I'm currently trying to find an effective way to select a mosfet with
the lowest power dissipation for the following circuit values:
Id=2A
Vd=270V
f=100khz
D=40%

Mosfet Driver Specs
Claims up to 2A peak gate drive
Tf = Tf = 14nS with 1nF load

Rdson is just a conduction loss.
Gate capacitance is the main cause of switching loss due to slowing
down driver rates. <<Not sure about that..

Rdson and gate capacitance are inversely proportional for mosfets?
Not sure about that too..
If so...
By finding the balance between Rdson and gate capacitance, I could get
less total power dissipation.

In that case...
Is there a figure of merit...Maybe like Ciss/Rdson? I just go for the
smallest number?

I've been looking at the SPP21N50
Rds(on) 0.19 ohms
Ciss = 2400pF
This mosfet has a great Rdson but I wonder if I could get less
dissipation with more Rdson and less Ciss?

What's a good selection procedure?

On Sun, 11 Feb 2007 07:28:12 GMT, D from BC
myrealaddress@comic.com> wrote:

I'm currently trying to find an effective way to select a mosfet with
the lowest power dissipation for the following circuit values:
Id=2A
Vd=270V
f=100khz
D=40%

Mosfet Driver Specs
Claims up to 2A peak gate drive
Tf = Tf = 14nS with 1nF load

Rdson is just a conduction loss.
Gate capacitance is the main cause of switching loss due to slowing
down driver rates. <<Not sure about that..

Rdson and gate capacitance are inversely proportional for mosfets?
Not sure about that too..
If so...
By finding the balance between Rdson and gate capacitance, I could get
less total power dissipation.

In that case...
Is there a figure of merit...Maybe like Ciss/Rdson? I just go for the
smallest number?

I've been looking at the SPP21N50
Rds(on) 0.19 ohms
Ciss = 2400pF
This mosfet has a great Rdson but I wonder if I could get less
dissipation with more Rdson and less Ciss?

What's a good selection procedure?

---
Rds(on) and Ciss aren't related except that the longer it takes to
charge and discharge the gate capacitance the longer it will take to
turn the MOSFET on and off. That means that during the time the
channel is in transition it'll be dissipating a lot more power than
it will be for the same time when it's either on or off.

Ciss doesn't dissipate any power in the MOSFET except in the ESR of
the gate capacitance, but the driver has to source and sink the
charge and discharge current, so there will be some power dissipated
in the driver.

So, what you want to do is get a MOSFET with the Rds(on) you
need/can afford and the lowest Ciss you can find, then drive the
gate as hard as you can so that the MOSFET will turn on and off
quickly.

Using a MOSFET driver will save you a lot of woe, and some good ones
are at:

http://www.micrel.com/page.do?page=product-info/mosfets.shtml

Sounds not so bad. Generally spoken, newer devices have lower losses (=smaller chip inside).

I use LTspice to run efficiency calcs. It's easy. Use digikey to find what is buyable.

I don't understand how Mot comes to the 50:50 rule.

- Henry

On Sun, 11 Feb 2007 08:52:50 -0600, John Fields
jfields@austininstruments.com> wrote:

Using a MOSFET driver will save you a lot of woe, and some good ones
are at:

http://www.micrel.com/page.do?page=product-info/mosfets.shtml

You mean do it like this?...
Let's say I can waste 2watts...

Then I solve for Rds(on).

Then I look at all the mosfets with that Rds(on) and pick the one with
the lowest gate charge.

Then ram the gate with powerful mos drivers and hope that Vg can rise
and fall fast enough to lower switching losses. (Slow operation =
heat.)

Hopefully the switching loss is not significant compared to the heat
from Rds(on).

What I'm looking into is:
Given Id, Vd, Igate, freq, duty; does a power dissipation minima exist
given a wide assortment of mosfet Rds(on) and gate charges? A sweet
spot...

On Sun, 11 Feb 2007 19:51:27 GMT, D from BC
myrealaddress@comic.com> wrote:

On Sun, 11 Feb 2007 08:52:50 -0600, John Fields
jfields@austininstruments.com> wrote:

Using a MOSFET driver will save you a lot of woe, and some good ones
are at:

http://www.micrel.com/page.do?page=product-info/mosfets.shtml

You mean do it like this?...
Let's say I can waste 2watts...

---
Where?

In the MOSFET?

In the driver?

Total from the supply?
---

Then I solve for Rds(on).

---
OK, that'll get you the steady-state dissipation if you know the
supply voltage and the current into the load.
---

Then I look at all the mosfets with that Rds(on) and pick the one with
the lowest gate charge.

---
Yup.
---

Then ram the gate with powerful mos drivers and hope that Vg can rise
and fall fast enough to lower switching losses. (Slow operation =
heat.)

---
Yup
---

Hopefully the switching loss is not significant compared to the heat
from Rds(on).

---
That's the trick!

The faster you can turn that booger on and off the lower your
switching losses will be, but you'll have to pay the piper in the
driver.
---

What I'm looking into is:
Given Id, Vd, Igate, freq, duty; does a power dissipation minima exist
given a wide assortment of mosfet Rds(on) and gate charges? A sweet
spot...

---
Sure, and there's nothing magic about finding it.

If what you're concerned about is steady-state power dissipation in
the MOSFET, get one with the lowest Rds(on) you can find.

If what you're concerned about is switching losses, get the one with
the smallest gate capacitance you can find and drive it with a
voltage source.

If what you want to do is to get the best of both worlds, then
you'll have to do the legwork to find out who minimizes both WRT
your application.

