Supply filters

[Phil Hobbs]

I'm doing a miniaturized PMT replacement product that has to fit a
pHEMT-bootstrapped front end, low noise PGA, variable ~50V MPPC bias
supply, TEC controller, MCU, and serial communications into a 22x22x50
mm envelope. (Fun.)

It needs supply polarity and overvoltage protection, but I haven't got
anything like enough space for a two-amp polyfuse, and couldn't stand
the voltage drop if I had.

Soooo, I'm using back-to-back PFETs controlled by the MCU for the
protection circuitry. Largish ceramic caps (4.7 uF 0603) look after ESD
sensitivity on the supply lines. (Turns out that you can get reasonably
decent ones, at least for low voltages--check out Kemet's
C0603C475K9PACTU, which is down only 30% at 2.8V. Most are far worse.)

The MCU itself is powered via an LP2951, which is OK to 30V and has
separate polarity protection from an SC70 PFET--no big worries there.

The current issue is the input filtering. Without the
conveniently-sized resistance of the polyfuse, it's difficult to control
the overshoot and ringing of the LC filter. It's not so bad at inrush,
because I can use a PWM to control the two PFETs. (The big inductor is
downstream of the PFETs, so it won't see the power-on inrush.) However,
if the supply is momentarily interrupted, the turn-on transient could
blow up quite a few things.

I'm planning to use a very small unidirectional 3.3V TVS in parallel
with the largest inductor (4.7 uH). That way most of the inrush goes
through the forward-biased diode, keeping the inductor current down, and
the worst of the ringing will be damped by the zener action.

It's not as comfortable as the TVS/polyfuse approach, for sure. Any
better ideas?

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Winfield Hill]

Phil Hobbs wrote...

... Largish ceramic caps (4.7 uF 0603) ...
(Turns out that you can get reasonably decent ones,
at least for low voltages--check out Kemet's
C0603C475K9PACTU, which is down only 30% at 2.8V.
Most are far worse.)

Hah, 2.2 million in stock at Digi-Key. That's
$80k worth, at 3.6 cents each. And they have
880k of the 10-volt version. The datasheet
says there's a 10uF version - no inventory.


--
Thanks,
- Win

 

On Wed, 25 Mar 2020 09:01:51 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

I'm doing a miniaturized PMT replacement product that has to fit a
pHEMT-bootstrapped front end, low noise PGA, variable ~50V MPPC bias
supply, TEC controller, MCU, and serial communications into a 22x22x50
mm envelope. (Fun.)

It needs supply polarity and overvoltage protection, but I haven't got
anything like enough space for a two-amp polyfuse, and couldn't stand
the voltage drop if I had.

Soooo, I'm using back-to-back PFETs controlled by the MCU for the
protection circuitry. Largish ceramic caps (4.7 uF 0603) look after ESD
sensitivity on the supply lines. (Turns out that you can get reasonably
decent ones, at least for low voltages--check out Kemet's
C0603C475K9PACTU, which is down only 30% at 2.8V. Most are far worse.)

The MCU itself is powered via an LP2951, which is OK to 30V and has
separate polarity protection from an SC70 PFET--no big worries there.

The current issue is the input filtering. Without the
conveniently-sized resistance of the polyfuse, it's difficult to control
the overshoot and ringing of the LC filter. It's not so bad at inrush,
because I can use a PWM to control the two PFETs. (The big inductor is
downstream of the PFETs, so it won't see the power-on inrush.) However,
if the supply is momentarily interrupted, the turn-on transient could
blow up quite a few things.

I'm planning to use a very small unidirectional 3.3V TVS in parallel
with the largest inductor (4.7 uH). That way most of the inrush goes
through the forward-biased diode, keeping the inductor current down, and
the worst of the ringing will be damped by the zener action.

It's not as comfortable as the TVS/polyfuse approach, for sure. Any
better ideas?

Cheers

Phil Hobbs

Power input protection is a chronic problem. Lately I use a 24 volt
sacrificial TVS and supply a current-limited 24 volt wall wart and
assume disasters will be infrequent.

Polyfuse and TVS is pretty good. It can be teased to fry the TVS, but
that takes creative malice.

What's your voltage and current? How much voltage can you lose?

One possibility is to use a depletion fet or an equivalent LED current
limiter and a coupled thermistor to shut off the load if it gets too
hot.

