Yesterday I was designing the board etc., for a 10V voltage reference I'm building.
For various reasons I want it to be battery powered, and because I need between 14 and 18 V input, I decided initially to use a pair of 9V batteries. This has the advantage of working until the batteries are *very* flat.
However, the box I'm building it in is about the size (internally) to fit 3 of these batteries, so (since the PCB will be the top of the box) I'm very limited in area. Most of it is actually taken up by the power switch and the nice gold plated banana sockets (bought because they're pretty, not for any really good reason).
OK, so it's getting squeezy.
I investigated what boost SMPS chips I have on hand, and I have nothing suitable.
A bit of googling shows up this site.
So I got to thinking... Can I make a simple oscillator using a Schmitt trigger inverter (you can get 2 of these in a SOT-23-6 chip) and drive a mosfet from them?
The answer is yes. And I managed to get 90V across the output (my load was 4k7). This was probably limited by the avalanche breakdown of the mosfet (60V) or the breakdown of the capacitor (63V).
To make this work, I creates a 50kHz oscillator using one schmitt trigger, and buffered it with another. The output is almost exactly 50% duty cycle.
The mosfet is rated at just over 100mA, and the inductor was either a 5 ohm 2mH choke, or a 750uH inductor (of lower resistance and higher current capacity). The results from both were almost identical.
The next trick was to regulate this. What I did was place a series resistor between the oscillator and the buffer, and then tie the output voltage via a 10V zener and a 220 ohm resistor to the input of the buffer.
At an input voltage of around 8V at 6.3mA (yeah, flat battery), I achieved a stable 14.5V at (calculated) 3.1 mA, so an efficiency of 89% -- which I think is pretty good.
Note that the output voltage is not well regulated, it is based on the input voltage from the battery since the schmitt trigger's switching points are also dependant on supply voltage.
I have to go out now. If anyone is interested I can post a schematic later.
For various reasons I want it to be battery powered, and because I need between 14 and 18 V input, I decided initially to use a pair of 9V batteries. This has the advantage of working until the batteries are *very* flat.
However, the box I'm building it in is about the size (internally) to fit 3 of these batteries, so (since the PCB will be the top of the box) I'm very limited in area. Most of it is actually taken up by the power switch and the nice gold plated banana sockets (bought because they're pretty, not for any really good reason).
OK, so it's getting squeezy.
I investigated what boost SMPS chips I have on hand, and I have nothing suitable.
A bit of googling shows up this site.
So I got to thinking... Can I make a simple oscillator using a Schmitt trigger inverter (you can get 2 of these in a SOT-23-6 chip) and drive a mosfet from them?
The answer is yes. And I managed to get 90V across the output (my load was 4k7). This was probably limited by the avalanche breakdown of the mosfet (60V) or the breakdown of the capacitor (63V).
To make this work, I creates a 50kHz oscillator using one schmitt trigger, and buffered it with another. The output is almost exactly 50% duty cycle.
The mosfet is rated at just over 100mA, and the inductor was either a 5 ohm 2mH choke, or a 750uH inductor (of lower resistance and higher current capacity). The results from both were almost identical.
The next trick was to regulate this. What I did was place a series resistor between the oscillator and the buffer, and then tie the output voltage via a 10V zener and a 220 ohm resistor to the input of the buffer.
At an input voltage of around 8V at 6.3mA (yeah, flat battery), I achieved a stable 14.5V at (calculated) 3.1 mA, so an efficiency of 89% -- which I think is pretty good.
Note that the output voltage is not well regulated, it is based on the input voltage from the battery since the schmitt trigger's switching points are also dependant on supply voltage.
I have to go out now. If anyone is interested I can post a schematic later.