What does it mean to design a Buck converter with frequency of 100kHz?

[innovationhut]

Hi,

Just a newbie question, I am getting confused!

Q: When somebody says that design a Buck converter with the switching frequency of 100 KHz, what does that mean?

 

[betwixt]

Re: Frequency

A. There are two basic types of converter, 'buck' and 'boost'. They work in the same way but buck is used when the output voltage is lower or the same as the input voltage and boost is used if it the same or above.

The idea is that the current path is switched off and on again very rapidly, that's where the 100KHz comes from, it's the 'chopping' rate. When switched on, some of the power is stored in an inductor as a magnetic field (it is possible but less common to use capacitors for storage too). When switched off, the energy released from the magnetic field is used to provide the output power.

In buck converters the energy released from the inductor is used to 'fill in the gaps' while the switch is off so it acts as a reservoir. In a boost converter, the energy is is added in series with the incoming supply so it adds to (boosts) the output voltage.

The output of these converters can be controlled by adjusting the relative on and off times of the switch so it provides a way of regulating the output. Normally the switch runs at a constant speed (100KHz in your question) but each cycles on and off period may be varied to set the output voltage. For example if the on and off times were equal it might produce half power, making it 25% on and 75% off would give less power.

The frequency is a compromise, if it is higher, the inductor and capacitor sizes become smaller so it is more compact and probably cheaper, but the demands on the switch components become more of a problem. Most converters use 25KHz to 150KHz.

These converters are very efficient compared to linear regulators because the switch is used in two states, off in which no current flows so no heat is produced and on in which the voltage dropped across the switch is almost zero so no heat is produced. Using Ohms law, a switch would dissipate I x V Watts and if the switch was perfect either I or V would be zero.

Brian.