No frills boost circuit

[ronv]

I see a lot of requests for boost circuits with a little more current that the little multi vibrator ones. Also seems to be a need for a circuit without a boost ic - only easy to find parts.
Soo... I've been playing with this one. It uses a 555 running at about 75Khz. It is regulated (I use the term lightly) by changing the frequency of the 555. Good for about 100ma at 75% it can use a low cost radial lead inductor good for 900 ma. I thought I would post it to get comments as I have no plans to build it. So maybe we can shoot holes in it.
Brief description:
Power up reset C4, R4 and R3 hold off the timer to allow the output cap to charge before switching begins in case the inductor is saturated by power on.
Q2 limits current thru the switching transistor to a safe level giving kind of a soft start and short circuit protection. A 1 amp fuse would still be a good idea.
Q1 is the feedback to the control voltage of the 555 to achieve some level of regulation.
Seems like less than $5 in parts. I know a chip is better, the question is will it work? What would make it better?

 

[4pyros]

What is your input voltage? 5 volts? What would you get with 3 volts input?

 

[ronv]

Yes, 5 volts. The 555 may not run on 3 volts. Could replace the 555 with a LMC555 and change the switch transistor to a logic level FET to run at 3 volts. Do you have a spec. in mind?

 

[crutschow]

The 120 ohm load is drawing 100mA but the current through the inductor is 300mA. That indicates to me that the zener is drawing the other 200mA and actually providing the regulation.

If you use Q1 as the regulator, then its base-emitter voltage is providing the reference voltage. That will give you about 0.3%/C change in voltage with ambient temperature, which may be a little high for some applications.

 

[4pyros]

Do you have a spec. in mind?

No just wouldering but I forgot about the supply limit on the 555 timer.

 

[ronv]

The 120 ohm load is drawing 100mA but the current through the inductor is 300mA. That indicates to me that the zener is drawing the other 200mA and actually providing the regulation.

I think the 300 ma is at 5 volts while the 100 ma is at 12.

If you use Q1 as the regulator, then its base-emitter voltage is providing the reference voltage. That will give you about 0.3%/C change in voltage with ambient temperature, which may be a little high for some applications.

Good point. I can't think of an easy way around that. At least the self heating is low. Any ideas?

 

[crutschow]

I think the 300 ma is at 5 volts while the 100 ma is at 12.



Good point. I can't think of an easy way around that. At least the self heating is low. Any ideas?

The average inductor current is equal to the load current. You need to adjust your feedback pot to give a lower output voltage so the zener is not conducting.

To reduce the temperature effect you could place a zener in series with the transistor base. The positive temperature coefficient of a 5.6V zener will just about cancel the negative temperature coefficient of the base-emitter junction. Of course with that you couldn't have a regulated output voltage below about 6.3V.

 

[ronv]

Actually in the sim the zener is a 15 volt one. It's a 2.4X boost converter.

The zener trick would work. Thanks!