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