On Sun, 11 Feb 2007 08:52:50 -0600, John Fields
jfields@austininstruments.com> wrote:

On Sun, 11 Feb 2007 07:28:12 GMT, D from BC
myrealaddress@comic.com> wrote:

I'm currently trying to find an effective way to select a mosfet with
the lowest power dissipation for the following circuit values:
Id=2A
Vd=270V
f=100khz
D=40%

Mosfet Driver Specs
Claims up to 2A peak gate drive
Tf = Tf = 14nS with 1nF load

Rdson is just a conduction loss.
Gate capacitance is the main cause of switching loss due to slowing
down driver rates. <<Not sure about that..

Rdson and gate capacitance are inversely proportional for mosfets?
Not sure about that too..
If so...
By finding the balance between Rdson and gate capacitance, I could get
less total power dissipation.

In that case...
Is there a figure of merit...Maybe like Ciss/Rdson? I just go for the
smallest number?

I've been looking at the SPP21N50
Rds(on) 0.19 ohms
Ciss = 2400pF
This mosfet has a great Rdson but I wonder if I could get less
dissipation with more Rdson and less Ciss?

What's a good selection procedure?

---
Rds(on) and Ciss aren't related except that the longer it takes to
charge and discharge the gate capacitance the longer it will take to
turn the MOSFET on and off. That means that during the time the
channel is in transition it'll be dissipating a lot more power than
it will be for the same time when it's either on or off.

Ciss doesn't dissipate any power in the MOSFET except in the ESR of
the gate capacitance, but the driver has to source and sink the
charge and discharge current, so there will be some power dissipated
in the driver.

So, what you want to do is get a MOSFET with the Rds(on) you
need/can afford and the lowest Ciss you can find, then drive the
gate as hard as you can so that the MOSFET will turn on and off
quickly.

Using a MOSFET driver will save you a lot of woe, and some good ones
are at:

http://www.micrel.com/page.do?page=product-info/mosfets.shtml

You mean do it like this?...
Let's say I can waste 2watts...
Then I solve for Rds(on).
Then I look at all the mosfets with that Rds(on) and pick the one with
the lowest gate charge.
Then ram the gate with powerful mos drivers and hope that Vg can rise
and fall fast enough to lower switching losses. (Slow operation =
heat.)
Hopefully the switching loss is not significant compared to the heat
from Rds(on).

What I'm looking into is:
Given Id, Vd, Igate, freq, duty; does a power dissipation minima exist
given a wide assortment of mosfet Rds(on) and gate charges? A sweet
spot...

D from BC

I've never hit a sweet spot. I filter on voltage it can cope with, then
current, then price, then buy a couple.
10nF, 1nF who cares? those gate C's are really big. Bang 'em with gate
driver chips. Only killer is operating frequency.
God know why but I spent 2 hours last month with paper and calculator
looking at switch losses on a design using a pair of 80A FETs. Bought the
FETs and the calculated heatsink, built it, run fine. Within 5 minutes and
for other reasons I'd changed the design to a higher Frequency and a
different switching arrangement. Those 2 hours now lost for all time. Lesson
learned was don't waste valuable time with the sums, just select on price
and keep an eye on the spice.

On Sun, 11 Feb 2007 23:32:38 -0000, "john jardine"
john@jjdesigns.fsnet.co.uk> wrote:

I've never hit a sweet spot. I filter on voltage it can cope with, then
current, then price, then buy a couple.
10nF, 1nF who cares? those gate C's are really big. Bang 'em with gate
driver chips. Only killer is operating frequency.
God know why but I spent 2 hours last month with paper and calculator
looking at switch losses on a design using a pair of 80A FETs. Bought the
FETs and the calculated heatsink, built it, run fine. Within 5 minutes and
for other reasons I'd changed the design to a higher Frequency and a
different switching arrangement. Those 2 hours now lost for all time. Lesson
learned was don't waste valuable time with the sums, just select on price
and keep an eye on the spice.

Interesting...
Are you saying that it's so complicated that hit and miss
determination is the faster method. Just pick the essential mosfet
specs and then compare in spice..

Select on price...Eye on spice
Nice electronic poetry...
D from BC

D from BC wrote:
On Sun, 11 Feb 2007 23:32:38 -0000, "john jardine"
john@jjdesigns.fsnet.co.uk> wrote:

I've never hit a sweet spot. I filter on voltage it can cope with, then
current, then price, then buy a couple.
10nF, 1nF who cares? those gate C's are really big. Bang 'em with gate
driver chips. Only killer is operating frequency.
God know why but I spent 2 hours last month with paper and calculator
looking at switch losses on a design using a pair of 80A FETs. Bought the
FETs and the calculated heatsink, built it, run fine. Within 5 minutes and
for other reasons I'd changed the design to a higher Frequency and a
different switching arrangement. Those 2 hours now lost for all time. Lesson
learned was don't waste valuable time with the sums, just select on price
and keep an eye on the spice.

Interesting...
Are you saying that it's so complicated that hit and miss
determination is the faster method. Just pick the essential mosfet
specs and then compare in spice..

Select on price...Eye on spice
Nice electronic poetry...
D from BC

If you look at the Rds losses and the gate capacitance losses you'll
realise you can calculate the whole lot dead easily. Then as you're
doing that you'll realise it depends not only on the frequency but also
upon the "on time" of the fet. So you can calculate the losses only as
long as you know the average on time. eg on time = 100% = no gate
capacitance losses and Rds is dominant. On time = 1% and Rds becomes
almost irrelevant and the switching losses dominate.

The Mot rule of Rds losses = to G cap losses works quite well when the
the average on time is 50%. But it's only a rule of thumb to give you a
starting point.