I found this circuit after doing some goggling, built it, tested it... and it worked wonderfully.

So I tweaked it a little bit by first powering it up with 12V instead of 9V, and then I changed the 220K resistor above the 1K pot with a 60K resistor... and voila! I got the 75V output that I had been looking for (but that's another story).

Unfortunately, I can only get about 15-20 mA out of it...

Question: what changes would be necessary for it to deliver up to 100ma without having to use an entirely different circuit? I'm trying to avoid using a transformer by all means possible.
Would adding another IR740 in parallel help? Or using a higher value inductor? or increasing the capacitor at its output from 4.7µ to 10µf?

Or all the above???

 

What are you using to power it ???

 

As I said, a 12V power supply... but I guess I didn't mention that the power supply is capable of delivering up to 5 amps...
I want to use this circuit to tweak a little with a piezo element...

 

The basic equation you need to consider is that Power In is always less than power out, sometimes much less.
75 volts * 20 mA = 1.5 Watts
Assuming an efficiency of 60% for starters, that means you need an input power of:
1.5 / .6 = 2.5 Watts
And
2.5 / 12 = 200 mA

OK so the input supply can deliver sufficient power on the input. I would look at the current carrying capacity of the inductor and how much time the switch spends in the linear region. You want to make sure you are not saturating the core material and that the wire used to wind the inductor is of sufficient current carrying capacity. It is not uncommon in boost regulators for the current in the inductor to be several times the input current. If the switch is getting hot my guess is that the 555 is not turning the switch on and off fast enough.

 

If the switch is getting hot my guess is that the 555 is not turning the switch on and off fast enough.

Thank you so much for the valuable input. What I'm using is an inductor RLB0914-101KL which has an IDC rated at 1 Amp max (RDC=0.28Ω, Q=30, and SRF=3.5Mhz... whatever that means)
No the MOSFET is not getting warm, and the 555 is normally running at 1.3 Khz (I checked it with my oscilloscope) when I connect the 75V output to ground via a 10K resistor. That means that I'm drawing 7.5 mA from the power supply when I do that, and at least at that point nothing gets warm (I'm only drawing a little bit more than 1/2 watt, after all)...
The BC547 is connected to the "control voltage" pin in the 555... how is that supposed to work? How does the 555 react when the input at the control voltage varies up or down? Is it working as a PWM or does it also change its output frequency?

 

IIRC, the RC time constant controls one of the threshold points. Pin 5 controls the reset point and between the two of them they control frequency and duty cycle.

1.3 kHz is not an especially good choice of frequency for a boost regulator. Better choices are 40 kHz to 400 kHz. Can you capture the current waveform?

RDC is the DC resistance of the wire used to wind the inductor. This seems high for an inductor in a boost regulator.
The Q of an inductor is the relationship between the reactance and the resistance at a particular frequency
SRF is the self resonant frequency. This is a frequency at which the inductive reactance is equal to the capacitative reactance of the windings.

What is the core material of the inductor and is it a torroid? The RLB0914-101KL is not an inductor I am familiar with.

 

The core material is Ferrite DR, according to its datasheet.

Are you telling me that the rest of the components in the circuit are not that important?
I understand that the maximum frequency that can be generated using a 555 is about 100 KHz. My area of expertise is digital electronics, specifically the 8051 architecture. If you think it would be better to use a much higher frequency I could easily generate it, and also control its PWM, using an Atmel MCU. The thing is I wouldn't know how to control the output voltage. Would using an analog comparator at the BC547's output be enough, so as to simply enable and disable the oscillator while trying to maintain the 75V output?

 

The core material is Ferrite DR, according to its datasheet.

Are you telling me that the rest of the components in the circuit are not that important?
I understand that the maximum frequency that can be generated using a 555 is about 100 KHz. My area of expertise is digital electronics, specifically the 8051 architecture. If you think it would be better to use a much higher frequency I could easily generate it, and also control its PWM, using an Atmel MCU. The thing is I wouldn't know how to control the output voltage. Would using an analog comparator at the BC547's output be enough, so as to simply enable and disable the oscillator while trying to maintain the 75V output?

No, I'm not telling you that. I'm trying to help you solve a problem of the output voltage and current not meeting you expectations. In my experience with switching regulators of all kinds the inductor is without a doublt the most important part. Even graduate engineers don't understand inductores very well for some reason. Poor inductor performance is the number 1 killer of switch mode power supplies.
That your inductor is a ferite core is a good thing. What I am now questioning is if it has been characterized for such a low frequency. Without looking at the waveforms, I can't give you much further direction.

