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# Paralleling Mosfets in smps for Cooler Operation

D

#### D from BC

Jan 1, 1970
0
Can a 100khz converter get less hot with paralleled mosfets?

I have my single mosfet max'd out due to internal Rg using a Tr=14nS
gate driver... The heat sink gets too hot in my app.. Can I improve
the efficiency by paralleling mosfets..?
(Not using more parts to share the total power loss but reducing the
total power loss..)

Paralleling mosfets can reduce the conduction losses to reduce heat..
But no so with the switching loss. It is just divided among the
paralleled mosfets..In other words...No efficiency gain.. correct?
(I suspect I'm wrong on this...but not sure..)

Also...
I've read that paralleled mosfets can oscillate..
A fix is to use gate resistors and or ferrite beads to prevent
oscillations.
Wouldn't this slow down the mosfets and create more switching heat?
Compensate with higher Vg?
How large are these gate resistors..I can only guess 10 to 30 ohms??

Do smps designers resort to paralleling while maintaining the same
switching frequency for a cooler box?
Maybe paralleled so much that no heat sink is required?
D from BC

J

#### Joerg

Jan 1, 1970
0
D said:
Can a 100khz converter get less hot with paralleled mosfets?

Yes.

I have my single mosfet max'd out due to internal Rg using a Tr=14nS
gate driver... The heat sink gets too hot in my app.. Can I improve
the efficiency by paralleling mosfets..?
(Not using more parts to share the total power loss but reducing the
total power loss..)

Yes

Paralleling mosfets can reduce the conduction losses to reduce heat..
But no so with the switching loss. It is just divided among the
paralleled mosfets..In other words...No efficiency gain.. correct?
(I suspect I'm wrong on this...but not sure..)

You need to make sure that the driver has enough gusto. If it doesn't
then the slopes flatten a bit and you might not gain anything, or worst
case even lose efficiency.

Also...
I've read that paralleled mosfets can oscillate..
A fix is to use gate resistors and or ferrite beads to prevent
oscillations.
Wouldn't this slow down the mosfets and create more switching heat?
Compensate with higher Vg?
How large are these gate resistors..I can only guess 10 to 30 ohms??

I won't get into that battle of opinions again. It's like arguing ground
planes. Or, ahem, politics.

While a gate resistor or bead can avoid oscillation they may not be
necessary with clever placement and layout. I usually was able to do it
sans resistance. A gate resistor will cost you in switching losses. To
me they often look like taking Maalox everyday (some people do that...)
instead of changing to a proper diet.

Do smps designers resort to paralleling while maintaining the same
switching frequency for a cooler box?

This here designer has done that. Many times. Not so much for
temperature reasons but for more efficiency.

Maybe paralleled so much that no heat sink is required?

Ahm, that would require more data from your side. A lot more data. If
this is a 5kW switcher I'd be inclined to say no, if it's 5W then
probably yes.

J

#### John Larkin

Jan 1, 1970
0
Yes

You need to make sure that the driver has enough gusto. If it doesn't
then the slopes flatten a bit and you might not gain anything, or worst
case even lose efficiency.

Switching losses will increase with paralleled fets, because more
capacitance means more switching energy dumped. How important this is
depends on the switching freq and such.
I won't get into that battle of opinions again. It's like arguing ground
planes. Or, ahem, politics.

10-30 is a reasonable range to kill birdies.
While a gate resistor or bead can avoid oscillation they may not be
necessary with clever placement and layout. I usually was able to do it
sans resistance. A gate resistor will cost you in switching losses. To
me they often look like taking Maalox everyday (some people do that...)
instead of changing to a proper diet.

Even single mosfets will often make rf bursts on rising/falling edges.
I'm not sure what determines this, but there's no obvious (to me)
relationship between "good" and "bad" layouts here.

John

J

#### Joerg

Jan 1, 1970
0
John said:
Switching losses will increase with paralleled fets, because more
capacitance means more switching energy dumped. How important this is
depends on the switching freq and such.

