PolarPAK is the latest products introduced in the semiconductor market by ST to fulfill the demand of more power per cubic inch of new dc-dc converters for high-end server and telecom applications. The new PolarPAK product series is the best choice when high current and low temperature became the two requirements of the design. In fact, thanks to the PolarPAK package, the heat can be dissipation from both, top and bottom path, allowing to handle high current without reach high temperature. This particular feature makes the PolarPAK the best SMD package in terms of power dissipation in the same outline. The superior thermal performance versus the other comparable packages can be easily understood by looking at Table 1.
As can be seen from the well known formula, thanks to the low Rthj-pcb, the power capability is boosted compared to the standard package.
The new package’s leadframe and plastic encapsulation are similar to those used for most standard power MOSFET packages, ensuring good die protection and easy handling in manufacturing. Yet compared to the standard SO-8, it is almost 50% thinner with height profile of just 0.8 mm, but the PolarPAK can dissipate heat through the top metal and so through the heat sink on top of it. This feature makes this package so efficient that it can handle up to twice the current of a standard package having the same size. This outstanding performance allows more compact design and higher power density with a reduction in the number of paralleled MOSFETs and so allowing a reduction of the system size. Another benefit coming from this new package is the absence of the wires to connect the silicon to the frame. This particular features, allows a reduction of the unwished noise on the circuit due to the parasitic inductors produced by the bonding wires. In addition to the performance mentioned above, the new PolarPAK Power MOSFET are designed with a fixed footprint and pad layout, that is independent of the die size used, to give manufactures maximum flexibility in order to avoid the need for re-design the print-circuit-board (PCB) layout when different PolarPAK MOSFET are used.
Benefit from the application point of view
In order to demonstrate the benefits coming-out from the use of this new package a test was performed on the L6732A ST demo-boards, working with and without heat–sink, assuming the following conditions:
Single phase
1 x HS, STK800
1 x LS, STK850
fsw = 500 kHz
VIN = 12V
VOUT = 1.5V
Tamb = 25 şC
A practical method to calculate the thermal behavior and in particular to calculate the Tj (junction temperature) of the device, is to characterize the internal body diode of the MOSFET. In fact, the Power MOSFET intrinsic body diode is the ideal sensor to measure the junction temperature, since the forward voltage Vf varies linearly with temperature, approximately -2.2mV/°C. Therefore, the basis of this method is to dissipate a known amount of power in the MOSFET, and from the variation of Vf obtained the amount of temperature rise of the junction, known the normalized curve of Vf Vs the Tj from the characterization data (Figure 1).
As can be seen, the estimated DTj, based on the Vf measurement, is around 70 °C (DTj=2.2*33), when the devices work with and without heat-sinks. To see how much this low junction temperature increase the overall system efficiency, we can refer to the efficiency curves in Figure 2.
Without air flow and with heat-sink, the impact on the overall efficiency is around 1.8% @ 36A that means a gain up to 1W at full load with respect to the condition without heat-sink and without airflow. If we consider the case with heat-sink and air flow (2.5m/sec), the PolarPAK package is even more efficient by dissipating more heat out of the topside and in this case we can reach up to 6% higher efficiency that means a gain up to 3.2W at full load. The performances are achieved with the the following devices: STK800 & STK850 that are the latest products introduced in the semiconductor market by ST to fulfill the demand of more power per cubic inch of new dc-dc converters for server and telecom applications. These Power MOSFETs are based on the 4th generation of ST’s proprietary STripFETTM planar technology, deliver extremely low conduction losses and switching losses and achieve very low figure of merit (FOM). The minimal capacitances and gate charge of STK800 and the low on-resistance of STK850 in conjunction with the double sided cooling path of the PolarPAK package, maximize the overall efficiency of customer’s application. To confirm as told above, is showed another efficiency comparison done versus two similar competitor devices where ST devices, thanks to lowest FOM, provide up to 2.5% highest efficiency (Figure 3).
Click here for the illustrations:
Table 1, Figure 1, Figure 2, Figure 3
