ESC with BEC or Separate BEC and ESC?

[Mad_Mechanic]

Hello everyone.

I suspect that what I'm about to ask may very well be a repost but my search box kung-fu is not very good it seems.

Here is my conundrum, and allow me to elaborate on some of my RC history.

About 7-8 years ago I got into RC Helicopters, this was about the time when multi-rotors (now just referred as 'quads' it seems) were really getting popular. At that time the industry was pretty well setup for fixed wing and helicopter support. Also at that time I recall that pretty much any ESC you looked at buying had a BEC built-in. But now, it seems that the industry has shifted in a big way. Now it seems to be hard core catering to the quad crowd. I'm not going to bash that, it's what is popular.

In a way I feel I'm echoing the sentiments of rmzalbar in his recent thread found here: https://forum.flitetest.com/index.php?threads/where-are-the-escs-for-fixed-wing.57196/

But here is the thing, I'm just trying to wrap my head around all the new options that are out there.

7 years ago it was common place to buy an ESC and a programing card with it. In fact, USB programmable ESCs were a pretty rare thing at the time. I think Caste Creations and the Turnigy SuperBrain were the only 2 I knew of that had USB programming ability. But now we have new firmwares, things like 'Simonk' and 'BLHeli'. Let's just say I'm feeling overwhelmed, but I'm digressing from my real question.

There are some really attractive looking ESCs on the market now but many of them do not include a BEC. 7 years ago I liked the simplicity of having the BEC included with the ESC, but it seems like for the modern 'really nice' ESCs, the BEC is no longer included on the board.

So at this point I have to ask: Is there an advantage in fixed wing craft to have a separate BEC and ESC? Are there weight savings? Cost Savings? Or some sort of performance advantage?

On the one hand I can see separating the two being a good thing because if you burn up 1 part (say your BEC), you just replace that one part. BUT at the same time it means you have two separate components and additional wires. This means more points of failure, and my engineer brain says that extra points of failure are generally not a good idea unless the advantages outweigh the disadvantages.

It's also entirely possible I'm way over thinking things and I should just do what evranch suggests in his reply posting to rmzalbar's thread and buy a bunch of Turnigy Plush ESCs and a programming card and get flying.

Thoughts?

 

[FDS]

I usually use an esc with a BEC, get one big enough for the peak load. Most FT stuff is pretty simple, for just flying around LOS with the basic motor set ups you shouldn’t need to reprogram an ESC. Most of the firmware stuff is designed for quads, even the fairly cheap ones will work on fixed wing. Generally all you do is calibrate it when you fire it up with the throttle stick so it knows where the start and end point is, otherwise the throttle requirements of a fixed wing are pretty basic. You shouldn’t have burn out problems if it’s all specd to match motors etc.

 

[Paracodespoder]

I prefer bec built in. I do have a few planes with separate bec, (super bee, scout wing on a balsa stick powered by an 1106, and a kf wing on 1106), it adds weight and makes wiring more complicated (especially when you have to solder 2 ESCs and a bec to one xt-60. Also, on airframes without a fuselage/electronics compartment, it makes keeping the wiring clean an issue.

 

[Namactual]

There are two types of BECs. Linear and switching.

Most onboard BECs are linear and most externals are switching.

The pros of onboard BECs are stated above. They are lighter and easier to use since there is no extra wiring. On the down side though, the linear power supplys step down voltage by turning the extra wattage into heat. This is not very efficient and forces your ESC to dissipate even more heat then it already generates.

With an external switch mode BEC you do not have the extra heat to dissipate which also makes it more efficient. But the down side is it's an extra unit to wire in and it's bigger and heavier. It also creates EMI and requires RF shielding.

I prefer switching BECs myself. The biggest plus is not overheating your ESC. Also using an external, if you do fry your ESC in flight you still have control of your aircraft as well.

 

[tamuct01]

For smaller models, the built-in BEC is probably fine. I'm finding that a lot of cheap ESCs have an equally cheap BEC that is incapable of driving larger loads without burning up. For example, my Durafly T-28 is decked out with 10 different servos for various tasks, but only a 3A BEC. I felt this was wholly inadequate and set it up to use a 2S LiFe receiver battery to power the control electronics. I picked up a larger 2 meter Sbach342 and I'm pretty sure I won't be using an ESC/BEC combo on the high-torque/high-speed servos I plan to install.

Short version: Check the specs on the BEC (switched vs. linear) and estimate the max current draw of the servo load and err on the side of having too much available current. If the built-in can't match that, go to a separate BEC or receiver battery pack.

 

[Namactual]

Good point about the load rating @tamuct01

One more thing to consider with linear BECs is the load rating is dependent on input voltage. An onboard linear BEC that is rated for 3 servos on 3s (11.1volts) will not support 3 servos running 4s (14.8volts). Reason being, the more volts the BEC has to step down, the more heat it has to dissipate. If you forget about this and upgrade your favorite 3s plane to 4s to hot rod around you can overheat your ESC very quickly or just burn out your onboard BEC altogether.

You do not have this problem with switch mode BECs.

 

[Mad_Mechanic]

To offer a slightly more 'technical' explanation, it sounds like a linear BEC uses (essentially) an 'old school' LM7805 fixed voltage regulator, which has a minimum required supply voltage in order to operate.

However, it sounds like a switching or 'switch mode' BEC uses a Buck converter or a Buck/Boost converter where it doesn't care (within reason) what the supply voltage is, it's going to spit out 5V for as long as possible until the batteries are just dead. This analysis does not take into account the fact that most (if not all BECs) at this point have battery protection logic included.

There is a part of me that just says "buy some cheap Turnigy Plush 30A ESC/BEC units and get flying", however, my engineer brain that likes redundancy, efficiency and reliability likes the idea now of using a separate switch mode BEC and find a nice light ESC only unit in 30A. Yes it means a couple extra components, but in my head I think the advantages are sound.

I certainly don't like the idea of a linear BEC pre-heating the ESC, that's a great way to encourage the ESC to overheat or worse, catch fire.

 

[makattack]

Heh heh, not add way more complexity to this discussion, but there are always pros/cons to everything, right?

Switching regulators, as efficient as they are, also generate more electrical noise. I've actually had to add additional filtering by way of ferrite rings to servo leads (a LC filter, or maybe just a big cap might be a more sophisticated way of filtering them out) due to the twitching that was induced by a switching regulator... actually, I combined the ferrite ring (3 turns) with twisting the servo wires... your mileage may vary, as they say. I may have had servos that were just more prone / sensitive to noise.

 

[Mad_Mechanic]

@makattack - you are absolutely correct, at least from what I've been reading. EMF/EMI seems to be the biggest downside to switching BECs with many different methods suggested/used to combat this issue.

Dimension Engineering (when they offered more BEC options) had a rule/guideline for using any of their BECs that the BEC be located at least 2 inches from any receiver/antenna.