True. It will increase the losses in the driver itself because it has to
charge more capacitance. What I meant were the losses caused by sluggish
transitions if the driver he has now is a bit too wimpy for more than
one FET.
10-30 is a reasonable range to kill birdies.

Agree. But for a big FET that carries a penalty.
Even single mosfets will often make rf bursts on rising/falling edges.
I'm not sure what determines this, but there's no obvious (to me)
relationship between "good" and "bad" layouts here.

A big sin is sloppy layout at the drain node. Long traces that are too
thin and stuff like that. Same for long gate traces, those can cause
real grief. Most "singing" that I noticed was due to that. It can also
happen when the driver is wimpy and can't hold the gate up while the
drain node roars down. In one case they didn't want to change the parts
placement and there wasn't any space to begin with except for a gate
trace of 1/2" or so. I put a pnp/npn follower there which made a huge
difference and also notched up the efficiency by a couple percent.

Another factor is the transformer/inductor. In most cases I wasn't happy
with catalog parts and we rolled our own. Surprisingly that often ended
up being cheaper for production than the catalog part (when made in Asia).

D

#### D from BC

Jan 1, 1970
0
Wow..that was a fast answer
Thanks...
This will keep me thinking for awhile..
Very interesting bit about the gate resistors too..

About the paralleling up to no heat sink:
It's for a 130watt Cuk converter running at 100khz.
Maybe it's possible to "make a heat sink out of mosfets"
I could prepare a LTspice file and put on my website for download?

Heat sinking question:
My estimated mosfet total power loss is 3Watts at 25C on paper.
I suspect my single mosfet heats up due to switching loss causing
Rds(on) to rise leading to more conduction loss.. Heat making heat.
Does this happen?
Efficiency depending on heat sink quality?

D from BC

J

#### Joerg

Jan 1, 1970
0
D said:
Wow..that was a fast answer
Thanks...
This will keep me thinking for awhile..
Very interesting bit about the gate resistors too..

About the paralleling up to no heat sink:
It's for a 130watt Cuk converter running at 100khz.
Maybe it's possible to "make a heat sink out of mosfets"
I could prepare a LTspice file and put on my website for download?

A schematic would be nicer but I won't have much time since I am in the
middle of a large design. With digital stuff in there, yuck...

But maybe someone else can take a look at it.

Heat sinking question:
My estimated mosfet total power loss is 3Watts at 25C on paper.
I suspect my single mosfet heats up due to switching loss causing
Rds(on) to rise leading to more conduction loss.. Heat making heat.
Does this happen?
Efficiency depending on heat sink quality?

I can't imagine that happening unless you are smoking them. But quite
frankly I wouldn't want to run that without any heat sink. Can't you
bolt them to the chassis via isolation layers? While maintaining safety,
of course.

J

#### John Larkin

Jan 1, 1970
0
Another factor is the transformer/inductor. In most cases I wasn't happy
with catalog parts and we rolled our own. Surprisingly that often ended
up being cheaper for production than the catalog part (when made in Asia).

Inductors and transformers are like sheet metal and heat sinks, often
a lot cheaper for custom parts than for catalog stuff. That's kinda
counter-intuitive.

Tried Minntronix? Excellent magnetics house. Ask for Butch.

John

J

#### Joerg

Jan 1, 1970
0
John said:
Inductors and transformers are like sheet metal and heat sinks, often
a lot cheaper for custom parts than for catalog stuff. That's kinda
counter-intuitive.

Tried Minntronix? Excellent magnetics house. Ask for Butch.

Not yet but after a recent post from you I added that link to my wiki.
It's always good to know a source that others were happy with. Right now
it's mostly optics stuff I am doing but there sure will be more
transformers. Then I'll ask for Butch. And whether there are any of

D

#### D from BC

Jan 1, 1970
0
A schematic would be nicer but I won't have much time since I am in the
middle of a large design. With digital stuff in there, yuck...

But maybe someone else can take a look at it.

I can't imagine that happening unless you are smoking them. But quite
frankly I wouldn't want to run that without any heat sink. Can't you
bolt them to the chassis via isolation layers? While maintaining safety,
of course.

