Converter In-Rush Thermistors etc, UPDATE -4

Salvo wrote:
Let's look at it another way. During the first 1/4 cycle, the capacitor charged to 75V. At that point in time the ac voltage is at 170Vpk.

The current at peak voltage is:

I = Vin - Vcap / R
I = 170V - 75V / 3 ohm = 32A

I see that I made an error. Charge current does not remain constant. Still, I see no way the current can peak at 86A.

Sal

This is my last post on this subject...

The current in the capacitor tapers off to zero when it reaches the 75 volt peak value which means the diodes are reverse biased...

Once the diodes become reverse biased, the capacitor discharges through the load to some value until the next set of diodes become forward biased which occurs during the next half cycle, and this transient condition continues until the capacitor reaches its steady state peak voltage value.

The peak current occurs when the capacitor is completely discharged and acting like a short.

The zero current/charge point occurs at the capacitor's voltage peak of 75V.

The current calculated previously was the average current, or charge, received by the capacitor during what I assume based on your narrative is the first quarter cycle.

The average current is the area produced by the right triangle shaped current pulse.

The area of a triangle is 1/2 times the base times the height (Ipk) so the peak is approximately equal to 2 times the average current.

You calculated the average current at 36 amps and I calculated at about 43.2 amps which is about 86.4 amps peak/instantaneous.

In real life the peak current will be about 10% less because the current pulse isn't a true triangle due to the exponential property of the capacitor charge.

I have no idea where the magical 3 Ohms you use comes from, the model you are using is not valid...

You need to calculate the resistance from a linearly independent equation if you want it to be valid and it needs to be when the capacitor is acting as a short with peak current or during steady state conditions and not during the transient state...

I'm sure you will tell me I'm wrong again, but I will consider the source and will have a good laugh...

EDIT: Just saw you posted while I was writing my response and you didn't disappoint me. :B

You truly are a legend and I bow to your superior knowledge and graceful way of expressing yourself...

:p

Salvo wrote:
And you are full of theateratics and BS. The absolute worst case current is when the capacitor is fully discharged and the converter is turned on right at the peak of the sine wave.

The absolute max current is I = Vpeak / R = 170V / 3ohm = 56.6A

Your 80 some amps are way off base and WRONG!

Sal

Your 32 amps x 0.9 (which is minus 10%) x 2 = 57.6 amps or just about what I stated for my current from which I used the actual 1/4 cycle period instead of your 5ms estimate...

:p :p :p