On Buck-Boost Converters

[srg2692]

I'm about to get a dna 75 mod and it'll be my first single 18650 device. I'm aware that for the most part resistance means nothing on a regulated mod, and I have a basic understanding of a buck converter, but I don't know much about the buck-boost converter that's in single 18650 regulated mods. I'm basically trying to figure out whether or not builds that require less voltage will be easier on the board.

I'm aware that buck converters are more efficient and therefor run cooler than buck-boost. Does that drop in efficiency only take place when it's actually boosting the voltage, or is it just the nature of the converter itself, and it runs at the same efficiency level no matter what? Basically my question is: if my resistance calls for 3.0 volts at 60 watts, will the board run cooler/more efficiently than if it required 5.0 volts at 60 watts? Also, depending on just how much the converter can boost the voltage, would it be possible to drain the batteries further without getting a "weak battery" warning if the converter isn't boosting?

I'm not sure if that makes any sense or not, this is right at the edge of my understanding so forgive me if I sound like an idiot.

 

[DaveP]

While you are waiting for answers here's an article that talks about the various ways to buck and boost voltage in ecig mods. A circuit that uses one battery will struggle more than a mod that uses two or three batteries in series. Voltage range is easier to achieve if the input voltage is higher vs the 3.7v input from one battery.

The Different Types of Power In E-Cigs

Most variable ecig mods use pulse width modulation to vary the output duty cycle. DC voltage is converted into a pulse train with variable on/off time. Basically, that means that the frequency and width of pulses is increased or decreased in frequency and/or duration to maintain power within a desired range. Battery voltage is converted from a DC voltage to a series of pulses that controls on time and off time to achieve something comparable to actually raising and lowering a DC voltage.

 

[srg2692]

While you are waiting for answers here's an article that talks about the various ways to buck and boost voltage in ecig mods.

The Different Types of Power In E-Cigs

Most variable ecig mods use pulse width modulation to vary the output duty cycle. DC voltage is converted into a pulse train with variable on/off time. Basically, that means that the frequency of pulses is increased in frequency and/or duration to maintain power within a desired range. Battery voltage is converted from a DC voltage to a series of pulses that controls on time and off time to achieve something comparable to actually raising and lowering voltage. This raises and lowers the average value of the voltage delivered to the atomizer as opposed to actually raising and lowering the voltage.

Thank you, I'll give it a read.

 

[mcclintock]

Basically my question is: if my resistance calls for 3.0 volts at 60 watts, will the board run cooler/more efficiently than if it required 5.0 volts at 60 watts?

1. Neither, around 4 volts would be better. 2. From my experience, the 5 volts is more efficient of those 2, but this was not with a TC mod. TC mods have to drive nickel wire which is super low Ωs so may fare better. Another exception would be a circuit without a boost portion could be more efficient, but mostly only mods designed for 2 or more cells in series skip that circuit and they can drive 5 volts easily.

 

[Mooch]

A well designed buck-boost converter is always more efficient when bucking. The input voltage must be a certain minimum voltage over the output voltage though to stay in buck mode. The lower the input, the lower the efficiency. A typical buck-boost regulator will be more efficient at higher input voltages. But depending on the regulator the efficiency levels at different input voltage levels might only differ by one or two percent when bucking.

The output of any regulated mod is a steady DC voltage with a small amount of ripple riding on top. The output is only PWM if it is a PWM mod (using a MOSFET with a pot to adjust the duty cycle). A switching converter does convert the input DC voltage to a pulsed waveform, at a frequency of tens of kilohertz to a couple thousand kilohertz, but that is filtered back to a DC voltage by the output capacitors.

Some e-cig regulators will pulse their outputs at high wattages but they are just chopping the DC output of the regulator.

If you are keeping the input voltage high enough to stay in buck mode then you never have to worry about getting a weak battery message as the battery voltage will never drop down low enough (assuming you're using a buck-boost converter).

 

[srg2692]

A well designed buck-boost converter is always more efficient when bucking. The input voltage must be a certain minimum voltage over the output voltage though to stay in buck mode. The lower the input, the lower the efficiency. A typical buck-boost regulator will be more efficient at higher input voltages. But depending on the regulator the efficiency levels at different input voltage levels might only differ by one or two percent when bucking.

The output of any regulated mod is a steady DC voltage with a small amount of ripple riding on top. The output is only PWM if it is a PWM mod (using a MOSFET with a pot to adjust the duty cycle). A switching converter does convert the input DC voltage to a pulsed waveform, at a frequency of tens of kilohertz to a couple thousand kilohertz, but that is filtered back to a DC voltage by the output capacitors.

Some e-cig regulators will pulse their outputs at high wattages but they are just chopping the DC output of the regulator.

If you are keeping the input voltage high enough to stay in buck mode then you never have to worry about getting a weak battery message as the battery voltage will never drop down low enough (assuming you're using a buck-boost converter).

Very informative, answers all my questions and a few I didn't think to ask. Thank you.

 

[DaveP]

Mooch, if you are still monitoring this thread, do you like series or parallel mods for low resistance applications? Series uses higher voltage to achieve wattage and parallel uses fixed battery voltage and is able to supply higher amperage through parallel connections. I'm betting on series applications as being the winner in a buck-boost circuit since all the implementations we see in ecig mods are variable voltage configurations that use pulse width modulation to achieve regulation.

