Indirect current monitoring in a buck converter. - Page 1
Forum section: electronics / projects

author: hamster_nz
author_rank: Super Contributor
timestamp: July 03, 2018, 05:19:42 am
content: Has anybody tried closely monitoring the switching of a buck regulator (e.g. a generic MP1584) to load deduce current, rather than using an in-line shunt?I want to be able to monitor very low sleep current (tens of nA range), as well as active current (up to 200mA) of some microcontroller designs, and a shunt that is useful for nA is useless for mA.At such low duty cycles (< 10% of rated output) I am guessing that current would be approximately linear to either the switching rate or duty cycle, or maybe the product of both.I am planning on switching in of known load resistance to allow calibration/reference at various ranges....

author: JS
author_rank: Frequent Contributor
timestamp: July 03, 2018, 06:30:26 am
content: Sure you can, you need to know the topology of your setup, input and output voltage at least, if you know all the circuit better as you can predict every parameter. Don't expect 24 bits from such a measurement but better than what you ent with a 200mA shunt.JS

author: Giaime
author_rank: Regular Contributor
timestamp: July 03, 2018, 06:41:27 am
content: With very low sleep currents, the DC/DC converter is going into pulse-skipping mode (that is, does some cycles and then switches off for a very long time, waiting for the output capacitors to discharge below a certain threshold). It is described in the MP1584 datasheet.In this condition it could be hard to syncronize to this rare event of the DC/DC turning on, to measure duty cycle.Keep also in mind that a duty cycle does not correspond univocally to an output current, since if you move between CCM and DCM mode the same duty cycle can correspond to different load conditions.You should take a look at source and measure unit instruments, they use multiple shunts with bypassing and range-changing amplifiers, to allow very wide range of measurements.As a start, an high-value shunt could be bypassed by some Zener diode in case of high current consumption, or a clever active MOSFET bypass... just some food for though.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 03, 2018, 10:20:51 pm
content: If you are using a "high efficiency at low power" type (e.g., TI's EcoBoost, or most any hysteretic mode, low power controller), then:1. Verify that the inductor peak current is constant over a reasonable range of load currents, including down to zero.2. Measure the switching frequency. It's producing one pulse every so often, to stay "topped up". So, there you go!If peak current is varying, then you still need to measure that, or infer it somehow. Like, the EcoBoost ones, I think, divide the frequency in half, progressively, as the load drops, so the peak current stays around a minimum at light load, but increases at heavy load (and full speed switching).An aside: it's stupid design, making nonlinear changeovers like that. Alas, it seems analog designers don't do SMPS controllers anymore. So whatever, make do with what you can find out there.Tim

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 03, 2018, 11:03:41 pm
content: The LTC3335 actually does that. It's limited to 50 mA output current though, so may not fit your requirements, but you may want to take a look or even evaluate it before diving in your own design. I haven't found any other buck converter that does this at the time, but maybe there are.I have actually designed a lab power supply based on this principle last year, around an FPGA which controls a switchable current source (which allows to get a more accurate estimation than a regular inductor-based buck converter), with a range of 500 mV to 9 V and ~10 mA to 1 A programmable current limit (and 5 W max on all settings). It's still a prototype (I have no definite plan to sell this, but I may in the future) but works fairly well. It's not open-source HW so I can't disclose the design at the moment, but I can answer questions to help with your design.You may be able to monitor the output of a buck converter that drives the gate of an external FET instead of implementing the converter yourself. Back when I worked on this I found this was not flexible nor accurate enough (for low currents) for my needs, but it may fit your bill.

author: hamster_nz
author_rank: Super Contributor
timestamp: July 03, 2018, 11:23:46 pm
content: Quote from: SiliconWizard on July 03, 2018, 11:03:41 pmOh, thanks for the heads-up. I might not be able to use the chip (I need 200mA), but it at least gives me a solid basis to work from.I'm thinking a couple of different current sources), one for driving the 'awake' loads, an one for driving 'sleep' loads, allowing the sort of "glup" and "sip" modes of operation.This is actually seeming maybe do-able.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 04, 2018, 12:30:45 am
content: If you use current sources as I did, just be aware that fast switching them can be a bit tricky, so you'll have some design work and simulation to do.I only used one current source though. If I get it well, you'd like to use several (or at least 2) and switch them in real-time? If so, the hard part would be to actually predict which one is to be used in real time, and this while keeping ripple to a reasonable level. Sounds tricky.All this is definitely doable though.

author: chris_leyson
author_rank: Super Contributor
timestamp: July 04, 2018, 12:41:33 am
content: A bit of a long shot, if you could measure the inductor voltage on a cycle by cycle basis you could use the volt time product to estimate incremental inductor current and hence stored and transfered energy on a cycle by cycle basis. In principle you could estimate load power from two non-invasive voltage measurements across the inductor. You might need a comparator on the switching node to detect CCM DCM boundaries. It's not something I've ever tried but if you model it in terms of energy transfer it would be a very useful non-invasive power measurement technique that you could apply to any topology.