How to prevent DC-DC Converter to enter into Discontinuous Conduction Mode (DCM)?

[JazzRei]

Hi,

I have designed a cascaded buck-boost converter also known as non-inverting buck-boost converter which is able to step up or step down the input voltage to produce a range of output voltage of 1V to 12V. But I have an quiry regarding how to prevent the circuit to enter into DCM as I do not wish the inductor current to go beyond zero.

Thanks if you can solve my problem.

 

[RAJAT AGGARWAAL]

have ur feedback to be sensed all the time and then u are done.
and if u say how ,this depends on controller u use
i use avr for these type of things

 

[JazzRei]

Ya.. a feedback is used to monitor the output voltage and display on pc using a window-based software.
What do u mean by avr?

 

[FvM]

Inductor ΔI is defined by inductance, voltages and switching frequency. For a given switching frequency and input/output voltage, the converter will necessarily enter DCM below a particular load current. So what are you exactly asking for? You didn't mention a minimum load current.

 

[JazzRei]

Hi...
For a switching frequency of 50KHZ and input voltage of 5V to produce the output voltage range of 1V to 12V. I have decided to use a load of 100ohm. Is it at output voltage of 1V, the converter will enter DCM?

 

[FvM]

You can select the inductor for the 1V/10mA output case, then the converter will operate in CCM. An inductance > 0.8 mH is required.

 

[JazzRei]

How you choose the inductance >0.8mH?
There is a boundary between CCM/DCM. How do I set to this mode?

 

[FvM]

Assuming CCM, you can calculate the inductor current ripple and switch duty cycles for each input/output voltage combination. The inductor has to been choosen such, that the inductor current stays positive, so CCM is assured.

 

[JazzRei]

Can I know where to find such formula to do the calculation and calculate the inductor value so that the inductor current will stay positive.

Thanks for ur quick reply. It has been very helpful.

 

[FvM]

Basically dI/dt = V/L or ΔI = 1/L ∫Vdt.
Application of the basic equations to DC/DC converter design is performed in chip application notes or text books.

I think, it also helps to sketch the waveforms or perform a simulation of a principle circuit (e.g. using ideal switches and diodes).

 

[JazzRei]

Hi...

I wanna find an inductor of 1mH to use in my circuit design. Can anyone recommend me the model of the inductor suitable for my cct?