How Continuous Can a Buck Converter Be without gradually Saturating the Inductor - Page 1
Forum section: electronics / projects

author: SuzyC
author_rank: Frequent Contributor
timestamp: August 04, 2025, 01:38:12 am
content: I would like to know, with someone giving me a engineering technical explanation, how to specify an inductor that will prevent inductor saturation in a PWM buck converter when near 100% duty cycle might be required.In a continuous mode buck converter, the inductor current never drops to zero during the off time of the switching Period, but although the inductor current never drops to zero, what is the maximum duty cycle that stillmagnetically dischargesthe inductor enough to prevent the inductor from gradually walking up to magnetic saturation?Case in point. I am charging 4 li-ion in series using a 18.6V computer wallwart. I want to achieve 4.15V/cell (16.6V total) while using a series connected schottky diode to prevent discharging the battery during non-charge/storage. This means that the 18.6V supply will have just ~1.6V overhead above the battery being charged and the duty cycle of the buck converter must approach 100%.

author: ajb
author_rank: Super Contributor
timestamp: August 04, 2025, 01:50:21 am
content: A converter could operate at 100% duty cycle without saturating the inductor, as long as the load current in that condition is less than the saturation current of the inductor. At that point you effectively just have an LC filter between the input and output.It's not like the non-zero current at the end of one cycle gets added to the next cycle -- exactly what happens will depend on the control mode the converter uses, but the inductor current will always oscillate linearly between a min and a max value based on the ratio of input to output voltage and load current. The worst-case peak current is one of the things you should be calculating as part of the converter design to ensure you select an inductor with a suitably high saturation current.

author: SuzyC
author_rank: Frequent Contributor
timestamp: August 04, 2025, 01:55:23 am
content: Ajb, a Continous Mode buck converter operating at 100% is not a switching converter it is a series-connected DC circuit using an inductor's resistance.

author: ajb
author_rank: Super Contributor
timestamp: August 04, 2025, 02:17:11 am
content: Basically, yes, as I said. The point is, you need to determine the operating conditions your converter will encounter and select components appropriately. Duty cycle isn't directly related to inductor current -- duty cycle is determined by the ratio of input to output voltage, current is determined by the load. If the DC-DC converter in question is directly charging the batteries, the worst case inductor current may not even be when the duty cycle is highest, since presumably the charging current drops off at that point. That's why you have to do the calculations and design the converter to suit the expected operating conditions. If you want some extra assurance, you can choose a converter with built-in current limiting, and select an inductor with a saturation current safely above that current limit.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 04, 2025, 02:18:00 am
content: Quote from: SuzyC on August 04, 2025, 01:38:12 amFlux walking is a fault behavior seen in some SMPS topologies but not in buck converters. So basically you don't have to worry.Flux walking is just a DC offset. Its usually due to mismatched power switches, whether diodes or transistors. The DC offset leads to linearly rising current and flux. If the resistance is low enough and the saturation field small enough this will eventually lead to saturation.This almost always occurs in transformer coupled supplies for two reasons. In a simple buck or boost converter the inductor current goes through the load in. This prevents it from increasing too much. In a transformer, DC offset on the primary is not transfered to the load so it can lead to a current build up. Second, transformers are not designed to store energy so their saturation flux is usually much less than the power inductors in buck/boost converters.

author: SuzyC
author_rank: Frequent Contributor
timestamp: August 04, 2025, 09:01:21 am
content: Ok, thanks ejefrey! The problem is no problem! Your reply answers my question, it is all I would need to know.I get it!The idea in choosing an inductor is just to choose an inductor that is, according to what I've read, has enough current capability to handle the the peak current during PWM on-time without saturating, with some margin.

author: PGPG
author_rank: Frequent Contributor
timestamp: August 04, 2025, 10:40:36 am
content: Quote from: SuzyC on August 04, 2025, 09:01:21 amHave in mind that:- peak current during start-up can be significantly higher than peak current during stable operation,- soft-start circuits (limiting that start-up current) can be not fully reset during short supply breaks (for example when hand connecting supply wires) and 'not-soft-start' can happen.Selecting inductor with saturation current higher than max IC current limit gives you guarantee to not saturate inductor.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 04, 2025, 11:51:34 am
content: Still, don't forget transient conditions! Even if your load draws only e.g. 1A, you probably do have an output capacitor which may have low enough ESR so that it can easily draw 10A and cause the saturation of the inductor during start-up. Or what if the user shorts the output? You don't want your converter to blow up on that.So even if flux walk is impossible in steady-state conditions, you still want the same protections.In ancient times, some circuits relied on timed "soft-start" which just increased output voltage setpoint (by tapping into the voltage reference circuit) slowly - and these died on output short, or any other random reason, or used massively overdimensioned components (inductors and transistors).A generic and reliable solution over all operating conditions and exceptions involves actually measuring the inductor current, and automatically terminating the cycle (turning the MOSFET off) if peak current limit is exceeded; the current limit chosen somewhat above the highest expected load current plus dI_L/2, so that it does not trigger normally. All decent and modern buck controller chips do this - either by sensing the Vds of the integrated power MOSFET (so using its Rds_on as shunt resistance), or with connections to external shunt resistor. They operate in constant-current mode by terminating each cycle as the inductor current (triangle ramp) reaches the peak close to inductor saturation. (It can be somewhat into saturation; it's not like saturation is totally abrupt; especially if the detection circuitry is fast, it can still operate with losing some inductance and hence increased ramp speed).Where driver ICs differ (and sometimes this is even configurable with pin strapping) is what they do if the limiting condition (clipping periods) is sustained; some continue operating happily, offering a crudely regulated constant-current mode (suitable for driving LEDs or charging batteries, for example, if the poor accuracy can be accepted), some enter hickup mode where they enable for a short train of pulses, turns off, and wakes up again in a second; some latch off until power is cycled.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 04, 2025, 03:55:56 pm
content: Quote from: SuzyC on August 04, 2025, 01:55:23 amWith that you've answered your own question.But as others have written here, there is more to consider such as start up behavior, and when charging (nearly) empty batteries. (Nearly?) all Buck converter IC's and battery chargers have a built in current limit. The saturation current of the inductor should always be higher then this current limit.