Boost converter with PWM fan control - Page 1
Forum section: electronics / projects

author: baoshi
author_rank: Regular Contributor
timestamp: December 09, 2012, 01:25:17 am
content: Hi,I'm designing a circuit. The purpose is to control a 12V fan using a MCU through PWM. Since the circuit is powered by single Li-ion battery, I also need a boost converter.I have the feeling that there's some redundancy here. I should be able to combine something.My first thought is to let Mcu output a pwm signal, through low pass filter and voltage divider and feed into a dc-dc's FB pin. At least I can save a power MOSFET driver at the fan side.Any other thoughts?

author: jeroen74
author_rank: Frequent Contributor
timestamp: December 09, 2012, 09:30:34 am
content: What about using the MCU as boost controller? You probably could get away with a fixed duty cycle, and regulate the output voltage/fan speed with relatively slowly with bangbang control;e.g. turn on PWM generator when o/p voltage it too low, turn if off when too high.

author: baoshi
author_rank: Regular Contributor
timestamp: January 16, 2013, 04:13:52 am
content: Hi Jeroen,I did some researches and finally decided go down the path as per your suggestion.I build a simple circuit and run some tests. The schematic is below. It is basically an open loop circuit. Output load is a fan, rated 12V / 0.7W.Since the load is more-or-less constant, my measurement is focused on efficiency-vs-duty cycle and efficiency-vs-output voltage.The result curves with varying frequency are attached. Both results show boost efficiency drops fast as duty cycle rises.As I understand boost converter loss is mainly due to the coil loss and forward diode drop. So why is the efficiency lost so much with constant frequency but increasing duty cycles?Thanks

author: Harvs
author_rank: Super Contributor
timestamp: January 16, 2013, 06:10:37 am
content: Hi baoshi,How are you measuring your efficiency? I've used your simulation file and see around 92-95% efficiency even up to very high duty cycles.But what I did change... Currently you've got a fairly small input cap with quite high ESR. This is feed by a perfect voltage source. So the result is the cap is doing nothing, and your input current is varying exactly with the inductor current. Clearly this isn't what you want, as it doesn't at all simulate real life. The only way to get an efficiency calculation with that is to integrate the current over a number of whole cycles.So, add a series resistance to your V1, I dropped in 0.3ohms. Then I changed C3 to a ultra low ESR 1000uF 50mOhms, which is typical for a small motherboard cap.Then if you plot efficiency as a trace:100*(V(out)*I(R2))/(I(V1)*V(in))You'll see it wobbling around the mid 90s for most reasonable duty cycles. It also allows you to play with the input capacitance and understand what sort of ripple you're going to be putting back on the power supply lines (i.e. Noise.)

author: baoshi
author_rank: Regular Contributor
timestamp: January 16, 2013, 06:40:08 am
content: Hi Harvs,I noticed the simulation result, which is pretty ideal compare to real world.The charts I did was measured from actual circuit. Though I do not have 4 multimeters like Dave does, I used two Agilent meter to measure input and output current and use scope to measure output voltage. The input voltage is from a 3.3v regulator. You see there are so many differences.I would think the inductor is making many losses as the one I'm using is pretty wimpy.I'll try replace the input capacitor and see there is any improvements.

author: Harvs
author_rank: Super Contributor
timestamp: January 16, 2013, 07:18:49 am
content: It'd be worth just measuring the DC resistance of the inductor you've got.Do you have a model number for it you could post here?For feedback, has the fan you're driving got a tach output? (i.e. is it a 3 or 4 wire computer case fan?)This would be idea for feedback, being able to measure the actual fan speed.

author: baoshi
author_rank: Regular Contributor
timestamp: January 16, 2013, 07:34:43 am
content: The inductor was salvaged from some crap. It looks similar to Panasonic ELC09D221Fhttp://sg.element14.com/panasonic/elc09d221f/choke-220uh-0-58a-10/dp/8094837,rated 0.58A/0.41ohm. I'll measure its actual DC resistance once I'm back at my bench.The FAN do have tachometer output. For the later part I'm planning to use tach reading to form a close loop so as to adjust the PWM signal. Guess that will be interesting.