2018.10.11 Si Power Device
From this article, we present examples of improvements to efficiency and other parameters in actual applications.
LED Illumination Circuits (boundary current mode PFC + DC/DC): Example of Efficiency Improvement and Noise Reduction Using MOSFETs
The following circuit is the relevant part of an actual LED illumination circuit. In this LED driver circuit, a DC/DC converter supplies power to an LED via boundary current mode (BCM) PFC.
In this circuit, the MOSFET which is the switch of the PFC section and the MOSFET which is the switch of the DC/DC converter section, and the gate resistor RG of the latter, are changed, and the efficiency and noise are compared.
Super-junction MOSFET (hereafter SJ-MOSFET) that was used in the original design is labeled "Original". For the Original MOSFET, the RG was set to 100 Ω, taking noise into consideration. With this as reference, the switches for the PFC and DC/DC converter sections were changed to three different SJ-MOSFET types, and RG values of both 100 Ω and 50 Ω were also tested. As the MOSFETs, R5207AND is a fast-switching MOSFET, while R6004END and R6007END are next-generation low-noise devices.
In the table below, yellow highlighting indicates efficiencies that are higher than for the Original case, and green highlighting indicates the highest efficiency. As a result, efficiency was best for the combination of an R5207AND in the PFC section and an R6004END in the DC/DC converter section, with an RG of 50 Ω (there are nine combinations of three device types, but those combinations with poor results are omitted). Compared with the Original case, the improvement is around 1% or so. The efficiency is that for the whole circuit.
In addition, noise characteristics of the DC/DC converter section are presented. Data is for the Original MOSFET and for R6004END/50 Ω, which resulted in the highest efficiency.
The Original MOSFET had a comparatively fast switching speed, and so an RG of 100 Ω was reasonable as a means to counter noise; but by using the low-noise R6004END and setting RG to 50 Ω, even at fast switching speeds, noise was lower than in the Original circuit, so that noise could be reduced simultaneously with an improvement in efficiency.
In order to determine the reason for this, we compare waveforms for the PFC section and for the DC/DC converter section. The following are the PFC section waveforms.
In the PFC section, efficiency was highest for the R5207AND, but over this range a determination was not possible, and so the display is enlarged.
The time for transition to the ON state is somewhat slower for the R6004END and the R5207AND compared with the Original device. Next we look at the transition to OFF.
We see that the transition to OFF is sudden and fast for the R6004END and the R5207AND. It is thought that this reduces the switching loss and so contributes to higher efficiency. Next, we consider the switching waveforms of the DC/DC converter section.
The Original device is compared with the R6004END and R5207AND, which exhibited good efficiency. With the R6004END, the waveform for an RG of 50 Ω is also shown. The ON and OFF waveforms are again expanded.
The transition to ON is fastest for the R6004END, and is faster still when RG = 50 Ω.
The transition to OFF is slowest for the R6004END when RG = 100 Ω, but when RG = 50 Ω the transition is fastest using this device. In this comparison, the transition was fastest for R6004END + RG = 50 Ω, and it is thought that for this reason the switching loss is reduced, contributing to improved efficiency.
When the R5207AND is used in the PFC section and the R6004END is used in the DC/DC converter, switching losses are reduced, albeit slightly, and an efficiency improvement of about 1% was achieved. Moreover, noise is also improved while reducing switching losses. In this way, by reviewing the characteristics and gate resistances of the MOSFET switches, it may be possible to improve both efficiency and noise characteristics. Thus similarly to the diodes that were discussed in the previous two articles, it is important to thoroughly examine the characteristics of MOSFETs as well.
・Efficiency changes depending on differences in the characteristics of the MOSFETs used as switches in the PFC section and the DC/DC converter section, and so it is important to thoroughly study the MOSFET characteristics.