I have used a thermal-limiting 3t regulator as a current limiter, with
the adjust pin open. An RRO opamp would work too for modest currents.
But I prefer to not have the first power input device be an IC.

We have used the TI e-fuse things, but I don't like them.

Or use a real fuse!



--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"

 

[Phil Hobbs]

On 2020-03-25 13:54, Joerg wrote:

On 2020-03-25 06:01, Phil Hobbs wrote:
I'm doing a miniaturized PMT replacement product that has to fit a
pHEMT-bootstrapped front end, low noise PGA, variable ~50V MPPC
bias supply, TEC controller, MCU, and serial communications into a
22x22x50 mm envelope. (Fun.)

It needs supply polarity and overvoltage protection, but I haven't
got anything like enough space for a two-amp polyfuse, and
couldn't stand the voltage drop if I had.

Soooo, I'm using back-to-back PFETs controlled by the MCU for the
protection circuitry. Largish ceramic caps (4.7 uF 0603) look
after ESD sensitivity on the supply lines. (Turns out that you can
get reasonably decent ones, at least for low voltages--check out
Kemet's C0603C475K9PACTU, which is down only 30% at 2.8V. Most are
far worse.)

The MCU itself is powered via an LP2951, which is OK to 30V and
has separate polarity protection from an SC70 PFET--no big worries
there.


Careful with LP29xx. I've had them go berserk under certain
conditions, such as if the input (drive side) impedance gets too
high.

Interesting. I've never had a problem with one--I nearly always power
small micros off the LP2951-33DRGR, with an NPN booster if necessary.
It's good enough for a voltage reference for the sort of undemanding
jobs that MCU ADCs are good for--checking the other power supplies,
current limits, sloppy digital feedback loops, lowish-performance
temperature controllers, and such like.

The current issue is the input filtering. Without the
conveniently-sized resistance of the polyfuse, it's difficult to
control the overshoot and ringing of the LC filter. It's not so
bad at inrush, because I can use a PWM to control the two PFETs.
(The big inductor is downstream of the PFETs, so it won't see the
power-on inrush.) However, if the supply is momentarily
interrupted, the turn-on transient could blow up quite a few
things.

I'm planning to use a very small unidirectional 3.3V TVS in
parallel with the largest inductor (4.7 uH). That way most of the
inrush goes through the forward-biased diode, keeping the inductor
current down, and the worst of the ringing will be damped by the
zener action.

It's not as comfortable as the TVS/polyfuse approach, for sure.
Any better ideas?


TVS have a large tolerance.

The TVS voltage isn't critical, because it's across the inductor rather
than across the supply. I just want it to be less than 5V
on the nastiest peaks. That way the circuit won't blow up if the power
supply opens up or we turn off the PFETs suddenly. (The plan is to PWM
them during turn-on and turn-off, but it'll probably happen during
software development at least.)

In critical situations I use a TL431-based shunt regulator. If
currents are low enough then a barefoot TL431. This is often
important because some high-speed logic and RF parts can go PHUT
when it's just a few hundred mV above abs max. Or worse, they only go
partially kaputt and have compromised noise behavior or something.

Yeah, that would be bad. The TL431 is schloooww, though. I suppose I
could just crowbar the supplies if they go more than half a volt out of
spec, but that would make me quite unpopular--some instruments will use
32 of these modules.

I thought about just using analogue control of the back-to-back FETs,
but they're so tiny that they won't stand much dissipation--the SOA
curve is practically indistinguishable from the Y axis.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Phil Hobbs]

On 2020-03-25 12:08, jlarkin@highlandsniptechnology.com wrote:

On Wed, 25 Mar 2020 09:01:51 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

I'm doing a miniaturized PMT replacement product that has to fit a
pHEMT-bootstrapped front end, low noise PGA, variable ~50V MPPC
bias supply, TEC controller, MCU, and serial communications into a
22x22x50 mm envelope. (Fun.)

It needs supply polarity and overvoltage protection, but I haven't
got anything like enough space for a two-amp polyfuse, and
couldn't stand the voltage drop if I had.

Soooo, I'm using back-to-back PFETs controlled by the MCU for the
protection circuitry. Largish ceramic caps (4.7 uF 0603) look
after ESD sensitivity on the supply lines. (Turns out that you can
get reasonably decent ones, at least for low voltages--check out
Kemet's C0603C475K9PACTU, which is down only 30% at 2.8V. Most are
far worse.)

