After reading my last blog you may have considered placing the LLC series resonant converter (LLC-SRC) into your next power supply. LLC-SRCs are usually hiding behind another well regulated power stage, such as the example in Figure1. This isn’t ideal in terms of circuit cost because the BOM cost of a two-stage PSU is always higher than a single stage PSU. What you want is to replace the Flyback converter in your old designs with an alternative topology that provides higher efficiency and lower EMI and avoid increasing BOM cost. Is the LLC-SRC truly ruled out by the single stage and low cost topology selection criterion?
The LLC-SRC needs another power stage in front of LLC-SRC due to its sensitivity to input voltage variation. Designers generally insert another power stage to provide a well-regulated input voltage for the LLC-SRC as it optimizes LLC-SRC efficiency. In general, LLC-SRC efficiency can be >94% in a two-stage PSU.
If you take a look at the PMP5141 design example in my last blog, you find that the LLC-SRC in PMP5141 can only operate within 280VDC to 400VDC input range. If we use PMP5141 LLC-SRC stage in a single stage AC-DC off-line PSU, we might only accept 240VAC+/-10% input. At this point, the LLC-SRC operational input voltage range now needs to be wider.
Higher Lm/Lr ratio leads to a narrower regulation range. Hence, if we lower the Lm/Lr ratio, the operational input voltage range has the capacity to be wider. PMP5141's ratio close to 5. If we reduce its Lm to make the Lm/Lr ratio 3, the minimum input voltage can be as low as 220VDC, as shown in Figure 2. The LLC-SRC parameter is now optimized for a single stage PSU with 176VAC to 276VAC input range.
The example above illustrates how lowering the Lm/Lr ratio making a single stage LLC-SRC PSU possible. A concern of reducing Lm is the smaller Lm makes the circulating current at primary side higher, which is a reduction of the converter efficiency. Keep in mind we are now a single stage PSU, we do not have the efficiency reduction on the power stage in front LLC-SRC as we had before.
PMP10283 is a single stage LLC-SRC PSU designed for 88VAC-132VAC or 176VAC-264VAC Input, +/-30V/200W output. When comparing the total efficiency of the two-stage PMP5141 and single stage PMP10283 PSUs in Figure 3, the performance of a single stage LLC-SRC PSU is significantly greater. Single stage LLC-SRC efficiency can be as good as a two-stage PSU and provide much lower Bom cost.
PMP8762 – 100VAC-132VAC input, 12V/9A output is another design example of single stage LLC-SRC for US line. Keep an eye out for more single stage LLC-SRC TIDesigns in the upcoming months