Whereas designing a undertaking that required 5v USB energy ports from a a lot greater voltage supply (eg. 12v or 24v), it occurred to me that there are circuits already designed to do that. Actually, not solely do they exist as low-cost, easy-to-purchase modules, however some of them have already got the USB port soldered onto the board.
Four Step-down converters with USB ports have been picked out and reviewed in an try to seek out “the best cheap USB switch-mode step-down converter” for 12v energy provides.
- 1 How does a Buck (step-down) converter work?
- 2 Analyzing the buck (step-down) converters
- 3 A fast overview of the four modules
- 4 Testing the step-down converter modules
How does a Buck (step-down) converter work?
Earlier than stepping into the testing, it’s in all probability prudent to cowl how a switch-mode step-down converter truly works. Whereas it might appear complicated, it’s truly slightly easy and is centered upon 1 single element to carry out the majority of the work: The Inductor.
Understanding the Inductor
In case you are unfamiliar with what an inductor is, it may be described merely as a wire coated in a non-conductive enamel after which that wire is (sometimes) wrapped round a metallic ring or rod. (The non-conductive coating is to stop short-circuits from winding the wire across the ring.)
Picture credit score: Wikipedia web page on Inductors
What occurs with an inductor is two-fold:
- When present begins passing via the wire operating across the ring, an invisible magnetic area is created. The inductor acts like a resistor with an initially excessive resistance. Because the magnetic area continues to develop, the resistance drops towards zero. So long as present is utilized to the inductor, the sector is maintained.
- The fascinating half is when the present is faraway from the inductor. The sector begins to break down, the inductor resists this modification, and all of the power saved within the magnetic area forces the electrons to start flowing once more, inflicting a rush (or spike) of present within the wire.
The necessary factor to know is that the inductor resists change in present passing via it and it resists by storing that power in a magnetic subject after which later giving up that saved power when wanted.
That is why a switch-mode converter is ready to be so environment friendly in comparison with converters like linear regulators or voltage dividers which simply resist present by changing unneeded power to waste warmth.
Understanding how the Inductor can step-down voltage
Now that one of the essential ideas of an Inductor might be thought of as an power storage system, let’s contemplate how we will use this to our benefit. If we quickly activate and off the present to the inductor, we will regulate how a lot power we need to put into the inductor and relying on how lengthy issues are on versus off we will successfully step-down the voltage to a desired quantity.
To do that, we’d like a really quick electrical change (transistor) generally known as a MOSFET.
Wanting on the circuit diagram above, we will see that we have now +12 volts related to the MOSFET (represented right here as a gap and shutting change) managed by a square-wave clock (labeled CLK) and instantly related to the Inductor (marked as 1H — 1 full Henry is definitely overly giant — this measurement was chosen purely for the simulation).
- When the clock (CLK) switches the MOSFET on, it fees up the inductor (storing power) whereas concurrently dropping the voltage flowing by way of the wire. Because the inductor continues to cost, the resistance continues to drop and the voltage continues to rise once more.
- When the clock (CLK) switches the MOSFET off, the inductor’s magnetic area collapses. At this level, the inductor turns into negatively charged and attracts present up from floor via the diode maintaining the present flowing. (The 100µF capacitor smooths out any ripples within the output.)
As seen on this simulation, the 12v enter is dropped to round 5v volts output.
Right here’s the take house: The frequency and period (duty-cycle) of switching on / off mixed with the dimensions of the inductor type the fundamentals of how a Step-Down converter works.
A complicated step-down converter has a posh IC/chip which regulates the bigger circuit utilizing suggestions in an clever method such that it could possibly continually adapt the switching frequency and duty-cycle to take care of a gentle voltage on output regardless of altering load or enter voltage.
Analyzing the buck (step-down) converters
For this review, four (four) totally different low cost Buck (step-down) modules have been chosen based mostly on what seemed like good design and had a USB port already built-in. Two totally different modules have been from Drok, one from SMAKN, and one other branded as Ailavi.