The MCU itself is powered via an LP2951, which is OK to 30V and
has separate polarity protection from an SC70 PFET--no big worries
there.

The current issue is the input filtering. Without the
conveniently-sized resistance of the polyfuse, it's difficult to
control the overshoot and ringing of the LC filter. It's not so
bad at inrush, because I can use a PWM to control the two PFETs.
(The big inductor is downstream of the PFETs, so it won't see the
power-on inrush.) However, if the supply is momentarily
interrupted, the turn-on transient could blow up quite a few
things.

I'm planning to use a very small unidirectional 3.3V TVS in
parallel with the largest inductor (4.7 uH). That way most of the
inrush goes through the forward-biased diode, keeping the inductor
current down, and the worst of the ringing will be damped by the
zener action.

It's not as comfortable as the TVS/polyfuse approach, for sure.
Any better ideas?

Cheers

Phil Hobbs

Power input protection is a chronic problem. Lately I use a 24 volt
sacrificial TVS and supply a current-limited 24 volt wall wart and
assume disasters will be infrequent.

Polyfuse and TVS is pretty good. It can be teased to fry the TVS,
but that takes creative malice.

What's your voltage and current? How much voltage can you lose?

+-5V, with up to a couple of amps from the +5 depending on the TEC load.
The negative supply is just running op amps and the front end
bootstrap, so 100 mA at the most. I really don't want to lose more than
200 mV all told. The FETs and the inductor together are contributing
about half of that.

The product is an improved plug-in replacement for a commercial PMT
module widely used in biomed instruments, which is why the weird
supplies. We'll probably do a follow-on that runs off +5 only. The
modules have voltage-programmable gain, which they get by changing the
PMT bias. We do it using the MPPC bias voltage and a PGA made from an
AD5273BRJZ dpot (1k, 64 taps of which we use 33), an ADA4899 (first
stage, gains 1, 2, 4, 8) and an ADA4860 CFA output stage (gains 1 or
8). The dpot is what limits the bandwidth to about 6 MHz.

Our faster MPPC box for the cathodoluminescence application uses a
4-stage switched attenuator instead of the dpot, which is good for 50
MHz or more. That's better than good enough for your average MPPC.

One possibility is to use a depletion fet or an equivalent LED
current limiter and a coupled thermistor to shut off the load if it
gets too hot.

The MCU owns the power supplies, so that wouldn't be hard to do. My
circuit is all super tiny packages, except that I'm splurging space on
0402 resistors. It's a pity that all the 1% 0804 quad packs are gone.
There's a Sensirion temperature/humidity sensor on the board, so that we
can calculate the dew point in case we want to build our own TEC package.

I have used a thermal-limiting 3t regulator as a current limiter,
with the adjust pin open. An RRO opamp would work too for modest
currents. But I prefer to not have the first power input device be an
IC.

We have used the TI e-fuse things, but I don't like them.

Or use a real fuse!

That would need a couple of gigundo TVSes that I don't have space for,
and I've only got about half a volt of headroom before I hit the abs max
spec on the TMUX1511s I'm using in the PGA. (Remarkable part BTW.)


Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Joerg]

On 2020-03-25 06:01, Phil Hobbs wrote:

I'm doing a miniaturized PMT replacement product that has to fit a
pHEMT-bootstrapped front end, low noise PGA, variable ~50V MPPC bias
supply, TEC controller, MCU, and serial communications into a 22x22x50
mm envelope. (Fun.)

It needs supply polarity and overvoltage protection, but I haven't got
anything like enough space for a two-amp polyfuse, and couldn't stand
the voltage drop if I had.

Soooo, I'm using back-to-back PFETs controlled by the MCU for the
protection circuitry. Largish ceramic caps (4.7 uF 0603) look after ESD
sensitivity on the supply lines. (Turns out that you can get reasonably
decent ones, at least for low voltages--check out Kemet's
C0603C475K9PACTU, which is down only 30% at 2.8V. Most are far worse.)

The MCU itself is powered via an LP2951, which is OK to 30V and has
separate polarity protection from an SC70 PFET--no big worries there.

Careful with LP29xx. I've had them go berserk under certain conditions,
such as if the input (drive side) impedance gets too high.

