Magnetic Saturation of High Power Medium Frequency Transformers due to Semiconductor On-State Voltage Drop and Switching Time Tolerances
GGOti. Ortiz, J. Mühle tha ler, J. W. Ko lar Swiss Federal Institute of Technology (ETH) Zurich Power Electronic Systems Laboratory www.pes.ee.ethz.ch Introduction
1 High-Power DC-DC Converters with MF Transformers
In Renewable Energy Generation High-Power DC-DC Converter TiifTransmission of energy in DC. iDC Frequency vs . Power Map Power flow control (Smart Grids). Reduction in size/weight.
In Traction Applications Reduction in size/weight. Increased efficiency.
2 The Dual Active Bridge
Unmatched turn-on/turn-off times Voltage is applied for a longer/shorter time in positive and negative semi cycles.
p Different output characteristics Different voltage levels are applied during the positive and negative semi cycles.
Result: Small DC component i n vol tages appli ed to th e transf ormer !
3 DC Magnetization
Equivalent circuit with independent DC and AC voltage sources sources
DC component of magnetizing current
DC component of magnetic flux density
Equivalent series resistance limits DC flux density
4 Example: 1MW / 20kHz Transformer
Core-Type Concept
Core Material – Vitroperm 500F (μr ≈ 20000 )
LV Winding – Loss Optimized Copper Foil LV winding Equivalent primary series resistance R = 6.8mΩ ( Primary) MV winding p,T (Secondary)
2 x U cut-core
Only 0.25ns of switching miss- match are enough to generate 0.2T of DC flux density bias !
Core&LV winding Water-cooled Heatsink
Potted isolation between LV and MV widiindings
5 Previously Proposed Methods
6 Magnetic Flux Density Balancing
system. Passive Series capacitor: any DC voltage on the transformer. However, tddit d ffi fit th d i reduces power densA seriesity an dcapacitor efficiency prevents of the
component in the magnetizing current generatesAir-gap in amagnetic smaller DCpath: flux density magneticcomponent. path reduces the equivalent ppermeabilitermeabilit ofy theof th coree Thereforecore. There a DCfore a DC An air-gap in the
7 Magnetic Flux Density Transducers
Saturation Detection E-core with air gap in external leg . [R. Patel 1980]
Reduced cross-section and a [J.A. Ferreira 1997]
dditional magnetic path.
Dynamic Flux Measurement Integration of applied voltag [D. Wilson 1981]
e by external RC network.
8 Magnetic Flux Density Transducers
Continuous Flux Density Measurement Measurement of magnetizing current. [J.W. Kolar 2000]
Superimposed orthogonal flux density with external coil. [S. Cuk 1982]98 ]
Direct flux density measurement with hall sensor.
9 Proposed Flux MtMeasurement CtConcept
10 Proposed Flux Measurement Method
Concept Shared magnetic path auxiliary core.
between main core and an mo auxiliary core winding.
Magnetic flux density through the main core changes properties of the shared magnetic path, modif iying, flfor example, thidhe inductance seen fhfrom the
Auxiliary Core Placement (a) Auxiliary flux parallel to main flux density. (()b) Auxiliary flux orthogonal to main flux densit y.
(a) (b)
11 Proposed Flux Measurement Method
Inductancethe main core increases. Measurement The inductance Laux measured on the auxiliary winding termin
This measurement was performed on a
N27 E55 Ferrite als decreases as theMetglas magnetization AMCC80 of
N27 Ferrite E core and on a
Metglas AMCC80 cut-core.
12 Proposedthe inductance Flux value. Measurement Method
Inductance Measurement Circuit Square-shaped voltage with 50% duty
hkiitiilti l t th ili l i d t t iTh ili k e peak auxiliary current is inversely proportional to the auxiliary inductance.
( ) The auxiliary current is rectified and fi cycle applied to auxiliary coil. vm t inversely proportional to
ltered to obtain an output voltage
13 Proposed Flux Measurement Method of the core.
Results The cores were magnetized with a square shaped voltage.
ht tth h tiTh t ti ti th d t i e magnetizing current shows that the core is driven to saturation.
Measurements show a clear relation vm(t) and the magnetization state
N27 E55 Ferrite Metglas AMCC80 between the measured voltage
14 Magnetic Flux Density Active Correction
aControl Scheme transformerThe signal fromcore usingthe magnetic the measurement from the proposed actively correct the volts-seconds applied to the transducer. tftransformer . transducer can be used to A central control unit adjusts the gating signals to achieve the desired transferre a t i thtdj d tit f t thustments are introduced to prevent the saturation of the
d power. Additionally, small
15 Conclusions
16 or Conclusionsdriving it into saturation.
Magnetic Cores Saturation in DC-DC Converters Small tolerances in the semiconductors output ch magnetization of a transfor the status of the magnetic flux This DC magnetization is limited by the eq mer in a DC-DC converter. As efficiency is a key aspect in high-power applicatio h h ihi k df hi Thih i or h i l i d i DC i d er to reduce losses. This in turn density increases inside th thee ris mainkaracteristics of reac core.hing and/or high DC swit magnetization in the core and thus
uivalent series resistance of the circuit. ching times can cause DC Proposed Magnetic Flux Density Measurement Concept The proposed flux density measurement concept is ba core and an auxiliary core. ns, a small equivalent series
A change change in in the the auxiliary auxiliary core core s’s inductance inductance was was sensed sensed by an by additional an additional circuit circuit giving, information giving information about about resistance is desired in
The proposed concept can be used to sed on sharing of magnetic density in in the transformer transformer core core.
path between the main implement a feedback control that
17 ensures a balanced magnetic flux