Control Engineering

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How does a buck regulator work?
March 31, 2008

A buck regulator, more properly called a buck convertor, is a dc-dc step-down power supply utilizing the fact that inductors react to electric-circuit fluctuations in such a way to keep the current flowing through them constant. This inductor property follows directly from Faraday’s Law: where I is the current flowing through the inductor, t is time, and E is an electromotive force generated by changes in the magnetic flux threading the inductor’s coils. In other words, whenever the current through the coil changes, that change modifies the magnetic field surrounding the coil, which changes the strength of the magnetic field passing through the coil’s interior, which induces an electromotive force (voltage) to oppose the change in current.

The buck convertor circuit makes use of this so-called “back EMF” to keep current flowing to the output after the electricity source has been temporarily disconnected. In the circuit below, controlled switch S connects a relatively high voltage dc source across diode D. Since D is back biased, no current flows through it, but current begins to flow through the inductor H to the load.

Because of the inductor’s back EMF, the current cannot rise instantaneously. Rise it does, however, as the source pumps energy into the inductor’s magnetic field. It rises asymptotically toward the current the load would draw if there were no inductor there with a time constant equal to the ratio of the inductance to the resistance. At the same time, the voltage across the load rises asymptotically toward the source voltage. 

When switch S is closed, current flows from the dc source through the inductor to the load. When opened, energy flows out of the magnetic field to keep current flowing to the load.

But, we don’t wait that long. This is a voltage divider circuit, so we open the switch when the load voltage reaches slightly over the target voltage.

Opening S breaks the circuit so no current can flow in the left-hand loop. The inductor, however, doesn’t want to let the current stop. As the current tries to drop, the inductor switches to a forward EMF to keep it going. The diode sees a forward bias and begins to conduct, completing the right-hand loop. The inductor now becomes the energy source by tapping the stored energy in its magnetic field. 

Load current rises when switch S is closed, and falls when S is open, giving a control circuit a way to hold the current within a tolerance band around a target value.

That stored energy, however, is finite, and tapping into it causes the magnetic field, and the current it supports, to sag. The load resistor, of course, doesn’t care where the current comes from. It just provides a voltage drop proportional to that current. As the inductor current sags, the load voltage sags in proportion.

When the load voltage sags below the target, we wake up and close the switch again. It reenergizes the circuit quickly, and as soon as the load voltage recovers past the target, we open S again.

This can go on forever. The average voltage is very close to the target voltage, while the excursions above and below the target can be as small as our ability to rapidly open and close the switch allows.

If the load starts drawing more current, we have to leave S closed a bit longer to make up for the extra loss. If the load becomes less demanding, we close S sooner.

Being smart controls engineers, we use feedback to control when S opens and closes, and a fast acting semiconductor switch for S. We might, for example, use an op-amp comparator as a trigger to close S when the voltage sags below a lower limit, and a second to open it when the voltage rises above an upper limit. For a switch, we might use a silicon controlled switch.

Adding a simple feedback circuit automatically controls the buck convertor. When load current sags below target, the green comparator switches high, toggling the SCS closed. When current rises above target, the red comparator goes low, toggling the SCS open.

This might seem to be a lot to go through for a voltage step-down regulator, but notice that the only dissipative element in the whole circuit is the load. All the other circuit elements are always biased on or off, dissipating minimal power in either state. The buck regulator is thus one of the most energy efficient voltage step down circuits available. 

Posted by Charlie Masi on March 31, 2008 | Comments (0)