The XL4015 from XLSEMI is a popular step-down (Buck) voltage regulator chip that is used in many cheap breakout boards.
Here are the key specs for this chip:
| Spec | Value |
|---|---|
| Input Voltage | 8V-36V |
| Output Voltage | 1.25V-32V |
| Max Output Current | 5A |
| Minimum Voltage Difference | 0.3V |
The XL4015 chip can handle a maximum of 5A only for output voltages below 10V. For higher voltages, the limit is 50W (75W with additional heat sink). Exceeding these values permanently destroys the board.
| Spec | Value |
|---|---|
| Switching Frequency | fixed 180kHz |
| Short Circuit Protection | yes |
| Constant Current Function | yes |
| Thermal Protection | yes |
| Max Junction Temperature | 125C |
| Design | PWM-Buck |
| Efficiency | up to 96% |
What does “Efficiency” mean?
Any voltage regulator converts a input voltage to a different output voltage, and this process has losses (causes heat). The efficiency states how much of the input energy is actually ending up at the output terminal, and how much is lost as heat:
An efficiency of 96% states that 4% of the energy is lost in heat. The amount of heat generated depends on the total amount of energy you feed into the regulator.
Examples
If you supply 12V and would like to get 5V and 3A, then these calculations apply:
- The total output energy is 5V x 3A = 15W.
- At 96% efficiency, 4% of input energy is lost. You need 15W x 100% / 96% = 15.625W at the input (and 0.625W dissipate as heat)
- When you drive the input with 12V, you need 15.625W / 12V = 1.3A
The XL4015 has a maximum efficiency of 96%. Efficiency is very different for different input and output voltage scenarios.
- When you reduce 24V to 12V and draw 4A at the output side (i.e. to run a car cooling box off a truck battery), efficiency is 93% (according to data sheet).
- When you reduce 12V to 5V and draw 5A at the output side (i.e. to operate USB devices from a car lighter jack or power a microcontroller), efficiency is just 87%.
Dissipate Heat
This illustrates another important consideration: heat sinks.
In the latter example, 5V @ 5A result in 25W power. The lost energy at 87% efficiency is ( 25W / 0.96) - 25W: 1.04W. So in this scenario, the voltage regulator also works like a 1W heater element. Make sure your housing and device design allows 1W worth of heat to be dissipated. If it is less, your device will continuously heat up until it breaks.
Affecting Efficiency
XL4015 efficiency is generally controlled by these scenarios:
- <200mA Output Current: when drawing currents less than 200mA, efficiency is worst and can drop to as low as 62%. This regulator is therefore not ideal to power small microcontroller circuits.
- >3A Output Current: Output currents above 3A hurt efficiency over-proportionally across all voltages.
- Voltage Difference: the higher the voltage difference between input and output, the lower the efficiency. Converting 36V to 5V drops efficiency to around 80% whereas converting 12V to 5V keeps efficiency above 90% most of the time.
- Very Low Output Voltage: the lower the output voltage, the less efficient. It is more efficient to reduce 24V to 12V (12V drop) than to reduce 12V to 5V (just a 7V drop, should be more efficient but the output voltage is very low
The XL4015 is best if operated in the mid-range. It suffers when you move towards its edge cases, either by drawing too little or too much current, or by using a very high input or a very low output voltage.
That said, even with a relatively lower efficiency, the XL4015 still performs well even in edge cases. Just don’t over-do it. The lesser the efficiency and the higher the current you draw, the better should be your heat sink.
Common Pitfalls
Here is a list of common misperceptions that can cause breakout boards to go off in flames:
- Maximum Current: The maximum current is 5A or the maximum handling power of the board - whichever is lower. The boards discussed here handle a maximum power of 50W (up to 75 when proper heat sinks are added).
- 50W allow a current of 10A at 5V output. The absolute maximum current for the XL4015 is 5A though. So at 5V output, the maximum current is 25W.
