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Role of Schottky Diode in XL4015 module

eagle1109

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Hello,

I'm wondering the role of a Schottky diode on the output of XL4015 buck converter module. I opened the datasheet of this diode and found different features and could of course have different uses. Now on this buck converter, what would be its role ?

XL4015_2 - Copy.png
 

betwixt

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Explaining as simply as possible, it fills in the gaps between current pulses from the XL4015.

When the internal switch between VIN and SW is closed, current flows straight through , then through L1 to the output. In doing so, a magnetic field builds up around the core of L1. When the internal switch opens the SW pin becomes disconnected inside the XL4015 and with no current passing from it, the magnetic field in L1 collapses. The polarity of voltage across L1 is reversed from then the field was created, this makes D1 conduct and the energy from the magnetic field is added to VOUT. All this happens very fast, the XL4015 switches at 10s of KHz. The diode is a Schottky type because unlike normal diodes, they store very little charge across their PN junction and that allows then to start and stop conducting very rapidly. Obviously this is essential when the switch is operating so fast.

Brian.
 

Easy peasy

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The diode, that diode is an essential element in a buck converter, when the driving switch is OFF, the established current in the buck choke still has a path to flow, through this diode, thus keeping the diode current continuous & preventing large voltage stresses across Vin and the switch o/p if the diode is not there - due to the ability of the buck choke to raise its voltage ( to quite high levels ) to keep its current flowing if it is interrupted ...
 

eagle1109

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it fills in the gaps between current pulses from the XL4015.
so the XL4015 is like the switcher IC that drive the MOSFET at hot area in SMPS. It provides the chopping voltage for current/voltage regulation.
And this one is more compact than the ones in SMPS units.

So now it chops the Vin to the Vout in PWM, there are gaps, how it fills the gaps ? there have to be gaps in PWM signals.

When the internal switch between VIN and SW is closed, current flows straight through , then through L1 to the output. In doing so, a magnetic field builds up around the core of L1. When the internal switch opens the SW pin becomes disconnected inside the XL4015 and with no current passing from it, the magnetic field in L1 collapses.
Yep, I know this sequence of charging the inductor and that it charges up with current and the magnetic field collapses in a reverse direction.

In this point I have a concern of what's the difference between the discharge voltage of the inductor vs capacitor ?

I know capacitor hold the voltage between metallic plates and it can hold the voltage charge for some time up to minutes or hours.
But my understanding of the inductor that, it's of course different than the capacitor, the electromagnetic field builds up between the rounds of wires, when the voltage or current source is disappeared, this electromagnetic field collapses in reverse direction according to the polarity of charge across it same like the capacitor.

The polarity of voltage across L1 is reversed from then the field was created, this makes D1 conduct and the energy from the magnetic field is added to VOUT.
My question here, is where the current passed through D1 goes ?
Also I didn't understand "the energy from the magnetic field is added to VOUT". Since the charge across the inductor has gone through the diode, which energy is added to the output ? Is it the voltage on the Cout ?

All this happens very fast, the XL4015 switches at 10s of KHz. The diode is a Schottky type because unlike normal diodes, they store very little charge across their PN junction and that allows then to start and stop conducting very rapidly. Obviously this is essential when the switch is operating so fast.

Brian.
Yep, got this part, so normal diodes are used in SMPS bridge rectifiers, because they run at 60Hz, but when considering frequencies more than kHz the Schottky diodes are the one that can be counted for.

Also this means I can use certain components in a wide range of operating frequencies below its max capable frequency.
--- Updated ---

thus keeping the diode current continuous
Why to keep the diode current continuous ? where should the diode pass a current ? from the circuit diagram, the current should go to ground rail.

due to the ability of the buck choke to raise its voltage ( to quite high levels )
who would the choke raise the output voltage if it has a collapse current in reverse direction ?

to keep its current flowing if it is interrupted ...
what would interrupt the current ?
 
Last edited:

Akanimo

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Wow! Lots of questions here. Will attempt to answer the much that I have time to do so.

About inductor current interruption:
During the ON time of the PWM, the main switch is turned ON. During the OFF time, the main switch is turned OFF. Turning OFF the main switch during this PWM OFF time interrupts the current that was flowing through the inductor during the ON time.

About "...the current should go to the ground rail":
Notice the polarity of the Schottky. It is conditioned to conduct only in the forward bias.

About "...is where the current passed through D1 goes ?":
For this also check the diode polarity and the polarity of the inductor voltage during the OFF time. For there to be current flow, there must be a closed loop. At any given instance, during the OFF time, the current flowing through the diode is the same current flowing through the inductor. For simplicity, we can say that the charges that constitute this current are dumped into the output capacitor (and load). This should, hopefully, answer the question on "the energy from the magnetic field is added to VOUT".

About "...what's the difference between the discharge voltage of the inductor vs capacitor ?":
At steady state, during the PWM OFF time, the voltage across the inductor would be approximately equal to the voltage across the output capacitor.

Overall, the Schottky is forced to conduct during the PWM OFF time so as to provide a complete path (or loop) for the inductor current (due to the collapsing magnetic field) to flow.
 
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