 

The DCR is higher and the current capacity is lower.

I've double checked the datasheet vs the info I posted, and it seems to be the same... unless you mean that what I posted is the maximum IDC, and not the rated one... my bad...
Anyway, if you give me a couple of days I'll get back to you with some images of various waveforms in different parts of the circuit, tested under two different types of loads, see what happens... So please stay subscribed to this thread, you've been of enormous help to me so far.

Thank you very much for your help and your time.

 

I can't get the link to your inductor to work for some reason, but the circuit should work.
You need to change the top resistor (10K) to about 1.5k and the bottom one (1K) to 10 K. This should give you a freqency of about 50Khz and a duty cycle of about 87%.
This setup with the control voltage of the 555 changes both frequency and duty cycle, but it does work. I usually get the voltage about right (higher than wanted) without the cv feedback then add it.
Here is a link to calculate the inductor and capacitor sizes. Your inductor needs to be good for a little more than an amp.
http://www.daycounter.com/Calculators/Switching-Converter-Calculator.phtml

 

i simualted the original circuit. The DC supply is 12V and the load is a 2.7K resistor.

I am able to get about 77V output with a current of 35mA. The diode is a schottky capable of 600V 1A and I have to put 3x 100uH inductors in parallel. On simulation I was suprised that the input current from 12V DC was so high. The 555 oscillates at about 300KHz and the spikes on the scope as seen on the scope is about 300V.

I would use a LM2577HV-ADJ version to make this booster DC-DC power supply.

Allen



[edit} the freq should be 30.3KHz. Sorry wrong calculation.

 

This is all very interesting stuff... thank you Ronv and Absf for your help. As you might already know, I don't understand inductance all that well... I thought that connecting them in parallel would decrease the inductance (and hence the generated voltage) although it would probably increase the current capacity... Ronv, I'll do the changes you're suggesting, and then I'll get back to you all... waveforms and all...

 

This is all very interesting stuff... thank you Ronv and Absf for your help. As you might already know, I don't understand inductance all that well... I thought that connecting them in parallel would decrease the inductance (and hence the generated voltage) although it would probably increase the current capacity... Ronv, I'll do the changes you're suggesting, and then I'll get back to you all... waveforms and all...

Yes you're right. I actually though that by connecting inductors in parallel would add up their inductance. And I was wrong.

After refering to the tutorials on the top of this forum, it should be the other way round. series to increase and parallel to reduce.

I have changed the inductor to a 300uH and increased the cap on 555 Pin 6 to 4.7nF. Now the frequency is reduced to 25KHz. I also changed the resistors as spoken by ronv to increas the duty cycle of the output from 555. Now the output voltage increased to 90+ volts with a 2.2K load.

Have fun on your breadboard.

Allen

 

I have changed the inductor to a 300uH and increased the cap on 555 Pin 6 to 4.7nF. Now the frequency is reduced to 25KHz. I also changed the resistors as spoken by ronv to increas the duty cycle of the output from 555. Now the output voltage increased to 90+ volts with a 2.2K load.
Allen

So what you're telling me is that you've made this circuit output 41 mA already? This would mean that you're getting a power output of 3.68 watts... that's practically half of what I'd like to get! What simulator are you using? is there any simulator out there that just so happens to be freeware? Maybe if I learned this simulator stuff I'd save tons of time before breadboarding anything at all...

 

I've used LTspice to work on an inverter like this, although I'm using a transformer because I'm looking for 100mA or more on the output, at 120V or more at 400Hz-2KHz. Using the simulator has been crucial.

 

LTspice is very competent and is free from Linear Technology. The Yahoo LTspice User Group has free downloadable libraries of many additional models.

 

Excelent, thanks wayneh and Alec_t for your recommendations. I'll start working on that and then I'll get back to this thread with the results, in case anyone else is interested. Thanks again.

 

MMhhh.... already downloaded LTspice, and it's interface is not the easiest one to understand... but what bothers me is that it's got a very limited library. I tried to add a UF4004 diode, and the 1N400N diode family is not even listed!
Would anyone happen to know where I could download a more extensive library?

 

Adding parts can be a pain. What I usually do is use a similar diode, transistor or FET from the existing library.
There are several sources for models. Most manufactures have some - TI has a bunch as does Diodes inc. There is also a users group with a yahoo site.
Be aware it will let you run a part well outside it's spec. So for example you can put 10 amps thru a 1 amp diode and only see that the voltage drop is kind high.

 

Here is my pass at your circuit without the control voltage adjustment.
This will help you get a start with it.