The current issue is the input filtering. Without the
conveniently-sized resistance of the polyfuse, it's difficult to control
the overshoot and ringing of the LC filter. It's not so bad at inrush,
because I can use a PWM to control the two PFETs. (The big inductor is
downstream of the PFETs, so it won't see the power-on inrush.) However,
if the supply is momentarily interrupted, the turn-on transient could
blow up quite a few things.

I'm planning to use a very small unidirectional 3.3V TVS in parallel
with the largest inductor (4.7 uH). That way most of the inrush goes
through the forward-biased diode, keeping the inductor current down, and
the worst of the ringing will be damped by the zener action.

It's not as comfortable as the TVS/polyfuse approach, for sure. Any
better ideas?

TVS have a large tolerance. In critical situations I use a TL431-based
shunt regulator. If currents are low enough then a barefoot TL431. This
is often important because some high-speed logic and RF parts can go
PHUT when it's just a few hundred mV above abs max. Or worse, they only
go partially kaputt and have compromised noise behavior or something.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Joerg]

On 2020-03-25 11:00, Phil Hobbs wrote:

On 2020-03-25 12:08, jlarkin@highlandsniptechnology.com wrote:
On Wed, 25 Mar 2020 09:01:51 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

I'm doing a miniaturized PMT replacement product that has to fit a
pHEMT-bootstrapped front end, low noise PGA, variable ~50V MPPC bias
supply, TEC controller, MCU, and serial communications into a
22x22x50 mm envelope. (Fun.)

It needs supply polarity and overvoltage protection, but I haven't
got anything like enough space for a two-amp polyfuse, and
couldn't stand the voltage drop if I had.

Soooo, I'm using back-to-back PFETs controlled by the MCU for the
protection circuitry. Largish ceramic caps (4.7 uF 0603) look after
ESD sensitivity on the supply lines. (Turns out that you can
get reasonably decent ones, at least for low voltages--check out
Kemet's C0603C475K9PACTU, which is down only 30% at 2.8V. Most are
far worse.)

The MCU itself is powered via an LP2951, which is OK to 30V and
has separate polarity protection from an SC70 PFET--no big worries
there.

The current issue is the input filtering. Without the
conveniently-sized resistance of the polyfuse, it's difficult to
control the overshoot and ringing of the LC filter. It's not so bad
at inrush, because I can use a PWM to control the two PFETs. (The big
inductor is downstream of the PFETs, so it won't see the power-on
inrush.) However, if the supply is momentarily interrupted, the
turn-on transient could blow up quite a few things.

I'm planning to use a very small unidirectional 3.3V TVS in parallel
with the largest inductor (4.7 uH). That way most of the
inrush goes through the forward-biased diode, keeping the inductor
current down, and the worst of the ringing will be damped by the
zener action.

It's not as comfortable as the TVS/polyfuse approach, for sure.
Any better ideas?

Cheers

Phil Hobbs

Power input protection is a chronic problem. Lately I use a 24 volt
sacrificial TVS and supply a current-limited 24 volt wall wart and
assume disasters will be infrequent.

Polyfuse and TVS is pretty good. It can be teased to fry the TVS,
but that takes creative malice.

What's your voltage and current? How much voltage can you lose?

+-5V, with up to a couple of amps from the +5 depending on the TEC load.
The negative supply is just running op amps and the front end
bootstrap, so 100 mA at the most. I really don't want to lose more than
200 mV all told. The FETs and the inductor together are contributing
about half of that.

The product is an improved plug-in replacement for a commercial PMT
module widely used in biomed instruments, which is why the weird
supplies. We'll probably do a follow-on that runs off +5 only. The
modules have voltage-programmable gain, which they get by changing the
PMT bias. We do it using the MPPC bias voltage and a PGA made from an
AD5273BRJZ dpot (1k, 64 taps of which we use 33), an ADA4899 (first
stage, gains 1, 2, 4, 8) and an ADA4860 CFA output stage (gains 1 or
8). The dpot is what limits the bandwidth to about 6 MHz.

Our faster MPPC box for the cathodoluminescence application uses a
4-stage switched attenuator instead of the dpot, which is good for 50
MHz or more. That's better than good enough for your average MPPC.

One possibility is to use a depletion fet or an equivalent LED current
limiter and a coupled thermistor to shut off the load if it gets too hot.

The MCU owns the power supplies, so that wouldn't be hard to do. My
circuit is all super tiny packages, except that I'm splurging space on
0402 resistors. It's a pity that all the 1% 0804 quad packs are gone.