Trendy step-down converters have a number of elements to concentrate to:
- The switch-mode regulating chip
- The inductor
- The smoothing capacitor(s)
1) The switch-mode regulating chip
If the inductor is the brawn, then the switch-mode regulating chip is the brains of this entire operation.
Analyzing the four step-down converters revealed that there have been three totally different regulating chips in play. Right here’s a fast desk displaying the module and the regulating chip.
The XL1509 chip is designed and manufactured by a Chinese language firm referred to as XLSemi and has a comparatively low fastened switching frequency of 150kHz. It doesn’t use MOSFET transistors to deal with the high-side switching, however Bipolar Junction Transistors (BJT) as an alternative.
The MP2307 chip is discovered on two totally different modules: one of the Drok modules and in addition the Smakn module. It’s designed and manufactured by a California firm referred to as Monolithic Energy Methods (MPS). The MP2307 chip was first producer in 2008, has a 340kHz fastened switching frequency, and MOSFETs with excessive and low aspect switching resistance of 100 milli-Ohms. (fascinating!)
The MP2315 (additionally by MPS) was first manufactured in 2014, has a 500kHz fastened switching frequency, and MOSFETs with 90 milli-Ohm (excessive aspect) and 40 milli-Ohm (low aspect) resistance. (much more fascinating!)
So what do these specs imply?
The switching frequency is the speed which the MOSFET is turned on and off. As switching frequency goes up, effectivity goes down. It will be straightforward to assume that the XL1509 with its comparatively low switching frequency can be probably the most environment friendly, however that’s not the entire story. In case you learn the spec sheets on every chip, it seems the XL1509 is the least environment friendly of the bunch.
Why is the XL1509 much less environment friendly regardless of having a decrease switching frequency?
Nicely, a 150kHz switching regulator chip is usually solely about three to five proportion factors extra environment friendly than a 500kHz chip (all else being equal.) The converters utilizing the MP2307 and MP2315 chips have MOSFETs with low Rds(on) resistances of beneath 100 milli-Ohms. In comparison with the XL1509, which makes use of BJT transistors for switching, the MP2307 and MP2315 chips are capable of change (or cross) energy extra effectively regardless of having a better frequency.
What’s the importance of the Rds(on) resistance?
The resistance of the circuit when a MOSFET is switched on contributes to the facility loss of the conversion: The decrease the Rds(on) resistance, the higher. The MP2307 has 100 milli-Ohms for each excessive and low aspect switching. The MP2315 is a bit higher having 90 milli-Ohms and 40 milli-Ohms for top and low aspect switching, respectively. This averages out to about 65 milli-Ohms (given an obligation cycle of 50%) which is an effective deal decrease than 100 milli-Ohms.
The XL1509? Nicely, it doesn’t use MOSFET transistors, however makes use of BJT transistors as an alternative. BJTs are typically much less environment friendly for energy switching than MOSFETs. On prime of this, the XL1509 doesn’t have low aspect switching. The step-down circuit depends on a diode to passively carry out the position of low aspect switching which makes the conversion much less environment friendly than one which has some type of transistor for low aspect switching.
2) The inductor
The selection of the inductor (kind of) goes hand-in-hand with the frequency of the switching regulator chip. Greater frequencies require inductors with smaller values. For instance, the one Drok module has a 150kHz switching frequency and a 47µH inductor. The tiny Ailavi module has a 500kHz switching frequency and a small four.7µH inductor.
Because the four.7µH inductor has a decrease worth, it has much less power storage which is right for the upper switching frequency. However the actual benefits are that all the module might be constructed to be smaller and price much less — each critical benefits!
In case you’re unfamiliar with the time period “ripple” in phrases of electronics, it may be summarized as “unwanted variation” in DC present. A battery outputs a gentle voltage with no ripple, however a DC-to-DC or AC-to-DC conversion will all the time have somewhat ripple within the output voltage. A tiny quantity of ripple is ok, however vital ripple may cause instability in some DC circuits. Computer systems, radios, and cellphones are sometimes vulnerable to ripple relying upon how dangerous it’s and if the switching frequency is near the working frequency of stated system.
three) The capacitor
Fortuitously, giant capacitors can clean out ripple. Greater switching frequencies coupled with smaller inductors also can produce smoother ripple.
