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Design >> High-Power Design >> switch vs diode in buck converter
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Message started by tg on Mar 8^th, 2007, 5:43am

Title: switch vs diode in buck converter
Post by tg on Mar 8^th, 2007, 5:43am

Hi,
My question relates to dcm in buck converter.
I know that a buck converter with a diode (connected between inductor and ground) would operate in dcm when inductors current become zero.
A switch (paralleled to diode) would allow such negative cuurent to flow to the ground.
If thats correct, this buck will operate in ccm regardless load resistance. (Am I right so far?)
I saw a design that does not allow such negative curent by diconnecting the switch when inductor`s current reaches zero (A diode is paralled to the switch). This way a dcm mode is forced. Why would anyone like to force dcm?
Please correct me if I'm wrong...
Thanx.

Title: Re: switch vs diode in buck converter
Post by Matthieu on Mar 13^th, 2007, 7:26am

Hi tg,

You are right in all you said. We would like to force DCM mode under light load conditions. The goal is to improve efficiency of DCDC converter. As soon as current begins to invert in NMOS transistor (flow to ground instead of from ground), efficiency begins to decrease in CCM mode.

In CCM mode D=Vout/Vin (D duty cycle). If you have a very ligth load, as DeltaI in inductor (Delta of current in inductor between NMOS and PMOS commutation) is always the same whichever load current you choose (in first approximation), Current have to go below zero.

As a good drawing is better than a long text, I have drawn inductor current in three cases for Vout=Vin/4:
- CCM, high load.
- CCM, light load.
- DCM, light load.

I hope this answer your question...

Matthieu

Title: Re: switch vs diode in buck converter
Post by tg on Mar 13^th, 2007, 11:16am

Thanx Matthieu, thats answer my question (without seeing any drawing ;))

Title: Re: switch vs diode in buck converter
Post by Matthieu on Mar 14^th, 2007, 7:09am

Yes, I should have forgotten to attach the file 8-)

If you want, maxim-ic website has a lot of interesting and hand on information about DCDC converters.
(http://www.maxim-ic.com/appnotes.cfm/appnote_number/2031/ for example)

Matthieu