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EEVblog #90 - Linear and LDO regulators and Switch Mode Power Supply Tutorial

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migsantiago:
Hi

I just saw this tutorial and there's something I didn't understand. In the step-down regulator, how does the diode provide current from GND? There's no negative source, how come the current can flow in that way?



Another question, in the linear regulator an ampop is shown configured as a comparator. If REF is greater that the voltage at the resistor, the ampop will output a Vss voltage (Vss as in +V supply). If the voltage at the resistor is greater than REF, the ampop will output 0V. How can the NPN transistor be regulated if there are only 2 different voltages that the ampop can output?



Thanks!!

jahonen:
In the step-down regulator, it is the inductor which provides the continuous current when the switch is off. Current is continuous in the inductor. Of course, the current will slowly decay to zero. Diode just provides the path for the current. You can see that arrow in the diode schematic symbol is in the same direction than the current in the loop, pointing to the inductor.

In linear regulator, opamp does not behave as a comparator, it "finds" linear operation point (assuming that loop is stable). Another way to say is that linear power supply is just error amplifier followed by high-current follower.

Regards,
Janne

s3c:

--- Quote ---In the step-down regulator, how does the diode provide current from GND? There's no negative source, how come the current can flow in that way?
--- End quote ---

When the transistor switches off there is some energy stored in the inductor, this energy can't just dissipate and has to go somewhere, recall that an inductor tries to avoid changes in current so it will try and up the voltage till current can flow in the direction it was flowing in before the transistor turned off. This will create the needed voltage drop across the diode to put it in the forward active region and complete the loop.


--- Quote ---If REF is greater that the voltage at the resistor, the ampop will output a Vss voltage (Vss as in +V supply). If the voltage at the resistor is greater than REF, the ampop will output 0V. How can the NPN transistor be regulated if there are only 2 different voltages that the ampop can output?
--- End quote ---

Recall that real op amps can't change their output instantaneously (look up slew rate), it would be more accurate to say that when REF is greater than the feedback voltage it increases it's output and decreases otherwise, this is a continual process that stabilizes around the output point that gives equilibrium.

s3c:
I'm new to switching regulators so could anyone comment on the frequency and waveform shape used by the oscillator? I'm assuming the frequency has something to do with the LC circuit?

migsantiago:
Inductors have always been a mistery to me. I quite don't understand how they can store energy just by being a single cable assembled into a loop. But science is science and I believe it hehe  ;D

Thanks Janne and S3C for your explanations!  ;D

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