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JimmyM
Joined: 27 Nov 2006
Posts: 3
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 Posted:
Mon Nov 27, 2006 1:34 am Post subject:
PWM regulator w/o LC |
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I'm trying to build a PWM regulator for a low voltage light bulb.
a) Vin will be from batteries.(6-30 volts depending on application)
b) Vout will only be a couple volts below Vin.
c) As Vin drops the PWM needs to increase duty cycle to keep Vout the same.
d) At some point the Vin will drop below the set point (as batteries die). At that time the duty cycle should just stay at 100% and Vout will drop with Vin.
e) The output load is a light bulb and therefore a square wave is acceptable.
f) I need low volt cutout, current limiting, and soft start.
Every PWM controller application note I find is for either a syncronous or non-sych SMPS with an LC output stage. I don't need the inductor, catch diode, capacitor because the load is completely resistive and also because size is a real problem.
Does anyone know of a resource where I can find something like this to use as a baseline for design. I'm concerned that the feedback to the error amplifier isn't going to deal well with a square wave.
Thanks for the help.
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neon
Joined: 25 Feb 2006
Posts: 590
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Mon Nov 27, 2006 3:20 pm Post subject:
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Why go through all this to light a simple lamp. The lamp is a resistance type of load all you need is a constant current load to light it up as battery dies as some point the regulator will disconnect. AT WHAT POINT it depends on your choise of current sources diodes transistor 1v lm337 probably 4v. SIMPLICITY is the mother of all virtues
I am amazed of why people consistenly try to regulate voltage as opposed to current. |
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JimmyM
Joined: 27 Nov 2006
Posts: 3
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Mon Nov 27, 2006 3:46 pm Post subject:
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The lamp when run at voltages above spec (overdrive) pushes it close to the point where it pops.
As an example: a 12V 90W halogen bulb running on a 19.2 V supply. The bulb will blow if just connected to the battery. It can run on 16.0 volts if the voltage is ramped slowly or current is limited during startup. The time it takes to ramp to the running voltage and then dissipate 3.2V @ 8A will overheat a linear device that's small enough to fit in the confines of the flashlight. The bulb, when cold, will draw 20-40 amps for a few milliseconds. I wanted to use PWM since the bulb doesn't care if the current is supplied via square wave at ~50kHz.
What do you suggest as a solution to limit current on startup (or ramp voltage) and then maintain a fixed voltage as supply voltage drops without excessive dissipation?
I'm not quite a complete noob, but close. So any feedback would be greatly appreciated.
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neon
Joined: 25 Feb 2006
Posts: 590
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Fri Dec 01, 2006 5:55 pm Post subject:
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| YOU din't think that i will need that info before? I have no idea of your application but let me assure you that MAXIM 8632 will do it. The response to loads variations are in the 100ns it has UVP,OVP AND DIGITAL soft start operates from 2to 28v I am sure that you can find a PWM that will suite you. Switching is in the mhz. the package is in the 10 mm |
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JimmyM
Joined: 27 Nov 2006
Posts: 3
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Fri Dec 01, 2006 8:20 pm Post subject:
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| Thanks for the info, neon. I 'll have a look. |
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