EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: F1GWR on December 06, 2019, 10:00:47 am
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Who could help in explaining how this SMPS buck regulator works :
- how is it powered (anode VR131, pin 5 U131, pin 3 U132, TP102)?
- "ground" return is it at a variable potential and pulsed?
- C131 has too low value to filter out VR131 voltage (t = C.V / I ~ 3 µs (switching frequency is around 80 kHz)
Christian |O
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This is not weird , this is how you do it if you don't have an insulated power rail . The power transistor Q102 is driven G-S , so the source is made virtual ground . The reservoir capacitor is C132 , it seems to be enough to hold the charge when the source goes high . It is charged through diode CR131 when source is low .
Usually , buck converter ICs , like LM2576 and many , many others use a P channel Mosfet which doesn't require this to be driven .
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It was also puzzling me, I didn't get why the other end of the +14VP wasn't referenced to the MOS source until I looked at the full schematic and saw it was also used elsewhere.
So it's as CDaniel said:
CR131, C131, C132 for the bootstrapped supply, for time constants given the values of C131, C132 the buck regulator must always have a low duty-cycle.
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This kind of driving topology is fairly common for half-bridge upper Mosfet , which source is also floating ... and works well , you don't need an insulated power rail just for that .