LED Switching Regulator

[Hero999]

Here's a floating version of this circuit.

This can be used in the car with a chassis return.

It also has the advantage of being short circuit proof and of being more efficient since the current driving the transistor isn't completely wasted.

 

[Hero999]

Another thing you should be aware of is that, if poorly designed, this circuit can fry LEDs if a brownout occurs. When the input voltage drops below a certain level, it will stop oscillating, this will mean the LEDs will overheat if they can't withstand the peak current continiously. The solution to this is to design the circuit so the peak current never exceeds the absolute maximum rating or add a low voltage disconnect circuit to protect it from a brownout.

 

[MrAl]

Then why bother with a specialised IC when a cheap comparator and transistor will do most of the time?

Hi there,


The idea is to light the LED and also get good regulation
with fluctuations in power supply voltage and LED voltage.
Many converters like this draw more current as the power
supply voltage rises, and sometimes we want to keep the
output power at some predetermined level even if the
voltage changes drastically.
Perhaps you can do a quick study to see how much the
LED current changes with power supply voltage, but
you also need to add in some resistance in series with
the inductor and in series with the power supply.

 

[Hero999]

The idea is to light the LED and also get good regulation
with fluctuations in power supply voltage and LED voltage.
Many converters like this draw more current as the power
supply voltage rises, and sometimes we want to keep the
output power at some predetermined level even if the
voltage changes drastically.
Perhaps you can do a quick study to see how much the
LED current changes with power supply voltage,

I've it at different voltage and you're right the current regulation isn't perfect. Generally the current drawn from the supply reduces with increasing voltage and the LED current increases slightly.

but you also need to add in some resistance in series with
the inductor and in series with the power supply.

I don't understand why you would want to do this, the inductor already has a series resistance and the current sense resistor adds to this; why would you put another resistor in series with the power supply which would only make it less efficient?

 

[MrAl]

I've it at different voltage and you're right the current regulation isn't perfect. Generally the current drawn from the supply reduces with increasing voltage and the LED current increases slightly.


[inductor and power supply series resistance addition]
I don't understand why you would want to do this, the inductor already has a series resistance and the current sense resistor adds to this; why would you put another resistor in series with the power supply which would only make it less efficient?

Yes, the regulator ic's usually have some form of regulation built in
to keep the output at some preset level, and it stays that way for
a wide range of input/output conditions.

The series resistor for the power supply is when you are using
batteries for the actual real life power supply, i apologize if you
are not doing this. The inductor series resistance also affects
regulation sometimes and even circuit start up. If it's low
however it wont be a problem, but i just thought i would mention
it.

 

[Ubergeek63]

Actually, I have considered that. I always try to grossly over rate Schottkys so the voltage loss is really low. My estimate was based on SPICE simulations and was just a rough guess, I didn't do an exact calculations. I jusr looked at the figures spat out by it.

The Schottky I've used in the simulation is rated for 3A and it only passes a maximum of 369mA which is 13.2% of the rated current.

The loss is only 288mV at the peak current draw and 261mV just before the transistor turns on which gives an average of only 275mV; this is <10% of one LED drop and <5% of the total LED drop in this circuit.

The transistor saturation loss is between 83.1mV and 103.2mV giveing an aveage of only 93.15mV.

The duty cycle is 63.5%.

The rise time is 6.7ns.

The fall time is 28.8ns.

The over all period is 6.309µs.

The inductor ESR was just set to 1mΩ which I know is far too low and needs increasing but my guess took account for this. I could do some more accurate calculations but there's no point with SPICE, it's much better to build the circuit.

Sorry I missed this before... I usually post quickies unless I have something well formed already or my wife is out.

So it sounds like you had that end right... it is just often even the dissipation in the rectifier is missed, never mind the inductor losses.

 

[Hero999]

Yes, the regulator ic's usually have some form of regulation built in
to keep the output at some preset level, and it stays that way for
a wide range of input/output conditions.

This circuit has regulation too, it just isn't perfect.

It uses a comparator as a schmitt trigger with preset on and off threasholds which vary very little as the power supply voltage changes.

The series resistor for the power supply is when you are using
batteries for the actual real life power supply, i apologize if you
are not doing this. The inductor series resistance also affects
regulation sometimes and even circuit start up. If it's low
however it wont be a problem, but i just thought i would mention
it.

I still don't see why you need a series resistor regardless of what the power supply is. It's true that you might want to incorperate over-current protection but a simple fuse or PTC resistor would do that.

 

[MrAl]

This circuit has regulation too, it just isn't perfect.

Yes, while many regulation chips are made to be very close over
a wide range of operating conditions. They have some extra circuitry
built in for the sole purpose of regulating some parameter. This
extra step is what puts them in a class where you start looking
at little things like percent voltage regulation with line and
with load. The dc accuracy is made to be very good on purpose,
not as a by product of some other function for the same amplifier.
This is often done with an integrator, added in just for that.

I still don't see why you need a series resistor regardless of what the power supply is. It's true that you might want to incorperate over-current protection but a simple fuse or PTC resistor would do that.

When batteries are used as power sources in real life there is always
some internal resistance, while in a spice environment you have to
add it yourself. The point is that some power converters work
very well with a zero impedance source, but as soon as you add
some real world series resistance they dont regulate as well.
This is why i suggested trying a circuit with some series resistance
if it is going to be run on batteries.