Do buck converters have a minimum load requirement? - Page 1
Forum section: electronics / beginners

author: logancane
author_rank: Regular Contributor
timestamp: May 04, 2021, 10:51:17 am
content: Hi everyone,First post! Whoop whoop!Anyway, as the subject suggests, I am wondering whether there is a minimum load needed for a DC/DC converter to maintain voltage regulation.Because the project that I am working on uses communications systems (Iridium and GSM), they have a bit of a range in current (i.e. the average standby current is around 150 mA but with momentary pulses of up to 1.5A. Obviously I need voltage regulation throughout).This is the buck converter that I am looking at using:https://fscdn.rohm.com/en/products/databook/datasheet/ic/power/switching_regulator/bd906xxefj-c-e.pdfSpecifically, I will be using the 2.5 A version (B90620-HFP).When reading through the datasheet (and those of other converters) I noticed that although the recommended operating procedures do not state a minimum (see page 5 of the datasheet above), there is a minimum in the application notes (see page 21). For example the 5V output application note of the part I want to use (2.5 A rated buck converter) says the output current is:Min 0.4 A / Typ 0.8 A / Max 1.5 A (side note: why is the maximum only 1.5 A!?)Does this mean I will need to have some kind of dummy load that maintains the correct current (something I don't really want to do given that it will be running on batteries), or have I looked at this the complete wrong way?I have searched both the internet and previous forum posts to no avail, which leads me to believe that either I've picked the wrong part, I'm worrying about something that is a non-issue, or that minimum loads are such a universally understood concept amongst people who use buck converters that no one has bothered to write about it.Thank you in advance

author: Circlotron
author_rank: Super Contributor
timestamp: May 04, 2021, 12:01:48 pm
content: Short answer is yes, you need a minimum load. If the inductor current decreases to zero the voltage at the "SW" pin of the regulator will not stay at zero during the entire "off" portion of the switching cycle.One solution would be to use a synchronous buck converter that uses a mosfet instead of the schottky diode. That guarantees the reg output voltage stays at zero volts during the "off" portion of the switching cycle so the inductor and capacitor can low pass filter the output even at zero load. No preload needed.

author: brucehoult
author_rank: Super Contributor
timestamp: May 04, 2021, 12:10:59 pm
content: Isn't a step-down regulator fundamentally different to a buck converter (which normally have higher output voltage than input), even if it's switching?I think those aren't the minimum and maximum loads you're allowed to apply, they're the variation in the part characteristics, when using the given external component values. I don't know whether you might want to adjust Co or L1 or maybe make sure you have more than 6V at the input. Or just use the 640 part. I suspect the other parts of the datasheet would let you work that out.I don't think a mere 10:1 variation in current draw would be a problem, at least if it changes slowly. It might be fun if it changes in time comparable to or less than the switching frequency.

author: wraper
author_rank: Unknown
timestamp: May 04, 2021, 12:22:40 pm
content: Quote from: brucehoult on May 04, 2021, 12:10:59 pmBuck converter = step-down converter. You are confusing it with boost converter. Resistors in voltage divider used for sensing output voltage often provide enough of minimum load.

author: langwadt
author_rank: Super Contributor
timestamp: May 04, 2021, 12:23:20 pm
content: Quote from: brucehoult on May 04, 2021, 12:10:59 pma buck is a step down, and the output is always lower than input

author: ogden
author_rank: Super Contributor
timestamp: May 04, 2021, 12:24:14 pm
content: Quote from: logancane on May 04, 2021, 10:51:17 amIt is nominal current range for particular application example. If you want to build higher current (than 1.5A) converter, most likely you need to use other inductor as minimum, likely adjust other components too. For lower current applications (than examples) you better use another switcher.Usually converters are designed in such a way that they can operate without no load at all. Thou if your application needs efficient converter for wide current range including no load, then you may look for pulse skipping buck converter instead.

author: brucehoult
author_rank: Super Contributor
timestamp: May 04, 2021, 01:07:07 pm
content: Quote from: langwadt on May 04, 2021, 12:23:20 pmDammit, I confused with boost converter. And as I just learned, you can even combine the two into one circuit.

author: tautech
author_rank: Super Contributor
timestamp: May 04, 2021, 01:11:00 pm
content: Quote from: brucehoult on May 04, 2021, 01:07:07 pmOr in the same device......We need more info from the OP about his DC supply as we only yet have what he needs to get out.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 04, 2021, 01:39:53 pm
content: Only required if the controller is unable to produce 0% duty cycle for whatever reason.Note that pulse width cannot go all the way to zero, there must be some minimum pulse width. And therefore the control can't cut off gracefully, but it has to somehow dither between fully off, and runt pulses. Consequently, at light load, ripple voltage will increase, and ripple frequency will decrease -- whether by this intrinsic property of the control, or explicitly as "pulse skipping" or "eco-whatever" as this behavior variously gets called. The inductor or capacitors may also emit an audible whine in this condition.For most purposes, the ripple isn't a problem; just something to keep in mind.Tim

