Hi!
I'm designing a bench power supply for hobby needs. And I wanted to make it small, efficient and low noise
. I found an idea of buck pre-regulator very interesting, but certainly it adds some noise. However, in my applications noise is only of concern for low-power loads. So, a small analog circuit may be sensitive to the noise, but my micro-dril consuming 3A l will be fine. Keeping this in mind, I decided to make the buck regulator "switchable", i.e., it can be disabled under light loads and activated only when the dissipating power is higher than what the heatsink can safely dissipate.
So, here it is, please see attachment (Vrect is unstabilized rectifier output, SiZ300DT is mosfet half-bridge, sorry for ugly representation). The idea is that the buck is managed by a MCU PWM signal. When we want to pass through the preregulator we just write "1" into PWM pin and buck's half-bridge is "always on". What do you think about this?
There are a few potential issues though. First, high-side mosfet is normally driven by a bootstrap circuit that will not work if there is no switching. I solved this by supplying required driving voltage into the driver (VHI pin on U7). But I've never seen such configuration in any datasheet. Do you think this will work?
Second, do I need a snubber(C27 R12)? I saw snubbers shunting bucks' diodes to reduce the noise. So I left one here. But may be I don't need it?
I would like to reduce switching noise as much as possible so I chose low frequencies (<100KHz) to drive the buck and slow mosfet switching speed (R5,R7,R20) to make it quieter. But in the datasheet for UCC27223 ([1]) there are no gate resistors. Does this mean they are not really helpful? The datasheet only tells about R20 to slow down the high-side mosfet. Does this mean gate resistor R5 is not needed for high-side? And what about low-side?
PS forgot to say, input is 15-18V, up to 3A. Output up to 5A, if this all matters.
[1]
http://www.ti.com/lit/ds/symlink/ucc27223.pdf
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