Need help to design a negative power supply from 5 Vdc.

Hey guys. I really need your help to design this power supply. I'm still blur about this thing.

Here are the details:

It must be powered from Vcc=5V and I may use an incoming clock signal.

Range of the output: -4V<Vout<-6V

I'm allowed to use only these components:
resistors, op-amp, comparators, transistors, resistors, c, i, diodes and simple logic gates.

This design must be able to power a non-inverting LM741 op-amp with a 1k ohm output load resistor.


I'm thinking I need to use smps, clamper, comparator etc but still confuse.

As a starting idea, this will give Vcc-1.5=5V-1.5=3.5V using 555 IC:
**broken link removed**

Since i can use a clock generator with 5Vdc as the input of my circuit, I reckon it is redundant to use the timer IC just to invert and reduce its amplitude as the IC's input can only accept Vdc input. Because, after that, I'll need to generate a signal(square wave).

or

Can I just connect the clock generator after using the timer IC?

or

If I start over again, can I use a smps buck to stabilise the input(5Vdc+clock) and use filters to generate a sine wave. Then, use a clamper to adjust the amplitude to a negative range values?

See this example with 7660 :



https://www.petervis.com/electronics guides/7660/7660.html

nurm. said:

...can I use a smps buck...

Click to expand...

No. That needs an inductor, which isn't in the list of things you're allowed to use. A simple charge pump with diodes and capacitors will do the job.

The pic below shows the simplest version. For higher output voltage, see here: Cockcroft–Walton generator

I don't think I can use 7660 in this project. but I'll try to ask my lab demonstrator tomorrow. Thanks btw.

---------- Post added at 00:08 ---------- Previous post was at 00:04 ----------

godfreyl said:

No. That needs an inductor, which isn't in the list of things you're allowed to use. A simple charge pump with diodes and capacitors will do the job.

The pic below shows the simplest version. For higher output voltage, see here: Cockcroft–Walton generator

Click to expand...

...I'm allowed to use only these components:
resistors, op-amp, comparators, transistors, resistors, c, i, diodes and simple logic gates...

sorry, my bad. the 'i' means inductor.
So, that means it's possible to use smps buck to invert to negative voltage right?

Yes, you can use a buck–boost converter. That will be more efficient, but a lot more complicated. It seems like overkill if you only need to power a 741.

godfreyl said:

Yes, you can use a buck–boost converter. That will be more efficient, but a lot more complicated. It seems like overkill if you only need to power a 741.

Click to expand...

Could you explain why buck-boost is complicated? Thanks mate.

---------- Post added at 10:34 ---------- Previous post was at 10:33 ----------

You've got an incoming clock signal. If you can use that to switch the transistor in the converter, then it's fairly easy. To start with, you need to know the frequency and duty cycle.

Best case:

• It's a fairly high frequency (say 100KHz), so you can use a small inductor.
• The duty cycle close to 50%, so you automatically get the right output voltage.

You can just choose an inductor value that won't give too much ripple, calculate the minimum load current, and put a resistor across the output that will draw that much current from the negative supply. Remember that with too little current drawn from the output, the output voltage will rise too high.

Harder:

• The duty cycle is wrong, say 25%.

Now you have two choices:
A) Use the given clock signal to generate a new clock with the duty cycle you want.
B) Make a converter that wants to create a higher output voltage than you want, and run it in burst mode. i.e. Monitor the output voltage, switch off the converter when the voltage goes above 5.5V, and switch it on again when the output voltage drops below 4.5V. This is messy because the output voltage is never steady, it's always rising and falling.

Also hard:

• The frequency is too low, say 1KHz.

That would require a very large inductor, which isn't very practical, so it would be better to make your own oscillator and use that to drive the converter.

One more thing:
When you switch the converter on, the output voltage may tend to rise too high before settling down to the correct value. The easiest way to deal with that is probably just to put a 5.6V zener across the output.