3.1.4 Cuk Converter Figure 3.5 shows the schematic of a Cuk converter [27]. Similar to the inverting. The weighted average efficiency of the dynamic-supply PA [calculated using Equation 1.3] is 4.43 times greater than the fixed-supply scheme, which translates into a battery life. The Cuk converters have low switching losses and the highest efficiency. Therefore Cuk converter is chosen as power conditioning circuit to track maximum power using Hybrid MPPT technique. The equation of the mathematical model is.
![Cuk converter design Cuk converter design](http://www.digikey.com/en/articles/techzone/2012/oct/~/media/Images/Article%20Library/TechZone%20Articles/2012/October/Generating%20Negative%20Output%20from%20Positive%20Input%20Voltage/article-2012october-generating-negative-output-fig4.jpg)
LT3581 Inverting Charge Pump Plus Boost The LT3581 has master/slave switches instead of a single power switch, and Schottky diode between pins SW1 and SW2 is used to isolate the switches so the current spike through coupling capacitor C1 (created when the power switch turns on) flows only through the slave switch and not the master switch (where the current comparator resides), thereby preventing the internal current comparator from falsely tripping. When the power switch turns off, the voltage at the switch node flies back to V IN +|V OUT| as energy is transferred to the output capacitor and load. Output disconnect is inherently built into this single inductor topology. For the inverting charge pump, the simplified duty cycle is given by: Duty Cycle (D) = 1 – (V IN/|V OUT|) Since|V OUT| is always greater than V IN, the duty cycle is near 0% when they are equal and increases as V OUT becomes more negative.
In the inverting charge pump configuration below, a resistor is added in series with the Schottky diode between the negative output and the D pin of the LT3483/LT3483A. The purpose of this resistor is to smooth/reduce the current spike in the capacitor C2 when the switch turns on. A 10Ω resistor works well in this application (Li+ battery to –22V@8mA), and the impact to converter efficiency is less than 3%. The resistor values recommended in the applications circuit also limit the switch current during a short-circuit condition at the output. LT3483 Circuit with Added Rs Resistor The Inverting Topology The inverting topology uses a single inductor and does not require a coupling capacitor; thus it requires fewer components as shown below. An example of the single inductor inverting topology is shown in figure 9 below using the LTC3863 inverting controller with external power switch. The LTC3863 has a 3.5V to 60V input voltage range, a low 70µA quiescent current, and it allows output voltages below –150V.
Since the power switch must see a negative voltage, the inverting topology is less versatile in that it can only be used for negative voltages. It also has higher peak current and output ripple than a Cuk converter with a similar output current.
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For the LTC3863, the external power switch allows the user to choose the best MOSFET for the peak current and output voltage desired. LTC3863 Inverting Converter The duty cycle for the inverting topology is the same as that of the Cuk converter, namely Duty Cycle (D) = V OUT/(V OUT – V IN) Similarly, given the same output voltage, input voltage and switching frequency, the circuits have the same duty cycle and the same inductor current slope (namely the ripple current, which equals V IN*t ON/L). Let's look at the current flow during switching cycles for each topology. Figures 10a and 10b show the current flow with the power switch closed and open.