max1771 question

[zapped_bug]

Hi there, I'm trying to build a nixie tube power supply using the max1771. I have a 6v supply that I will boost up to 179v to supply 72mA

I'm having some difficulties with two components in this circuit. The first is the Rsense resistor. I would like to limit the current to 80mA, but do not understand what value current sense resistor I need. From looking around on the web, it seems the formula is
I = 100mV/Rsense or 100mV/I = Rsense
But, it looks like the I value is the current drawn at 6V vs the output 172 volts? That doesn't seem right!

I would be grateful if someone could explain this!

 

[ronsimpson]

The MAX1771 measures current at 0.1 volts.
The MAX1771 measures input current not output current.
I think you want 13 watts out. The input must be slightly larger than 13 watts. Input current is about 2.5 amps. You need to use a resistor that produces 0.1V at 3A. 0.033 ohms

 

[Dragon Tamer]

The data sheet knows best, it says for Rsense:

Determining R​

​

​

​

SENSE​

Use the theoretical output current curves shown in
Figures 4a–4d to select R​

​

​

​

SENSE. They were derived
using the minimum (worst-case) current-limit comparator
threshold value over the extended temperature
range (-40°C to +85°C). No tolerance was included for
RSENSE. The voltage drop across the diode was
assumed to be 0.5V, and the drop across the power
switch rDS(ON) and coil resistance was assumed to be​

0.3V.

http://www.datasheetcatalog.org/datasheet/maxim/MAX1771.pdf

 

[zapped_bug]

Thank you ronsimpson.

Rsense is connected after the inductor (through the mosfet) to gnd so I really expected it to be measuring the current at the output voltage vs the current at the input.

Why is this not the case? I'm still new to switching regulator designs so perhaps I am missing something fundamental?

 

[Diver300]

You have to understand that the current through an inductor will increase steadily if there is a positive voltage across it and decrease steadily if there is a negative voltage. The bigger the voltage, the faster the current changes.

When the MAX1771 turns on the MOSFET, there is 6 V across the inductor, so the current increases. The current is flowing from the supply, through the inductor, through the transistor, and through the resistor to ground. The purpose of this is only to build up current in the inductor.

When the current is large enough, the voltage across the resistor gets up to 0.1 V, the MAX1771 turns off.

The current in the inductor cannot change instantaneously, so it carries on flowing from the 6 V supply, through the inductor and the diode and onto the output. So there is never any current flowing to the output at the same time as there is current in the resistor.

Because there is a lot more voltage across the inductor (172 - 6 = 166 V) as opposed to 6 V, the current in the inductor falls much faster than it increased. So although the peak current out is the same as the peak current in, the average current is about 166/6 = 28 (ish) times smaller than the average input current.

You should allow for a peak current in the inductor of about 4 amps.

However, I would question how much current your Nixie tubes need. This big Nixie Clock https://www.kosbo.com/index.php?option=com_content&view=article&id=53&Itemid=61 takes less than 4 W.

If you really do need 12 W or so, why are you going for a 6V supply? I would suggest a larger supply voltage if you do need that much power.