**3. Power electronic converters for PEM fuel cells**

Power electronic converters are used in fuel cell systems to convert the DC electrical power generated by the fuel cell into usable AC or DC power through power electronic circuits. The power electronic converter plays an important role on the interface of the fuel cell system as power generating system and many solutions are already presented in the literature [14-30]. The output voltage of the fuel cell varies normally in the range of 20 V to 50 V and the possible converter topologies that can be used are such as; DC-DC together with DC-AC, DC-AC interfacing directly the fuel cell to the grid, or DC-DC together with AC-AC isolated by a transformer.

Figure 8 shows that the DC-DC power converters can be divided according to the operation mode into tree types: 1) the linear mode, 2) the switching mode and 3) the soft switch‐ ing or resonant mode. The main difference between them is caused by efficiency. The softswitching or resonant has some advantages compared to the linear like; the high switching frequency, which enables the use of a small ferrite transformer core, it may operate in a much larger DC input voltage range than the linear regulators, and it often has a higher efficiency. However, there are some drawbacks associated too, the noise at the supply may be increased according to different power switching techniques, and the control circuitry is more complicated compared to the linear one. Figure 8 also shows that the switchingmode topologies are divided into two types, the isolated and the non-isolated. Nonisolated DC-DC converter topologies are the Buck, Boost and Buck-Boost converters; and further, the Cuk converter. For many applications, isolation between the input and the output is a necessary requirement within the converter. By inserting isolation transform‐ ers into the four basic non-isolated switching topologies presented above, four singleended isolated switching DC-DC converters can be obtained, namely; Forward, Boost, Flyback and Cuk converters. Nonetheless, the single switch topology is not an ideal solution for higher power converters, since these converters need a higher power trans‐ former. Therefore, another group of DC -DC isolated converters utilizing more than one switch are identified: Push-pull, Half-bridge and Full-bridge converters.

**Figure 8.** DC-DC power converter family tree.

*2.2.3. Constraints imposed to the converter*

338 New Developments in Renewable Energy

transformer.

Regardless the topology of the converter selected, the constraints imposed by the PEM fuel cell should be respected that is, the minimum and maximum values of the voltage, the current

**Ifc (A) Vfc (V) Power (W)**

2.8 23.71 66

24 19.11 492

Power electronic converters are used in fuel cell systems to convert the DC electrical power generated by the fuel cell into usable AC or DC power through power electronic circuits. The power electronic converter plays an important role on the interface of the fuel cell system as power generating system and many solutions are already presented in the literature [14-30]. The output voltage of the fuel cell varies normally in the range of 20 V to 50 V and the possible converter topologies that can be used are such as; DC-DC together with DC-AC, DC-AC interfacing directly the fuel cell to the grid, or DC-DC together with AC-AC isolated by a

Figure 8 shows that the DC-DC power converters can be divided according to the operation mode into tree types: 1) the linear mode, 2) the switching mode and 3) the soft switch‐ ing or resonant mode. The main difference between them is caused by efficiency. The softswitching or resonant has some advantages compared to the linear like; the high switching frequency, which enables the use of a small ferrite transformer core, it may operate in a much larger DC input voltage range than the linear regulators, and it often has a higher efficiency. However, there are some drawbacks associated too, the noise at the supply may be increased according to different power switching techniques, and the control circuitry is more complicated compared to the linear one. Figure 8 also shows that the switchingmode topologies are divided into two types, the isolated and the non-isolated. Nonisolated DC-DC converter topologies are the Buck, Boost and Buck-Boost converters; and further, the Cuk converter. For many applications, isolation between the input and the output is a necessary requirement within the converter. By inserting isolation transform‐ ers into the four basic non-isolated switching topologies presented above, four singleended isolated switching DC-DC converters can be obtained, namely; Forward, Boost, Flyback and Cuk converters. Nonetheless, the single switch topology is not an ideal solution for higher power converters, since these converters need a higher power trans‐

and the power, which for the PEM Mark 1020 are listed in the Table 1 below.

**Table 1.** Constraints imposed to the power system by PEM MARK 1020.

**3. Power electronic converters for PEM fuel cells**

In switched-mode topologies, finite duration of the switching transitions will cause high peak pulse power dissipation in the devices, degradation of the converter efficiency and, also can lead to transistor damage during the turn-off transition. Employing load-line snubbers can reduce this problem. When using snubbers the stress of the switches are minimised, as shown in Figure 9. However, with the appearance of new power electronic converters based on softswitching technologies, [15,17,19,20-22,30], the reduction of switching losses and the continual improvement of power switches allow at being able to increase the switching frequency. In this type of converter the turning on and turning off of the converter switches appears when the switch voltage or the switch current is zero, as shown in Figure 9 [23].

