ELE Times March 2017 ELE Times | Page 36

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Microchip ' s dsPIC33EP ' GS '

Devices

Aids to Improve Loop Gain Performance in Digital
Power Supplies
Alex Dumais Pr . Applications Engineer Microchip Technology Inc .
With the consistent push for higher performance and higher power density power supplies , switching frequencies are rising , which is causing digital controllers to adapt to market trends . Microchip ’ s dsPIC33EP ‘ GS ’ series of Digital Signal Controllers aimed for power supply applications is one primary example . These devices have introduced new features that reduce the execution time of the linear difference equation ( LDE ) and reduce overall system delays . These features help achieve higher sampling rates of the control loop ( s ) and mitigate phase erosion , thereby leading to improved loop gain performance .
In a digital power supply unit ( PSU ) there are several factors that
over the existing dsPIC33FJ devices . If we take a control loop that
impact loop gain performance that are specific to the
executes at a rate of 250kHz and is 60 instructions , this consumes
microcontroller . These factors include maximum sampling rate ,
a total of 15MIPS or 30 % of the available resources on the
time required to execute compensator algorithm ( s ),
dsPIC33FJ devices . This same control loop code consumes only
sampling / conversion time of analog to digital converter ( ADC ),
20 % of the available CPU resources on the new dsPIC33EP
and microcontroller operating speed . For peak current mode
processor when executed at the same sampling frequency . If the
control converters , the speed of the comparator and the
same percentage of MIPS is consumed after migrating to the new
accuracy / speed of the control digital to analog converter ( DAC )
dsPIC33EP devices then this allows the control loop to be
will also have an impact on the PSU loop gain performance . All of
executed at a rate of 350kHz . Further analysis shows that phase
these items need to be considered when selecting a
erosion is reduced by 29 % at a given crossover frequency . See
microcontroller for a given application . Let ’ s take a look at how
Figure 1 for calculation of phase erosion due to sampling vs . cross
Microchip ’ s dsPIC33EP ‘ GS ’ devices can help improve loop gain
over frequency .
performance for the next generation of power supplies .
The most noticeable feature of the new dsPIC33EP devices is the
increase in operation frequency . The operating frequency of the
new dsPIC33EP devices has increased to 70MHz , which is a maximum increase of 20 million instructions per second ( MIPS )
Figure 1 : Phase Erosion due to Sampling
ELE Times | 36 | March , 2017