I used a crappy temporary heat sink on the mosfet (1/2"x 4"x 1mm
aluminum strip) for out of the box bench testing. No grease.
The top of the TO220FP case almost reached 100C after 1minute.
Then I unplugged it..So that's go me freaked..
The control is stable.
It does get bolted to a thick aluminum extrusion once fully assembled.
I was just trying a crude way of evaluating efficiency with the temp
heat sink.
I did think about dumping the whole smps in water and measuring a
temperature rise..

I should be able to research the rest confident that it's not a dead
end road. I'll work on the math for optimum mosfet specs for
paralleling and make sure gate drive as good as it gets..

Maybe post some of that yucky digital stuff on here
Thanks again..
D from BC

J

#### Joerg

Jan 1, 1970
0
D said:
I used a crappy temporary heat sink on the mosfet (1/2"x 4"x 1mm
aluminum strip) for out of the box bench testing. No grease.
The top of the TO220FP case almost reached 100C after 1minute.
Then I unplugged it..So that's go me freaked..

Well, that sure ain't much of a heat sink. Maybe a wider piece?

The control is stable.
It does get bolted to a thick aluminum extrusion once fully assembled.
I was just trying a crude way of evaluating efficiency with the temp
heat sink.
I did think about dumping the whole smps in water and measuring a
temperature rise..

Your best friend is the scope. Check out what happens upon turn-on and
turn-off. Also across the Isense resistors (hoping there is one). That
gives you a good indication of whether something is out of whack. Make
sure you don't fry it with any spikes. If in doubt use a resistive
divider so some more sane voltage level.

Most of the efficiency problems I saw were due to sluggish turn-on and
core saturation.

I should be able to research the rest confident that it's not a dead
end road. I'll work on the math for optimum mosfet specs for
paralleling and make sure gate drive as good as it gets..

Maybe post some of that yucky digital stuff on here

It's not ready yet. I am wrestling with ADC, DAC, SPI, addressing and
all that. It is like eating oatmeal soup. I don't like oatmeal soup...

T

#### Terry Given

Jan 1, 1970
0
D said:
I used a crappy temporary heat sink on the mosfet (1/2"x 4"x 1mm
aluminum strip) for out of the box bench testing. No grease.
The top of the TO220FP case almost reached 100C after 1minute.
Then I unplugged it..So that's go me freaked..
The control is stable.
It does get bolted to a thick aluminum extrusion once fully assembled.
I was just trying a crude way of evaluating efficiency with the temp
heat sink.
I did think about dumping the whole smps in water and measuring a
temperature rise..

calorimetry can be a lot harder than you expect.

its often easier to measure the DC input & output power - if you have
reasonable input & output filters. if the current is all nasty and
pulsating, this is not quite so easy.
I should be able to research the rest confident that it's not a dead
end road. I'll work on the math for optimum mosfet specs for
paralleling and make sure gate drive as good as it gets..

Maybe post some of that yucky digital stuff on here
Thanks again..
D from BC

one gotcha with FETs is the positive tempco of Rdson. If you dig up a
copy of the Siliconix MOSPOWER applications book, there is a paper on it
in there (by Rudy Severns IIRC).

in a SMPS the FET current tends to be independant of Rdson, os Pfet =
I^2*Rdson.

causing dTj

causing Rdson to increase

causing more Pfet....

there is a critical heatsink thermal resistance above which you get
thermal runaway (little dutch timebomb, tick tock boom)

below Rtheta_kaboom, it doesnt run away, *but* the junction temperature
will end up higher (sometimes a lot) than you expect.

For a prototype, just use a monster heatsink until you have the whole
thing running, and can make an efficiency measurement.

Then try the real heatsink, and make another measurement.

