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From: Tim Williams on 10 Aug 2010 22:45
"Jon Elson" <jmelson(a)wustl.edu> wrote in message
news:CpqdnatqVfc3XfzRnZ2dnUVZ_sCdnZ2d(a)giganews.com...
> The effect of D-G capacitance and a 200 V drain swing is GREATER than
> the gate charge alone!

Ah, but it *is* included in total gate charge.

Big FETs might be 50-100nC (~260 for the old ones like IRFP250), which
means 10ns swing with 5-10A drive. You need a fast enough driver (TC4420
is only 6A and 30ns, one of those 20A 10ns stripline drivers is called
for), and you need a low enough gate spreading resistance (most are around
1 ohm, when they're rated at all), which might be a bigger problem over
~10A drive.

> You can dream of 10 ns switching, but it is a fantasy without about 20
> Amps of gate drive current. Then, the lead inductance of the transistor
> start to become significant, and you can't maintain safe G-S voltages
> with these kinds of di/dt into the gate.

Definitely the domain of stripline, ground planes, oodles of bypass caps,
and very careful design!

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


From: sarah kent on 11 Aug 2010 15:25
On Aug 9, 9:45 am, "Tim Williams" <tmoran...(a)charter.net> wrote:
> "sarah kent" <sarah6...(a)gmail.com> wrote in message
>
> news:e7ddfee9-2223-4f54-872c-ea3c042d51ba(a)e15g2000yqo.googlegroups.com...
>
> > > > (14nC x 30 V) / 3 = 140nsecs. Are my calculations accurate and in
> > > > right direction?
>
> > > No, C == A.s, so 14nAs * 30V = 420nVAs == nJ (energy). Divide by amps
> > > gets you Vs, which is flux, not time.
>
> > > Since C == A.s, 14nC / 3A = 4.67ns. This is less than the rated t_r so
> > > it will be chip-limited, not capacitance-limited.
> > I am unable to understand that why we do consider Gate to Source
> > Charge (14nC) to calculate the time?
>
> Quoted above.
>
> Tim
>
> --
> Deep Friar: a very philosophical monk.
> Website:http://webpages.charter.net/dawill/tmoranwms

Hi,

I am planning to turn OFF and ON the transistor using PWM of 100KHz.
The time that I calculated 140nsecs. What exactly does it tell. Does
it tell that I can turnON and OFF the transistors at 140nsecs or
somthing else.

Thanks

Sarah
From: Tim Williams on 11 Aug 2010 15:43
"sarah kent" <sarah6541(a)gmail.com> wrote in message
news:63713e9a-993d-42fe-9882-c19bbaf937e8(a)c10g2000yqi.googlegroups.com...
> I am planning to turn OFF and ON the transistor using PWM of 100KHz.
> The time that I calculated 140nsecs. What exactly does it tell. Does
> it tell that I can turnON and OFF the transistors at 140nsecs or
> somthing else.

Then you can drive transistors with 14nC gate charge quite easily.
14nC/140ns = 0.1A.

A gate drive transformer and controller chip will do this, e.g. TL598
(200mA output drive).

Small transistors are also a cheap alternative, like the drive circuit
used here:
http://myweb.msoe.edu/williamstm/Images/Discrete_Boost.png
Q4/Q5 emitter follower drives Q6 with plenty of speed.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


From: sarah kent on 11 Aug 2010 16:45
On Aug 11, 3:43 pm, "Tim Williams" <tmoran...(a)charter.net> wrote:
> "sarah kent" <sarah6...(a)gmail.com> wrote in message
>
> news:63713e9a-993d-42fe-9882-c19bbaf937e8(a)c10g2000yqi.googlegroups.com...
>
> > I am planning to turn OFF and ON the transistor using PWM of 100KHz.
> > The time that I calculated 140nsecs. What exactly does it tell. Does
> > it tell that I can turnON and OFF the transistors at 140nsecs or
> > somthing else.
>
> Then you can drive transistors with 14nC gate charge quite easily.
> 14nC/140ns = 0.1A.
>
> A gate drive transformer and controller chip will do this, e.g. TL598
> (200mA output drive).
>
> Small transistors are also a cheap alternative, like the drive circuit
> used here:http://myweb.msoe.edu/williamstm/Images/Discrete_Boost.png
> Q4/Q5 emitter follower drives Q6 with plenty of speed.
>
> Tim
>
> --
> Deep Friar: a very philosophical monk.
> Website:http://webpages.charter.net/dawill/tmoranwms

Hi,

How can it be calculated that the driver chip (HIP4081A, A4940) and
the transistor (IRF540) can be worked at 100KHz pulse width modulation
frequency? IRF540 is NMOS transistor, I am looking for its equivalent
PMOS , any suggestions!

Thanks

Sarah
From: Tim Williams on 11 Aug 2010 17:12
"sarah kent" <sarah6541(a)gmail.com> wrote in message
news:b13a78de-1777-4342-b7e1-c64c3abc2731(a)t20g2000yqa.googlegroups.com...
> How can it be calculated that the driver chip (HIP4081A, A4940) and
> the transistor (IRF540) can be worked at 100KHz pulse width modulation
> frequency?

Because the datasheet says so (maximum clock frequency etc).

> IRF540 is NMOS transistor, I am looking for its equivalent
> PMOS , any suggestions!

9540

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


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