Even though you may have to be careful about noise, and assuming the
operating voltages are not exotic, you can probably find an
off-the-shelf switching supply that will fit where the transformer and
linear supplies were, with lots of room to spare, and significantly
reduced heat dissipation. I am thinking about doing that with the 5370,
considering how hot the rear heat sink runs.

Didier KO4BB

Bill Janssen wrote:

Didier Juges wrote:

Didier

Replacing the HP5370 power supply with switching regulators will be a
somewhat challenging exercise in ensuring that switching regulator noise
doesn't degrade the 5370 performance. Even verifying this will be
challenging.  If you don't burn yourself on the external heatsink there
should be no problem with reliability of the linear regulators.

Bruce

Didier Juges wrote:

I have already looked at the space available and I think that is "do--able".

Bill K7NOM

Dr Bruce Griffiths wrote:

Bruce,

I am not too concerned about reliability. Power transistors are designed
to run hot. I am concerned about burning myself on the heat sink (it
measures well over 60 degree C here), and overall power consumption and
heat dissipation in the shack, because the instrument is typically
running a lot of time. I have installed it on top of the short rack I
made for my main instruments to maximize air flow, but as a result, the
heat sink is quite exposed. Also, my shack is above the garage, and
therefore the air conditioning is not too effective there (3 walls,
ceiling and floor face non A/C spaces).

I have not looked in detail, but I suspect the voltage drop across the
linear regulators is a little too high and probably could be reduced.

I may also look at simply putting a switching pre-regulator in front of
the existing linear regulators (or at least the worst offender, I am
sure the 5V supply must be loaded pretty well) to reduce the voltage
drop and the heat, while keeping the good regulation and low noise of
the linears. I have a box of LT1074 integrated switching regulators
which require only a handfull of components to make a complete switching
supply.

A small fan would address the high temperature, but would not help with
power consumption and dissipation. That would be an easy fix though.

Didier

Didier Juges wrote:

Didier

Try using a Murata BNX002 filter between the switching supply output and
the linear regulator input.
These should eliminate the High frequency noise (0.5MHz - 1GHz) from the
switching regulator output leaving the linear regulator to deal with the
low frequency noise.
However for these to be effective a low impedance ground plane is required.

Worst case linear regulator design allowing for low mains input and some
margin for the ripple from the rectifier certainly increases the series
pass element dissipation considerably over that when the input to the
regulator can be held within 0.1V or so with low ripple.

The traditional preregulator employs SCRs in a phse controlled rectifier
arrangement, the resulting low frequency output noise from such
preregulators is easily dealt with by the linear regulator.

Bruce

Dr Bruce Griffiths wrote:

Bruce,

I work for a company that designs and builds switchmode power supplies,
so I have access to lots of power supply and filter components, and for
better or for worse, we are used to dealing with switcher noise.

I will check the Murata filters though, I am not familiar with those.

Most switching supplies operating up to 200kHz make significant noise up
to 30 to 50 MHz, and much less above that. Of course, if you design a
receiver, you may still be able to detect much lower level emissions up
to 300-400 MHz.

Another pitfall of cheap switchers is line emissions, where noise is fed
back into the mains and may affect other equipment (by conduction or
radiation from the AC line itself). This is normally controlled by
regulations, such as FCC regulations here in the US for commercial
equipment, and via MIL-STD-461 for military equipment.

It has been a long time since I have seen an SCR preregulator. They have
almost the same complexity as a switcher (from a parts count
standpoint), without any of the advantages, other than slightly lower
noise levels at higher frequencies. However, they are bad for line noise
(conducted emissions) and power quality (power factor, harmonic
distortion) and require oversized transformers, inductors and
capacitors, even compared to a normal linear supply. These are now
completely abandoned (except maybe at very high power levels, as used in
industrial processes and I believe in rail transportation) because they
can't meet the power quality requirements and they cost too much/are too
big and heavy.

In the past, we have been using what we call linear-switchers (switcher
followed by a linear regulator) in some cases when the customer had high
expectations for cleanliness and efficiency, but nowadays, we can meet
all our customers needs with high frequency switchers, using good design
practices to minimize noise in the first place, and effective filters to
take care of the rest..

Didier

Man with hammer says: "Mmmm, nails!"

Conduction-angle preregulators were used in some of the HP/Harrison benchtop
power supplies (my 6263B has one), as well as some of the smaller Carver
audio amplifiers in the 80s.  Definitely one of those 'dead-end' branches of
the great evolutionary tree of electronics...

-- john, KE5FX

Didier Juges wrote:

Didier

SCR's are still used in very high power (Megawatt) high voltage (0.5
Megavolts or more) inverters where they replaced grid controlled mercury
arc devices (some of these have been in operation for 40 years or more).

I am sceptical that any affordable switcher has an output noise as low
as a well designed linear regulator (<10uV pp DC -20MHz).

Bruce

Dr Bruce Griffiths wrote:

They are not, but system designers, as far as I can tell, have overcome
this problem by providing local filtering/decoupling/regulation at the
most sensitive points and therefore do not require the entire power
budget to a system to be low noise.

Most of our power supplies (we only make custom military supplies) have
many outputs (one we make right now has 34 outputs, all independently
regulated with floating grounds), sometimes several outputs of the same
voltage, so that the customer can isolate various sections of the
system, and provide as much filtering at the point of load as needed for
each subsystem. Also, being a 100% military house, few customers call
our supplies affordable :-)

For instance, on the 5370, a lot of the power is used to drive logic
circuits, TTL and ECL. For the most parts, these are not very sensitive
to noise, certainly not as much as the OCXO, the VCOs and the input
comparators, which probably (I have not checked the manual yet) run from
a different voltage.

Modern high integration chips have advanced power/thermal management
functions which turn off parts of the chip when not in use. This results
in very severe load transients. Often, the noise (periodic and random
deviation, PARD) on the 5V, 3V and lower rails is driven by the very
fast load transients of the logic chips more than by the switcher
itself. A specification we are reviewing right now has a load change
requirement from 55 to 110A on a 3V rail in less than 1uS at the end of
a 56 inches bus bar. With that kind of load transient, the ripple does
not matter much compared to the transient response through the bus bar,
even assuming a perfect power supply with infinite output capacitance.

Didier