Boost Topologies for 1500V MultiString Solar Inverters
This article investigates the performance and cost of different boost topologies for 1500V multistring solar inverters.
Designers are seeking for a higher level of integration, which means the mounting of the boost inductors on the printed circuit board (PCB). Besides this fact, also efficiency and cost are important aspects to be considered.
This case study investigates the optimal solution to meet cost and performance requirements at the same time. It introduces a new threelevel boost topology named flying capacitor boost and shows that this topology outperforms in cost and performance.
Comparing Boost Topologies
Three different types of boost topologies will be compared: twolevel, threelevel symmetric and threelevel flyingcapacitor circuit.
Figure 1: Twolevel boost circuit
Figure 2: Threelevel symmetric boost circuit
Figure 3: Threelevel flyingcapacitor boost circuit
The threelevel topologies comprise an additional third voltage level. This third voltage level reduces the voltage across the boost inductor, boost switch and diode to half the value required for twolevel topologies. Less voltage across the boost inductor has the advantage that the required inductance for a given ripple current is only half of the required inductance at twolevel. In consequence, the overall inductor volume, weight, and cost is reduced. This benefit is not considered in the following power module cost benchmark and should, therefore, be kept in mind. In the symmetric boost topology, the third voltage level is created by splitting the boost circuit into a positive and negative part. Input and output capacitors are split as well and connected to the neutral point, which provides the additional third voltage level. The pulsewidth modulation (PWM) pattern needs to be corrected in order to ensure symmetry of the neutral point. The flyingcapacitor boost topology creates the third voltage level – as the name indicates – by a floating or flyingcapacitor (CFC). The flyingcapacitor shall be charged to half of the output voltage. It is noticeable that only one boost inductor is required.
Cost and Performance Benchmark
In this case study, the cost and performance is compared for a 21 kW boost leg of a 1500 V multistring solar inverter. Following topologies and chipsets are benchmarked:
Chipset  TwoLevel  ThreeLevel Symmetric  ThreeLevel FlyingCapacitor 
Si/SiC hybrid  not examined as efficiency doesn’t meet the requirement 
1200 V fast Si IGBT 1200 V SiC diode 

full SiC  1700 V SiC MOSFET 1700 V SiC diode  1200 V SiC MOSFET 1200 V SiC diode 
From the efficiency benchmark in Figure 4, it can be seen that the full SiC twolevel boost stage has the lowest efficiency (red dotted line) and the highest price (Figure 5). The hybrid chipset would even have lower efficiency in this frequency range and therefore is not taken into account.
Figure 4: Module efficiency benchmark – conditions: Vin = 760 V, Vout = 1200 V, Pdc = 21 kW
The flyingcapacitor (FC) boost topology (solid and dashed blue line) shows always higher efficiency as the symmetric boost (solid and dashed green line). Both use the same components and therefore have the same module price. The first conclusion to be drawn is that the flyingcapacitor topology has the best price and performance ratio compared to symmetric and twolevel boost topology.
Figure 5: Module cost benchmark
Up to 50kHz, the hybrid flyingcapacitor topology (solid blue line) has the highest efficiency. Above 50 kHz the full SiC flying capacitor boost circuit (dashed blue line) has the highest efficiency, but also a higher price than the hybrid circuit.
Figure 6: Operation mode 1 – excitation
Figure 7: Operation mode 2  freewheeling
Figure 8: Operation mode 3 – excitation
The twolevel boost and threelevel symmetric boost topologies are already well described in the literature and considered as state of the art. In the following section, the flyingcapacitor topology is discussed.
