To experimentally verify the impact of the MOSFETs placement,
ADI's high efficiency, four-switch synchronous buck-boost
controller demo boards LT8390/DC2825A and LT8392/DC2626A
are used. As shown in Figure 7a and Figure 7b, the DC2825A has a
straight MOSFETs placement and the DC2626A has a 90° shape
MOSFETs placement. To make a fair comparison, the two demo
boards are configured with the same MOSFETs and decoupling
capacitors and tested at 36V to 12V/10A, 300kHz step-down
Figure 7: (a) LT8390/DC2825A hot loop with straight MOSFETs placement; (b) LT8392/DC2626A hot loop with 90° MOSFETs placement; (c) V
IN
ripple
waveforms at M1 turn-on. (Source: Analog Devices)
operation. Figure 7c shows the tested V
IN
AC ripple during the M1
turn-on moment. With the 90° shape MOSFETs placement, the V
IN
ripple has a lower magnitude and higher resonant frequency,
hence validating the smaller PCB ESL due to a shorter hot loop
path. On the contrary, because of the longer hot loop and higher
ESL, the straight MOSFETs placement results in much higher V
IN
ripple magnitude and slower resonant frequency. A higher input
voltage ripple also causes a more severe EMI emission, according
to the EMI test results in the study by Cho and Szokusha.
15
Advanced Power Solutions for Efficiency and Robustness | ADI
