PFM dc-dc boost converter operating in discontinuous conduction mode

[R4sbee]

Hello folks! I'm currently working my undergrad study about "current limited—PFM dc-dc boost converter that operates in discontinuous conduction mode". But it's really hard for me to find the exact values for my inductor and capacitor to start my cmos ic design. Please helppp me... i don't know what formula i should use to find it's value.

I have this boost converter that operates in 1.8V that wants to have an output of 5V with 100uA to 1mA load. Since i'm dealing with light loads I want it to operate in DCM to maximize my efficiency. But i don't know the formula to get the inductor and capacitor. What shoul i do?

 

[KlausST]

Hi,

you should at least have a clue about the frequency range.
And wheter you use a catch diode (vs sync rectifying), what type and what forward voltage vs forward current. ... what efficiency to expect.
(I mean: in numbers. Not text like "best efficiency")

Please tell what documents you have read so far. Post links. Ask detailed questions.

*****
Inductor, capacitor:

There are the most basic formulae: like
* how the current behaves over time and applied voltage
* what´s the stored energy
(no need for a dedicated SMPS document so far. They are even provided by standard capacitor and inductor informationns like Wikipedia)

For example: W = 0.5 * L * I * I.

The problem is: There is no document that tells YOUR application requirements, nor YOUR ideas how it should operate.
You need to decide YOUR requirements on your own.

For sure there are already made limits. (Example, because we don´t know your application details)
With an input voltage of 1.8V and an (inductor) output voltage of 5.5V (5V + 0.5V diode) you already know the max duty cycle can be 3:1 or 75% ( : 25%).
You need to add some headroom.

***
Please don´t expect that one of us does the whole work for you.
This is not how a forum works and not how a school work is meant to be.

Usually wou show your ideas, your calculations and we help to rectify problems.

Now I gave hints, formulae, math .... but indeed this is what we expect from you.
Simple taks like dividing 5.5 by 1.8 to get 3. (Even if you used 5V instead of 5.5V ... we are here to help you)

Klaus

 

[BradtheRad]

I haven't worked with 1.8v powered CMOS devices...
I have seen schematics of boost converters which contain one or two transistors that operate under 1V. These generally allow inductor current to fall to zero. That seems like a proper definition of DCM. Or, a certain length of 'dead time' (as seen via oscilloscope) indicates you're in DCM.

The boost converter starts to switch your inductor conducting and Amperage ramps up (in your case to 5 or 10 mA). The action is governed by the inductive time constant L/R. The switch is turned off and the stored inductor energy is sent to your output stage. A suitable formula which applies is Webers calculated as Amps * L.

In a boost topology the supply voltage is added to whatever comes through the inductor.

A simulator is helpful to reveal what frequency tends to be adequate with what Henry value of inductor. Experimental setups are flexible. When you obtain an inductor, you can try different frequencies and duty cycles, to find desired operation. At your low current output you only need to fabricate your own inductor and try it with different frequencies.