These ought to be small enough:

https://industrial.panasonic.com/cdbs/www-data/pdf/AOC0000/AOC0000C14.pdf

Just don't try to hand-solder them

There's a Sensirion temperature/humidity sensor on the board, so that we
can calculate the dew point in case we want to build our own TEC package.

I have used a thermal-limiting 3t regulator as a current limiter, with
the adjust pin open. An RRO opamp would work too for modest currents.
But I prefer to not have the first power input device be an
IC.

We have used the TI e-fuse things, but I don't like them.

Or use a real fuse!

That would need a couple of gigundo TVSes that I don't have space for,
and I've only got about half a volt of headroom before I hit the abs max
spec on the TMUX1511s I'm using in the PGA. (Remarkable part BTW.)

All this almost screams for a shunt regulator. Let it shunt brief spikes
and if you suspect longer overvoltages you could do this:

Detect whether the shunt draws current which can be done with a simple
BJT plus small resistor. If it draws current for more than xxx
microseconds cut the power by shutting off the series FETs.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Joerg]

On 2020-03-25 11:18, Phil Hobbs wrote:

On 2020-03-25 13:54, Joerg wrote:

[...]

In critical situations I use a TL431-based shunt regulator. If
currents are low enough then a barefoot TL431. This is often important
because some high-speed logic and RF parts can go PHUT when it's just
a few hundred mV above abs max. Or worse, they only go
partially kaputt and have compromised noise behavior or something.

Yeah, that would be bad. The TL431 is schloooww, though.

Too slow for a power rail? Then you seriously need more bypass
capacitance

... I suppose I
could just crowbar the supplies if they go more than half a volt out of
spec, but that would make me quite unpopular--some instruments will use
32 of these modules.

With a shunt regulator set a few hundfred mV above 3.3V there wouldn't
be a crowbar effect, it just swallows the peak and then goes back into
its guard house.

I thought about just using analogue control of the back-to-back FETs,
but they're so tiny that they won't stand much dissipation--the SOA
curve is practically indistinguishable from the Y axis.

If the spike comes from a simple connection issue there shouldn't be a
SOA concern. Unless of course the cause is a loose contact.

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Phil Hobbs]

On 2020-03-25 14:49, Joerg wrote:

On 2020-03-25 11:18, Phil Hobbs wrote:
On 2020-03-25 13:54, Joerg wrote:


[...]

In critical situations I use a TL431-based shunt regulator. If
currents are low enough then a barefoot TL431. This is often important
because some high-speed  logic and RF parts can go PHUT when it's just
a few hundred mV above abs max. Or worse, they only go
partially kaputt and have compromised noise behavior or something.

Yeah, that would be bad.  The TL431 is schloooww, though.


Too slow for a power rail? Then you seriously need more bypass
capacitance

I don't have control over the input supply. People sometimes power our
stuff from truck batteries. It's the peak of the ring waveform I'm most
concerned about.

                                                   ...  I suppose I
could just crowbar the supplies if they go more than half a volt out of
spec, but that would make me quite unpopular--some instruments will use
32 of these modules.


With a shunt regulator set a few hundred mV above 3.3V there wouldn't
be a crowbar effect, it just swallows the peak and then goes back into
its guard house.


I thought about just using analogue control of the back-to-back FETs,
but they're so tiny that they won't stand much dissipation--the SOA
curve is practically indistinguishable from the Y axis.


If the spike comes from a simple connection issue there shouldn't be a
SOA concern. Unless of course the cause is a loose contact.

The PFETs are DMP4047LFDE's, which actually aren't super delicate, so
you're probably right that I'm being a nervous Nellie about that. I'll
see if I can magic up enough board space.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Phil Hobbs]

On 2020-03-25 15:44, Joerg wrote:

On 2020-03-25 11:00, Phil Hobbs wrote:
On 2020-03-25 12:08, jlarkin@highlandsniptechnology.com wrote:
On Wed, 25 Mar 2020 09:01:51 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

I'm doing a miniaturized PMT replacement product that has to fit a
pHEMT-bootstrapped front end, low noise PGA, variable ~50V MPPC bias
supply, TEC controller, MCU, and serial communications into a
 22x22x50 mm envelope.  (Fun.)

It needs supply polarity and overvoltage protection, but I haven't
 got anything like enough space for a two-amp polyfuse, and
couldn't stand the voltage drop if I had.

