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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I am hoping someone knowledgable about mosfets can clear up some questions about the mosfet module IXFN56N90P
It is rated for 56A @ 25c. The datasheet then states Idm = 168A, pulse width limited by Tjm. Does this mean the peak current can reach 168A if the pulse width is small enough? If the temperature can be maintained at 25c, why is there a current limit at all? What causes failure from too high current?
Are we talking about RMS or peak current?
It then says that Ia = 28A @ 25c. What is Ia?
So, when can the current be higher then 56A? If the temperature is kept at 25c, is the current max twice as much if it is a 50% duty? Is it 4x as much if it is a 25% duty? Let's assume the temperature is kept at 25c.
Registered Member #2481
Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134
Tj means "junction temperature" which is the actual temperature of the silicon where power dissipation occours. You can control case temperature (Tc) and you can keep that temperature as low as you like (with a big sink and/or forced cooling) but there is no way to control Tj, since you cannot change the junction-case thermal resistance (impedance). So, even if you are able to keep the case at 25°C, junction temperaure will increase with power dissipation because of non-ideal heat transfer from mosfet channel (silicon) and metallic case.
So, 56A should be the maximum continuous current when the case is at 25°C.
168A is the maximum pulsed current, and pulse width should be small enough in order to keep junction temperature below the maximum value (usually 150°C). Actually, from power dissipation (Ron*Ipeak^2) and thermal impedance you can estimate the maximum pulse lenght.
Registered Member #2481
Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134
56A refers to DC (continuous) current. From a point of view of thermal dissipation due to conduction losses, DC current and rms current play a very similar role.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
The MOSFET appears like a resistance Rds(on) when it is switched on. Therefore it's conduction losses are proportional to I². This means that it is the RMS of the current waveform that determines the heating in a MOSFET. Therefore it is the RMS of the current waveform that the datasheet is referring to.
Just remember that in practice it is very difficult to get near to the datasheet's absolute current ratings in practice. Datasheet figures are often quoted for junction temperatures of 25'C. Once a device is dissipating considerable power the silicon die inside the package will be many degrees hotter than the backplate, which will in turn be hotter than your heatsink. Even if you could keep the heatsink at 25'C the die will be much hotter when carrying significant current.
Those current figures also assume that conduction losses are the only source of heat generation. They are essentially a DC only fully turned on steady state test. If your MOSFETs are switching then you need to also include the heating effect of:
1. Turn-off switching losses from simultaneous V/I overlap 2. Turn-on switching losses from simultaneous V/I overlap 3. Capacitive turn-on losses from discharging of Cds into the channel 4. Leakage current losses in the off state 5. Avalance losses from voltage spikes above Vds(max)
In general a well designed power electronics system will have roughly equal conduction losses and switching losses. So you can assign roughly half of the thermal budget to conduction losses and half to switching losses.
If in doubt then parallel up many cheaper devices rather than push a small number of expensive ones close to the datasheet ratings.
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Mosfet issues and datasheet terms
