50-200VDC to 48VDC converter. - Page 1
Forum section: electronics / projects

author: microherb
author_rank: Contributor
timestamp: January 23, 2024, 06:38:08 am
content: I’ve been asked to look into the feasibility of a system that takes the output from a BLDC motor (3 phase AC) used as a generator, rectifies the AC to DC, then steps down and regulates the voltage to a 48VDC bus. The rectified output from the BLDC motor can be anywhere from around 50VDC at 1000RPM to about 200VDC at 4000RPM. This is high power system as well, about 4000W, so around 80A at 48V.My initial thoughts are it could be done with a buck LC circuit driven from a PWM controller, but was wondering if anyone can point me in the right direction as a starting point.

author: Slh
author_rank: Regular Contributor
timestamp: January 23, 2024, 06:49:27 am
content: Sounds like a good start. You might want to look at interleaved buck converters. Basically two (or more) buck converters run at 180 degrees to each other. This doubles the ripple frequency in the capacitors so they can be half the capacitance for the same total ripple.Do you care about the efficiency? You might want a motor inverter instead of the rectifier to have a good power factor.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 23, 2024, 09:09:10 am
content: As an exercise in laziness I entered, Input 50-220V Output 48V@90A into TI's WEBENCH® POWER DESIGNER.It came up with four designs:webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=171B9814DDD3C6B2webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=EFA13103B7760EBFwebench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=37E7679C28B07E54webench.ti.com/appinfo/webench/scripts/SDP.cgi?ID=7AF864CCE0AF9F92Worth a look. The BOMS cannot be entirely trusted. eg The rectifiers are very wrong.

author: microherb
author_rank: Contributor
timestamp: January 23, 2024, 10:03:36 am
content: Quote from: Terry Bites on January 23, 2024, 09:09:10 amThanks Terry, great minds think alike, I did a similar thing and got the same results (for some reason it wouldn't let me log in to take a closer look, it said "Try again later" so I'll try again tomorrow). Not lazy, it's a way of checking possible solutions.Cheers.

author: uer166
author_rank: Super Contributor
timestamp: January 23, 2024, 01:57:44 pm
content: You may also look into a proper BLDC controller to regen boost the output of motor to some fixed voltage (say 210V or something slightly above max), before feeding your DCDC stage. Then you can convert that to 48V with a non-regulated bus converter at a very high efficiency, and more importantly maintain power factor close to 1 on the motor.With a dumb rectifier into a cap you'll have decently high I2R losses since RMS current is higher than it needs to be.

author: langwadt
author_rank: Super Contributor
timestamp: January 23, 2024, 02:13:28 pm
content: Quote from: uer166 on January 23, 2024, 01:57:44 pmrectifying three-phase you don't need much capacitance

author: uer166
author_rank: Super Contributor
timestamp: January 23, 2024, 02:28:45 pm
content: Quote from: langwadt on January 23, 2024, 02:13:28 pmGood point, I looked up and looks like it's 0.9 PF on 3-phase rectification. So there's something to gain but not much.One advantage of a BLDC drive is you use the windings as the main power conversion inductor, so the external converter becomes much smaller. May or may not matter here..

author: microherb
author_rank: Contributor
timestamp: January 23, 2024, 09:39:43 pm
content: Quote from: uer166 on January 23, 2024, 01:57:44 pmSounds Interesting, thanks, but I don't really understand how this would work. Can you explain a bit more or send me some links that would help me understand what you mean?

author: uer166
author_rank: Super Contributor
timestamp: January 23, 2024, 09:50:49 pm
content: Quote from: microherb on January 23, 2024, 09:39:43 pmIt's not really any different from a normal BLDC motor controller, you're just setting the current to reverse direction (a.k.a. regenerative braking, that's something to to google!). Whenever you regen brake a BLDC motor, the windings act as inductors as part of a boost converter formed via the FETs and the back-EMF voltage (the 60 to 200V you're talking about), and can therefore output a voltage higher than the BEMF. Another term is "synchronous rectified boost converter", which is what a BLDC motor during regen is.This is likely how good MPPT wind turbine controllers are designed, but I haven't taken any apart to confirm.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: January 23, 2024, 09:56:04 pm
content: Quote from: microherb on January 23, 2024, 06:38:08 amA simple Buck limits to Vin being some margin above output. If your PWM can go to 100%, Vout can follow Vin minus any polarity-diode/switcher drops, but Vin has ripple here.The other issue here is the simple rectify will draw large peaks from the 3 phase AC, which may cause excess motor heating, or exceed the motor parts ratings. 4000W is quite high power.Power factor correction circuits are used in Mains SPMS to broaden those peaks, so the load looks more simply resistive. Less peaks means less loss.

author: microherb
author_rank: Contributor
timestamp: January 25, 2024, 01:50:08 am
content: Quote from: uer166 on January 23, 2024, 09:50:49 pmAhh, thanks, ok I think i get it now, something like as described here:https://ww1.microchip.com/downloads/en/devicedoc/regenerative%20braking%20of%20bldc%20motors.pdfGives me more to investigate , thanks.