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super cap charging dilemma

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larryg

Electrical
Feb 23, 2001
25
I'm looking for a clever and simple way to current limit charge some super caps and also allow them to be available to provide pulse power. Let me explain further:

I'm providing 3.6V from a regulated output to a GSM/GPRS modem. During transmission, I need to suppply up to 2A peak for about 1.15 ms. My supply can only provide about 800 mA. To get around that,I'm using two supercaps in series (4.7F, 2.5V)to provide the pulse current. What I want to do is to trickle charge (50-100 mA)the caps while allowing the 3.6V rail to power up the other components(microcontroller,etc).Once charged, the caps need to be available to provide boost on the 3.6V rail to the modem. I don't have any type of control signal available to switch the caps in/out of the charging circuit. Am I making sense?

Any help would be appreciated. Thanks
 
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Hi, a resistor in parallel with a schotky diode between the cap and supply.
 
You also need to bypass the two caps to equalize leakage, otherwise the voltages will not be equal and you'll destroy one or the other of them.

You might also consider whether your supply will tolerate overcurrent or a short for 10 seconds. Most of them are fairly happy. The uProcessor may have an undervoltage reset and so it may not kick in until you hit slightly under 3.6 volts.


DspDad
 
You have not said whether the 1.15ms current pulse is a "one-off" or is repetitive. A resistor and diode are a useful way of doing this circuit, but if the pulse is repetitive have you done the maths on the duty cycle / voltage variation allowed on the supply to the modem between pulses?
 
Hi all,

Thanks for the posts. BrianG brought up a good point I had overlooked. The radio transmission sequence is on a 25% DC (1.154 ms/4.616 ms) and is repetitive. The voltage drop calculation for the burst of 2A comes out at around .8V, which is too low for the radio to handle. So...To get around that, I will move the supercaps to the input side of the 3.6V regulator (5V input). That will take care of the voltage drop problem. However, I still need a way to slow-charge the caps without dragging down the 3.6V regulator in the process. I was thinking of putting a resistor in line with the caps from the +5v rail to control the charge, but am not sure how to connect the caps to the input of the +3.6V reg (which is also where the +5V rail is connected). That connection would nullify the resistor. Any further thoughts?

Thanks to all for your posts

larryg
 
What you want to do gets a little complicated without having voltage drops across a diode or a resistor.

I suggest the following more complex solution. Add a second 3.6 volt regulator just for charging the super caps. Select the regulator for the maximum charge current of the caps. Add a comparator controlling a P-MOSFET such that if your 3.6V system supply falls below the 3.6V of the supercap, the 3.6V cap supply is connected by the MOSFET to the 3.6V system supply.

You will have to examine carefully the current limit specs of your regulators. Also, you may have to adjust supplies so the system supply is nominally slightly higher than the other supply - probably 50 mV.
 
If you have a 25% duty cyle with a 4.6 msec repeat interval, then the caps are way too big (by a factor of 10000). Choose the cap size based on allowable voltage drop during the 1.15 msec 'on' time.

I * delta time = C * delta voltage

250 uF would give you 0.1 volts drop.

DspDad

 
For a burst of only 1.15mS every 4.6mS, you don't need that much capacitance. A 330uFd to 470uFd cap should be fine. However, for long term performance, perhaps you should consider a "low ESR" type as these handle pulses better and they are not much more money.
 
Cooper Electronics for their "PowerStor Aerogel" capacitors has a energy storage worksheet that takes into account pulse time, capacitance, and maximum allowable voltage drop. This is an on-line Excel worksheet that you can save to your hard drive and modify.

You can find it at:
 
Hi All,

Thanks for all the informative posts. I forgot to mention one important part of the design. In addition to providing pulse power, I need to hold up the 3.6V rail for 30-40 seconds in the event of a power failure. I apologize for not providing this information earlier. So, it seems that I will indeed need to use the super caps and not downsize as was suggested (which would have been nice due to space constraints). So, I am back to the original problem...

Thanks
LarryG
 
Then you have to charge the super cap through a current limited device. A transistor with a current limiter or a current limited voltage regulator IC.
 
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