Fast Recovery Diode

[Anand P. Paralkar]

Hi Everybody,

Could someone please explain the use of a Fast Recovery Diode? I am
particularly interested in:

1. How is this diode different from a regular rectifier diode.
2. The role of these diodes in MOSFET circuits.

(Sorry, there is no Wiki for this.)

Thanks for your help.

Regards,
Anand

 

[Jamie]

Hi Everybody,

Could someone please explain the use of a Fast Recovery Diode? I am
particularly interested in:

1. How is this diode different from a regular rectifier diode.
2. The role of these diodes in MOSFET circuits.

(Sorry, there is no Wiki for this.)

Thanks for your help.

Regards,
Anand

The difference is, a fast switching diode will become non conductive
a lot sooner than a non-fast switching diode.

In non fast recovery diodes, the path between the anode and cathode
is still some what conductive after the current that placed them in
that state is gone. What this means is, reverse polarity on this diode
will cause a conductive path in the reverse direction for a per scribed
amount of time just after a completion of a forward path on the diode
if not sufficient time given to have it recover before this reverse path
appears across the nodes of the diode.

It acts like a cap that has been charged and when reverse voltage
appears across it, it'll exert current. But don't get confused, it's
not the same thing. It's all about electrons and the way they impact
the surface of the materials between the anode and cathode used in the
diode. Fast diodes are made a little differently to limit this problem.


TO get a better understanding, look up Ultra Fast Diodes or Switching
diodes..


P.S.
Transistors of many kinds are also spec'ed this way.

 

[Tim Williams]

It acts like a cap that has been charged and when reverse voltage
appears across it, it'll exert current. But don't get confused, it's
not the same thing. It's all about electrons and the way they impact
the surface of the materials between the anode and cathode used in the
diode. Fast diodes are made a little differently to limit this problem.

Actually, it acts more like, no, it acts exactly like a battery. The
voltage is almost constant until the charge is gone -- that means the
diode is actually producing energy (positive voltage, negative current) as
the charge leaves.

FYI, a simulation of 1N4007 driven at If = 1A and turning off at -100A/us
reaches a peak current I_rr of 6.978A in 69.7ns before turning off. A
proper high speed diode like MUR420 does it in 1/3 the time, and
accordingly about 1/9th the charge (Q_rr).

Real recovery time of a 1N4007 is probably a lot worse (4us+?).

Tim

 

[Tim Williams]

Try this some time: apply 48 volts in the forward direction to a
1N4007 or 1N4009. The current will ramp up pretty linearly to numbers
like 50 amps in 100 ns or so. Now apply a lot of reverse current, like
50 amps, through a small inductor. It will conduct in the reverse
direction for maybe 50 ns then snap off, making a huge (kilovolt
maybe) spike across the inductor, just a couple ns wide. Lotsa EMI.

Yep, lotsa EMI. Some tube amp builders have noticed this. So, instead of
simply applying an RC damper, they go with high speed diodes. Usually the
bigass FREDs. What they need 6A diodes for, I don't have a clue. The
ones with deeper pockets even buy SiC schottkies.

The mind reels.

Tim

 

[Clifford Heath]

Try this some time: apply 48 volts in the forward direction to a
1N4007 or 1N4009. The current will ramp up pretty linearly to numbers
like 50 amps in 100 ns or so.

How the hell do you do that, for just the 100ns? Surely any device
that could deliver the 48 volts and 50 amps would produce heaps of
EMI all by itself?

Clifford Heath.

 

[ehsjr]

They are all online!

http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/hpjindex.html


Here's the Boff diode one:

http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1960-01.pdf


I wonder if the GR Experimenters are available somewhere. Tek had a
journal, too.

John

Nice links! Thanks.

Ed