Re: [SI-LIST] : Diodes, EMC, ESD Protection networks

khardin@lexmark.com
Tue, 12 May 1998 07:51:11 -0400

Peter,

On your problem:

1) With regard to diodes in Switch Mode Power Supplies. Do Ultra Fast
Recovery Diodes produce more EMI than Schottky Diodes? I've heard
conflicting stories.

They definitely can but it depends on the connected circuitry. Fast
recovery diode tend to exhibit
a phenomena called snap off (or on). This happens during the process of
decreasing forward current
until the diode is no longer forwarded biased. At the impending 0 current
the diode instantaneously
stops conducting causing a large di/dt. If the diode is connected to a
transformer that has a
resonate tank circuit between the winding inductance and parasitic winding
capacitance and
some part of the system has an antenna at this frequency, then the radiated
emissions can get a lot
worse. One fix is to use a ferrite bead to limit the di/dt.

2) I have a design in which a buffer IC, 74VHC574, is being clocked at
100MHz. It creates a lot of 2nd harmonic radiation at 200MHz which
radiates badly on all input pins (and probably output pins). I have a
cable from an external device comming in via a connector to all 8 of
the 74VHC574's input pins. As you can imagine, this cable radiates a
lot of EMI. My solution is to put a ferrite clamp on the cable which
works and I pass emissions testing.
Would running that track (2"//50mm) from the connector to the IC's
input pin as a strip-line (internal) make any difference over
micro-strip (surface) or is this a problem that can only be solved by
filtering?

Do you have same emissions when the grounds are the only connections made
to the cable?
(I am assuming that the 100MHz is still running.)
If yes, then your stuck with the ferrite or connect the PCB ground to a
close chassis ground point.
If no, then try another vendor for the 74VHC574 or add caps to the inputs.

The strip-line is not likely to work but you could implement it with some
copper tape and mylar
to see.

Good Luck,

Dr. Keith Hardin
Senior Engineer
Lexmark International Inc.

To: SI-LIST%SILAB.Eng.Sun.COM@interlock.lexmark.com
cc: (bcc: Keith Hardin)
bcc: Keith Hardin
Subject: [SI-LIST] : Diodes, EMC, ESD Protection networks

Hi,
Sorry if this is not strictly SI issues.
1) With regard to diodes in Switch Mode Power Supplies. Do Ultra Fast
Recovery Diodes produce more EMI than Schottky Diodes? I've heard
conflicting stories.
2) I have a design in which a buffer IC, 74VHC574, is being clocked at
100MHz. It creates a lot of 2nd harmonic radiation at 200MHz which
radiates badly on all input pins (and probably output pins). I have a
cable from an external device comming in via a connector to all 8 of
the 74VHC574's input pins. As you can imagine, this cable radiates a
lot of EMI. My solution is to put a ferrite clamp on the cable which
works and I pass emissions testing.
Would running that track (2"//50mm) from the connector to the IC's
input pin as a strip-line (internal) make any difference over
micro-strip (surface) or is this a problem that can only be solved by
filtering?
3) A Multiple VCC Supplies//Partial Power Down question. Think of it
as a Bus Analyser sitting on a CPU bus.
I have a CPU circuit (50MHz clocks...) and a monitoring Bus Analyser
(not a product as such but a circuit I am developing). If power is
applied to both devices, all is fine. However, if power to the Bus
Analyser fails (or is turned on after the CPU circuit is turned on),
it
then loads the data lines on the CPU circuit corrupting operation. The
loading is primarily done by the Bus Analyser's Input IC's ESD
protection network, clamping the input to 0.7 volts above VCC, which
in
this case is 0 volts. The Bus Analyser's power supply has a very low
DC
resistance when the power is switched off due to other things.
The only solutions I have seen (so far) is from Texas Instruments
Application Information.
A) Use large value series resistors in series with the inputs. I can't
put 4K7 resistors (for example) in series with the Bus Analyser's
inputs as that in conjunction with the ICs input capacitance/PCB
strays.. will kill high frequency response (at 50MHz).
B) Put a diode in series with the Bus Analyser's input IC power supply
pin. This then moves the problem from the input IC to the following IC
(IC's) in sequence after the input IC. The input IC is free to "float"
but is clamped by the following IC's ESD clamp diodes once again.
C) Adding a diode in series with the input (cathode on the CPU bus,
anode to the Bus Analyser's input pin) and a pull up resistor from
anode to VCC. This would need a fast/low capacitance schottky diode
(Vf=0.4volts) which is fine. I've tried this and it does work but is
a bit messy.
My question is:
There must be thousands of circuits out there doing exactly what I'm
trying to do. What do others do?
Thanks
Peter Baxter
pbaxter@cochlear.com.au