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Buck peak current mode has no PWM deadtime

Question asked by Chris Hossack on Jan 15, 2017
Latest reply on Jan 19, 2017 by Chris Hossack

Hi,

I’m trying to implement Peak Current Mode on a KIINETIS CPU (it’s a MKV46F256). Peak current mode uses a fixed duty which is cleared by the comparator when the peak current reaches the 6 bit DAC value. It’s explained in a bit more detail in App Note AN4485, section 3.1.7 Current-limit PWM. (its a different CPU but same eFlexPWM unit)

 

 Current limit/peak mode

 

It’s suggests using the eFlex PWM fault inputs for the submodule

 

where the comparator output will be used as the fault input of the PWM module”

 

Also this question "PWM termination using the analogue comparator" also states using the fault inputs.

"The CMP output signal can be as a fault signal, which can be connected to fault signal of eFlexPWM module via crossbar and disable PWM signal automatically."

 

But the fault inputs only take affect after the deadtime insertion logic. 

 High level output PWM generation Block Diagram

This means when you implement a synchronous buck you will get shoot through on the FETs (e.g. they will both be on at for a small amount of time when they switch).

 

I think I might be able to use the Force Out logic and connect the comparator output to the EXT_SYNC to select OUT23 on it’s rising edge, e.g. turn off the PWM before the deadtime  insertion logic.

 Force Out logic

But there is no automatic means of re-enabling the PWM23 output, e.g. changing SEL23 back to 0 and generating another FORCE_OUT clock.

 

Since this chip is marketed as a digital power supply chip and it states the following…

 

“This PWM module can generate various switching patterns, including highly sophisticated waveforms. It can be used to control all known Switched Mode Power Supplies (SMPS) topologies”

 I’m guessing I must have missed something.

Any help in this matter would be greatly appreciated.

 

Chris

 

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