Re: Auto switching power supply between USB power and external supply


ganeshok...@xxxxxxxxx wrote:
> Dear Friends,
> We are on to designing a communication board based on the NEC
V850
> microcontroller that will have a USB port for communicating with a
PC.
> The board requires a supply voltage of 5V and draws at the most 300
ma
> of current. We want to design the board to use two sources of supply
-
> one is from an external power supply if available and the other is
from
> the USB port. Power should be drawn from the USB port only if the
> external power supply is not present. The circuit should be able to
> sense this automatically. We have some ideas but each of them has one
> or the other hurdle.
>
> 1) Use a connector for connecting the external power supply which
will
> mechanically disconnect the USB ports supply voltage when plugged in.
> We are looking for a connector assembly (similar to the circular ones
> present on most cell phones) that can do this job. The catch here is
we
> do not know a source. If we get to know a source, this is the most
> acceptable option.
>
> 2) Use a simple diode based switch over circuitry where in the diode
on
> the USB ports line will reverse bias when the external supply voltage
> is present and hence disconnect the USB supply. The problem we have
he
> is that the USB power can go as low as 4.4V (for bus powered hubs) as
> per the specs and a further forward voltage drop of around 0.4 volts
> will leave us with an effective 4.1 V for the circuitry. We have an
> isolated DC-DC converter (5V-5V) at the input section which requires
a
> minimum of 4.5 volts to function. So the 4.1 V input will not
guarantee
> a 5V at the output of the DC-DC. We are trying to locate a DC-DC
which
> has a wider input range, preferably 3.5 V onwards which will resolve
> this problem. If this DC-DC is found, this is the second desired
> option.
>
> 3) Use a mechanical switch which the user sets to one of the two
power
> sources. This requiring end user intervention is not desired.
>
> I would like to request the readers:
> a) Let us know of any source for the type of connectors mentioned
> above.
> b) Let us know of any source of DC-DC that can output 5V with input
> voltage as low as 3.5 V. The DC-DC should be rated for atleast 500ma.
> We have searched under C&D Technologies, Tyco Power and Datel but
could
> not find anything suitable.
> c) Let us know of any other trick to achieve this.
>
> Thanks and Regards,
> Ganesh Okade
> Sunlux Technologies Ltd.
> Bangalore, India
> www.sunlux-india.com

For option 2), get a standard 6VDC @ 1A wall wart, a couple of 1N4002
diodes, and a Power One DFC6U5S5 single output DC-to-DC converter. It
accepts input voltages from 3.5 to 16VDC, and will give you 5.0VDC at
up to 1.2 amps. It also offers 700V isolation.

Option 1) is lacking because if the wall wart becomes unplugged from
the wall while it's plugged in to your blivet, your device will lose
power. Remember, a cell phone has a battery, and will still operate if
this happens. If that's a problem to you, try something else. You're
right, option 3) is lame.

By the way, no minimum load is required on the DFC6, but dynamic
regulation suffers at below 5% load (60 mA). Also, when you're running
at less than 4.6V for Vin (USB through the diode), you have to start
derating, but if you look at the data sheet, you'll see you have
nothing to worry about at 500mA or so of load.

http://www.power-one.com/resources/products/datasheet/dfc6s.pdf

Good luck
Chris

.

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