- At 24V output, 5A would be 120W which massively exceeds the 50W. To stay within 50W, at 24V output the maximum current is 2A.
- Short Circuit Protection: While the XL4015 comes with short-circuit protection, you cannot skip a fuse nor can you keep the output terminals shortened for an extended period of time. The XL4015 reduces switching frequency from 180kHz to 48kHz when short-circuited but will not disconnect the output. It will ultimately run into its thermal protection which wears down and can eventually damage the chip. Short-circuit protection is meant to protect from very short and momentary events.
- Input Voltage > Output Voltage: this is a Buck regulator. It reduces the input voltage. Once you set an output voltage, this limits the range of allowable input voltages:
- The regulator supports input voltages in the range of 8-36V. If you set the regulator to an output voltage of 12V, the input voltage now is in the range of 12.3-36V (add 0.3V to the output voltage: this is the minimum voltage difference found in the specs above).
What does “Switching Frequency” mean?
The XL4015 uses a fixed switching frequency of 180kHz which is considered to be relatively low.
Occasionally vendors claim a switching frequency of 300kHz. This frequency refers to the XL4005 regulator. The XL4015 switching frequency cannot be modified and is always 180kHz during normal operation.
Benefits of Low Switching Frequency
Low switching frequencies must store more energy per switch so they must increase component size (for coils and inductors so that they can actually store more energy) which increases board size, weight, and cost.
At the same time, this can improve efficiency though, especially when larger currents are required: each time energy needs to be transferred back and forth, losses occur. With lower frequency, losses happen less frequent.
Another positive effect of low switching frequencies is less EMI (electromagnetic interference). Using regulators with low switching frequencies like the XL4015 are well-suited for DIY projects where makers seldom pay attention to proper EMI protection, and where miniaturization is not critical.
Benefits of High Switching Frequency
There are advantages for high switching frequencies, too: Regulators with high switching frequencies (up in the range of several MHz) are much smaller.
They need to switch only very small amounts of energy because due to the high frequency, they switch much more often.
This can result in less output voltage ripple. For the same reasons, regulators with high switching frequency can respond faster to changes in load and output impedance. In general, high switching frequencies produce more stable output voltages (steady-state). They are ideal for highly integrated devices such as smartphones.
Pins
Usually the XL4015 is part of a ready-to-go breakout board that comes equipped with all the other necessary supporting components like coils, capacitors, and MosFETs.
You can of course also use the chip by itself and tailor your own Buck converter:
| Pin | Name | Description |
|---|---|---|
| 1 | GND | Make sure you connect it outside a supporting Schottky diode to prevent switching current spikes to backfire as noise into the chip |
| 2 | FB | Feedback. Typically a resistor voltage divider senses the actual output voltage and reports it through this pin (feedback threshold voltage is 1.25V) so that the chip knows whether adjustments are needed |
| 3 | SW | Output positive voltage |
| 4 | VC | Connects a 1uF bypass capacitor to VIN |
| 5 | VIN | Input positive voltage. Use large capacitor to GND to control noise |
CV 50/75W Buck Converter
Even though the XL4015 comes with constant current functionality built-in, some breakout boards do not utilize this functionality and provide constant voltage (CV) only. You can easily recognize these boards by the one trim potentiometer (instead of two):
This Buck converter is perfectly suitable to supply power to your DIY devices as this typically requires a constant voltage only. Its newer revisions come with decent EMI (electromagnetic interference) protection, too.
Whenever your use case requires constant voltage though (i.e. to drive LED or use it as a battery charger), this breakout board is unsuitable.
Hardware Design
These very simple Buck converters feature through-hole connectors for input and output power on both ends, and a printed arrow on the back side indicates the direction of power flow.
Often, you can get these converters for cheap in packs of 3 or 5 from places like Amazon.
A small LED beneath the trim potentiometer lights up when powered.