**1.** If the parts are chosen so that Cs and Ld are very small and have minimal effect on the circuit action. With Ls and Cd forming an LC series combination, the transistor operation

Methodology of Designing Power Converters for Fuel Cell Based Systems: A Resonant Approach

http://dx.doi.org/10.5772/54674

341

**3.** It is also possible to use all four parts to support ZVS and ZCS action together, called multi-

ZCS topologies can eliminate the switching losses at turn-off and reduce the switching losses at turn-on. If a relatively large capacitor is connected across the output diode during resonance, the converter operation becomes insensitive to the diode's junction capacitance. The major limitations associated with ZCS when Mosfet's are used are the capacitive turn-on losses. Thus, the switching loss is proportional to the switching frequency, during turn-on, considerable rate of change of voltage can be coupled to the gate drive circuit through the Miller capacitor, thus increasing switching loss and noise. Another limitation is that the switches are under high current stress, resulting in high conduction loss. ZVS eliminates the capacitive turn-on loss. It is suitable for high-frequency operation. For single-ended configuration, the switches could suffer from excessive voltage stress, which is proportional to the load. The output regulation of the ZCS and ZVS resonant converters can be achieved using variable frequency control. The ZCS [20-22] operates with constant on-time control, while ZVS [24] operates with constant off-

For the selection of the converter topology the following requirements are considered in order to ensure the maximum efficiency and minimum cost of the power generation system.

A comparative analysis of the major topologies of DC-DC converters described above is

Power electronic converters in general and DC-DC converters in particular have a great importance on the performance and efficiency of energy production process based on fuel cells. The control of the operation point of the fuel cell requires appropriate use of static power converters, capable of providing accurate support to the control methods. The main objective to beachievedwhenapplyingtheconverterstofuelcellsisobtainingthemaximumefficiencyusing the most appropriate control strategies, taking into account requirements described above. As

can take advantages of current zero crossing for ZCS.

resonance, but this is not a common technique.

**3.1. Requirements for selecting the converter topology**

**1.** Control of output voltage according to a given reference;

**2.** Deliver current with little ripple and harmonic contend

**3.** High efficiency in the whole operating range

**5.** Incorporated filtering and storing possibilities

**4.** Properly operation in all conditions

**3.2. DC-DC converter topologies**

time control.

presented below.

**2.** If the values of Cs and Ld are small, then the transistor supports ZVS [24].

**Figure 9.** Switching loci trajectories of the different converter types.

Figure 10 provides an arrangement for a soft-switching resonant converter. An inductor -Ls and a capacitor-Cs have been added to help the switch action. A similar *LdC*d pair is added to the diode. In any of these soft switching cases, switch action at a zero crossing cuts off the ringing resonant waveform. This technique is often called quasi-resonance.

**Figure 10.** General structure of a resonant converter, where ZVS or ZCS can be obtained.

To create conditions for the ZCS or ZVS in DC-DC converters, the resonance or soft switching approach can be used. The ZVS or ZCS can be obtained by re-arranging the resonant compo‐ nent Figure 10, whose combinations offer several possibilities for resonant action as follows:


ZCS topologies can eliminate the switching losses at turn-off and reduce the switching losses at turn-on. If a relatively large capacitor is connected across the output diode during resonance, the converter operation becomes insensitive to the diode's junction capacitance. The major limitations associated with ZCS when Mosfet's are used are the capacitive turn-on losses. Thus, the switching loss is proportional to the switching frequency, during turn-on, considerable rate of change of voltage can be coupled to the gate drive circuit through the Miller capacitor, thus increasing switching loss and noise. Another limitation is that the switches are under high current stress, resulting in high conduction loss. ZVS eliminates the capacitive turn-on loss. It is suitable for high-frequency operation. For single-ended configuration, the switches could suffer from excessive voltage stress, which is proportional to the load. The output regulation of the ZCS and ZVS resonant converters can be achieved using variable frequency control. The ZCS [20-22] operates with constant on-time control, while ZVS [24] operates with constant offtime control.
