Author: Jeff Zhang, Jian Li, Ya Liu, ADI
Design points dn571 – Introduction
For high-voltage I / O applications, the inductorless switched capacitor converter (charging pump) can significantly improve efficiency and reduce solution size compared to the traditional step-down or step-up topology based on inductors. By using a charging pump instead of an inductor, a “jumper capacitor” can be used to store energy and transfer it from input to output. The energy density of capacitor is much higher than that of inductor, so the power density can be increased 10 times by using charging pump. However, due to the traditional application of low power, stable voltage and grid pump, it is limited to start-up and charging.
ADI’s ltc7820 overcomes these problems and achieves a high power density, high efficiency (up to 99%) solution. This fixed ratio, high voltage, high power switched capacitor controller has four n-channel MOSFET gate drivers built in to drive the external power MOSFET to produce a voltage divider, voltage multiplier or negative output converter: specifically, 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 from up to 36V input Out conversion. Each power MOSFET performs switching at a constant preset switching frequency with a duty cycle of 50%.
Figure 1 shows a 170W output voltage multiplier circuit using ltc7820. The input voltage is 12V, the output voltage is 24V under the condition of load current up to 7a, and the switching frequency is 500KHz. Sixteen 10 μ f ceramic capacitors (type X7R, size 1210) act as a jumper capacitor to transmit output power. As shown in Figure 2, the approximate size of the solution is 23mmx16.5mmx5mm, and the power density is up to 1500W / in3.
Figure 1: a high efficiency, high power density 12vvin to 24V / 7a voltage multiplier with ltc7820
Figure 2: estimated solution size
Since inductors are not used in the circuit, soft switching is performed on all four MOSFETs, which greatly reduces the loss caused by switching. In addition, low rated MOSFET can be used in switched capacitor voltage doubler, which significantly reduces the conduction loss. As shown in Figure 3, the converter can achieve 98.8% peak efficiency and 98% full load efficiency. The balance of power consumption is realized among the four switches, the heat dissipation is propagated, and the work of reducing heat in the intelligent layout is simplified. The temperature recorder in Figure 4 shows that the temperature rise of the hot spot is only 35 ° C at an ambient temperature of 23 ° C and free air flow.
Figure 3: efficiency and load regulation rate of 12vvin to 24V / 7a voltage multiplier at 500khzfsw
Tight load regulation
Although the voltage multiplier based on ltc7820 is an open-loop converter, the high efficiency of ltc7820 maintains strict load regulation. As shown in Figure 3, the output voltage at full load only drops by 0.43v (1.8%).
In voltage multiplier applications, the ltc7820 can be started without withstanding capacitor surge charging current if the input voltage rises slowly from zero. As long as the input voltage ramp up slowly (lasting for several ms), the output voltage can track the input voltage, and the voltage difference between the capacitors remains small, so there is no large surge current.
The conversion rate control of input can be realized by using a disconnected FET on the input or using a hot swap controller, as shown in the typical application section of ltc7820 product manual. In Figure 1, a break FET is used at the input. Unlike the voltage divider solution, the voltage multiplier must be started from zero input voltage each time, but it can be started directly under heavy load. Fig. 4 shows the starting waveform under 7a load condition.
Figure 4: startup waveform under 7a load condition
Ltc7820 is a fixed proportion high voltage high power switched capacitor controller of ADI company. It has built-in gate driver to drive external MOSFET, which can achieve very high efficiency (up to 99%) and high power density. The 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.