“This paper analyzes the loss composition of the inverter, compares the performance of the fourth generation IGBT and the seventh generation IGBT through thermal simulation, and finally verifies the conclusion through experiments. Under the same working conditions, the loss and junction temperature of IGBT7 are significantly lower than those of IGBT4, so that the volume of the inverter can b...
Design Essentials DN571 – Introduction
For high voltage input/output applications, inductorless switched capacitor converters (charge pumps) can significantly improve efficiency and reduce solution size compared to traditional Inductor-based buck or boost topologies. By replacing the inductor with a charge pump, a “crossover capacitor” can be used to store and transfer energy from the input to the output. The energy density of capacitors is much higher than that of inductors, so using a charge pump can increase the power density by a factor of 10. However, charge pumps have traditionally been limited to low-power applications due to challenges in startup, protection, gate drive, and regulation.
ADI’s LTC7820 overcomes these issues, enabling high power density, high efficiency (up to 99%) solutions. This fixed-ratio, high-voltage, high-power switched capacitor controller incorporates 4 N-channel MOSFET gate drivers for driving external power MOSFETs to create a voltage divider, voltage doubler, or inverting converter: specifically That is, a 2:1 step-down ratio from up to 72V input, a 1:2 step-up ratio from up to 36V input, or a 1:1 negative output transition from up to 36V input. Each power MOSFET switches with a 50% duty cycle at a constant preset switching frequency.
Figure 1 shows a 170W output voltage doubler circuit using the LTC7820. The input voltage is 12V, the output is 24V at up to 7A load current, and the switching frequency is 500kHz. Sixteen 10μF ceramic capacitors (type X7R, size 1210) act as a jumper capacitor to transfer the output power. As shown in Figure 2, the approximate dimensions of this solution are 23mm x 16.5mm x 5mm, and the power density is as high as 1500W/in3.
Figure 1: A High Efficiency, High Power Density 12V VIN to 24V/7A Voltage Doubler Using the LTC7820
Figure 2: Estimated solution size
Since no inductors are used in this circuit, soft switching is performed on all 4 MOSFETs, greatly reducing losses due to switching. Additionally, low voltage rated MOSFETs can be used in switched capacitor voltage doublers, which significantly reduces conduction losses. As shown in Figure 3, the converter achieves a peak efficiency of 98.8% and a full load efficiency of 98%. Power balance between 4 switches spreads heat dissipation and simplifies heat reduction in smart layouts. The temperature logger in Figure 4 shows that at an ambient temperature of 23°C and free air flow, the temperature rise of the hot spot is only 35°C.
Figure 3: 12V VIN to 24V/7A Voltage Doubler Efficiency and Load Regulation at 500kHz fSW
tight load regulation
Although the LTC7820-based voltage doubler is an open-loop converter, the high efficiency of the LTC7820 maintains tight load regulation. As shown in Figure 3, the output voltage drops only 0.43V (1.8%) at full load.
In voltage doubler applications, if the input voltage is slowly ramped up from zero, the LTC7820 is able to start up without experiencing a capacitor surge charging current. As long as the input voltage ramps up at a slow rate (several ms duration), the output voltage tracks the input voltage and the voltage difference between the capacitors remains small, so there are no large inrush currents.
Slew rate control of the input can be accomplished by using a breakout FET on the input or using a hot-swap controller, as shown in the Typical Applications section of the LTC7820 data sheet. In Figure 1, a breakout FET is used on the input. Unlike the voltage divider solution, the voltage doubler must start from zero input voltage every time, but it can start directly with heavy loads. Figure 4 shows the start-up waveform under 7A load condition.
Figure 4: Startup Waveforms at 7A Load
The LTC7820 is a fixed-ratio, high-voltage, high-power switched capacitor controller from Analog Devices with built-in gate drivers to drive external MOSFETs for very high efficiency (up to 99%) and high power density. Robust protection features make the LTC7820 switched capacitor controller suitable for high voltage, high power applications such as bus converters, high power distributed power systems, communication systems and industrial applications.