Operating Modes

29.4.1 Operating Modes

ulation scheme, the switches of the multilevel HF converter follow SPWM, and the ac–ac converter switches are switched In this section, we discuss the modes of operation of the based on the power-flow information. Unlike the first modula- inverter in Fig. 29.12 for 120-V and 240-V ac output and for tion scheme, which modulates the ac–ac converter switches at an input voltage in the range of 42–60 V (i.e., N = 4.3). The HF, in the second modulation scheme, ac–ac converter oper- modes of operation less than 42 V (i.e., N = 6.5) remain the ates at line frequency. The switches are commutated at HF same. Figures 29.14 and 29.15 show the waveforms of the five only when the polarities of output current and voltage are dif- operating modes of the phase-shifted HF inverter and a posi- ferent. Usually this duration is very small, and therefore the tive primary and a positive filter-inductor current. Modes 2 and switching loss of the ac–ac converter is considerably reduced

4 show the zero-voltage switching (ZVS) turn-on mechanism

778 S.K. Mazumder

Q1 Q 2 Q 4 Q 3

Modulating signal for the HF inverter

Inverter output voltage

Inverter output current (a)

Logic Operator

Logic operator

FIGURE 29.11 (a) and (b) Schematic waveforms [3] for the HF dc–ac converter on the primary side of the transformer and the ac–ac converter on the secondary side of the transformer. (c) Overall control scheme for the two-stage HF inverter.

for switches Q 3 and Q 4 , respectively. Unlike conventional con- Five modes of the inverter operation are discussed for pos- trol scheme for ac–ac converter [12], which modulates the itive primary current. A set of five modes exists for a negative switches at HF, the outlined ac–ac converter operates at line primary current as well. A similar set of five modes of opera- frequency. The switches are commutated at HF only when the tion for the 240 V ac exists for input voltage of more than 42 V polarities of the output current and voltage are different [12]. (N = 4.5). Again, the mode of operation for input voltage of For unity-power-factor operation, this duration is negligibly less than 42 V (N = 6.5) remains the same. small, and therefore, the switching loss of the ac–ac converter is

considerably reduced compared with the conventional control Mode 1 (Fig. 29.14a): During this mode, switches Q 1 and method [13].

Q 2 of the HF inverter are ON, and the transformer

29 High-Frequency Inverters 779

Dc–ac converter

Ac–ac converter

Dc–ac converter

Ac–ac converter

FIGURE 29.12 Circuit diagrams [1] of the proposed fuel-cell inverter for (a) 120 V/60 Hz ac outputs and (b) 240 V/50 Hz ac outputs. A single-pole- double-throw (SPDT) switch enables adaptive tapping of the transformer.

primary current I p1 and I p2 is positive. The load cur- Mode 2 (Fig. 29.14b): At the beginning of this interval, the rent splits equally between the two cycloconverter

gate voltage of the switch Q 1 undergoes a high-to-low modules. For the top cycloconverter module, the

transition. As a result, the output capacitance of Q 1 load current I out /2 is positive and flows through the

begins to accumulate charge and, at the same time, the

switches pair S 1 and S 1 ’, the output filter L f1 and C f1 ,

output capacitance of switch Q 4 begins to discharge.

Once the voltage across Q 4 goes to zero, it is can be Similarly, for the bottom cycloconverter module, the

switches S 2 and S 2 ’, and the transformer secondary.

turned on under ZVS. The transformer primary cur- load current 0.5 ×I out is positive and flows through

rents I p1 and I p2 and the load current I out continue the switches pair S 5 and S 5 ’, the output filter L f2 to flow in the same direction. This mode ends when

the switch Q 1 is completely turned OFF and its output secondary.

and C f2 , switches S 6 and S 6 ’, and the transformer

capacitance is charged to V DC .

780 S.K. Mazumder

Cycloconverter

On PCB

HF inverter switches

switches

V FC V Cf

UC3895 Phase-shift

Adaptive K p

selector Compensator

HF inverter switches

Cycloconverter

switches

y FC V grid

i LF

UC3895 Phase-shift

Adaptive K p

FIGURE 29.13 (a) and (b) Schematics for converter operation [1], respectively, at 120 V ac and 60 Hz and 240 V ac and 50 Hz. (c) and (d) Control schemes of the inverter in grid-parallel and grid-connected modes.

capacitance of switch Q 3 begins to discharge as shown The transformer primary currents I p1 and I p2 are still

Mode 3 (Fig. 29.14c): This mode initiates when Q 1 turns OFF.

in the Fig. 29.14d. The charging current of Q 2 and the

discharging current of Q 3 together add up to the pri- Fig. 29.14c. Also the load current continues to flow in

positive, and free wheels through Q 4 as shown in

mary currents I p1 and I p2 . The transformer current the same direction as in Mode 2. Mode 3 ends at the

makes a transition from positive to negative. Once the

voltage across Q 3 goes to zero, it is turned ON under Mode 4 (Fig. 29.14d): At the beginning of this interval, the

commencement of turn off Q 2 .

