Benm
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- Aug 16, 2007
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If you buy a finished product you're stuck with whatever the manufacturer chose to do.
There are some examples of lasers that can take both 1 18650 or 2 18350 cells, and hence have circuitry that can deal with a wide range of input voltages somehow.
With switching regulators you get a bit of a difficult situation if the output voltage is close to the input, and especially if the input is a battery that is over the desired output voltage when full, but under it when running low but still some power remaining.
This situation is something you would get running blue diode lasers of 2 lithium cells: Say the laser diode (plus wiring losses etc) require 7 volts to operate. If you power it from 2 lithium cells you get a bit of a problem: When fully charged they provide 8.4 volts or so, but you want to run them all the way down to a 6 volts (3.0 volts cutoff).
As during the discharge curve you need to switch from lowering to increasing the voltage, you cannot just use a buck or a boost converter in this case. A linear driver also doesn't work when the battery voltage goes under the desired output voltage.
At that point you basically have two options:
- connect the batteries in parallel instead of series, and use a boost topology
- use a sepic topology so you can buck and boost with the same circuit
And there is a third option: buck or boost the battery voltage to some fixed voltage, and boost/buck that to the desired output. This requires 2 switchmode conversions though and will reduce efficiency. Something like that is usually only used in cases where input voltage is totally unpredictable.
There are some examples of lasers that can take both 1 18650 or 2 18350 cells, and hence have circuitry that can deal with a wide range of input voltages somehow.
With switching regulators you get a bit of a difficult situation if the output voltage is close to the input, and especially if the input is a battery that is over the desired output voltage when full, but under it when running low but still some power remaining.
This situation is something you would get running blue diode lasers of 2 lithium cells: Say the laser diode (plus wiring losses etc) require 7 volts to operate. If you power it from 2 lithium cells you get a bit of a problem: When fully charged they provide 8.4 volts or so, but you want to run them all the way down to a 6 volts (3.0 volts cutoff).
As during the discharge curve you need to switch from lowering to increasing the voltage, you cannot just use a buck or a boost converter in this case. A linear driver also doesn't work when the battery voltage goes under the desired output voltage.
At that point you basically have two options:
- connect the batteries in parallel instead of series, and use a boost topology
- use a sepic topology so you can buck and boost with the same circuit
And there is a third option: buck or boost the battery voltage to some fixed voltage, and boost/buck that to the desired output. This requires 2 switchmode conversions though and will reduce efficiency. Something like that is usually only used in cases where input voltage is totally unpredictable.
