# Performance and scalability of isolated DC-DC converter topologies in low voltage, high current applications

##### Väisänen, Vesa (2012-12-18)

Väitöskirja

Väisänen, Vesa

18.12.2012

Lappeenranta University of Technology

Acta Universitatis Lappeenrantaensis

**Julkaisun pysyvä osoite on**

http://urn.fi/URN:ISBN:978-952-265-351-2

#### Tiivistelmä

Fuel cells are a promising alternative for clean and efficient energy production. A fuel cell is

probably the most demanding of all distributed generation power sources. It resembles a solar

cell in many ways, but sets strict limits to current ripple, common mode voltages and load

variations. The typically low output voltage from the fuel cell stack needs to be boosted to a

higher voltage level for grid interfacing. Due to the high electrical efficiency of the fuel cell,

there is a need for high efficiency power converters, and in the case of low voltage, high current

and galvanic isolation, the implementation of such converters is not a trivial task.

This thesis presents galvanically isolated DC-DC converter topologies that have favorable

characteristics for fuel cell usage and reviews the topologies from the viewpoint of electrical

efficiency and cost efficiency. The focus is on evaluating the design issues when considering a

single converter module having large current stresses.

The dominating loss mechanism in low voltage, high current applications is conduction losses.

In the case of MOSFETs, the conduction losses can be efficiently reduced by paralleling, but in

the case of diodes, the effectiveness of paralleling depends strongly on the semiconductor

material, diode parameters and output configuration. The transformer winding losses can be a

major source of losses if the windings are not optimized according to the topology and the

operating conditions. Transformer prototyping can be expensive and time consuming, and thus

it is preferable to utilize various calculation methods during the design process in order to

evaluate the performance of the transformer. This thesis reviews calculation methods for solid

wire, litz wire and copper foil winding losses, and in order to evaluate the applicability of the

methods, the calculations are compared against measurements and FEM simulations. By

selecting a proper calculation method for each winding type, the winding losses can be

predicted quite accurately before actually constructing the transformer. The transformer leakage

inductance, the amount of which can also be calculated with reasonable accuracy, has a

significant impact on the semiconductor switching losses. Therefore, the leakage inductance

effects should also be taken into account when considering the overall efficiency of the

converter.

It is demonstrated in this thesis that although there are some distinctive differences in the loss

distributions between the converter topologies, the differences in the overall efficiency can

remain within a range of a few percentage points. However, the optimization effort required in

order to achieve the high efficiencies is quite different in each topology. In the presence of practical constraints such as manufacturing complexity or cost, the question of topology

selection can become crucial.

probably the most demanding of all distributed generation power sources. It resembles a solar

cell in many ways, but sets strict limits to current ripple, common mode voltages and load

variations. The typically low output voltage from the fuel cell stack needs to be boosted to a

higher voltage level for grid interfacing. Due to the high electrical efficiency of the fuel cell,

there is a need for high efficiency power converters, and in the case of low voltage, high current

and galvanic isolation, the implementation of such converters is not a trivial task.

This thesis presents galvanically isolated DC-DC converter topologies that have favorable

characteristics for fuel cell usage and reviews the topologies from the viewpoint of electrical

efficiency and cost efficiency. The focus is on evaluating the design issues when considering a

single converter module having large current stresses.

The dominating loss mechanism in low voltage, high current applications is conduction losses.

In the case of MOSFETs, the conduction losses can be efficiently reduced by paralleling, but in

the case of diodes, the effectiveness of paralleling depends strongly on the semiconductor

material, diode parameters and output configuration. The transformer winding losses can be a

major source of losses if the windings are not optimized according to the topology and the

operating conditions. Transformer prototyping can be expensive and time consuming, and thus

it is preferable to utilize various calculation methods during the design process in order to

evaluate the performance of the transformer. This thesis reviews calculation methods for solid

wire, litz wire and copper foil winding losses, and in order to evaluate the applicability of the

methods, the calculations are compared against measurements and FEM simulations. By

selecting a proper calculation method for each winding type, the winding losses can be

predicted quite accurately before actually constructing the transformer. The transformer leakage

inductance, the amount of which can also be calculated with reasonable accuracy, has a

significant impact on the semiconductor switching losses. Therefore, the leakage inductance

effects should also be taken into account when considering the overall efficiency of the

converter.

It is demonstrated in this thesis that although there are some distinctive differences in the loss

distributions between the converter topologies, the differences in the overall efficiency can

remain within a range of a few percentage points. However, the optimization effort required in

order to achieve the high efficiencies is quite different in each topology. In the presence of practical constraints such as manufacturing complexity or cost, the question of topology

selection can become crucial.

##### Kokoelmat

- Väitöskirjat [825]