That'll tell you straight away if its thermal runaway (kaboom) or
thermal step-away (efficiency drops, Tj higher than you expect)

Cheers
Terry

D

#### D from BC

Jan 1, 1970
0
On Wed, 14 Feb 2007 18:28:31 -0800, Joerg

[snip]
Your best friend is the scope. Check out what happens upon turn-on and
turn-off. Also across the Isense resistors (hoping there is one). That
gives you a good indication of whether something is out of whack. Make
sure you don't fry it with any spikes. If in doubt use a resistive
divider so some more sane voltage level.
[snip]

Huhhh...I wish ...I made a damn offline smps.
Can't scope ground it or poof! But it's gotta be done...
(Using a Hickok scope from 1968!! Only 8 knobs
I think I have to buy a >100W power transformer (or equivalent
transformer combo) to isolate the smps supply under test. That's going
to cost $$. Or brainstorming ideas such as: Make an optocoupler probe.. Make an op amp differential probe.. Make HF transformer probe.. Buy an isolated digital scope D from BC D #### D from BC Jan 1, 1970 0 calorimetry can be a lot harder than you expect. its often easier to measure the DC input & output power - if you have reasonable input & output filters. if the current is all nasty and pulsating, this is not quite so easy. one gotcha with FETs is the positive tempco of Rdson. If you dig up a copy of the Siliconix MOSPOWER applications book, there is a paper on it in there (by Rudy Severns IIRC). in a SMPS the FET current tends to be independant of Rdson, os Pfet = I^2*Rdson. causing dTj causing Rdson to increase causing more Pfet.... there is a critical heatsink thermal resistance above which you get thermal runaway (little dutch timebomb, tick tock boom) below Rtheta_kaboom, it doesnt run away, *but* the junction temperature will end up higher (sometimes a lot) than you expect. For a prototype, just use a monster heatsink until you have the whole thing running, and can make an efficiency measurement. Then try the real heatsink, and make another measurement. That'll tell you straight away if its thermal runaway (kaboom) or thermal step-away (efficiency drops, Tj higher than you expect) Cheers Terry Those sneaky mosfets.. I'll try a Pin Pout measurement/comparison to see if I'm getting mysterious losses.. The continuous mode Cuk has easy waves to work with.. Triangle Iin& Iout currents and Vin has 120Hz ripple and Vout is nearly DC. Thanks D from BC T #### Terry Given Jan 1, 1970 0 D said: On Wed, 14 Feb 2007 18:28:31 -0800, Joerg [snip] Your best friend is the scope. Check out what happens upon turn-on and turn-off. Also across the Isense resistors (hoping there is one). That gives you a good indication of whether something is out of whack. Make sure you don't fry it with any spikes. If in doubt use a resistive divider so some more sane voltage level. [snip] Huhhh...I wish ...I made a damn offline smps. when I test smps I power them up from a lab supply (to smps startup)- this lets you check your feedback loop (use another supply to crank up the output, at the desired setpoint your duty cycle should drop to zero), gate drive, oscillator etc. Then use another lab supply to slowly wind up the DC bus. If you scalethe switching frequency down, you can organise for the right peak current in your magnetics, and therefore test for saturation. or get really keen and get a variac (I bought a big 3-phase variac, dismembered it and sold one phase for slightly more than I paid for the entire unit). light bulbs are also very handy, and cheaper than exploding FETs Can't scope ground it or poof! But it's gotta be done... (Using a Hickok scope from 1968!! Only 8 knobs I think I have to buy a >100W power transformer (or equivalent transformer combo) to isolate the smps supply under test. That's going to cost$$$. its low power, so scab a pair of transformers from something, and hook them up sec-to-sec. I've got a 2.5kW isolation transformer I bought from a 2nd hand store for about$25.

Or brainstorming ideas such as:
Make an optocoupler probe..
Make an op amp differential probe..
Make HF transformer probe..

waste of time really. besides, when looking at a nasty waveform, how can
you tell if its the circuit or your probe?
Buy an isolated digital scope

the new Tek scopes are fabulous in this regard.
D from BC

Cheers
Terry

D

#### D from BC

Jan 1, 1970
0
D from BC wrote: [snip]
Or brainstorming ideas such as:
Make an optocoupler probe..
Make an op amp differential probe..
Make HF transformer probe..

waste of time really. besides, when looking at a nasty waveform, how can
you tell if its the circuit or your probe?
Buy an isolated digital scope

the new Tek scopes are fabulous in this regard.
D from BC

Cheers
Terry

I didn't like any of my probe ideas either..
So it's down to surplus pwr xformers or new scope..
Mmmm chunk of iron...or new toy...

