Hi Everyone,

I'm currently reading Electronic Principles 7th Edition by Albert Malvino. In chapter 24 (pg 978), the book discusses a buck regulator. I attached an image so that you can better understand what I'm asking.

In discussing the action of this circuit, it is stated that the comparator controls the duty cycle of the PWM. When the power is first turned on, there is no output voltage and no feedback voltage on R1-R2. Thus the comparator output ir large and duty cycle is near 100%. However, as the output voltage builds, the feedback voltage reduces the comparator output, thereby reducing the duty cycle.

How can the comparator's output be reduced? My understanding was that a comparator produced either a high or a low saturated signal.

 

By reducing the duty cycle or the amount of time that is in in the on state.

 

Switching mode supplies are more efficient because the control element is either on or off.

On state has almost 0 volts and maximum current.

Off State has max volts and practically 0 current.

Either of those mean there is almost no lost power in the control transistors.

Compare that to any linear regulation which has all the current and all surplus amounts of voltage in the pass transistor or control IC. Space heaters.

 

Hi Everyone,

I'm currently reading Electronic Principles 7th Edition by Albert Malvino. In chapter 24 (pg 978), the book discusses a buck regulator. I attached an image so that you can better understand what I'm asking.

In discussing the action of this circuit, it is stated that the comparator controls the duty cycle of the PWM. When the power is first turned on, there is no output voltage and no feedback voltage on R1-R2. Thus the comparator output ir large and duty cycle is near 100%. However, as the output voltage builds, the feedback voltage reduces the comparator output, thereby reducing the duty cycle.

How can the comparator's output be reduced? My understanding was that a comparator produced either a high or a low saturated signal.

I agree the function of the comparator makes little sense. I would think an error amplifier would be more credible.

 

I agree with that. I read up on an error amplifier, and that seems much more relevant.

 

Keep in mind however that it is possible to use "bang-bang" (hysteretic) control methods which would make sense in relation to your initial post. Perhaps that was the writer's intention - if not explicitly stated as such.

 

I think the author was using the term "comparator" in a general analog sense (not the typical digital output comparator) where the two inputs are "compared" and the analog output is proportional the the difference, i.e an error amp.

 

I agree the function of the comparator makes little sense. I would think an error amplifier would be more credible.

That's a perfectly normal and workable buck regulator using a comparator. It is commonly used in many buck regulator ICs.

I'm not sure why anyone is seeing a problem with it?

 

That's a perfectly normal and workable buck regulator using a comparator. It is commonly used in many buck regulator ICs.

I'm not sure why anyone is seeing a problem with it?

A point to which I alluded in post #6.