Buck Converter vs rectified PWM Topology for DC-DC Stepdown

[ngmedaboard]

Never pulled apart the theory on this one to have a good answer, but can anyone speak to the advantages of the buck over inductorless PWM?

Option 1:
Buck Converter (Most commonly used in DC-DC step down supplies)
-PWM modulated based on feedback
-Uses dI/dt over inductor for energy conversion

Option 2:
PWM Converter (Most commonly used in Motor Control)
-PWM modulated based on feedback
-No inductor just duty cycle and rectification​

I can see the inductor is necessary to trade off voltage for current. For instance you will be able to pull more current at the lower output voltage than your higher input voltage so long as your total power * efficiency isn't greater than your input power. Anything beyond that?

 

[FvM]

No inductor just duty cycle and rectification

I think you misunderstand PWM motor control. It's using the motor inductance as storage inductor, in so far it's almost identical to "Option 1". As a difference you omit the fílter capacitor and accept a certain amount of load current ripple and additional losses by the non-ideal inductor.

 

[D.A.(Tony)Stewart]

PWM with R only is a variable resistor and still lossy but if load is inductive like a motor, then losses are small if RdsOn < R coil

PWM with L is a variable current source stored in L with lower losses from ESR and load capacitance during dv/dt

 

[ngmedaboard]

I think you misunderstand PWM motor control. It's using the motor inductance as storage inductor, in so far it's almost identical to "Option 1". As a difference you omit the fílter capacitor and accept a certain amount of load current ripple and additional losses by the non-ideal inductor.

FvM, thanks for the read on this. I'll do some more research on the impact of motor inductance on the respective drive circuit.

But getting back to your comment about PWM motor controllers not having a cap. Let's say assume for the purposes of comparing Option 1 to Option 2 that Option 2 does have a substantial capacitor (and thus low impedance) with low ESR (lets say it were a Tantalum). Would that affect your comparison? Would we still see a lot of ripple in Option 2?

-NG