-35V, which isn’t quite what I was looking for, but
the price was right so I thought I’d take a chance. This is what I found.

I had a good look at the circuit board. It’s based on the UC3843 chip, which is a pretty old device
(I think it dates back to 1984) and is often found in PC power supplies. However, its age and
ubiquity means that documentation on it is readily available. I traced out the circuit, so here’s
the schematic diagram:

You can also have it as a PDF file: 150W_ boost.

It’s a pretty straightforward boost converter topology with a MOSFET switching transistor and
a variable resistor in the feedback loop to set the output voltage. There is no over-current,
 over-voltage or reverse polarity protection at all, and the chip isn’t designed for low power
 consumption so this module wouldn’t be suitable where very low standby power is a
requirement. There are a couple of interesting features, though.

The circuit includes an arrangement with an NPN transistor which feeds some bias to the
 current sense feedback loop. According to the UC3843 datasheet, this improves the
 stability of the converter at duty cycles higher than 50%.

The control supply for the UC3843 is derived from a 9V regulator, so it’s independent
of the input or output voltage. This is convenient.

The UC3843 is designed to operate from fairly high supply voltages, and won’t start up until
its supply voltage reaches 8.4V. That was a bit of a problem for my application, where the
input voltage was only 5V. However, there’s nothing to say that the chip power supply has to be
 the same as the power input. In fact, the module already has a handy 9V regulator which
feeds the control chip. Looking at the circuit diagram, there are even a pair of resistors
 (I’ve labelled them R1 and R2) which select whether that regulator is fed from the input or the
 output. As supplied, R2 was fitted, so the control chip was fed from the output. Here’s a closeup
of the relevant part of the board showing R1 and R2.

My application happened to have a low-current 12V supply available, which would be
 perfect for powering the UC3843. I simply removed R2 and connected my 12V supply to
the point where the black arrow is in the photograph. The boost converter now worked
perfectly with a 5V input.

I also had to modify it a little to be able to reduce the output voltage below about 11V. R3,
 labelled in the photo, is part of the feedback network. I simply removed it and replaced it
 with a piece of wire. Now the output voltage was variable down to 5V, and I was able to
set it to the 8V I wanted.

The module seemed very comfortable delivering 3.3A at around 8V, and drew about 5A from
the the 5V input. The heatsinks only got slightly warm.

Unfortunately, the power supply I wanted to run the converter and its load from didn’t like starting
 up with it all connected. This is quite often a problem with boost converters, since the inrush
current at startup can be very large as the controller tries to bring the output up to voltage
as quickly as possible. I solved this by adding a soft-start circuit to the module. More on that later