The power density of DC/DC converters is usually limited by bulky magnetic components, especially in applications with relatively high input and output voltages. The size of the inductor/transformer can be reduced by increasing the switching frequency, but the loss caused by the switching will also reduce the converter efficiency. A better method is to use a non-inductive switched capacitor power supply (charge pump) topology to completely eliminate magnetic components. Compared with traditional DC/C power supplies, charge pumps can increase power density by as much as 10 times without sacrificing efficiency. The flying capacitor replaces the inductor to store energy and transfer it from the input to the output.Despite the advantages of charge pump design

The power density of DC/DC converters is usually limited by bulky magnetic components, especially in applications with relatively high input and output voltages. The size of the inductor/transformer can be reduced by increasing the switching frequency, but the loss caused by the switching will also reduce the converter efficiency. A better method is to use a non-inductive switched capacitor power supply (charge pump) topology to completely eliminate magnetic components. Compared with traditional DC/C power supplies, charge pumps can increase power density by as much as 10 times without sacrificing efficiency. The flying capacitor replaces the inductor to store energy and transfer it from the input to the output. Despite the advantages of charge pump design, switched capacitor power supplies have traditionally been limited to low-power applications due to challenges in startup, protection, and MOS transistor gate drive.

LTC7820 is a fixed conversion ratio, high voltage, high power switch capacitor power supply control chip, which can provide a small, cost-effective solution with fault protection for high power, non-isolated intermediate bus applications. Features of LTC7820 include:

Thin profile, high power density, can provide more than 500 W power
Maximum VIN for voltage divider (2:1): 72 V
Maximum VIN for voltage doubler (1:2)/inverter (1:1): 36 V
Wide bias VCC range: 6 V to 72 V
Soft switching: 99% peak efficiency and low EMI
Soft start into steady state
Input current detection and overcurrent protection
Integrated MOS tube driver
Output short circuit/overvoltage (OV)/undervoltage (UV) protection with programmable timer and retry function
Thermally enhanced 28-pin 4 mm × 5 mm QFN package
48 V to 24 V/20 A voltage divider with a power density of 4000 W/In3

High-efficiency, high-density switched capacitor power supply for high-power applications

Figure 1 shows a voltage divider circuit using LTC7820’s 480 W output. The input voltage is 48 V, the output is 24 V, and the load current is up to 20 A. Sixteen 10 F ceramic capacitors (1210 size) are used as flying capacitors to transmit power. As shown in Figure 2, the approximate size of the solution is 23 mm × 16.5 mm × 5 mm, and the power density is as high as 4000 W/in3.

Figure 1. 48 V to 24 V/20 A voltage divider with a power density of 4000 W/In3.

High-efficiency, high-density switched capacitor power supply for high-power applications

Figure 2. Estimated solution size (maximum height is 5 mm).

high efficiency

Since no inductor is used in this circuit, all 4 MOSFETs can achieve soft switching, which greatly reduces the loss caused by switching. As shown in Figure 3, the converter can achieve high efficiency, with a peak efficiency of 99.3% and a full load efficiency of 98.4%. The thermal image in Figure 4 shows a balanced thermal design. At an ambient temperature of 23°C and without forced air cooling, the hot spot temperature is approximately 82.3°C.

High-efficiency, high-density switched capacitor power supply for high-power applications

Figure 3. Efficiency at 48 V input, 24 V output and 200 kHz switching frequency.

High-efficiency, high-density switched capacitor power supply for high-power applications

Figure 4. Thermal test results at 48 V input, 24 V/20 A output and 200 kHz switching frequency.

Pre-balance to avoid inrush current

In addition to outstanding efficiency and thermal performance, the LTC7820 also uses a proprietary pre-balance method designed to minimize inrush current in voltage divider applications. Before performing the switching operation, the LTC7820 control chip detects the voltage of the VLOW_SENSE pin and compares it with VHIGH_SENSE/2 internally. If the voltage on the VLOW_SENSE pin is much lower than VHIGH_SENSE/2, a current source will inject a current of 93 mA on the VLOW pin to pull up VLOW. If the voltage on VLOW_SENSE is much higher than VHIGH_SENSE/2, another current source will draw 50 mA of current from VLOW to pull it down. If the voltage on VLOW_SENSE is close to VHIGH_SENSE/2 (that is, within the pre-programmed window), both current sources are turned off and the LTC7820 starts to perform switching operations.

Figure 5 shows that without pre-charging, there will be a huge input surge current at startup, which is enough to damage the MOSFET and capacitor. In contrast, after applying the pre-balance method, no excessive surge current was observed (as shown in Figure 6).
Figure 5. Start-up waveform when pre-balance is not used, showing that there is a huge inrush current.

High-efficiency, high-density switched capacitor power supply for high-power applications

Figure 6. The start-up waveform using LTC7820 pre-balance shows that the inrush current is eliminated.

High-efficiency, high-density switched capacitor power supply for high-power applications

Good load regulation

Although the voltage divider based on the LTC7820 is an open-loop control power supply, it can still get a good load regulation rate thanks to its high efficiency. As shown in Figure 7, the output voltage at full load drops only 1.7%.

Figure 7. Load regulation rate.

High-efficiency, high-density switched capacitor power supply for high-power applications

Protective function

The LTC7820 has protection functions to ensure high reliability. Overcurrent protection is realized by a current sampling resistor on the high voltage side. The precision rail-to-rail comparator is responsible for detecting the differential voltage between the ISENSE+ pin and the ISENSE- pin. These two pins are connected to the current sampling resistor in Kelvin. When the voltage on ISENSE+ is 50 mV higher than ISENSE-, an overcurrent protection is triggered, the FAULT pin is pulled down to ground, and the LTC7820 stops switching operations and starts the retry mode according to the timer pin settings.

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