eagle1109
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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.
thus keeping the diode current continuous
who would the choke raise the output voltage if it has a collapse current in reverse direction ?due to the ability of the buck choke to raise its voltage ( to quite high levels )
what would interrupt the current ?to keep its current flowing if it is interrupted ...
is a good answer ...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
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".
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.
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.
Recall that if you charge an inductor, the magnetic field created is in such a direction that when it collapses it induces a current that is in the same direction that the charging current was. So in your bottom diagram, the inductor current is supposed to flow in a direction opposite to what you indicated.
Another way to look at it is that the MOSFET is turned OFF during the OFF time and the so the voltage source is disconnected and the inductor becomes the source as it discharges. Current flows from the positive polarity of the source into the load.
I'm sorry, I didn't get the question....
But in AC SCR circuits where the load is inductive one, there showing negative spikes with the output signal ? so how to differentiate that with the case of DC chopping with inductive element ?
...
It will be useful to remember the basic ideas about capacitors and inductors:
1. For an inductor, const current casues zero voltage drop across it.
2. For a capacitor, const voltage across it causes zero current to flow.
Is it the lagging of voltage when connecting an inductor to AC circuit ?3. For an inductor, if the current is increasing, it acts like a voltage source (with a negative sense). Voltage sources have zero resistance.
the current is sourced from negative to positive, right ?4. For a capacitor, if the voltage is increasing, it is acting like a current source. Current sources have infinite resistance.
is the voltage supplied to the load al the same voltage that caused the inductor to be filled with current ? or it is a spike voltage that x100 the source voltage ?5. When the current is falling in an inductor, it is a voltage source with a positive sense (can supply positive voltage to a load).
6. If the voltage is falling across a capacitor, the capacitor acts like a current sink.
If the voltage across an inductor is const (non zero), the current increases to infinity.
If a capacitor is fed with a const current (non zero), the voltage across it increases to inifinity.
A circuit with a capacitor OR an inductor must have a resistance in series to limit current or voltage.
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