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From: "Fritz Schlunder"
Subject: Re: 87% All that vector calculus paid off
Date: Mon, 23 Dec 2002 07:39:25 -0700
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"Chris Carlen" wrote in message
> Fritz Schlunder wrote:
> > I rather doubt you are actually getting real spikes to 600-700V at the
> > at MOSFET switch off. If we look at figure 12d from the IRF730a
> > (http://www.irf.com/product-info/datasheets/data/irf730a.pdf), we can
> > roughly estimate that the anticipated avalanche voltage would be around
> > at an avalanche current of around 1.5A (whatever the maximum inductor
> > current at switchoff is, but in your case in the vicinity of 1.5A).
> > Assuming you are reallying using an IRF730a (by International
> > and assuming you aren't dealing with a major fluke on your hands, it is
> > unlikely that you are actually getting 600-700V spikes.
> Perhaps you are right, since I wasn't measuring this spike with proper
> treatment of the high frequencies involved. It was a quick and dirty
> look at the drain with a 20MHz B&K scope probe, with the long ground
> wire (gasp!).
> I was afraid to hook up my Tek DPO to the thing. I need to build or get
> my hands on some probes rated to more than 300V so I can use the DSO.
> > BTW, for clarity this isn't a transformer isolated flyback converter is
> > This is a simple discontinuous conduction mode boost converter (which
> > refer to as a boost flyback) topology, yes?
> Yes. There is only one winding, only an inductor.
> > I also find it most unlikely that a HER105 could possibly have a forward
> > recovery voltage anywhere near 700V-351V = 349V. Unforuntately diode
> > datasheets virtually never publish information about forward recovery
> > characteristics, but in my experience forward recovery on an ultrafast
> > doesn't normally exceed a few volts maximum (even with very high dv/dt
> > di/dt).
> Hmm. This is interesting. Ok, you got motivatied me to measure it
> I just measured the drain with my 100MHz scope probe, with the RF tip
> (no long ground wire, just the barrel touching the ground bus near the
> FET source, and the pin touching the drain lead.)
> Now I see a totally different picture. The flyback swings from 0V to
> about 380V in 40ns. Then it settles to 325V in another 20ns or so. So
> it seems the diode is behaving consistent with its 50ns reverse recovery
> time specification.
Err... Not really, reverse recovery time of a diode has pretty much nothing
to do with its turn on/forward recovery characteristics. Reverse recovery
time being the time it takes for the current to drop to some set level
(usually 250mA in most test setups for testing reverse recovery time) when
the diode goes from an intial state of forward conduction (usually 1A in
most manufacturers' test setups) to reverse bias. As soon as the diode
becomes reverse biased the current through the diode reverses direction and
isn't significantly limited by the diode until the excess carriers get
absorbed/get swept away after a finite amount of time. The reverse recovery
time is a strong function of the forward current just prior to reverse bias
(trr increases as the forward current increases). AFAIK forward recovery is
the time it takes for the diode's forward voltage to drop to near (110%? for
at least some manufacturers?) the steady state forward voltage after an
initially reverse biased (and fully blocking, IE not in reverse recovery)
diode becomes forward biased.
These numbers (although still not very ideal) are allot more believable than
the last ones. That extra 380V-325V = 55V of overshoot can probably mostly
be attributed to the parasitic loop inductance going from one end of your
inductor, through the diode, through the output capacitor, back around
through your supply rails, through the input filter capacitor, and back to
the other side of the boost inductor. On a breadboard if meticulous
attention isn't paid to minimize this inductance it can become relatively
substantial (but don't be discouraged breadboards can still be used quite
effectively for testing out SMPS designs).
It is a bit hard to calculate exactly what voltage overshoot impact this
inductance will have since it involves allot of variables and time dependant
functions (IE, turn off time of MOSFET, current in inductor, parasitic
capacitances such as MOSFET drain-source capacitance). But for a quick and
dirty analysis lets assume your diode current experiences a turn on di/dt of
about 100A/us or 100,000,000 Amps/sec. Let us also assume that 5V of your
55V overshoot can be attributed to the diode forward recovery
characteristic. Then using the formula:
Where E is the electromotive force produced by inductor L with a given
change in current versus time of (di/dt).
500nH or parasitic inductance is relatively large, but probably not too
difficult to obtain on a breadboard. From the original layout from the
pictures of your original circuit on your webpage, it doesn't look like
layout is all that spectacular (all those probes and test leads hooked up,
etc.), and 500nH might not be unrealistic.
> The forward recovery voltage is a new concept for me. I'm glad you have
> made me aware of it. I hope I understand it better in time.
Unfortunately most diode datasheets completely ignore forward recovery
voltages and times. This is probably because forward recovery times and
voltages are usually very small and therefore make a relatively negligible
impact on circuit performance in most applications (unlike reverse recovery
characteristics which can have a tremendous impact). ST seems to have some
datasheets with at least a little forward recovery characteristic
information, for one example:
For this diode we see that the peak forward recovery voltage (Vfp) is
typically 3V for a di/dt of 50A/us.
My understanding of diodes (which may be slighly erroneous since good
information about forward recovery characteristics seems to be virtually
nonexistant) is that they turn on virtually instantaneously, with maximum
turn on speed mainly limited by ESL, but for a brief moment while the diode
is turning on it may have an inflated forward voltage from that of its
steady state value. The increased voltage is usually minor. The "t on"
parameter in the IRF730a datasheet
(http://www.irf.com/product-info/datasheets/data/irf730a.pdf) under the
diode characteristics tends to agree with this idea (granted a MOSFET isn't
exactly a regular discrete diode).
> Good day.
> Christopher R. Carlen
> Suse 8.1 Linux 2.4.19
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