Power Supplies Tutorial No.6 (Switch-mode Buck Converter)

23.06.18

Buck Converter

The Buck Converters is a type of switch mode power supply that can be used to convert DC voltages from a higher voltage to a lower one. The Buck converter does this in a much more effecient manner than Linear regulators. I have put together a video on the subject but more information is below as well.

 

First of all, lets take a look at a simplified circuit. As the name "switch mode" indicates, the power supply operates in a switching mode, rather than dropping the excess voltage across a device like a linear regulator. The basic components are: the switch - normally a BJT or MOSFET, an inductor, a Diode and a Capacitor as shown below.

If we look at the operation with the switch closed first.

With the switch on the current flows from the battery, through the inductor and through the load resistor. As the current flows through the inductor, it generates a magnetic field in the inductor. The capacitor is also changed by the battery in this stage. The diod is reverse biased, so does nothing at this point in time.

Now lets look at when the switch is opened.

In this situation the magnetic field in the inductor begins to colapse. This generates a back EMF with positive to the right of the induction. The current flows from the induction, through the load resistor and now through the positively biased diode, back to the inductor. As th emagnetic field colapses the back EMF voltage reduces and at some point the capacitor will start suppling current to the load as well.

So the premise is, if the switch is cycled frequently enough, the load will be provided with a contsant smoothed voltage and by varying the duty cycle for the switching we can control the actual voltage.

So lets look at how the voltage could be stabilised.

In this version, we see the same basic circuit, however we have added the control component. This consists of an error amplifier and driver circuit. The input of the control component is the actual measured output voltage and this is used to vary the duty cycle of the switch. Pretty simple hey!

 

So next lets look at a real world example.

Here we have the LM2575-5.0 Buck Converter IC. We can see the normal diode, inductor and capacitor from out earlier circuit, and in the case of this device, it contains the controller and the switching element. You can also see the feedback signal being taken directly from the output voltage.

If we take a look at the internals of the IC we can even work out how the control component works.

If we look at Vin we can see it feeds across to a BJT (transistor) which is the switching device. It also feeds a internal regulator for the chip itself.

If we next look at the Feedback pin, we can see it feeds directly to a voltage divider formed by two resistors, with the voltage divider output is feed to an error amplifier. The other imput of the error amplifier comes from a nice stable internal voltage reference. In effect the error amplifier is comparing the output voltage to a standard to determine where it needs to be increased or decreased and setting its output accordingly. The error amplifier feeds one imput of a comparitor.

The other comparitor imput comes from an oscillator. This oscillator waveform will be a ramp, so the comparitor will turn on when the error output is higher than the ramp waveform. This is a simple way of producing a pulse width modulated waveform based on the feedback voltage.

The compator pulses are fed via a NOR gate and driver to the actual switching transistor. There is other control imputs for current and thermal shutdown however that is the basis of the controller workings.

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