# Ripple Voltage from Constant Power Load

Discussion in 'Electronic Design' started by D from BC, Feb 11, 2008.

1. ### D from BCGuest

Here's something I'm stuck on...

120VAC----Bridge
60Hz ----Rectifier-----+------+ <<<What's the ripple voltage?
| | |
| 180uF 90W Constant
| | |
+----------+------+

A constant power load will draw whatever current to maintain 90Watts
of dissipation.
As the capacitor discharges, the current increases to maintain the
power dissipation.

I could spice for an answer but..
I haven't figured out yet how to model a constant power load in
LTSpice..

D from BC
British Columbia

2. ### leggGuest

For a capacitive rectifier, the droop in the capacitor will be the
same for constant power as it will be for equal average power.

The problem is determining the exact intersect between the rectified
sine and the capacitor droop to get the discharge period. That sounds
like integral calculus to me, but its probably just algebra and trig.

For rough estimation you can assume 2mSec charging in a 6mSec period,
that leaves ~4mSec to discharge.

C/2 x ( V1^2 - V2^2) = delta j delta j = P x t j = CV^2/2

1.8E-4/2 x ( 160^2 - V2^2) = 90 x 4E-3

root{ 160^2 - ( 180 x 4E-3 / 1.8E-4 ) = V2 = 147V

ppk ripple = 166 - 147 = 20V

Worst case ripple occurs at low line as the initial energy stored in
the caps reduces.

RL

3. ### D from BCGuest

I figured out a model to confirm.
470kb jpeg
Got Vripple = ~21.77Vpp
Those pesky diodes...

Good math.. Your rough estimate was accurate enough for my app.

Thanks

D from BC
British Columbia