Boost converter minimum load - Page 1
Forum section: electronics / projects

author: shadewind
author_rank: Frequent Contributor
timestamp: July 01, 2011, 09:57:24 pm
content: 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 µH for 744 KHz and 50 mA min load current).What will happen if it goes into DCM? Will it still work properly?

author: dopplershift
author_rank: Contributor
timestamp: July 02, 2011, 04:13:36 am
content: Hard to tell what it will do from the datasheet. It does say "[the CCM] mode of operation has higher efficiency and usually lowerEMI 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!

author: Zero999
author_rank: Super Contributor
timestamp: July 02, 2011, 07:27:44 am
content: 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.

author: shadewind
author_rank: Frequent Contributor
timestamp: July 02, 2011, 08:38:08 pm
content: Quote from: Hero999 on July 02, 2011, 07:27:44 amI 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?

author: shadewind
author_rank: Frequent Contributor
timestamp: July 03, 2011, 12:57:53 am
content: Quote from: AcHmed99 on July 02, 2011, 10:57:21 pmThanks 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.

author: shadewind
author_rank: Frequent Contributor
timestamp: July 03, 2011, 01:38:06 pm
content: I've been looking through the datasheet for the NCP3065 and I don't quite understand how the COMP pin is to be used for PWM dimming when it's already used for current regulation feedback? If get this correctly, one applies a voltage higher than the comparator voltage which then turns off the output?

author: shadewind
author_rank: Frequent Contributor
timestamp: July 03, 2011, 07:29:14 pm
content: A question about the TPS40211, how is PWM dimming done. Judging from the application circuit, it uses the COMP pin but how does this work?

author: thmjpr
author_rank: Regular Contributor
timestamp: July 04, 2011, 12:20:06 am
content: Quote from: shadewind on July 03, 2011, 07:29:14 pmWhich app circuit?http://focus.ti.com/lit/an/slyt302/slyt302.pdf:"The brightness can be programmed by altering R1, current injection into the FB pin, or by PWM dimming."edit: I see it now page 34 of thedatasheet.Good question, it seems overly complex. Not sure why you can't just use the EN/DIS input.

author: shadewind
author_rank: Frequent Contributor
timestamp: July 04, 2011, 12:39:34 pm
content: Yeah, I found the demo board manual description. Since the COMP pin is simply the output of the error amplifier into the logic, what they're doing is pulling this down to ground to inhibit switching? The diodes seem to be for raising the voltage just a little bit above ground and not for any rectification. Thus, a logic level FET would be enough for my purposes. Can anyone confirm my thinking?Also, in the same application circuit in the data sheet, what is the purpose of the resistor R13 which goes from the feedback pin to the current sense resistor. Is that necessary?