Switching PSU output capacitor and inrush current

[whitecollar]

I am building a 4.8v to 24v switching power supply based on the MAX1771 IC.

I wish to charge a 22,000 uF capacitor bank to about 24v.

My question is, can I simply replace the output smoothing capacitor C4 of my MAX1771 circuit (diagram 2b on page 8 of datasheet here) with my 22,000 uF capacitor bank?

I assume not, because the inrush current would be very high. Please tell me if this assumption is correct.

The only way I can think of charging the 22,000 uF capacitor bank is to design the switching power supply completely separately, with an output current of say 250 mA, and then use a series resister of 96 Ohms to limit charging current of my capacitor bank to this value. However, at the instant the capacitor bank starts charging, my current limit resister will have to dissipate 5w of power (assuming a 96 Ohm resister). This seems too high to me.

Either my calculations and assumptions are wrong, or there must be a batter way of doing this.

Many thanks,
Andrew.

 

[BradtheRad]

If you are using a boost converter in the simple configuration, then the coil and capacitor get immediate startup surge from the 4.8V power supply.

As you know startup surge occurs in many power supplies. It causes us to have to rate components high enough to handle it if we want maximum power. The alternative is to install resistors, etc., which limit power.

It should be okay to put your large capacitor as the smoothing capacitor. It will reach 24 volts after a couple of seconds.

The power supply must put a few amps through the coil during each cycle, to achieve what you want.

The startup surge may automatically be limited by components such as:

> Does your 4.8V supply come from a transformer? It will automatically tend to limit current surge.

> Does your supply have a smoothing capacitor? It may drop a couple of volts at startup.

> DC resistance of your coil will automatically limit current to some extent.