Modifying XL4015 Buck Converter with an Adjustable Current Limiter

The post explains a simple way to enhance the the XL4015 DC to DC buck converter with an adjustable current limiter, which seems to be missing in the original module.

About XL4015

The XL4015 is a 180 KHz fixed frequency PWM buck (step-down) DC/DC converter, specially designed for operating a 5 V, 5 Amp load with good efficiency, minimal ripple and exceptional line and load regulation.

Built using very few number of additional parts, the regulator module is easy to work with and consist of built-in frequency compensation along with a fixed-frequency oscillator.

The PWM control circuit features adjustable duty ratio at a constant rate from 0 to 100%. The IC XL4015 also features an in-built over-current protection functionality.

When a short circuit is detected at the output, the operating frequency is instantly lowered from 180 KHz to 48 KHz, causing an immediate drop in the output voltage and current.

The chip has a fully integrated compensation block, without depending on any external components.

XL4015 IC Main Features

1. Broad 8V to 36V Input Voltage Range
2. Output voltage is Adjustable from 1.25V to 32V
3. Maximum Duty Cycle can be as high as 100%
4. Output Drop-Out is merely 0.3V
5. Switching Frequency is fixed at 180 kHz
6. Output current is constant at 5A.
7. In-built Power MOSFETs ensure high voltage/current optimization
8. Operating efficiency is very impressive at 96%
9. Line and load regulation are extremely good
10. IC features an internally controlled thermal shutdown function
11. Likewise it also features an in-built current limit function
12. Needless to say, the chip also includes an output short protection feature.

Major Disadvantage

Although the XL4015 module is loaded with many excellent features a buck converter needs to have, it lacks one major facility.

The module has no arrangement for adjusting the output current to preferred levels, as per the load specifications.

So if you want to charge a Li-Ion battery with a XL4015 module, say at 2 amp rate, you won't be able to do that, due to the above mentioned drawback.

Similarly, if you wanted to drive a 3.3 V LED at 3 amp maximum current rate, you'd be disappointed likewise, since the module is rated at a fixed 5 amp current.

How XL4015 Works

The basic working schmetaic of the XL4015 buck converter is shown below:

The circuit is configured to produce a fixed 5 V at a constant 5 amp current output in response to a supply input of 8 V to 36 V. The input power specifications has to be higher than the output power, meaning the input supply wattage capacity must be higher than 5 V x 5 A = 25 W.

Therefore, if an input supply of 36 V is used, then the input current should be higher than 25 / 36 = 0.7 Amps. If 8 V is used then the input current may be higher than 25 / 8 = 3 Amps, and so forth.

The internal circuitry of the IC XL4015 consists of the basic elements such as an oscillator and an error amp. The well calculated and controlled 180 kHz oscillator frequency is generated at pin3 (SW) for feeding the external buck converter configuration consisting of the diode, inductor, and the capacitor. This enables the buck stage to process the input supply to a precise 5 V, 5 A output.

The pin2 (FB) functions as the input for the error amp feedback. A minimum of 1.25 V input at this pinout is enough to begin the shut down process for the IC.

This pinout can be seen configured with a potential divider R1, R2, which ensures that the output voltage can never go beyond the 5 V range, which then causes a voltage higher than 1.25 V to develop at the FB pin initiating the shut down process for the IC, thereby preventing the output from crossing the 5 V level.

This also implies that the output voltage could be adjusted to other voltage levels, such as 12 V or 15 V, by suitably varying the R1/R2 feedback divider values.

The R1/R2 can be also fixed using the following formula, for getting the desired output voltage:

Vout=1.25 x (1+R2/R1)

Current Limit Adjust

As we can see from the schematic the XL4015 module does not include a current limiting feature which apparently is a major limitation of the module.

However, the module does include a shut down pinout FB that could be configured with an external current limiter circuit, for accomplishing the feature. This could be implemented as indicated in the following diagram:

The RX may be calculated using Ohm's law:

RX = 0.2 / Current Limit

Since the two transistors are wired with a very high gain output, a potential difference of just 0.2 V across RX should be enough to trigger the FB pin of the IC and initiate the current limiting action.

As soon the current tends to exceed the desired limit, causes the required minimum potential to develop across RX causing the NPN to conduct, which in turn triggers the PNP BJT hard. The action supplies the intended the positive DC on the FB pin, initiating the shut-down.

When this happens the output current drops below the set limit, turning OFF the BJTs and restoring the earlier condition, wherein the current yet again begins exceeding the set limit switching ON the BJTs. The cycle keeps repeating, ensuring the current always remains within the set limit.

With this arrangement, the XL4015 becomes equipped with the very useful adjustable output current limit feature.

XL4015 Alternative (Equivalent Circuit)

Although, the XL4015 module is easily available from most online stores, the IC is not manufactured by reputed brands, and may be prone to becoming obsolete anytime.

Therefore, having an alternative 5 V adjustable buck converter circuit using discrete components appears to be a much better option.

The following diagram shows a very efficient 5 V buck converter using the popular TL494 chip:

The example above shows a simple yet extremely handy, precision 5 V buck converter equivalent for the XL4015.

Here, it shows a solar inverter buck converter application, which can be adopted for any other desired DC to DC converter purpose.

The use of TL494 ensures that the design will not get obsolete easily and the replacement for the IC will be readily accessible whenever required.

Here too, an error amp feedback loop determines the output current by setting up the potential divider network built around R8/R9.

The current can be adjusted by tweaking the R13 resistor appropriately.

R13 = 0.2/Max Current Limit

Another great advantage of using the above discretely built buck converter is the output current level, which is not limited to 5 amps, rather could be upgraded to much higher levels simply by upgrading the transistors, the inductor wire thickness and the R13 resistor value.