Just an update. I’ve built the second prototype board (I skipped over the first
design), and it’s powering my tbolt right now.
The design calls for 15v in (though it would also work with 13.8v). The +12
output comes from a D2PAK 7812. For +5, there is an AP1509 buck converter to
make around 6.5 volts, then a DPAK 7805. For -12, there is an MC34063
configured as an inverter to make around -13.75 volts and then a DPAK 7912.
Steady-state, the system appears to be working just fine. The AP1509’s inductor
and the D2PAK 7812 are just warm to the touch.
I checked for noise and ripple on the outputs and it’s somewhere around ±2 mV
or so generally. From what I can see on the scope, there’s no ripple - it’s all
high frequency noise. I am not absolutely certain that the noise measurement
represents real noise or the limits of my measuring ability. I’m just using the
scope probes the scope came with, and 2 mV/div is its lowest range.
I haven’t compared the noise with the ex laptop supply that I was using before,
but I’d have to believe it’s cleaner. I don’t really have a way to check the
oscillator’s before and after ADEV. My only other reference is an FE5680A, and
I think the thunderbolt’s going to be far better at lower tau (where this all
I know also that ±2 mV is still one and perhaps two orders of magnitude higher
than some have called for. But before I attempt to reduce the noise further,
I’d like to know that there are real gains to be had. Would someone with a
Thunderbolt and better output noise measuring wherewithal be willing to take a
prototype and compare it with something that does have µV levels of noise and
ripple so I can get an idea of what there is to gain? If you like, you can make
such comparisons public - no secrets here.
> On Aug 30, 2016, at 10:37 PM, Nick Sayer <nsa...@kfu.com> wrote:
>> On Aug 30, 2016, at 8:48 PM, Cube Central <cubecent...@gmail.com> wrote:
>> I would be interested, I think. Planning ahead for if the one I have for my
>> Thunderbolt fails, I guess. Are there different models or would a photo of
>> the input ports on mine be useful?
> Actually, what I had in mind is to just put a SIP4 header on the board for
> the output and people could wire the “last mile” themselves. The input is a
> 2.1mm barrel connector. You use whatever 15W 12VDC wall wart is handy and
> plug it right in.
> What it really amounts to is that you get +12 volts directly from the input,
> then there’s a buck converter to drop the +12 down to +5 and an inverter to
> generate -12 from the +12. Those 3 voltages, plus a ground go to the SIP4.
> So it’s just two switching power supplies to turn a +12 volt only supply into
> the three-way that the Thunderbolt wants.
> It’d be good for around 1500 mA @ 5V and around 50 mA @ -12 (the +12 spec is
> whatever is left from the source supply’s power spec) - more than enough for
> a Thunderbolt. Probably enough for a hard disk or a smallish PC.
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