Flyingcapacitor boost topology in details
The two semiconductor switches T1 and T2 in the flyingcapacitor topology are controlled on phase opposition (180° phase shift), but with identical on time (duty cycle). During normal operation, T1 and T2 are never turned on at the same time. In continuous conduction mode (CCM) the duty cycle (D) can be calculated to
$$D = 1  \frac{V_{in}}{V_{out}}$$
The operation is divided into three modes. In mode 1 the lowside switch T2 is turned on and the inductor current is rising. The current is flowing through D2, the flyingcapacitor (CFC) and T2. Once T2 is turned off mode 2 is entered. In mode 2 the current is freewheeling through D1, D2 and the output capacitor. In mode 3 the floating switch T1 is turned on and the inductor current is rising again. The current is flowing through T1, the flyingcapacitor and D1. After turning off T1, mode 2 is entered again and the sequence starts from the beginning. The normal operation mode sequence is:
1 → 2 → 3 → 2 → 1
Figure 9 shows for the sake of clarity the gate signals of T1 and T2 together with the boost inductor current IL. The inductor current period is defined from one rising slope to the next rising slope. The period of the PWM fundamental switching frequency is defined from the rising edge of T1 or T2 to the next rising edge of the same gate signal. It then shows that the inductor current period is half of the PWM period or in other words, the inductor current frequency is double of the PWM frequency.
Figure 9: Gate signals of T1 (VG1), T2 (VG2) and boost inductor current I(L)
The flyingcapacitor boost topology allows for an existing choke design with fixed inductivity and ripples current to halve the PWM frequency (case 1 in Figure 10). In the first approximation, this will also half the semiconductor switching losses and increase the power module efficiency. Figure 4 illustrates case 1 for 40 kHz inductor current frequency: The efficiency can be increased from 98.8% (symmetric boost stage) to 99.24% (flying capacitor boost stage).
Figure 10: Potential for optimization with flyingcapacitor boost topology
Another potential for optimization is to keep the power module losses constant (same module efficiency), but increase the inductor current frequency (case 2 in Figure 10). Figure 4 shows for case 2 that the efficiency is not changing when the inductor current frequency is doubled from 40 kHz (symmetric boost stage) to 80 kHz (flyingcapacitor boost stage).
An investigation of the required boost inductance shows that moving from twolevel to threelevel cuts the inductance to half and the additional doubling of the inductor current frequency further halves the required inductance for the same ripple current. Boost inductance of twolevel can be calculated to
$$L_{2L} = \frac{U_{L} \cdot t_{on}}{\Delta I}$$
L_{2L}: boost inductance of twolevel boost
U_{L}: voltage across boost inductor
t_{on}: boost switch ontime
ΔI: inductor current ripple
Moving from twolevel to threelevel will reduce the voltage across the inductor to half value:
$$L_{3Lsym} = \frac{\left(\frac{1}{2} \cdot U_{L}\right) \cdot t_{on}}{\Delta I} = \frac{1}{2} \cdot L_{2L}$$
L_{3Lsym}: boost inductance of threelevel symmetric boost
L_{2L}: boost inductance of twolevel boost
Moving from symmetric boost to flyingcapacitor boost topology will double the inductor frequency or half the ontime:
$$L_{3LFC} = \frac{\left(\frac{1}{2} \cdot U_{L} \right) \cdot \left(\frac{1}{2} \cdot t_{on} \right)}{\Delta I} = \frac{1}{2} L_{3Lsym} = \frac{1}{4} L_{2L}$$
L_{3LFC}: boost inductance of threelevel flyingcapacitor boost
L_{3Lsym}: boost inductance of threelevel symmetric boost
L_{2L}: boost inductance of twolevel boost
Conclusion and outlook
This article discussed the benefits of flyingcapacitor boost topology in regards of cost and performance. The costs for symmetric and flyingcapacitor boost topology are the same, but the flyingcapacitor boost circuit comprises higher efficiency at the same inductor current frequency. Alternatively, it can even double the inductor current frequency while keeping the module efficiency. In the required frequency range, the twolevel full SiC boost circuit can’t meet the required efficiency and cost. This article will be continued by taking a closer look into the flyingcapacitor topology, discussing its challenges e.g. balancing strategy of the flyingcapacitor and further highlighting the systemlevel benefits.
References
 "Symmetrical Boost Concept for Solar Applications up to 1000V", Temesi, Frisch, 01/2009
This article originally appeared in Bodo's Power Systems magazine.