Setting Output Voltage
To set the desired fixed output voltage, connect a multimeter to the output, set its range to the 200V range (or any range that safely covers your input voltage), then connect the input power.
The multimeter shows the actual output voltage. This voltage cannot be higher than your input voltage.
Now turn the trim potentiometer in either direction and watch the voltage change at your multimeter.
Specs
| Feature | Value |
|---|---|
| Input | 4-38V |
| Output | 1.25-36V |
| Maximum Current | 5A |
| Maximum Power | 50W |
| With Heat Sink | 75W |
| Reverse Voltage Protection at Input | yes |
| Reverse Voltage Protection at Output | no |
| Reverse Polarity Protection | no |
(always double-check the data sheet or manual of *your board. There are many different variants available that all look and behave similar but not necessarily identical)*
These boards work best when not pushed to their limits. When you test them with various loads and voltages, simply make sure they do not heat up.
CV CC 50/75W Buck Converter
This is a very popular breakout board that is available in a number of slight variations. Boards work fundamentally the same. Here are the differences:
| Item | Difference |
|---|---|
| Board Color | red, black |
| LED Colors | red,green,blue / red,red,blue / all red |
| Schottky Diode | TS1010, SS54, SS56 |
| LED description printed on backside | yes / no |
| Coil placement | upright / flat |
Hardware Design Review
All board variants use the same fundamental components.
XL4015 Buck Regulator
The five-leg XL4015 main regulator chip is located close to the two input screw terminals.
Next to it and close to the input terminal, is one of two electrolyte capacitors with a printed voltage. This capacitor is connected directly to the input power, so make sure the printed maximum voltage tolerance is higher than the maximum input voltage.
Schottky-Diode
In-between the capacitor and the coil is a Schottky diode located that provides reverse voltage protection for the input terminal.
There is no reverse voltage protection diode for the output terminals. If you plan to use the board as a charger you should add a Schottky diode to the output. Else, the board can drain connected batteries when not powered.
The type of diode varies among board variants (TS1010, SS54, SS56). All of these diodes support the maximum current of 5A.
LM358 OpAmp
At the side of the two output terminals, a LM358 operational amplifier is found. It is used to implement the constant current functionality.
78L05 Voltage Regulator and TL431 Voltage Reference
Next to the OpAmp, you find a 78L05 voltage regulator that provides a stable 5V power source capable of 100mA.
Next to the voltage regulator, a TL431 provides a programmable voltage reference.
Shunt-Resistor
On the backside, a R050 shunt resistor is mounted that is connecting the negative input and output terminal. The voltage drop across the shunt is connected to the XL4015 feedback pin to control the output voltage.
Should the back of your module be dirty with residue like in the picture, clean it with isopropanol alcohol before first use.
Using the Module
Status LED
The board comes with three LEDs, two located at the output terminals, and third one next to the XL4015.
- Full: the (green or red) LED closest to the output terminals lights up when the input terminals are connected to power but no load is drawing energy from the output terminals. This is the equivalent of a battery being fully charged and no longer drawing energy.
- Charging: the (blue or red) LED next to the latter lights when there is a load connected to the output terminal that draws current. This is the equivalent of a battery being charged.
- Constant Current: the red LED next to the XL4015 lights up when the constant current mode is active. This is the case whenever the output current is going to exceed the maximum current you set with the trim poti closer to the output terminals.
The original board design comes with three differently colored LEDs and is clearly preferrable. You can identify it by the printed LED description on the board back side. The next best choice is the black board from Tenstar that uses one blue and two red LEDs. All other tested boards use all red LEDs which are much harder to interpret.
Setting CV and CC
To set the desired constant voltage and constant current, there are two blue multi-turn 10K (W103) trim potentiometers:
Setting Constant Voltage
The trim pot next to the XL4015 controls the constant voltage: turn it clockwise to increase the voltage. The maximum voltage cannot be higher than the input voltage you are supplying.