D from BC

K

#### Klaus Kragelund

Jan 1, 1970
0
Well, that sure ain't much of a heat sink. Maybe a wider piece?

Your best friend is the scope. Check out what happens upon turn-on and
turn-off. Also across the Isense resistors (hoping there is one). That
gives you a good indication of whether something is out of whack. Make
sure you don't fry it with any spikes. If in doubt use a resistive
divider so some more sane voltage level.

Most of the efficiency problems I saw were due to sluggish turn-on and
core saturation.

I think the issue of oscillation using paralling MOSFETs is relevant
when you operate in the linear mode of the MOSFET. When you drive it
hard on, then there should be no problem (the internal of a trench
MOSFET is parallel devices, isn't it?)

In a current design we are using 8 small MOSFETs in parallel instead
of one big MOSFET. The losses are lower even though the resulting
RDSon is the same, since the MOSFETS cool to the PCB (so there is less
thermal resistance) and there is no hotspot on the PCB (there was
before since the PCB has significant thermal resistance). Since the
hotspots are gone, the individual MOSFET runs cooler. Remember RDSon
is about 2 times larges for a MOSFET running hot). Lastly, we can
avoid the heatsink, since the MOSFETs are distributed on the PCB to
allow the entire PCB to act as a heatsink.

Regards

Klaus

W

#### Winfield Hill

Jan 1, 1970
0
Can a 100khz converter get less hot with paralleled mosfets?

I have my single mosfet max'd out due to internal Rg using a Tr=14nS
gate driver... The heat sink gets too hot in my app.. Can I improve
the efficiency by paralleling mosfets..?

It would be helpful if you told us the MOSFETs and drivers
you are using, and some of the other operating parameters.

A

#### Andrew Edge

Jan 1, 1970
0
I used a crappy temporary heat sink on the mosfet (1/2"x 4"x 1mm
aluminum strip) for out of the box bench testing. No grease.
The top of the TO220FP case almost reached 100C after 1minute.
Then I unplugged it..So that's go me freaked..
The control is stable.
It does get bolted to a thick aluminum extrusion once fully assembled.
I was just trying a crude way of evaluating efficiency with the temp
heat sink.
I did think about dumping the whole smps in water and measuring a
temperature rise..

I should be able to research the rest confident that it's not a dead
end road. I'll work on the math for optimum mosfet specs for
paralleling and make sure gate drive as good as it gets..

Maybe post some of that yucky digital stuff on here
Thanks again..
D from BC

At the frequency you are working at it, it is normal usage to place a
Zener between the gate and source in bridge based switchers to reduce
the oscillation problem.
If overheating worries you so much
use MOSFETS with inbuilt protection circuitry. The PHILIPS TOPFET
range (BUK100 ) has thermal protection incorporated, and probably
protection against short circuits and other stuff too.

For paralleled use though derate the current rating by 20% for 2 MOS's
and as you add more increase that. Differences between them will lead
to different currents flowing in them. Though this isn't a big problem
in most cases because of the positive temperature coefficient of the
drain to source gates, be prepared for the worst.

Andy

G

#### Gary Reichlinger

Jan 1, 1970
0
Can a 100khz converter get less hot with paralleled mosfets?

Take a look at the multiphase converter topology. International
Rectifier (www.irf.com) and Texas Instruments (www.ti.com) make parts
for this approach. Basically, you have a separate inductor for each
mosfet so the current is evenly divided among them. The multiphase
part refers to the fact that they are sequenced to be on at different
times so that the magnitude of the ripple current is less.

J

#### John Larkin

Jan 1, 1970
0
At the frequency you are working at it, it is normal usage to place a
Zener between the gate and source in bridge based switchers to reduce
the oscillation problem.

How does a zener do that?

John

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