Electronics > Projects, Designs, and Technical Stuff

Boost converter minimum load

(1/2) > >>

shadewind:
I'm designing a boost converter using the LM3478 PWM controller and looking through the equations, the minimum inductor value is determined by the minimum load current. You can also say that the inductor value determines the minimum load current. If I understand this correctly, dropping the load current lower than this moves the operation into DCM, discontinuous conduction mode. This is going to be used for bosting 12V to 40V for driving high power LEDs which will be dimmed by PWM so chances are high that I cannot avoid dropping below this minimum load current unless I want a very large inductor (i.e. 56 ┬ÁH for 744 KHz and 50 mA min load current).

What will happen if it goes into DCM? Will it still work properly?

dopplershift:
Hard to tell what it will do from the datasheet. It does say "[the CCM] mode of operation has higher efficiency and usually lower
EMI characteristics than the DCM"

So, I think it will still work, but will be relatively inefficient and produce a noisy output. Usually the manufacturers like to show off all the great features of their parts.... the fact that DCM is not shown in the datasheet may tell us that the output is not pretty!

Zero999:
Going from what you've said, this seems to be the wrong approach. To drive LEDs you need a constant current source. You could of course boost the voltage to 40V and use a resistor to limit the current but this is inefficient and if you're going to be doing PWM the current will periodically drop to zero.

shadewind:

--- Quote from: Hero999 on July 02, 2011, 07:27:44 am ---Going from what you've said, this seems to be the wrong approach. To drive LEDs you need a constant current source. You could of course boost the voltage to 40V and use a resistor to limit the current but this is inefficient and if you're going to be doing PWM the current will periodically drop to zero.

--- End quote ---
I need the higher voltage to drive LEDs in series so the boost controller is only for getting the 40 volts. I'm of course going to be using a LED driver for the LEDs themselves. I thought it was obvious in my post that I was not asking about the whole solution, only about the boost converter?

shadewind:

--- Quote from: AcHmed99 on July 02, 2011, 10:57:21 pm ---You can control the current through the LEDS by sensing the output current through feedback instead of the voltage. That way you don't need a separate driver. The boost converter in CC mode can be used to regulate the current independent of the Vf of the LED's,much less dissipative then a linear driver.

A lot of SMPS CC controllers can also be strobed (dimming). You could use your controller LM3478 in CC mode by taking feedback from the led strings current. The error amp doesn't care what your controlling all it wants to do is keep the inverting input equal to the positive input which is Vref. Usually though you use a controller with a reference of a couple hundred millivolts  to keep power dissipation low in the sense resistor. Read through the data sheets I linked too.

Oh and to answer your question your converter will still regulate when transitioning from CCM to DCM under light load. You could run unto problems when you design for DCM and go into CCM; CCM has RHPZ that needs to be taken into account,slope compensation is also needed for peak current mode control.An explanation of RHPZ and slope compensation as well as a boost LED  driver is in the tps40211 datasheet full design example for 35V 700mA LED driver.

If you dont want to  bother with RHPZ and slope compensation the NCP3065 doesn't require any.

--- End quote ---
Thanks for the great answer, this is what I was looking for. I think I'll take the plunge and learn about RHPZ since I've been reading a lot about complex frequency and Laplace tranforms lately. It's probably good knowledge to have anyway since I'm doing this for learning.

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks WYSIWYG Editor
Powered by SMFPacks Advanced Attachments Uploader Mod