Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Max1771 for 180v supply operation questions

Status
Not open for further replies.

spark_hands

Newbie level 2
Joined
Nov 5, 2011
Messages
2
Helped
0
Reputation
0
Reaction score
0
Trophy points
1,281
Location
Reading, PA
Activity points
1,306
Hello all,

I'm building a 7v to 180v supply (for nixies) using a max1771 IC. At 180v, the least amount of current I will draw is 2mA and the most I will draw is 94mA. I've read through the datasheet a few times, but I had a few questions as I am quite a novice when it comes to switching power supplies.

1. The description states it can control 30mA to 2A loads. Is this the output load it can handle (i.e. 2A at 180v) or the load it can handle going into (2A@5v) the circuit? I'm inclined to believe it relates to the output load.

2. In order to choose an inductor, they suggest using the following formula:
L >= (Vin(max) * 2mS) / ILim

Is ILim the current at 7v or 180v? And why? I suppose I do not understand the origins of the formula, but would like to. :)

2. How do I calculate the inductor current rating and saturation current rating? This person is using a 2A inductor for 50mA loads at 220v:
Nixie HV Switching PSU

This is how I believe they calculated that value, but I am not sure:
Their max output voltage and current is 50mA @ 220v so that's 11W
Their min input voltage is 9v and that would mean at a 11W, a 1.22A inductor is needed. Atop of that, the rating was increased to allow for a safety tolerance and to account for the fact the switcher is not 100% efficient. Is my mindset correct? Now what about the saturation rating?

3. There is an Rsense resistor that limits the current. This seems to be dependent on the input voltage, output voltage and the inductor according to the graphs. I'm completely lost on how to figure this out! :sad:

Any help is appreciated! Thanks all
 

1. Power Switch (MOSFET ) external, hence I can handle 180V, 2Amp. If fact it can handle even more depending upon the power curcuit, as this IC is just controller.
2. I would calculate for these two values(7v & 180V) and few intermediate points as well, so that Inductance is choosen such that the peak current through the inductor does not exceed the rated over current rating. Idea is prevent current shutdown occuring in normal operation.
Rsense should be chosen such that at trip current, voltage drop across the Rsense resistor is 100mV.
 
Oh, that helps quite a bit, thank you. :)

So Rsense should be calculated as 100mV/I, correct? This seems to match up with the schematic such as on this page:
Nixie HV Switching PSU

I.e. using 0.1Ohm resistor to limit current to 1A, 0.05 for 2A, and 0.025 for 4A.... But it appears the limit is for the current at Vin vs. Vout. That's quite strange. Am I correct here too?
 

MAX1771 boost circuits can be designed using the equations in the MAX863 data sheet. But, you'll need to substitute in the tOFFmin and tONmax specifications for the MAX1771, of course.

About 16 years ago, the Apps Engineer tried to put the full set of design equations in the datasheet but they were taken out by clueless morons in Maxim management. There was even a design spreadsheet. Heaven forbid that anyone makes it easy for customers to design with the products.

Anyway, the equations will spec out the saturation current rating of the inductor, filter capacitor values, etc.
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top