USB inrush current issues - Page 1
Forum section: electronics / projects

author: depeje
author_rank: Contributor
timestamp: June 04, 2016, 06:00:34 pm
content: I'm having either measurement issues or too much inrush current when plugging in a USB thing I made. Just to be complete, the schematic can be found herehere, but the relevant part of the schematic is included in this message.The issue is that if I plug in the device (using one USB port, the second one doesn't seem to be necessary for operation), there is an inrush current exceeding 100mA and more than 10µF. I approximate it to 25µF. It works in all my own PC's, but I want it to work just everywhere. I measured the inrush current usingthis, and Dave's µCurrent or just a 1 Ohm shunt. Plugging in the USB cable yields the measurements in attachment. Channel one is always the current into the USB cable. Plogging in the cable from the A-end doesn't differ from pugging in via the B-end. It also doesn't matter whether I use a bench power supply or a laptop running on battery with nothing else connected.To investigate which is the culprit, I started assembling a fresh board component by component and do the measurement. I populated the components in this order: USB1, R21, C15, R20, L1, C2, C3. Until C2, I couldn't measure any inrush current. When I added C3 (datasheet), the inrush current became what is visible in the screenshots. This means an estimated 25µF. Now, because I only populated resistors and capacitors yet, I used my agilent U1252b to measure the capacitance and surprise-surprise: it only measures 5µF.What might be happening here?

author: Monkeh
author_rank: Super Contributor
timestamp: June 04, 2016, 09:19:03 pm
content: How exactly are you arriving at this 25uF figure? You haven't got a full waveform to evaluate.

author: georges80
author_rank: Frequent Contributor
timestamp: June 04, 2016, 09:22:35 pm
content: What are your calculations?Consider Q = CVand 1C = 1A x 1SSo, with 4.7uF charged to 5V you have 23.5uCIf your charge time (per your scope capture) is about 20uS you would see around 23.5 x 10-6 / (20 x 10-6) which would be 1.2A (if a 20uS pulse, yours is actually shorter and YES, your scope is also clipping the peak).Now, the 100mA USB limit is for continuous current draw (prior to enumeration). USB allows for a 10uF input capacitance as seen at plug in time, so the USB spec ALLOWS you to draw more than 100mA during that short charge time. The host side has sufficient capacitance to deal with the device's 10uF input capacitance and that initial and short duration current surge.cheers,george.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 05, 2016, 06:52:39 am
content: So?USB is designed to deliver 500mA pretty much all the time, with an option for more.Whether any of these functions are implemented is up to the manufacturer. Often, not even the current limit is present, which is.The minimum I've seen used: just a polyfuse. This opens at about double the rated current, and only after about a hundred milliseconds. In the mean time, that's tens to a hundred milliseconds of fault current available, limited mainly by the polyfuse's cold resistance. So, maybe tens of amperes.Good equipment uses a semiconductor switch with active current limit and thermal reset characteristics.This is only a concern for USB host applications. Peripherals simply need to withstand whatever current they draw by themselves (given that they may be plugged into such shitty ports as the polyfused kind), and protect against voltage surges (not usually reverse polarity, but transient overvoltage can occur due to line inductance).Tim