DATA CENTRES  ANALOG DEVICES


Figure 3. A 48V to 12V/25A hybrid converter using the LTC7821


Figure 4. Possible layout for a complete bus converter uses the top and bottom sides of the board, requiring only 2.7cm2 of the topside of the board


 400kHz fSW


48V TO 12V AT 25A HYBRID CONVERTER FEATURING 640W/IN3 POWER DENSITY Figure 3 shows a 300W hybrid converter using the LTC7821, switching at 400kHz. The input voltage range is 40V to 60V and the output is 12V at loads up to 25A. Twelve 10μF (1210 size) ceramic  and CMID. The relatively small size 2μH inductor (SER2011-202ML, 0.75in × 0.73in) can be used because of the high switching frequency and the fact that the inductor only sees half of VIN at the switching node (small volt-second). The approximate solution size is 1.45in × 0.77in, as shown in Figure 4, resulting in a power density of about 640W/in3. As the bottom three switches always see half the input voltage, 40V rated FETs are used. An 80V rating FET is used for the very top switch because it sees the input voltage at the beginning of the  switching). During steady state operation, all four switches see half of the input voltage. Therefore, the switching losses in a hybrid converter are much smaller compared to a buck converter in which all switches see the full input voltage. Figure 5 shows    performance is very good, as shown in the Figure 6 thermograph. The hot spot is 92°C at an ambient


16 September 2024 Irish Manufacturing


Figure 6. Thermograph of the hybrid converter solution in Figure 2


Figure 7. The pre-balancing period in the LTC7821 start-up avoids high inrush currents


Figure 8. LTC7821 start-up at 48V input, 12V output at 25A (no high inrush current)


  CMID pre-balancing technique, which prevents input inrush current during start-up. During initial   voltages are not at VIN / 2, the TIMER capacitor is allowed to charge up. When the TIMER capacitor voltage reaches 0.5V, internal current sources are   charged to VIN / 2. The TRACK/SS pin is pulled low during this duration and all external MOSFETs are  VIN / 2 before the TIMER capacitor voltage reaches 1.2V, TRACK/SS is released, and a normal soft start


Figure 9. Connection of key signals of LTC7821 for a 2-phase design.


begins. Figure 7 shows this pre-balancing period and Figure 8 shows the VOUT soft start at 48V input, 12V output at 25A.


1.2KW MULTIPHASE HYBRID CONVERTER


The easy scalability of the LTC7821 makes it a  found in telecom and data centres. Figure 9 shows the key signal connections for a 2-phase hybrid converter using multiple LTC7821s. The PLLIN pin of one LTC7821 and the CLKOUT pin of another LTC7821 are tied together to synchronise the PWM signals.


For a design with more than two phases, the PLLIN pin and CLKOUT pin are connected in a


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