Soooo, I'm using back-to-back PFETs controlled by the MCU for the
protection circuitry.  Largish ceramic caps (4.7 uF 0603) look after
ESD sensitivity on the supply lines.  (Turns out that you can
get reasonably decent ones, at least for low voltages--check out
Kemet's C0603C475K9PACTU, which is down only 30% at 2.8V.  Most are
far worse.)

The MCU itself is powered via an LP2951, which is OK to 30V and
has separate polarity protection from an SC70 PFET--no big worries
 there.

The current issue is the input filtering.  Without the
conveniently-sized resistance of the polyfuse, it's difficult to
control the overshoot and ringing of the LC filter.   It's not so bad
at inrush, because I can use a PWM to control the two PFETs. (The big
inductor is downstream of the PFETs, so it won't see the power-on
inrush.)  However, if the supply is momentarily interrupted, the
turn-on transient could blow up quite a few things.

I'm planning to use a very small unidirectional 3.3V TVS in parallel
with the largest inductor (4.7 uH).  That way most of the
 inrush goes through the forward-biased diode, keeping the inductor
 current down, and the worst of the ringing will be damped by the
zener action.

It's not as comfortable as the TVS/polyfuse approach, for sure.
Any better ideas?

Cheers

Phil Hobbs

Power input protection is a chronic problem. Lately I use a 24 volt
sacrificial TVS and supply a current-limited 24 volt wall wart and
assume disasters will be infrequent.

Polyfuse and TVS is pretty good. It can be teased to fry the TVS,
but that takes creative malice.

What's your voltage and current? How much voltage can you lose?

+-5V, with up to a couple of amps from the +5 depending on the TEC load.
  The negative supply is just running op amps and the front end
bootstrap, so 100 mA at the most.  I really don't want to lose more than
200 mV all told.  The FETs and the inductor together are contributing
about half of that.

The product is an improved plug-in replacement for a commercial PMT
module widely used in biomed instruments, which is why the weird
supplies. We'll probably do a follow-on that runs off +5 only.  The
modules have voltage-programmable gain, which they get by changing the
PMT bias.  We do it using the MPPC bias voltage and a PGA made from an
AD5273BRJZ dpot (1k, 64 taps of which we use 33), an ADA4899 (first
stage, gains 1, 2, 4, 8) and an ADA4860 CFA output stage  (gains 1 or
8).  The dpot is what limits the bandwidth to about 6 MHz.

Our faster MPPC box for the cathodoluminescence application uses a
4-stage switched attenuator instead of the dpot, which is good for 50
MHz or more. That's better than good enough for your average MPPC.

One possibility is to use a depletion fet or an equivalent LED current
limiter and a coupled thermistor to shut off the load if it gets too
hot.

The MCU owns the power supplies, so that wouldn't be hard to do.  My
circuit is all super tiny packages, except that I'm splurging space on
0402 resistors.  It's a pity that all the 1% 0804 quad packs are gone.


These ought to be small enough:

https://industrial.panasonic.com/cdbs/www-data/pdf/AOC0000/AOC0000C14.pdf

Just don't try to hand-solder them

They're all 5%, though.

There's a Sensirion temperature/humidity sensor on the board, so that we
can calculate the dew point in case we want to build our own TEC package.

I have used a thermal-limiting 3t regulator as a current limiter, with
the adjust pin open. An RRO opamp would work too for modest currents.
But I prefer to not have the first power input device be an
IC.

We have used the TI e-fuse things, but I don't like them.

Or use a real fuse!

That would need a couple of gigundo TVSes that I don't have space for,
and I've only got about half a volt of headroom before I hit the abs max
spec on the TMUX1511s I'm using in the PGA.  (Remarkable part BTW.)


All this almost screams for a shunt regulator. Let it shunt brief spikes
and if you suspect longer overvoltages you could do this:

Detect whether the shunt draws current which can be done with a simple
BJT plus small resistor. If it draws current for more than xxx
microseconds cut the power by shutting off the series FETs.

I've already got voltage dividers so the MCU can look at the input
supplies, so I can just use a dual OC comparator to overrule the MCU
signals. Look, ST makes an LM393 in 2x2 DFN!

Thanks for the steer.

