Buck or Step Down Converter Basics

Buck or step down converter is switching mode power supply (SMPS) DC-DC converter.

Go to buck (step down) DC-DC converter simulation.

There is widespread request for power supply voltage adjustment for particular needs. For example, if there is micro processor with supply voltage of 3.3V and we have voltage source of 5V or 12V, somehow this input voltage has to be stepped down to required 3.3V. One and not so practical way to achieve this is to use linear regulator for decreasing input voltage. Trouble with linear voltage regulator it that all load current is flowing through regulator, dissipating a lot of power. Approach in switching converter is to supply the load with power pulses. Look at the figure bellow.

buck_fig01

With this simple scheme, just by controlling duty cycle, it is easy to control power delivered to the load. For example, if duty cycle is 50%, it means that only for 50% of time input voltage is connected to the load. So average output voltage in this case is

buck_eq01

This solution is acceptable only if we need to deliver less power to the load, without concerning about voltage level. If bulky capacitor would be attached to the load, output voltage would be less and more flattened but still very far from desired all flat DC output. And yet, during “ON” time, there would be high current peaks, since voltage source would be directly connected to the capacitor. One passive and non dissipative element should be putted between active switch  and the load with the capacitor. This non dissipative element that can limit the current is the inductor. The scheme now looks like this.

buck_fig02

We are nearly done, but not yet. Suggested circuit wouldn’t work, since as soon as main switch would be turned “OFF”, it would meant that inductor current would be cut-off. It isn’t possible, and according to Lentz law, huge negative voltage would be induced distracting the switch. What we need is just a clamping diode to provide discharging path for inductor current. Finally, we have basic buck or step down converter in figure bellow.

buck-dc-dc-converter

Now during “ON” time current flows from voltage source to the load through inductor with increasing slope. When main switch is “OFF”, diode is clamping and provides current path, and current still flow to the load, but with decreasing slope. If next “ON” comes before inductor current drops to zero, buck converter is operating in so called continuous conduction mode (CCM). Otherwise, buck converter is operating in discontinuous conduction mode (DCM). Both modes are presented in figure bellow.

ccm_dcm

If buck DC-DC converter operates in CCM, output voltage is correlated with input voltage with

buck_eq02

In here, D is duty cycle defined (for fixed switching frequency) as “ON” time over T ratio, where T is switching frequency period.

buck_eq03

So, output voltage of step down converter can be controlled almost linearly, just by adjusting duty cycle D.

Controlling of DC-DC converters is wide subject that will be presented later on this site in details.

Go to buck (step down) DC-DC converter simulation.

External links:

Buck Converter Basics on Wikipedia
Buck Converter Basics on services.eng.uts.edu.au
Buck Converter Basics on Analog
Buck Converter Basics on Linear

Posted in electronic

Leave a Reply

Your email address will not be published. Required fields are marked *

*