FWIW, here's an interesting NASA document explaining the various configurations of voltage regulator circuits and how they interrelate and compare in various applications. There's lots of info and some good explanations of the various regulator circuits. Some of it is straightforward and parts are pretty in-depth descriptions using higher level math, but there are some great schematics that show simple operational block diagrams of circuit sections.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19740008693.pdf

 

[Mooch]

Mooch, if you are still monitoring this thread, do you like series or parallel mods for low resistance applications? Series uses higher voltage to achieve wattage and parallel uses fixed battery voltage and is able to supply higher amperage through parallel connections. I'm betting on series applications as being the winner in a buck-boost circuit since all the implementations we see in ecig mods are variable voltage configurations that use pulse width modulation to achieve regulation.

FWIW, here's an interesting NASA document explaining the various configurations of voltage regulator circuits and how they interrelate and compare in various applications. There's lots of info and some good explanations of the various regulator circuits. Some of it is straightforward and parts are pretty in-depth descriptions using higher level math, but there are some great schematics that show simple operational block diagrams of circuit sections.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19740008693.pdf

Sitger setup can work well but series allows for lower current for the same power level and this allows for smaller gauge wiring and lower resistive losses through all the components, keeping things cooler. As you noted, higher input voltages to a boost converter is more efficient too. Series-connected batteries can even allow you to just use more efficient buck converters.

Interesting document! They've come a long way in dc-dc converter design in the last 43 years but there is still a lot of good basic stuff in that paper. Thanks!

 

[DaveP]

Sitger setup can work well but series allows for lower current for the same power level and this allows for smaller gauge wiring and lower resistive losses through all the components, keeping things cooler. As you noted, higher input voltages to a boost converter is more efficient too. Series-connected batteries can even allow you to just use more efficient buck converters.

Interesting document! They've come a long way in dc-dc converter design in the last 43 years but there is still a lot of good basic stuff in that paper. Thanks!

The reason I ask is that I'm seeing various battery life levels in different series mods and wondering why, for instance, my RX200S eats three VTC4 or 25R batteries faster than my dual battery Koopor plus 200W mod using the same battery in different married sets for each. Both mods vape at the same 10W level.

I haven't seen any schematics available for either, so it's a black box circuit from my standpoint. You'd think that three 18650's in series would last longer than two at the same wattage level. The voltage is higher in the three battery configuration and the combined current capacity is also higher with three equal 18650's.

 

[DaveP]

While we're talking about such circuits ...

The older variable mods like the Zmax (33hz and 66hz output freq) and the early Provari V2 (800hz output freq) are still good mods, but have been superseded by higher frequency converters. I don't hear much about the specs anymore now that buck-boost converters in ecigs have matured. Anyone have info on what frequencies are common and what they are running at these days in particular mods like the RX200 series and the Koopor 200W?

 

[DaveP]

Sitger setup can work well but series allows for lower current for the same power level and this allows for smaller gauge wiring and lower resistive losses through all the components, keeping things cooler. As you noted, higher input voltages to a boost converter is more efficient too. Series-connected batteries can even allow you to just use more efficient buck converters.

Interesting document! They've come a long way in dc-dc converter design in the last 43 years but there is still a lot of good basic stuff in that paper. Thanks!

!! I had to search to find the date ... 1973! Yep that document was ancient. I guess I was impressed that a NASA document on buck-boost converters was published for layman eyes. It's old ... that's why!

 

[Mooch]

The reason I ask is that I'm seeing various battery life levels in different series mods and wondering why, for instance, my RX200S eats three VTC4 or 25R batteries faster than my dual battery Koopor plus 200W mod using the same battery in different married sets for each. Both mods vape at the same 10W level.

I haven't seen any schematics available for either, so it's a black box circuit from my standpoint. You'd think that three 18650's in series would last longer than two at the same wattage level. The voltage is higher in the three battery configuration and the combined current capacity is also higher with three equal 18650's.

The cutoff voltages and delays can be different between mods, greatly affecting run time since the discharge curves are pretty flat. A small change in voltage = big change in run time.

 

[Mooch]

While we're talking about such circuits ...

The older variable mods like the Zmax (33hz and 66hz output freq) and the early Provari V2 (800hz output freq) are still good mods, but have been superseded by higher frequency converters. I don't hear much about the specs anymore now that buck-boost converters in ecigs have matured. Anyone have info on what frequencies are common and what they are running at these days in particular mods like the RX200 series and the Koopor 200W?

Most run at, a minimum, tens of kilohertz. The Sig 213 runs at 161kHz, a typical value. The higher the frequency, the smaller the components but the higher the power losses. The designer decides which is more important. Commercial converters can be up at 2mHz.

 

[DaveP]

Most run at, a minimum, tens of kilohertz. The Sig 213 runs at 161kHz, a typical value. The higher the frequency, the smaller the components but the higher the power losses. The designer decides which is more important. Commercial converters can be up at 2mHz.

At those Freqs you're talking about virtually clean DC levels after filtering. Years ago, even in the Provari there was little "rattlesnake" sound in the coil at 800hz pulsated DC. The Provari was filtered prior to the coil and showed up on a scope as a virtually pure DC signal. Synthesized DC voltage has certainly come a long way since then.