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 04, 2021, 02:31:31 pm
content: Hi,If you assume that the switch, diode, inductor and output capacitor have no losses, ideal components, the transfer function of a buck regulator is:Vout = duty-cycle x VinThis is only true if inductor is continuous. At light loads, to maintain continuous current, the current in the inductor is negative. This requires a buck regulator capable of synchronous operation.The Rohm part chosen by the OP is not a synchronous part and does not operate in the forced continuous mode.The part can regulate the voltage, but at light loads the switching may by erratic. In some applications it is desirable to have the power supply running at a fixed frequency.Do you need low noise?Jay_Diddy_B

author: tszaboo
author_rank: Super Contributor
timestamp: May 04, 2021, 02:44:25 pm
content: Quote from: T3sl4co1l on May 04, 2021, 01:39:53 pmA lot of controllers will just go into pulse skipping mode.They reduce the frequency, while pulse length is the same.I recently used this low power switcher, that was switching at 30Hz (not a typo), when the microcontroller was sleeping and everything was off, few hundred KHz otherwise. I had to check the measurement setup 3x because I didn't believe my eyes.

author: logancane
author_rank: Regular Contributor
timestamp: May 04, 2021, 09:18:24 pm
content: Quote from: tautech on May 04, 2021, 01:11:00 pmThe source is a 2sXp set of 18650 cells, so input voltage is between 6 and 8.4 V (7.4 Vnom.)

author: logancane
author_rank: Regular Contributor
timestamp: May 04, 2021, 09:33:13 pm
content: Thank you for your help everyone. Despite having done an entire course at university in buck/boost converters, it seems I still know very little. What I'm getting from this is that when choosing a converter, I should pay special attention to the minimum duty cycle. I also need to learn what is meant by synchronous operation and pulse skipping.

author: tautech
author_rank: Super Contributor
timestamp: May 04, 2021, 11:16:04 pm
content: Quote from: logancane on May 04, 2021, 09:18:24 pmOK then why not KISS and just use a LM7805 or a 317 ?If the duty cycle for 1.5A is low you won't even need to heatsink it.

author: logancane
author_rank: Regular Contributor
timestamp: May 05, 2021, 01:12:15 am
content: Quote from: tautech on May 04, 2021, 11:16:04 pmI am still concerned about efficiency. The project I am working on is a high altitude balloon, so weight is an incredibly important factor. The design uses four of these regulators to power other loads with similar power requirements. Do you think the loss in efficiency will necessitate more batteries being required, or would a linear regulator only add a negligible amount of extra power.

author: tautech
author_rank: Super Contributor
timestamp: May 05, 2021, 01:24:22 am
content: Grab the datasheets and see.What's your expected total uptime ?As the equipment is likely to be lost providing it doesn't run the batteries flat before crashing it might not matter.You can get these parts in SMD in order to keep the weight down.

author: Peabody
author_rank: Super Contributor
timestamp: May 05, 2021, 02:18:07 am
content: I would just ask if you are sure you need regulators at all. I just wonder if some combination of Energizer Ultimate Lithium primary AA batteries (nominally 1.5V) might let you operate directly from the batteries. And those batteries would also be the most tolerant to cold.

author: viperidae
author_rank: Frequent Contributor
timestamp: May 05, 2021, 05:18:15 am
content: It can get pretty cold up there, 18650 cells are not much good at all below -20c.A random data sheet says capacity for a 2600mah cell is 1800mah at -20c, and not rated to function below that.

author: brucehoult
author_rank: Super Contributor
timestamp: May 05, 2021, 06:03:41 am
content: Quote from: viperidae on May 05, 2021, 05:18:15 amIf it's not there for long at all then thermal inertia can be enough. We take gliders with wings full of water up in -20 and colder temperatures for a number of hours but it takes a lot longer than that for it to freeze.Insulation can help with keeping the rate of temperature loss down.A heater can prevent it entirely. You can easily get more useful energy out a battery that is keeping itself warm than from a battery that is allowed to get cold.

author: logancane
author_rank: Regular Contributor
timestamp: May 05, 2021, 08:39:04 am
content: It's a full on final year project that I am working on with 4 other guys, so it will have the works. Heating, comms, rechargeable batteries, a flight computer etc... I'm thinking a synchronous buck converter is the way to go. I need four of them for the various different circuits. Thank you for all of your help everyone.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 05, 2021, 03:29:28 pm
content: To answer the original question: it would be a very odd buck converter that could not handle 10% of the normal operating current without acceptable performance. Especially one with a relative low Vin/Vout ratio like you need. It is always good practice to do the calculation for yourself -- its easy to calculate the duty cycle if you know Vin, Vout, and L, but it shouldn't be excessive. In DCM mode, the duty cycle only drops as ~sqrt(Io). This is because the peak inductor current is proportional to the duty cycle, and the energy delivered is proportional to 0.5 * L * Ipk^2.If you do reach the minimum duty cycle of the controller it will have to enter pulse skipping mode or whatever the equivalent is, that will increase ripple voltage and decrease frequency, but it will still work.