A fixed ratio high voltage high power switched capacitor controller

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 "span capacitor" can be used to store energy and transfer energy from the input to the output. Capacitors have a much higher energy density than inductors, so a charge pump can increase power density by a factor of 10.

However, charging pumps have traditionally been limited to low power applications due to challenges in startup, protection, gate drive and regulation .

The LTC7820 overcomes these problems and enables high power density, high efficiency (up to 99%) solutions. This fixed-scale, high-voltage, high-power switched-capacitor controller has four N-channel MOSFET gate drivers that drive an external power MOSFET to create a voltage divider, voltage doubler, or negative output converter: specifically It is said to achieve a 2:1 step-down ratio from up to 72V input, a 1:2 boost ratio from up to 36V input, or a 1:1 negative output conversion from up to 36V input. Each power MOSFET performs a switching operation with a 50% duty cycle at a constant preset switching frequency.

Figure 1 shows a 170W output voltage multiplier 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 16μF ceramic capacitors (X7R type, 1210 size) function as a jumper capacitor to deliver output power.

Figure 1: A high efficiency, high power density 12V VIN to 24V/7A voltage doubler using the LTC7820

2, the approximate dimensions of the solution is 23mm x 16.5mm x 5mm, and a high power density of 1500W / in 3.

Figure 2: Estimated solution size

Since the inductor is not used in this circuit, soft switching is performed on all four MOSFETs, greatly reducing losses due to switching. In addition, low voltage 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% with a full load efficiency of 98%.

Figure 3: Efficiency and load regulation of a 12V VIN to 24V/7A voltage doubler at 500kHz fSW

Achieve power balance between the four switches, propagate heat dissipation, and simplify heat reduction in smart layouts. The temperature logger in Figure 4 shows that at 23 ° C ambient temperature and free air flow, the temperature rise of the hot spot is only 35 ° C.

Figure 4: Thermal test at 12V VIN, 24V VOUT, 7A load, TA = 23°C, free air

Although the LTC7820-based voltage doubler is an open-loop converter, the LTC7820's high efficiency 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 ramps up slowly from zero, the LTC7820 can be started without being subjected to capacitor surge charging current. As long as the input voltage ramps up at a slow rate (for a few ms), the output voltage tracks the input voltage and the voltage difference between the capacitors remains small, so there is no large inrush current.

The slew rate control of the input can be achieved by using a disconnect FET at the input or by using a hot-swap controller, as shown in the Typical Applications section of the LTC7820 Product Manual. In Figure 1, a disconnect FET is used on the input. Unlike a voltage divider solution, the voltage doubler must be started from zero input voltage each time, but it can be started directly with heavy loads. Figure 5 shows the startup waveform under 7A load conditions.

Figure 5: Startup waveform under 7A load conditions

The LTC7820 is a fixed ratio high voltage, high power switched capacitor controller with a built-in gate driver to drive an external MOSFET for very high efficiency (up to 99%) and high power density. Rugged protection makes 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.