MOSFET transient dissipation

I used the Alt key code for the ohm symbol here and it looked ok while composing, but it's turned into a question mark after posting. So please read m? as milliohm.

I'd go for the FETs that are used on motherboards in VRM - for confersion of 12V to 1.something at 100 amps for CPU core.

Those things are cheap and plentiful and should have low Rdson...

Get the data sheet for the FET and there will be a graph for safe operating area.

1. The surge current will only be transitory and not have much effect once the lamp heats up and should not have any significant effect. Most devices can handle much more transient effects than steady state.

Your lamp is drawing about 4A. If, say, the mosfets Rds_on is 50mOhms then that is just 0.8W dissipation max in the mosfet(assuming it is always on, which in your case it's not, if it's 50% duty then thats only about 0.4W)

Thats well under under what the mosfet can handle. In fact your mosfet is too big. It has a 62C/W for no heat sink. Hence you do not need to use a heat sink. It will get hot of course but it can easily withstand double what you are using assuming normal ambient temperatures.

To get a better idea, suppose you are pulling a max of 4A steady state(ignore any short term transients from turn on since they will not have any long term effect) and your Rds_on is 25mOhms. If a 50% duty cycle then that is 0.2W. 0.2*65 = 13C above ambient. This is not much. Of course you need to derate a little and design for worse case..

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news:c76ih5lqg5193ombg21q6gqc9skjlhjnhs@4ax.com

JF

Of course. But how much more for how long, that's the question. The transitory period is brief but finite.

I'm not concerned about the steady dissipation once the lamp is fully heated up. The math is simple and, as I said in my initial post, it should make the transistor only slightly warm to the touch even without a heatsink.

I'm using this transistor because 1) I want to be sure it can handle the turn-on surge, and 2) it's cheap and easily available here(I got them for the equivalent of $0.37 US each).

_That_ is the question. *Can* the transient be ignored? Instinct says it can, but instinctive assumptions are always correct. I want to make a bullet-proof design as far as is practicable.

Already covered.

Regarding the turn-on surge, I've drawn a load line on the MOSFET characteristics curve with an assumed cold filament resistance of

0.25 ohms. It intersects the 10V Vgs curve at about 45A Id and 0.7V Vds. That's more than 30W dissipation for a brief moment (ignoring gate drive rise time). The fall to the steady-state dissipation of less than 0.4W will be non-linear. What I'm concerned about is the thermal inertia during that brief period.

On a sunny day (Fri, 4 Dec 2009 20:48:36 +0530) it happened "pimpom" wrote in :

I would mount those MOSFETs on the metal case of the box it is in. This is what I do, same MOSFET, I use a mica washer. It may live without anything to conduct the heat away, but having some heatsink makes you worry less.

(bottom page, for diagram) (that switch is no in this box). The battery is fused with 2 x 30A fuse.

If you look at the safe operating area graph you will see the FET can withstand about 50A with 1V drop across it.

It has a thermal resistance junction to ambient of 62C/Watt and a junction temp limit of 175C so should cope with 2.4W continuous without a heatsink. With an RDS of 0.0175 ohms 2.4 Watts equates to

11.7A continuous.

So start worrying somewhere between 11.7A and 50A.

Hello,

Steady state dissipation is about 0.4W, as Rthjc = 62C/W, this results in 25K temperature rise. So the steady state doesn't require a heat sink.

Assuming a cold resistance of 12 times below hot resistance, the mosfet has to supply about 50A. This would result in about 63W (0.025 Ohm), but for a very short time as lamp temperature rises rapidly and dissipation is proportional with I^2. Let us assume that 63W is dissipated during 0.1s, that is 6.3J. Look to figure 8 of the datasheet and extrapolates the graph to 100ms. Then you will see that you are close or above the SOA limit (for Dutch speaking readers, Safe Operating Area).

When the inrush current reduces to 60% of peak value well within 0.1s, you are within the safe operating area (at 60%, the dissipation drops to 36% of max. dissipation).

Do you need a heat sink for the inrush current? Assuming 0.8 gram of copper in the tap results in an average temperature rise of 20K in case of 6.3J added heat. For the transient energy, a heat sink is not required.

So my first conclusion is: heatsink is not required as long as inrush current to reach 60% is well below 0.1s (based on 50A peak current). When inrush current > 0.1s, use another mosfet. I think you should measure the inrush current versus time, or try to get reliable data. AFAIK, halogen head lights have more inrush current.

Best regards,

Wim PA3DJS

without abc, the address is valid.

That's what I would normally do without asking myself or the group if it's necessary. But my almost-finished pcb design will be considerably simplified if the MOSFETs didn't have to be provided with extra thermal inertia. Oh well.

This unit is going to drive 14 separate loads (not all at the same time). I intend to provide a profusion of fuses. Thanks for your input.

Thanks for the input. I also considered biasing the lamps with a pre-heating current to just below incandescence (a dim red glow wouldn't interfere with the application), but that would add complexity and also consume considerable standby power from the battery.

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Pimpom, did you miss my earlier post?  Click on the link above.

JF

Nothing happens when I click on it. That kind of link is new to me. What's supposed to happen? I'm using OE6 and IE8.

RAMP the MOSFET's ON ?? ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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         You can never be too prepared for the REPRESSION!

Or a current limit at maybe 2X operating current... avoids the typical

10X+ surge. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
         You can never be too prepared for the REPRESSION!

Well, a linear ramp would raise the MOSFET's turn-on dissipation - just the thing I want to limit. Pulsed drive with a rising duty cycle will stretch out the lamp's turn-on time which is also unacceptable. Since the lights are used for timing the racers, they have to be lit up as nearly instantaneously as possible.

What I meant by pre-heating was either by pulsing the MOSFETs at perhaps 25% duty in standby or with a resistor in parallel with the MOSFET. But either scheme would waste battery power.

How about slowing down the turn-on pulse, and "warming up" the filament (and raising its resistance) more gradually?

Run your gate drive through an RC with a reasonable time constant, and then feed it to the gate through a gate-stopper resistor. Depending on the time constant of the RC, and the thermal time constant of the filament, I imagine that you can probably bring the lamp up to full brightness in (e.g.) 100 milliseconds or so, without the current rising to more than double its steady-state value at any point. This might result in a better-looking power dissipation curve. You may increase the service life of the bulbs, too.

If you do decide to bang the MOSFET all the way on as fast as possible, it wouldn't surprise me if the MOSFET's ability to get rid of the transient heat pulse is limited by the rate of heat conduction through the package and tab. The current surge may be over, and the dissipation settled down to its steady state, before much of the heat has been conducted out to the far side of the tab. If that turns out to be the case, adding a heatsink (for additional thermal inertia at this point) might not buy you much.

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That will slow down the lamp's build up of light which is unacceptable. Even the normal turn-on time of an incandescent bulb is not completely satisfactory - at least in theory, but the error it creates in the racers' recorded elapsed time should be minor. We'll just have to accept that for the coming event. For later races, it should be possible to adapt the control unit to LEDs with little or no modification.

A slow build-up of light is unacceptable for the reasons I gave in the reply I just posted to Jim Thompson's post. Please see that.

That's what I'm concerned about and is really what this thread is about. Hmmm