Download Highly Integrated Switched-Mode Power Converters Employing

School of Electrical, Computer and Energy Engineering
M.S. Final Oral Defense
Highly Integrated Switched-Mode Power Converters Employing CMOS and GaN
Technologies for Distributed MPPT
by
Kiran Kumar Krishnan Achary
November 9th, 2015
9:00 am
Room 555A
Committee:
Dr. Jennifer Kitchen (Chair)
Dr. Sayfe Kiaei
Dr. Bertan Bakkaloglu
Abstract
The photovoltaic systems used to convert solar energy to electricity poses a
multitude of challenges including energy conversion efficiency, partial shading effects
and power converter efficiency to list a few. PV generated power is a non-linear function
of voltage/current which is temperature and Irradiance sensitive which calls in the
requirement for a Maximum Power Point Tracker (MPPT). MPPT is essentially a power
converter with a non-linear control algorithm that ensures the operation at the true MPP
even if there are multiple maxima due to partial shading. Research prove that sub-panellevel DMPPT result in yearly energy yield improvements of 14.5% over String-level
MPPT which is common in central-inverter based installations today.
This research aims at implementing a highly efficient power management system at
sub-panel level with focus on cost and form-factor. Smaller form-factor motivated
increasing the switching frequency of converter which can significantly reduce the size of
passives and substantially improve transient performance. But currently available power
MOSFETs put a constraint on the highest frequency switching possible due to increased
switching losses. The solution is GaN HEMTs. Gallium Nitride based power device
delivers figure of merit (FOM) performance at least an order of magnitude better than
existing silicon MOSFETs. Low power loss, high power density, low cost and small die
sizes are few of the qualities that makes GaN superior to Si counterpart. With careful
design, GaN can enable a 20-30% improvement in efficiency compared to converters
with Si MOSFET.
A high efficiency, 20MHz, small form-factor, GaN HEMT based DC-DC MPPT for
a 12V/5A sub-panel is the key design objective for this research. Hard and soft switching
Boost converters are explored. The stringent drive requirements for GaN and lack of
efficient gate driver at these high frequencies enabled the requirement of a custom
integrated gate driver. Research proved that Ripple Correlation Control (RCC), which
uses inherent current and voltage ripple; will be the optimum control method that can be
used for this particular application because of its fast convergence, inherent perturbation
and simple analog integration. The gate driver and RCC algorithm is integrated in AMS
0.18µm HV process.