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

Question

Hello s.e.d,Here's a puzzle I've been thinking about for a few days.  I am testinga bunch of LCD montiors that draw around 19W or 20W of power on a12Vdc input.  One of the tests is a ripple test wherein I supply themonitor(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 aroundcan't source 1.6ish amps.  I've been thinking about amplifying the ACportion through a simple 4-resistor BJT bias circuit, but thetransistors had too low of a saturation voltage.  I've been thinkingthat in a few weeks I'll have to do the same thing but for monitorsthat draw 30W and even 60W.  I've been thinking about equipment thatalready exists that will do this, but I don't know it.  Shouldn't thisbe an easy enough circuit?-drewbob

Rich Grise · Accepted Answer

Yes, it should be almost trivial. Is this a homework question?How about a current-boosted LM317 with your reference modulated?Good Luck!Rich

Tam/WB2TT · Answer

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

I Throw Thumbers · Answer

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

Jan Panteltje · Answer

On a sunny day (19 Apr 2007 19:46:34 -0700) it happened I Throw Thumbers wrote in:Small transformer secondary in series.On its primary for example a DL92 tube amp or a trans-sister.

Tam/WB2TT · Answer

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

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

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