Always set the constant voltage first. This sets the maximum possible output voltage.
To set the constant voltage, connect the input to a power source, and connect the output to a multimeter in Voltage mode. Make sure the multimeter voltage range is high enough to measure voltages up to your input voltage.
The multimeter should display the actual voltage at the output terminal now.
If this voltage is equal to the input voltage, turn the trim pot counter-clockwise until the multimeter starts to display lower voltages. It can take multiple counter-clockwise turns of the trim pot until you see the output voltage change.
Setting Constant Current
The trim pot next to the output terminals controls the constant voltage: turn it clockwise to increase the current.
Do not short-circuit the output terminals to set the constant current. The XL4015 is not short-circuit proof for long periods of time and can be destroyed this way.
To set the constant current, turn the trim pot all the way counter-clockwise until each new turn just produces a click sound. This sets the constant current to the lowest setting. Then connect the input to a power source, and connect your load to the output terminals.
The red constant current LED next to the XL4015 lights up. Turn the trim pot now clockwise until your load (i.e. LED) receives sufficient power. If your load is over-current sensitive (like LEDs) you may want to add a multimeter in Ampere mode in series to your load to read the actual current.
If your load is not sensitive towards over-current, after you connected it to the output terminals, simply turn the trim pot clockwise until the red constant current LED turns completely off (no flicker or dim light). Now the current is limited to just a bit over the current that your load currently draws.
If you own an electronic load, setting constant current can be extra safe and precise: you can set the constant current before you actually connect any real load, and the electronic load provides you with precise information about the current.
Connect your electronic load to the output terminals. Next, set the load to the desired current (never ever exceed the board specs: 50W or 5A, whichever is lower). Turn on the load now.
It draws precisely the set amount of current now. If the constant current LED next to the XL4015 is on, turn the trim pot clockwise until the LED switches off completely (no flicker or dim light). If the LED is off, turn the trim pot counter-clockwise until the LED is on, then turn it clockwise again until the LED is completely off.
How do Constant Voltage and Constant Current Really Work?
A Buck regulator like the XL4015 is a voltage regulator that can do just one thing: lower the voltage below the input voltage.
- Constant voltage keeps the output voltage strictly fixed at the voltage you set.
- Constant current is also controlling the voltage, but not to a static value: while constant voltage sets the voltage to one fixed value no matter what, constant current is automatically decreasing the voltage until the current drops below the threshold.
From this it becomes evident that there can only be one mode active at a time: either constant voltage, or constant current.
Typically, constant voltage is active by default, and with it you set the upper voltage limit: the voltage will never be higher than this limit. Constant current can lower the voltage, though: whenever the current exceeds the threshold you set, lowering the voltage lowers the current.
Version With Display Shield
There is also a special version of this breakout board available that comes with a mounted display shield to display the voltage and current.
The display shield uses a modified Buck board design where the trim potentiometers can be adjusted from the side (instead of from top).
Using the Display Shield
Connect input power and load as you would with any other board. The display shield is active automatically and shows voltage and current. The shield is electrically connected to the Buck regulator board via the four screws that mount the shield on top of the regulator board. There are no wires to connect.
The display shield comes with a two- and a three-pin connector next to the LED displays. These connectors are without function. You can remove them if they are in the way.
Using For Charging
Specs
| Feature | Value |
|---|---|
| Input | 5-32V |
| Output | 0.8-30V |
| Maximum Current | 5A |
| Maximum Power | 50W |
| With Heat Sink | 75W |
| Reverse Voltage Protection at Input | yes |
| Reverse Voltage Protection at Output | no |
| Low Battery Protection | yes |
| Reverse Polarity Protection | no |
| Size | 26.5x51.5x14.0mm |
(always double-check the data sheet or manual of *your board. There are many different variants available that all look and behave similar but not necessarily identical)*
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(content created Mar 09, 2024)