# Creating a 12Vdc + 1Vpp 1MHz ac 20W power supply?

Discussion in 'Electronic Design' started by I Throw Thumbers, Apr 20, 2007.

1. ### I Throw ThumbersGuest

Hello s.e.d,

Here's a puzzle I've been thinking about for a few days. I am testing
a bunch of LCD montiors that draw around 19W or 20W of power on a
12Vdc input. One of the tests is a ripple test wherein I supply the
monitor(s) with 12Vdc supply that has an AC ripple on it of 1V peak-to-
peak at a frequency of 1MHz and verify functionality.

The basic question is: how do I do create this kind of supply?

I've been thinking about op-amps, but the ones I had laying around
can't source 1.6ish amps. I've been thinking about amplifying the AC
portion through a simple 4-resistor BJT bias circuit, but the
transistors had too low of a saturation voltage. I've been thinking
that in a few weeks I'll have to do the same thing but for monitors
that draw 30W and even 60W. I've been thinking about equipment that
already exists that will do this, but I don't know it. Shouldn't this
be an easy enough circuit?

-drewbob

2. ### Rich GriseGuest

Yes, it should be almost trivial. Is this a homework question?

Good Luck!
Rich

3. ### Tam/WB2TTGuest

I don't think the LM317 will come close to having enough bandwidth. The most
straight forward way I can think of is to put the secondary of an RF
transformer in series with the DC output, and apply the RF signal to the
primary. This might take some doing if the monitor input is well bypassed,
and requires driving 1 V p-p of 1 MHz into almost a short circuit (probably
dozens of .1 uF capacitors) , and take gobs of power. Or suppose the supply
was switched at high level between 12 and 13 Volts; now, the thing is, after
you connect the monitor you might see that the voltage is only switching
between 12.49 and 12.51 V if there are no series inductors in the monitor DC
input.

How sure are you that you really want to do this at 1 MHz?

Tam

4. ### I Throw ThumbersGuest

Not homework, just part of my job that I'm no expert at . Maybe
that's even worse on this forum...
Ah. And a good place/vendor to look for an RF transformer?
I did diode-or 11.5V supplies and 12.5V supplies at 1MHz, and the
switching time on that FET switch I used was far too fast for the
monitor.
I will have to double check in the morning what the input capacitances
are. I suppose that at 1MHz it's unreasonable to expect a simple
circuit that will be able to drive the input capacitances. There is
an inductor (ferrite) in series.
I'm not. I may be able to convince the powers-that-be that 1MHz is
unreasonable for my feeble brain/abilities and that something in the
low kHz may have to do.

thanks, db

5. ### Jan PanteltjeGuest

Small transformer secondary in series.

On its primary for example a DL92 tube amp or a trans-sister.

6. ### Tam/WB2TTGuest

I would think that you worry about 60 Hz on the DC feed, and RF pickup on
the signal cable. Seems like your systems engineers should tell you what you
need to do. For starters, pass UL and FCC part 15.

Tam