Benjamin Jackson, Manager of Automotive Power Switch Products at International Rectifier considers performance power packaging for automotive through-hole applications
Efficiency matters. In fact efficiency is a key deliverable on the design of many new automotive power electronics systems. Every watt of wasted electricity can be traced back to an extra drop of fuel that must be put in the tank or a gram of CO2 exhaled from the exhaust; both of these are increasingly the subjects of higher and higher taxes. But how do automotive semiconductor suppliers help customers achieve better efficiency?
The package of the semiconductor is a series element in the circuit, the electric current must flow through the package uninhibited and let the heat out to the cooling system.
The system is therefore only as strong as the weakest link in the chain; when the RDS(ON) of a typical MOSFET was in the order of 8mOhm then around one mOhm extra in the package was acceptable. But when the resistance of the silicon is less than the package there is clearly a need for improvement in the package.
Surface mount packages have risen to this challenge; the typical D2Pak will only add around 0.5mOhm of resistance while packages like the DirectFET only contribute a mere 150µOhm to the on resistance. But what about the through-hole packages? Here, there has been limited innovation.
A popular package in automotive applications is the TO-262 – the long leaded cousin of the D2Pak, this is a package that is frequently chosen in high power applications where to achieve good cooling, the power components are placed on a separate substrate from which the heat can be more easily extracted. The irony is however that despite being used in high power systems the TO-262 is a poor performer when it comes to package resistance. The main limitation is not in the wire bonding technology but in the leads themselves. Typically the total resistance of both the source and drain leads alone adds up to be around 1mOhm!
Consider a 40V TO-262 MOSFET with an RDS(ON) of 2mOhm stated on the data sheet. The data sheet value is comprised of the summed resistance of the MOSFET die and the package, but it excludes the leads.
So in a worst case when the full length of the leads are used in the system, the total resistance from tip to tip of the leads is in fact 3mOhm. In the application this has several results; the high lead resistance results in self-heating in the leads which in turn heats up the rest of the MOSFET and adds to the cost of cooling.
The resistance of the package also leads to higher conduction losses and poorer efficiency. To this end a simple improvement on the standard TO-262 has been made; this is the WideLead TO-262.
The width of the leads of the new device are substantially wider than previous generations. As a result the resistance of the leads is reduced by around 50% when compared to its parent package, the standard TO-262.
The lower lead resistance combined with improved technology inside the package results in a maximum current rating that can now be increased to 240A much greater than the 195A normally associated with the leading TO-262 packages on the market.
Finally the body and form factor of the package remains the same as the traditional TO-262, so no major mechanical redesign is needed when switching to the WideLead package.
At a system level, the benefits can be seen in Figure 1; this chart plots the temperature of the leads against DC current for both the standard and WideLead TO-262 packages, in both cases the silicon inside the packages is identical. At 60A the WideLead was 39% cooler; this gives several system level benefits, reduced heating and therefore reduced reliability concerns.
Alternatively by having less heat generated, less has to be extracted so maybe the cooling arrangements can be downsized or perhaps a lower grade of PCB material used, one that does not have to be rated at such a high temperature.
Alternatively for a given operating temperature up to 30% higher current can be achieved – higher current with the same silicon. Either way improved performance can be achieved with cost reduction, as the package performance becomes less of a limitation on the semiconductor inside.
Energy has been a plentiful resource for decades, but increasingly it’s becoming in short supply. Something that is not in short supply is CO2 and indeed so much so governments are aggressively starting to tax emissions to halt their flow.
This challenge is bringing out the very best in automotive engineering, and power semiconductors have to rise to the challenge at both the semiconductor and package level.
Today the through-hole package is playing catch-up, but with modern innovations it is reaching closer and closer to the performance of the silicon.