Hello I'm working with DC High Voltage supplies to power a Geiger tube and am having a little bit of trouble understanding them. I really just have 3 questions:

Example Circuit:

1.) When using a diode ladder like in this example circuit, how do you determine what is the maximum voltage it can output? I mean, in general, you are charging the first capacitor to some maximum voltage Vo, and then each step on the ladder doubles it. I am having a good deal of difficulty figuring out what that first initial voltage is. The power line for the circuit is +5v, and that is applied across an inductor which is switched to ground to get an AC current flowing to be used to climb the ladder. How do you calculate the peak AC voltage that will be multiplied as it goes up the ladder?

2.) This circuit is a rather wierd one as it takes feedback from the lowest part of the ladder (thats their way to make sure the not much current leaks through the larger resistor voltage divider back to ground). Most circuits that I have seen take feedback from the highest point of the ladder. I have no idea how the MAXIM ciruit controlls the voltage at the top of the ladder, maybe it just limits the peak AC Vo by the resistor divider and the low level of the ladder and assumes the voltage will multiply up the ladder in perfect doubles?

3.) In our design, there is no Schmidt Trigger or comparator. We just have a single IC that takes in a reference voltage (From a resistor divider from the top of the ladder). And when that reference voltage drops below a certain value, it sends out a 45kHz square wave (0-Vcc) to the base of the MOSFET and hence toggles the mosfet to generate the AC voltage to climb the ladder. This way, if we want the voltage to stay at 400, the divider at the top of the ladder is set such that it puts in the reference required to turn on the IC-oscillator when the top of the ladder reaches 400V.

In the MAXIM circuit, this feedback is somehow used to create an oscillating square wave to the base of the MOSFET.
How does this particular method oscillate the MOSFET?



Any explaination on this stuff would be greatly appreciated.

 

Inverter A6 is used as an oscillator. Boost regulator is setup using the mosfet.

Equation V = -Ldi/dt decides the voltage induced in the inductor and the duty cycle of the oscillators decides the voltage induced in the inductor.

 

Ok, so A6 is just a single Schmit trigger wired externally to work as an oscillator? I would assume some combination of resistor and cap would set the frequency of oscillation?

As for the Inductor voltage. I would have assumed you use that formula, but I'm not too sure how to evaluate the di/dt term. I know its asking a lot, but do you think you could explain with an example? I don't really care about the actual values so you could make up whatever. Ideally I would like to have say an inductor voltage of 50v.

To aquire this, would I assume di/dt is Δi/Δt and that the duty cycle is just Δt. I'm not sure how to determine Δi. I would assume it would be the peak current?? But I have no idea how to get the peak current. It should be a somehow related to the voltage source +5v, and the series resistance of the MOSFET Rds(on), Inductor Resistance, and Inductor Impedance..

Thanks for your response so far, I'm slowly making progress.

 

Hello,

On this page from EE-Times the circuit is discribed:
http://www.eetimes.com/design/power...s-supply-powers-low-power-Geiger-Mueller-tube

Bertus

 

Hello,

On this page from EE-Times the circuit is discribed:
http://www.eetimes.com/design/power...s-supply-powers-low-power-Geiger-Mueller-tube

Bertus

That is exactly what is on the Maxim-IC website. Doesn't help me for what I'm looking for, which is a detailed electrical analysis of the inductor voltage. Thanks though.

 

Hello,

A6 is a PWM oscillator.
The 10K resistor with the diode make the low time.
The 100K resistor with the voltage coming from the opamp (max4162) make the up time.
The up time can change depended on the feedback voltage,
compared with the voltage from the zener diode (max6008).

I have attached the datasheets for the MAXIM components.

Bertus

 

Here's a good link for that oscillator. It's on the last page.

John

View attachment 26637

 

Thats definitely some good info. I think I'm now familiar with how the oscillation of the MOSFET is set by the comparator and schmitt trigger. Now I need to determine how to calculate the inductor voltage given the switching frequency of the MOSFET. Thats the part that is most important to my design.