

15th September 2009, 13:35 #1
Member level 5
voltage drop formula
what factors effect the voltage drop on PWM dcdc converter?

15th September 2009, 13:35

15th September 2009, 19:20 #2
Super Moderator
why current drop in pwm
The voltage drop inputoutput? That's the stepdown
ratio which becomes roughly the PWM duty cycle,
VOUT/VIN. The drop is (duh) VINVOUT.
The voltage droop, or load regulation, is pretty much
just the error amplifier (and powertrain) composite
gain as a chopped transconductor. If you are current
mode controlled then you have a voltsin, currentout
relation somewhere in there (say, 1A/V) and a volts/volts
error amp gain (say, 60dB=10,000). So at the error amp
input you would see for a 1A load step, a 1/10,000 (or 0.1
mV) input deflection. Now scale that up by the feedback
network ratio (say, 5:1 for a 28V to 5V buck) and you get
a 0.5mV output deflection. All happy, right?
Well, probably not. A major, major term in load regulation
is the current induced offsets in the ground and the feedback
lines, which develop currentdependent error voltages. If
the load current return and the PWM ground are offset,
this may go right into the error amp (prime reference is
chip ground, != package, !=board ground). You would like
a Kelvin ground and load sense, but this is not always a
practical thing.