Need a voltage regulator circuit

[AndreasC]

I would need to boost about 4,8 V to 9 V and be able to draw up to 3 A of current. I have 4 NiMh cells in series and I want to up the voltage to 9 v. I looked up some regulators but they doesn't seem to be able to supply as much as 3 A for my load even though the regulator is rated for 3 A. I thought about the possibility of using a mosfet or something that can provide higher current than the regulator can provide. Any help on a suitable circuit would be much appreciated.

Reason for voltage regulation is that I'm concerned about the voltage. When batteries are fresh, they meassure higher voltage which then drops when they drain. I could design for the higher voltage, say that I used 8 NiMh in series to get about 12 V, but after they've drained for a while, the voltage drops and then I'm concerned that the circuit will perform to lesser satisfactory. Any tips how I could get around this without the need of voltage regulation?

 

[(*steve*)]

You're far better off starting with a higher voltage (say 12V) and regulating it down to 9V that to use a lower voltage and a boost SMPS to increase the voltage.

Even at 100% efficiency, you'd be looking at 6A or so from the batteries to get 9V at 3A. If this causes the voltage to fall (as it would) things only get worse.

These regulators already use mosfets, so adding another one isn't going to solve your problems. The limiting factor may well be the inductor used. If you try to draw oo much current from a boost SMPS and the inductor saturates, bad things happen.

 

[AndreasC]

You're far better off starting with a higher voltage (say 12V) and regulating it down to 9V that to use a lower voltage and a boost SMPS to increase the voltage.

Even at 100% efficiency, you'd be looking at 6A or so from the batteries to get 9V at 3A. If this causes the voltage to fall (as it would) things only get worse.

These regulators already use mosfets, so adding another one isn't going to solve your problems. The limiting factor may well be the inductor used. If you try to draw oo much current from a boost SMPS and the inductor saturates, bad things happen.

You basically mean that I'll get quite a lot of power loss this way? Batteries deliver around 6A but I can only get 3 A on the regulator output?

I thought about using 8 cells in series and design for that voltage and when circuit start to function poorly, I would just need to recharge them.
Any other advice though?

 

[(*steve*)]

You basically mean that I'll get quite a lot of power loss this way? Batteries deliver around 6A but I can only get 3 A on the regulator output?

At 100% efficiency power out = power in.

If you want 9V @ 3A that is 27W.

from 4.8V, 27W is 27/4.8 = 5.625A

However you're likely to get maybe 75% efficiency, so your input current will be closer to 100/75 * 5.625 = 7.5A

If the voltage from the batteries drops to 4V under this load, the current demanded will be 27/4 * 100/75 = 9A (and possibly more as the efficiency will likely drop)

You *can* do this, but it's a lot easier to get a higher voltage battery and regulate it down to the voltage you require.

A 3A buck (voltage reducing) regulator is a lot easier to find. In addition you have the option of a simple like a linear regulator which. while not as efficient, can be a lot easier to get working.