In the context of control loop stability of DC-DC Converters,
A switch mode power supply is essentially a sampled-data system, therefore the theoretical maximum bandwidth is one half the switching frequency. Practically the phase and transport lag there make it impossible to close the loop there, so 1/5 to 1/10th the switching frequency is a good rule of thumb.
Could some one tell me what does the words in blockquote mean when it says , "there make it impossible to close the loop there" ? What does "close the loop" mean in electrical terms ?
Best Answer
This is a control theory concept and simply means that you measure some physical quantity you want to keep at a certain value and then use this measurement to tweak some sort of control to cause this quantity to get closer to the desired value.
Let's be a little less abstract... think about a thermostat in a house air conditioner. This is a closed loop because the thermostat will measure the room temperature and if the room is hotter than desired it will turn on the air conditioner so that the temperature drops and get closer to the desired value. Once the temperature is low enough the air conditioner turns off to prevent the room from becoming too cold. If you simply turn on the air conditioner with the fan at a certain speed and leave it on all the time regardless of the actual room temperature, then you have an open loop.
Now back to your DC-DC converter... closed loop means that you'll measure the output voltage and will increase or decrease the duty cycle of the switcher if the output voltage is too low or too high in an attempt to keep the output voltage at the desired value.
My understanding is that "there" means "at frequencies close to the switching frequency".
The idea is that in a switched circuit you only have the opportunity to "fix things" once per cycle. If you want the output voltage to go up, you increase the duty cycle (I'm thinking about a Buck converter) for that particular period; if you want the output voltage to go down, you decrease the duty cycle for that particular period. This means that the switching frequency imposes a limitation on how fast you can change the output voltage - you can never react faster than the switching period. That's why the text suggests using 1/5 or 1/10 of the switching frequency as an upper limit in terms of how fast you should design your control circuit to react in order to keep the output voltage constant.