what is the formula of PWM dc-dc voltage drop- load current?

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pianomania

Joined: 15 Jun 2006
Posts: 55

15 Sep 2009 13:35 voltage drop formula

what factors effect the voltage drop on PWM dc-dc converter?

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dick_freebird

Joined: 04 Mar 2008
Posts: 309
Helped: 45
Location: USA

15 Sep 2009 19:20 why current drop in pwm

The voltage drop input-output? That's the step-down
ratio which becomes roughly the PWM duty cycle,
VOUT/VIN. The drop is (duh) VIN-VOUT.

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 volts-in, current-out
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 current-dependent 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.