Looking for a nice transformer driver chip

[WillHuang]

I'm working on a small project where I need to power a device from a PC's USB.

This is the device I that I am working with:

It's an ESD tester to see if people are properly grounded. It runs on a 9V alkaline battery.

I previously added external circuitry to include a uC to get readings off of the device. The circuit also has opto-isolators to isolate the uC from the device. However, what happens now is that after a couple of months the battery does not have enough juice to drive the opto-isolators and so the uC won't be able to get the correct readers.

So, now I'm thinking of powering the device from a PC's USB port. I need to double the voltage from ~5V to ~10V to power the device while also supplying about 100mA. I also need to have the two isolated which pops transformers into my head which means I will need a transformer drive IC .

I've been having troubles looking for a good IC. The first one that popped up into my search was TI's SN6501, but it doesn't seem to be able to provide enough current at ~9V. The SN6505 and LT3439 looks like they can do the job, but.. they are quite expensive!

I was hoping any of you guys would know of any good chips that will help do this job. Or if there's any other suggestions, that would be great!

[SeanB]

How about an isolated DC DC converter, like some variant on the 0509 which used to live on every 10M network card with a BBNC connector. 5V in and 9V out, and isolated to 2kV internally.

First result on fleabay....

http://www.ebay.com/itm/DC-DC-1W-isolated-converter-5V-IN-9V-OUT-MORNSUN-F0509S-1W-/130659182819?hash=item1e6be4e8e3:g:uBgAAOxyFiRR3g15#shpCntId

[WillHuang]

Oh hey that looks pretty good too, thank you! It looks very easy to implement as well. Seems like I just need to slap on a couple of bypass caps to smooth out the ripple voltages? And maybe a LDO to regulate the ouput?

[SeanB]

47uF on input and on the output will work, they were designed to run off a 5V rail in any case, though the current draw might be more than a proper USB port with an actual current sense and switch ( vanishingly rare these days, they mostly use a 2A polyfuse as it is cheaper) will want to supply without proper USB enumeration. But cheap, small and good isolation, though they will not reliably start up into too large an output capacitance.

[WillHuang]

Hm very interesting, I never knew about the enumeration of USB ports. I googled a bit, and it appears that the default current output to the USB is 100 mA without proper enumeration. If I treat the DC/DC converter as a transformer in terms of power, then on the USB side will be 500mW. Maintaining that power on the output will leave me at 50mA at 10V output.
If I need to enumerate the USB, I'm assuming this is done in some sort of driver file on the PC side?

[Skashkash]

Are you  just using the USB for power? Or is there an actual USB device to enumerate within your gizmo?

[WillHuang]

In this case, I would like to use the USB for power, although I do have a FT232R chip connected to my external uC to perform serial communication between the PC and uC.

[Siwastaja]

Hm very interesting, I never knew about the enumeration of USB ports. I googled a bit, and it appears that the default current output to the USB is 100 mA without proper enumeration. If I treat the DC/DC converter as a transformer in terms of power, then on the USB side will be 500mW. Maintaining that power on the output will leave me at 50mA at 10V output.
If I need to enumerate the USB, I'm assuming this is done in some sort of driver file on the PC side?

PC side driver for this simple task is already integrated in all operating systems.

But you need an IC that communicates with said driver. There are integrated power management IC's that you just design in and they negotiate the power for you - sorry, can't remember part numbers.

This being said, many PC's are just fine if you draw 500 mA without communicating anything - some are fine at even much higher currents, i.e., the port is only protected by a fuse. But I guess you can't depend on this as you might want the product to work on any PC.

[Skashkash]

In this case, I would like to use the USB for power, although I do have a FT232R chip connected to my external uC to perform serial communication between the PC and uC.

I'm still a bit unclear on that, but never mind.

 If you don't need USB communication, just use a dumb USB cell phone charger. 1 amp typical output.
 But that will vary.

If you have an FTDI chip in there...

 The FT232 has a eeprom programming utility available for download that allows you change the default chip behavior and some of the USB descriptor strings.

This allows you to specify the max current draw of the device (up to 500ma I believe) and to set bus vs self powered.

 You might also be able  to program one of the IO pins to enable high current mode (can't remember specifics).

 

[WillHuang]

Very good information, thank you! I tested an old pc today to see how much current I can draw. I managed to get about 300ma and stayed within the 4.5 to 5.5v range.

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