Cheers

Phil



--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[piglet]

On 25/03/2020 1:01 pm, Phil Hobbs wrote:

I'm doing a miniaturized PMT replacement product that has to fit a
pHEMT-bootstrapped front end, low noise PGA, variable ~50V MPPC bias
supply, TEC controller, MCU, and serial communications into a 22x22x50
mm envelope.  (Fun.)

It needs supply polarity and overvoltage protection, but I haven't got
anything like enough space for a two-amp polyfuse, and couldn't stand
the voltage drop if I had.

Soooo, I'm using back-to-back PFETs controlled by the MCU for the
protection circuitry.  Largish ceramic caps (4.7 uF 0603) look after ESD
sensitivity on the supply lines.  (Turns out that you can get reasonably
decent ones, at least for low voltages--check out Kemet's
C0603C475K9PACTU, which is down only 30% at 2.8V.  Most are far worse.)

The MCU itself is powered via an LP2951, which is OK to 30V and has
separate polarity protection from an SC70 PFET--no big worries there.

The current issue is the input filtering.  Without the
conveniently-sized resistance of the polyfuse, it's difficult to control
the overshoot and ringing of the LC filter.   It's not so bad at inrush,
because I can use a PWM to control the two PFETs. (The big inductor is
downstream of the PFETs, so it won't see the power-on inrush.)  However,
if the supply is momentarily interrupted, the turn-on transient could
blow up quite a few things.

I'm planning to use a very small unidirectional 3.3V TVS in parallel
with the largest inductor (4.7 uH).  That way most of the inrush goes
through the forward-biased diode, keeping the inductor current down, and
the worst of the ringing will be damped by the zener action.

It's not as comfortable as the TVS/polyfuse approach, for sure.  Any
better ideas?

Cheers

Phil Hobbs

I use a resistor across the L to damp ringing but thanks for the TVS
idea - cool!

piglet

 

[Phil Hobbs]

On 2020-03-25 17:09, Winfield Hill wrote:

Phil Hobbs wrote...

The PFETs are DMP4047LFDE ...

They're in huge so8 packages, there are
some nice sot-23-6 duals, e.g., BSL314PE.


Maybe I have the wrong p/n. The ones I'm using are in 2x2 DFNs.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Winfield Hill]

Phil Hobbs wrote...

The PFETs are DMP4047LFDE ...

They're in huge so8 packages, there are
some nice sot-23-6 duals, e.g., BSL314PE.


--
Thanks,
- Win

 

[Klaus Kragelund]

A capacitor coupled to the input with a diode and a pull up on the cap to keep the diode reverse biased normally

During a pulse the cap clamps the surge

Add a resistor to bleed the cap

 

[Phil Hobbs]

On 2020-03-25 17:58, Klaus Kragelund wrote:

A capacitor coupled to the input with a diode and a pull up on the cap to keep the diode reverse biased normally

During a pulse the cap clamps the surge

Add a resistor to bleed the cap

No space unfortunately.

Thanks

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Joerg]

On 2020-03-25 13:04, Phil Hobbs wrote:

On 2020-03-25 15:44, Joerg wrote:
On 2020-03-25 11:00, Phil Hobbs wrote:
On 2020-03-25 12:08, jlarkin@highlandsniptechnology.com wrote:

[...]

One possibility is to use a depletion fet or an equivalent LED current
limiter and a coupled thermistor to shut off the load if it gets too
hot.

The MCU owns the power supplies, so that wouldn't be hard to do. My
circuit is all super tiny packages, except that I'm splurging space on
0402 resistors. It's a pity that all the 1% 0804 quad packs are gone.


These ought to be small enough:

https://industrial.panasonic.com/cdbs/www-data/pdf/AOC0000/AOC0000C14.pdf

Just don't try to hand-solder them

They're all 5%, though.

The Yageo line comes in 1% but long lead times:

https://www.yageo.com/upload/media/product/productsearch/datasheet/rchip/PYu-YC_TC_group_51_RoHS_L_9.pdf

[...]

--
Regards, Joerg

http://www.analogconsultants.com/

 

[Winfield Hill]

Phil Hobbs wrote...

On 2020-03-25 17:09, Winfield Hill wrote:
Phil Hobbs wrote...

The PFETs are DMP4047LFDE ...

They're in huge so8 packages, there are
some nice sot-23-6 duals, e.g., BSL314PE.

Maybe I have the wrong p/n. The ones I'm
using are in 2x2 DFNs.

Aw, I had the wrong suffix. Three really
quite different parts, by changing a few
letters at the end.