ZVS. The load current flows in the same direction as

gate voltage of Q 2 undergoes a high-to-low transition.

in Mode 3 but makes a rapid transition from the bidi-

rectional switches S 1 and S 1 ’ and S 2 and S 2 ’ to S 3 and accumulate charge and, at the same time, the output

As a result, the output capacitance of Q 2 begins to

S 3 ’ and S 4 and S 4 ’, and during this process I out /2 splits

29 High-Frequency Inverters 781

Mode 1

Mode 2

Dc–ac converter

Ac–ac converter

Dc–ac converter

Ac–ac converter

1 i sec1 S +

i sec1 +

4 i sec1 S − sec1 − 2’

i sec2 +

Load

i sec2 + Load S 5 S 7 S 5 S 7

Dc–ac converter

Ac–ac converter

Dc–ac converter

Ac–ac converter

i sec1 +

i 2’ sec1 −

sec1 −

i sec2 +

Load

i sec2 + Load S 5 S 7 S 5 S 7

1:N

1:N

i pri2

S i sec2 6’ − sec2 −

Gating Q 1 Q 4 Q 1

Pulses

Mode 5

Dc – ac converter

Ac–ac converter

i sec1 +

ν pri

i pri1

sec1 i −

sec2 +

Load

S 5 S 7 i sec1 −

S 8 S 6 i sec1 +

S 8’

S 6’

i sec2−

Mode 1

Mode 2 and 3

Mode 3

Mode 4 and 5

(e)

(f)

782 S.K. Mazumder

Mode 1

Mode 2

Dc–ac converter

Ac–ac converter

Dc–ac converter

Ac–ac converter

i sec1 +

i sec1 −

i sec2 +

Load

i sec2 + Load S 5 S 7 S 5 S 7

i sec2 −

Dc–ac converter

Ac–ac converter

Dc–ac converter

Ac–ac converter

1 sec1 +

sec1 S

ipri1

V dc 1:N

a C f1 V dc ’ ipri1 1:N S 1 S 3 V ’ n ν v a a C f1

pri

L f1 Vpri

i 2’ sec1 −

sec1 −

i sec2 +

Load

i sec2 + Load S 5 S 7 S 5 S 7

i S sec2 6’ − sec2 −

Dc–ac converter

Ac–ac converter

v pri

S 1 i sec1 + Q 1 Q 3 S 3

S 1 ’ S 3 dc ’ ipri1 v

V 1:N

V C f1 n

C 1 pri L f1 i pri1

Q 4 Q 2 S 4 S 2 i out

S 2’

i sec1−

sec2 +

Load

S 5 S 7 i sec1 −

i pri2

ν b i sec1 +

i sec2−

Mode 2 and 3

Mode 4 and 5

29 High-Frequency Inverters 783 between the two legs of the cycloconverter modules as

29.4.2.2 Optimization of the Transformer Leakage

shown in Fig. 29.14d. Mode 4 ends when the switch

Inductance

Q 2 is completely turned OFF, and its output capaci- The leakage inductance of the HF transformer enhances the tance is charged to V DC . At this point, it is necessary ZVS range of the dc–ac converter but reduces the duty ratio to note that because S 1 and S 2 are OFF simultaneously, of the converter, which increases the conduction loss. Thus, the

each of them support a voltage of V DC .

leakage inductance of each transformer is designed to achieve Mode 5 (Fig. 29.14e): This mode starts when Q 2 is completely the highest efficiency, as illustrated in Fig. 29.18. For the sinu-

turned OFF. The primary currents I p1 and I p2 are neg- soidally modulated dc–ac converter, the ZVS capability is lost ative, while the load current is positive as shown in twice in every line cycle. The extent of the loss of ZVS is a func- Fig. 29.14e.

tion of the output current. The available ZVS range (t ZVS ) as a percentage of the line cycle (t LineCycle ) is given by [1]