Wanting on the four modules on this review we will see good examples of this. The Smakn module has an enormous 1000µF capacitor. This could be capable of clean out any ripple nicely.
The Ailavi module has the very best switching frequency of the bunch (and in addition the smallest worth inductor as properly.) As such, it ought to have minimal ripple as properly. Because it already has minimal ripple, a lot smaller capacitors can be utilized to clean out any ripple remaining.
A fast overview of the four modules
The Drok with LED show module
The Drok module with the LED show is the primary module on this review. It includes a + and – screw terminal for straightforward wiring and a couple of buttons to regulate the operation of the unit. The highest button turns the show on and off. The button button allows and disables energy output. The LED show exhibits the enter voltage. It has a pleasant 47µF cap on the output rail and separate solder factors if the consumer wishes to attach a wire on to the outputs as an alternative of through the USB port.
The Drok with no show
The Drok module with no show is far smaller than its massive brother. There’s so much much less to this unit in phrases of options. Unusually, the design put the + and – rails proper subsequent to one another (as proven within the image.) This appears sort of unsafe in that it wouldn’t take a lot to a brief circuit. Apart from that it additionally has some small ceramic capacitors on the enter and output rails. General it appears properly designed.
The SMAKN module
The SMAKN module is a bit of a thriller as virtually the complete module is encased in a plastic enclosure. It has screw terminals just like the bigger Drok for straightforward connecting. It additionally has separate enter and output terminals for soldering. It additionally has a somewhat giant 1000µF capacitor on the output rail.
The Ailavi module
Yup, it’s actually that small relative to the opposite three modules. The + and – enter rails are unlabeled for some unknown purpose. Fortuitously, the module has a reverse polarity safety diode in case you by chance wire it up backwards. It additionally has 1 ceramic capacitor on the enter rail and a couple of on the output rail and an over voltage safety clamp. And an LED to point energy to the circuit. (A good contact.)
Testing the step-down converter modules
Voltage and voltage droop
Since all four of the step-down converters are designed to energy/cost USB units, the primary check was to examine the voltage out. Since they’re USB, they’re presupposed to be +5v (plus or minus 5%), in accordance with spec. Moreover, many energy provides could have their voltage droop when beneath load.
As may be seen within the above desk, the Drok with the LED show outputs precisely 5.00v and droops right down to four.93v volts underneath a 1 amp load which is properly inside USB specs.
The opposite Drok (with out the LED show) and the Ailavi each output 5.1 volts (quite common for USB energy provides) and droop to round 5.03 volts underneath a 1 amp load. This is a perfect design as the facility provide ought to keep inside USB spec underneath all load circumstances — even a 2 or three amp load.
The SMAKN module outputs 5.35 volts. That is technically an issue since USB spec permits for the voltage vary to be between +four.75 and +5.25 volts. The SMAKN module is exceeding spec by zero.10 volts. Even underneath a 1 amp load, the availability solely drops to five.33v.
Realistically, that is in all probability not an issue for many units. Many smartphones have their very own DC-DC conversion circuits on-board which may *in all probability* deal with 5.35 volts simply nice. All units on-hand have been capable of hand this voltage with none noticeable drawback. That stated, over-volting is extra more likely to injury a tool over time than under-volting it.
Studying the spec sheets for every chip advised that the XL1509 ought to get hold of round 80% effectivity, whereas the MP2307 ought to come near 90% and the MP2315 ought to hit 95% effectivity. Testing these claims was the subsequent order of enterprise.
To finish this check, a digital ammeter was positioned in-line with the enter to measure present draw. Then a USB digital ammeter was positioned on the output port to measure how a lot was being drawn out.
