thanks, LLC topology is not much suitable for variable output voltage. Or am i wrong?
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In general, I think you're correct there. LLCs do tend to suffer from a slightly narrow range of control and at least the control dynamics change rather a lot if you push them to the limits. There are things that are possible with mixing PWM and frequency control to extend their control range... but its a bodge really. LLCs make for a better front end, as you can keep the losses relatively low and they are less sensitive to transformer parasitics and afterall, in a lab supply you can suffer a slightly lower efficiency by using a second converter, especially if it keeps the output well controlled.
There are of course plenty of MCUs that make themselves well suited to direct PSU control, and you can do things to prevent un-determined startup conditions and faults from locking the PWM high etc (like using a watchdog to inhibit the gate drivers), but, yeah, I burned through a lot of FETs when debugging a control loop! Best to just use a DAC to set the reference to an analogue controller.