The ever decreasing ‘on’ resistance of modern MOSFETs means that they stay cool even when handling lots of amps. It is possible to dispense with a heatsink and rely on the copper of the PCB to dissipate the heat. This ability to surface-mount power devices is what makes new packages like LFPAK and Power SO8 so popular.
So, how much power can a SMD MOSFET handle? A good example is the Nexperia PSMN0R7- 25YLD. This has a typical ‘on’ resistance of 0.57mΩ and a sturdy, internal ‘clip’ construction that can carry a lot of current. A quick glance at the datasheet indicates that this tiny SO-8 size device can dissipate 158W and the low RDS(ON) suggests a current handling capability in the hundreds of amps!
But hold on – that figure of 158W only holds true if the mounting base (the solder tab) is kept at 25°C. What does that imply? Well, getting rid of 158W requires a lot of heatsink, probably water cooled, and is not practical in the real world. However, it does give us a maximum figure to work to. The datasheet shows that if the mount- ing base gets hot, the power dissipation must be reduced. For example, at 75°C we must de-rate to 95W.
When the device is working hard the silicon will get hot. The maximum allowable for this device is 150°C. At that temperature, the RDS(ON) increases by a factor of 1.6 to 1.15 mΩ. We can use this to calculate the current that will generate 95W and therefore the maximum current.
The power is given by: P = I2R
Rearranging: I2 = P/R
That is: I2 = 95/0.00115
Taking the square root: I = 287A
This is an absolute maximum figure and suggests that a target of 200A is realistic. What kind of PCB can carry 200A? Clearly, not a conventional one. The thickest PCB copper around is 0.5mm (14 oz). Reference to the online PCB trace-width calculator shows that even with that thickness of copper, a trace 20mm wide is needed to keep the temperature rise of the PCB down to 20°C.
To demonstrate 200A capability, I chose to use solid copper bus bars 25 x 3mm thick. Two pieces, one for the source and one for the drain were glued to a third to form a rigid structure. The glue used was ‘self shimming’ and thus provides an electrical insulating barrier.
Soldering the source and drain of the PSMN0R7-25YLD directly to the copper provides the lowest possible thermal resistance. The thick copper,
apart from carrying the high current, ‘spreads’ the heat over a large area and cools the MOSFET by natural convection.
MOSFETs are designed to operate as switches. When they are ‘on’ the current is high but the voltage drop is small. When they are ‘off’ the current is very small but the voltage drop is high. In both these conditions the power dissipation given by V x I is low. It is when the device is switching that the danger comes. In the intermediate state the V x I product is large and therefore it is essential to switch as fast as possible.
A TC4422CPA gate driver is used to drive the gate. A 12-volt supply ensures the lowest possible ‘on’ resistance. The TC4422CPA driver can provide up to 9A to charge the gate capacitance and thus guarantee fast switching times.
All circuits have inductance. This causes problems when switching off because the current is forced to keep flowing through the MOSFET after it is
turned off. This is known as avalanche mode. In avalanche mode, the voltage across the device is usually just above the Vds rating. Avalanche mode can be fatal to the MOSFET because the V x I product is large. For instance, in the case of PSMN0R7-25YLD switching 200A, the V x I product is 25 x 200 = 5000W! This is acceptable only if the duration is very short – in the order of nanoseconds.
In the demonstrator, nichrome wire is used as a resistive load and the loop inductance is kept low by running the flow and return cables close together to reduce the loop area.
In use, the push button on the top of the demonstrator switches on the MOSFET via the gate driver. Two DC-DC converters provide 100A each to the Nichrome wire resistors which begin to glow red after a few seconds. The clamp-on current meter indicates just over 200A. A thermocouple attached to the copper near to the MOSFET shows the temperature rising slowly. After about thirty seconds it steadies at around 60°C.
In conclusion, SMD power devices have an advantage over their leaded counterparts because they can be directly soldered to a heatsink. This property, combined with remarkably low ‘on’ resistance, enables them to switch immense currents. The rugged ‘clip’ design of Nexperia’s LFPAK packages takes advantage of this high current capability.
In practice, SMD MOSFETs are soldered to far thinner copper layers than are used in this demonstrator. The high current rating of LFPAK is still valuable to enable devices to survive short-term overload conditions such as motor start-up or locked rotor situations.