--
Thanks,
- Win

 

[George Herold]

On Wednesday, March 25, 2020 at 4:53:47 PM UTC-4, piglet wrote:

On 25/03/2020 1:01 pm, Phil Hobbs wrote:
I'm doing a miniaturized PMT replacement product that has to fit a
pHEMT-bootstrapped front end, low noise PGA, variable ~50V MPPC bias
supply, TEC controller, MCU, and serial communications into a 22x22x50
mm envelope.  (Fun.)

It needs supply polarity and overvoltage protection, but I haven't got
anything like enough space for a two-amp polyfuse, and couldn't stand
the voltage drop if I had.

Soooo, I'm using back-to-back PFETs controlled by the MCU for the
protection circuitry.  Largish ceramic caps (4.7 uF 0603) look after ESD
sensitivity on the supply lines.  (Turns out that you can get reasonably
decent ones, at least for low voltages--check out Kemet's
C0603C475K9PACTU, which is down only 30% at 2.8V.  Most are far worse.)

The MCU itself is powered via an LP2951, which is OK to 30V and has
separate polarity protection from an SC70 PFET--no big worries there.

The current issue is the input filtering.  Without the
conveniently-sized resistance of the polyfuse, it's difficult to control
the overshoot and ringing of the LC filter.   It's not so bad at inrush,
because I can use a PWM to control the two PFETs. (The big inductor is
downstream of the PFETs, so it won't see the power-on inrush.)  However,
if the supply is momentarily interrupted, the turn-on transient could
blow up quite a few things.

I'm planning to use a very small unidirectional 3.3V TVS in parallel
with the largest inductor (4.7 uH).  That way most of the inrush goes
through the forward-biased diode, keeping the inductor current down, and
the worst of the ringing will be damped by the zener action.

It's not as comfortable as the TVS/polyfuse approach, for sure.  Any
better ideas?

Cheers

Phil Hobbs


I use a resistor across the L to damp ringing but thanks for the TVS
idea - cool!

piglet

Is there any difference between a TVS and a couple of power zeners
in series? (Well besides the obvious.. one versus two parts.)

George H.

 

[Phil Hobbs]

On 2020-03-26 20:25, George Herold wrote:

On Wednesday, March 25, 2020 at 4:53:47 PM UTC-4, piglet wrote:
On 25/03/2020 1:01 pm, Phil Hobbs wrote:
I'm doing a miniaturized PMT replacement product that has to fit a
pHEMT-bootstrapped front end, low noise PGA, variable ~50V MPPC bias
supply, TEC controller, MCU, and serial communications into a 22x22x50
mm envelope.  (Fun.)

It needs supply polarity and overvoltage protection, but I haven't got
anything like enough space for a two-amp polyfuse, and couldn't stand
the voltage drop if I had.

Soooo, I'm using back-to-back PFETs controlled by the MCU for the
protection circuitry.  Largish ceramic caps (4.7 uF 0603) look after ESD
sensitivity on the supply lines.  (Turns out that you can get reasonably
decent ones, at least for low voltages--check out Kemet's
C0603C475K9PACTU, which is down only 30% at 2.8V.  Most are far worse.)

The MCU itself is powered via an LP2951, which is OK to 30V and has
separate polarity protection from an SC70 PFET--no big worries there.

The current issue is the input filtering.  Without the
conveniently-sized resistance of the polyfuse, it's difficult to control
the overshoot and ringing of the LC filter.   It's not so bad at inrush,
because I can use a PWM to control the two PFETs. (The big inductor is
downstream of the PFETs, so it won't see the power-on inrush.)  However,
if the supply is momentarily interrupted, the turn-on transient could
blow up quite a few things.

I'm planning to use a very small unidirectional 3.3V TVS in parallel
with the largest inductor (4.7 uH).  That way most of the inrush goes
through the forward-biased diode, keeping the inductor current down, and
the worst of the ringing will be damped by the zener action.

It's not as comfortable as the TVS/polyfuse approach, for sure.  Any
better ideas?

Cheers

Phil Hobbs


I use a resistor across the L to damp ringing but thanks for the TVS
idea - cool!

piglet

Is there any difference between a TVS and a couple of power zeners
in series? (Well besides the obvious.. one versus two parts.)

George H.

A unidirectional TVS is a power zener. Bidirectional ones look like two
zeners back-to-back.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com