(Energy out ÷ Energy in) × 100 = Effectivity %
For this check, numerous widespread USB masses have been tried with a 12v supply:
- 250mA (finish of cost)
- 500mA (sluggish cost)
- 1000mA (quick cost)
- 1500mA (quicker cost)
The Drok with the LED show is the least environment friendly measuring between 60 and 75% relying on load. It’s most effective (75%) at 1000mA load. Not excellent.
The Smakn begins out poorly (65% effectivity) underneath mild load, however manages to hit 85% effectivity beneath a 1500mA load.
And the Drok with no show and the Ailavi are each extremely environment friendly beginning at ~80/81% effectivity underneath mild load (250mA) and hitting 86/87% effectivity underneath heavier load (1500mA).
The Ailavi with its barely newer chip edges out the Drok by a constant 1 to 2 %, besides at 1500mA the place they appear to be about the identical.
USB cost fee
Simply because a step-down regulator can technically provide sufficient present, doesn’t imply that your telephone or gadget will truly take that a lot. Because it seems, there’s signalling on the USB knowledge strains to point how a lot present a tool is allowed to take. Sadly, this isn’t constant throughout all units. Apple has three totally different signatures, Samsung has their very own signature, after which there’s the USB commonplace for indicating whether or not a tool is allowed to quick cost or not.
The signaling desk is as follows:
Supply: APC Magazine: Android USB charging secrets and techniques
As proven, Apple makes use of three permutations of 2.8v and a couple of.zero on the D+ and D- knowledge strains, whereas Samsung units each knowledge strains to ~1.2v, and the USB normal simply shorts the 2 strains collectively to point high-speed charging. Floating (un-grounded) knowledge strains appear to be universally understood as sluggish velocity (500mA) charging.
Testing the USB cost fee
And positive sufficient, every step-down conversion module has its personal particular method of signaling on the info line. And apparently sufficient, the units that we utilized in testing typically selected the very best present price that they might cost at and different occasions, they selected decrease present charges than what was out there. Here’s a desk of the precise measurements of the info line voltages:
The Drok with LED readout appears to point sluggish charging. The Drok with no show signifies Apple Quick charging, whereas the Ailavi units each strains to 2.eight which I’ve learn can imply 2400mA charging. The Smakn appears confused because the D+ rail is 3v and the D- rail seems to be floating.
When hooked up to units various things truly occurred, as summarized on this desk:
With testing concluded, it was straightforward to see that there was one clear winner:
The Ailavi DC-DC Step-Down Buck Converter 5V/3A USB Output Module ($7.25)
The Ailavi offers:
- Very best voltage (~5.05v) even underneath 1.5 amp load
- Helps steady present draw as much as 2.1 amps (three.zero amps max with heatsink.)
- Correctly alerts most cost fee to Android, iPhone, and non-phone units
- Pretty extensive enter vary: 6v – 24v
- Probably the most environment friendly (85 – 87%) @ 12v enter
- The smallest of all of them
The Drok with out the show was a reliable competitor being almost as small and almost as environment friendly (84 – 86%). Sadly, it solely appeared to sign most cost fee to Apple units correctly and it prices $eight.25 ($1 greater than the Ailavi).
The Drok with the LED enter voltage show was much less environment friendly (70 – 75%) and bigger than the remaining, however had the widest enter voltage vary of four.5v to 40v. It additionally appeared to sign 1000mA (quick) cost price to all units and often sells for $7.00.
The Smakn module is a humorous one as a result of it makes use of the identical chip because the smaller Drok module. And but, it doesn’t carry out as properly at decrease masses (65 to 75% effectivity). However at larger masses, it approaches 85% effectivity. Sadly, it doesn’t sign to units correctly, so both units like Android will cost slowly or like iPhone by no means. It additionally performs just a little “fast and loose” with the voltage kicking it as much as 5.35v. Whereas that is in all probability okay in apply, it’s technically outdoors of USB spec. It additionally sells for round $7.00.
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