AN4407

AN4407 STMICROELECTRONICS | Alldatasheet

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Technical content

Datasheet sections

• 1 Description
• 1.1 Approach to the study
• 1.2 Parasitic inductance effects
• 1.2.1 Turn-off
• 1.2.2 Turn-on
• 1.3 Comparison analysis between 3 pins and 4 pins devices in efficiency and
• 2 Eon / Eoff energy: experimental waveforms
• 3 Conclusions
• 4 References
• 5 Revision history

Advantage of the use of an added driver source lead in discrete Power MOSFETs Antonino Gaito, Marc Laudani, Massimo Nania, Cristiano Gianluca Stella Introduction In modern power supply design, more and more attention is given to the electrical efficiency of an overall system and to the junction temperature of semiconductor devices handling all the power to be converted into a usable form. Among all the semiconductor devices, transistors are by far the most important category; almost all of them are three pin devices (MOSFET, BJT, IGBT) and, as opposed to diodes, they have a driving section which makes them more sensitive to issues related to the interaction between power to handle and input signal. Even if this document focuses on Power MOSFETs, in some cases the results could be extended to other power transistors depending on the current level to be switched and on the switching speed. The aim of this document is to illustrate the limitation related to a 3-pin device and detail the advantages of using a fourth driving source pin, also known as Kelvin source, besides the traditional power source. In a 3-pin device, during every switching (turn-on or turn-off) cycle, the stray inductance of the source wire bonding coupled with the slope of the current being interrupted always generates a voltage signal opposite to the driving signal (V GS) of a MOSFET, in our case. The effect of this opposing signal is to slow down the switching cycle, which in turn increases the cycle by cycle switching loss. The introduction of the Kelvin source pin allows separating the path of the power from that of the driving signal and to refer the driving network to the Kelvin source, where no current is supposed to flow. In this situation, it is possible to obtain a driving signal which is immune to any disturbance deriving from the verylarge and fast current variation flowing though the power path. This results in the reduction of the overall power loss in the transistor, and consequently a lower operating temperature and potentially more reliable and longer lasting power systems.

1 Description

package with an additional driving pin. Figure 1. Package silhouette and electrical symbol (4 pin) switching operations. Table 1shows the principal electrical specifications. Table 1. Electrical characteristics

Figure 2. Demo board of PFC 2 kW using MOSFETs in TO247-4 (4 pin)

1.1 Approach to the study

output signal is used to regulate the conduction time of the power switch. along the lines of the solution proposed by STMicroelectronics.

Figure 3. Schematic of a push-pull driver for power device current control, a resistor is put in series with the source of the power device ( Figure 4). Figure 4. Schematic of a push-pull connection usingan L6536 as the driver

247 MOSFET with the 4-pin version (TO-247-4), we risk short-circuiting the sensing resistor

and losing any information coming from it (Figure 5).

Figure 7. Section of the driving network related to the demo board current flowing through the sensing resistor Rs. Hence, a 1Ω resistor was chosen.

1.2 Parasitic inductance effects

deriving from the connection of the lead with the die (Figure 8 and Figure 9).

Figure 12. Relationship between stray inductance parasitic effect and current

1.2.1 Turn-off

off switching power dissipation. Figure 13. Impact of the parasitic effect during the turn-off operation

pin solution and the 4-pin alternative. In Figure 14, this specific aspect is shown. Figure 14. Turn-off comparison between 3 pin and 4 pin solution GATE is changed, accelerating or delaying the switching operations. Figure 15. Turn-off energy comparison @ different Rg values

1.2.2 Turn-on

increased total switching power dissipation (Figure 16).

Figure 16. Impact of the parasitic effect during the turn-on operation standard 3-pin solution and the 4-pin alternative in Figure 17 and Figure 18.

1.3 Comparison analysis between 3-pin and 4-pin devices in

power levels for thermal and energy saving aspects. using both the 3-pin and 4-pin solutions.

2 Eon / Eoff energy: experimental waveforms

Figure 21. Waveforms and power losses during Figure 22. Waveforms and power losses during Figure 23. Waveforms and power losses during Figure 24. Waveforms and power losses during

3 Conclusions

The introduction of the fourth (driving) pin strongly improves the dynamic performance of Power MOSFET devices, which is even more evident when the switching current levels and the output power levels are higher during both device turn-off and turn-on. The test results show that the MDmesh™ M5 technology device assembled with the driving source pin provides significant benefits when the switching current is higher than approximately 10 A at turn-off, but the advantages are present at all power levels. Different increments in efficiency were measured for different power levels, and in a market where power levels are always increasing, each small efficiency improvement represents a large advantage for energy saving systems demanded in today’s very competitive market. For our demo board, the energy saving was almost 7 W at maximum output power (2 kW). The most important benefit, however, lies in the thermal behavior: in fact, a small magnitude of power loss is translated into a difference of many degrees (around 20°C) on case temperatures for the power device. Lower working temperatures allow the use of smaller heat sinks which occupy less area on board or, more importantly, can increase the reliability of the entire system.

4 References

applications, and design”, Second edition, John Wiley & Sons, New York, 1995. 2. Won-suk Choi, Dong-kook Son, Markus Hallenberger and Sungmo Young"Driving and Layout Requirements for Fast Switching MOSFETs.” Evaluation of Interconnect Parasitic Inductance on MOSFET Switching Characteristics” 4. Jon Mark Hancock, "Super junction FETs Boost Efficiency in PWMs", in Power Electronics Technology, July 2005, pp. 20-29. MDmesh™ 2nd Generation Devices Improve Efficiency in PFC Applications”, Proceedings of the 4th International Conference on Integrated Power Electronics Systems, CIPS 2006, 7-9th June 2006, Napoli, Italy, pp. 195-199. 6. C. Adragna Application Note AN1792, “Design of fixed-off-time-controlled PFC pre- regulators with the L6562", STMicroelectronics, November 2003. 7. STMicroelectronics, AN2951 "3 kW fixed-off-time (FOT) power factor correction”, June 2010.

5 Revision history

Table 2. Document revision history 29-Jan-2015 1 Initial release.