Class D power Dissipation - PWM rise fall time

[AJAB]

How does the rise and fall time of PWM affect the power dissipation in the output filter circuit of Class D amplifier?

 

[smijesh]

1) Rapid rise and fall times keep the inductor current constant hence more power dissipation in the output filter.

 

[AJAB]

But that will depend upon the duty cycle right?? How does it depend upon rise and fall time?

 

[Audioguru]

When the rise and fall times are fast then the output transistors dissipate low heating since they are fully turned on or fully turned off for MOST of the time.
When the rise and fall times are slow then the output transistors get hot because they are turned on for MOST of the time.

 

[AJAB]

When the rise and fall times are fast then the output transistors dissipate low heating since they are fully turned on or fully turned off for MOST of the time.
When the rise and fall times are slow then the output transistors get hot because they are turned on for MOST of the time.

Correct. This is true for switches. But for output filter (LC) if we consider the rise time and fall time, instead of a square wave, the PD will be less than square wave. Because due to rise and fall time the PWM will not be a square wave, and the time for which PWM stays high will be less than perfect square wave. Am I correct?

 

[Audioguru]

The L of the output filter has a very low resistance so it dissipates low power. The C does not dissipate power.

 

[goldsmith]

How does the rise and fall time of PWM affect the power dissipation in the output filter circuit of Class D amplifier?

Hi AJAB

As Audioguru mentioned , if rise time and fall time be increased thus the mosfet won't be totally turned on in some short times so it will work as a linear device in those short times , so it would be affected in more dissipation across the mosfets and it will create some overlapping problems which would be cause of more dissipation .

And about the out put filter , rise time and fall time , will add even harmonics into the spectrum , and thus the filter will has to , do it's job instead of more signals , so the reactive power will be increased , but there is small value of active power which is absolutely insignificant ( resistive effect of inductor and ESR of capacitor ) . and other thing is that because there are more harmonics which are being applied to the inductor so the core would have to tolerate more magnetic current , so it can be saturated too ( it would deal with it's risk factor ) and if the core be saturated thus there would be more reactive dissipation and the filter won't be able to do it's best as we're expecting for .

Best Wishes
Goldsmith

 

[AJAB]

How does the Tr and Tf fit into dissipation equation?

 

[Audioguru]

High frequencies are present in the rise and fall times. The LC filter does not pass them so they do not cause high current and do not cause heating.

 

[crutschow]

Even with a relatively slow rise and fall time, any addition ripple current from this will be much smaller than the load current through the inductor, which is the primary cause of inductor losses in the output filter. As noted by others, the primary concern with slow rise and fall times is the power loss in the switching devices, not the possible tiny increase in power dissipated in the filter.

 

[AJAB]

ok. so I can safely ignore the rise and fall time effects for calculating the power dissipated in inductor.

Thank you all for the help. :grin: