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Solid-State Relays with optimized superjunction FET technology

By eeNews Europe


Enabling a new generation of cheap, reliable, and feature-rich solid-state relays that match conventional EM relay performances

Introduction

Despite being based on a century-old technology, electromechanical relays (EMR) are still widely used for switching AC and DC loads. While providing great power density, EMRs face limitations in terms of controllability and reliability.

Today’s solid-state relays are based on TRIACs (silicon controlled rectifiers, SCRs for AC systems) or standard planar MOSFETs (for DC systems) as the switching element and optically isolated drivers for control. Known limitations of state-of-the-art SSRs are the achievable maximum output current, high power dissipation, size, and total solution cost.

The controllability of standard SSRs is limited. SCR-based relays can only support random-phase switch-on, zero-crossing switch-on, and zero-current switch-off. Due to the high forward voltage drop of SCRs, the minimum power dissipation is 0.6-1 W/A.

Standard MOSFET-based relays use devices with an RDS(on)∙A figure of merit (FOM) of 2-3 Ω∙mm², leading to large and costly semiconductors dies. For higher currents, standard MOSFET-based SSRs are single-ended and DC only.

Infineon has recently introduced the new 600 V CoolMOS™ S7 family, optimized for cost-sensitive static switch applications. The technology enables the implementation of a new generation of solid-state relays for 250 VAC/DC, matching the performance of conventional electromechanical relays.

The advantages of the new technology are numerous:

  • The reduced chip size leads to higher packaging density, especially in a back-to-back configuration of two FETs for both AC and DC switching.
  • The controllability of MOSFETs enables smart SSRs with comprehensive control features, including phase-control, section control, zero current switching, and zero voltage switching.
  • The increased robustness and protection features of the FET-based SSRs can eliminate the necessity of external fusing elements.

Superjunction (SJ) technology: making the impossible, possible

To reduce the specific RDS(on)∙A (Ω∙mm²) FOM, Infineon introduced in 1999 the CoolMOS™ technology, which first enabled a novel drain structure employing the superjunction concept [1].

With the introduction of the latest generation of CoolMOS™ SJ MOSFETs optimized for static switching applications, the 600 V CoolMOS™ S7 high-voltage superjunction MOSFET family achieves an unprecedented low RDS(on)∙A FOM of 0.6 Ω∙mm². This enables the implementation of solid-state relays in previously impossible power classes and packaging density.

The improvements enable products with RDS(on) values down to 22 mΩ in a TOLL SMD package. Products of the 600 V CoolMOS™ S7 family with 10 mΩ, 22 mΩ, 40 mΩ and 65 mΩ in different packages are already available.

Power dissipation budget and selection of proper SJ FET

In EMRs, two parts contribute to the power dissipation, coil dissipation, and conduction losses caused by the contact resistance.

Out of these two, the coil dissipation is independent of load current and not insignificant: even high-sensitivity EMRs usually have a coil dissipation of several 10s or 100s of mW.

FET-based solid-state relays only require a very small switching energy and no holding power at all. Furthermore, conduction losses do not increase over their lifetime.

Thus, depending on the power dissipation budget, a designer can choose the transistor pair with the required RDS(on). Figure 1 shows a diagram of the maximum total RDS(on) (i.e., the sum of both back-to-back FETs), depending on load current for four different power dissipation budgets.

As can be seen, the currently available portfolio of the 600 V CoolMOS™ S7 family enables the implementation of low-power-loss relays for different power classes.

 

Safe operating area (SOA), gate driving, and robustness

The continuous shrink of chip sizes enables devices with ultra-low on-resistance in a small package. When utilizing new power SMD packages such as QDPAK, previously impossible low RDS(on) values down to 10 mΩ are possible.

However, these advantages also come with certain conditions that a circuit designer must consider, especially when constructing a solid-state relay.

The overall smaller chips have reduced thermal capacity and increased thermal resistance compared to standard FETs of similar RDS(on) classes. This is best summarized in the safe operation diagram [2], which is narrower than standard MOSFETs [3].

As standard MOSFETs have a larger SOA diagram in general, they are less sensitive to gate driving with small output current circuits. Photovoltaic isolators (PVI) are often used due to their simplicity: the driver does not need an external, isolated power supply.

To ensure safe and reliable operation, either a buffered output driver or a standard, isolated gate driver with a separate supply is necessary for SSRs. The output buffer can also be implemented with discrete components, with only minimal added complexity.

Conclusion

Recent advancements in superjunction FET technologies enable the implementation of low-cost, low RDS(on), high voltage MOSFETs. With the introduction of the latest CoolMOS™ family, the S7 static switching products, circuit designers now have the essential building blocks to build small, cheap, reliable solid-state relays to improve their market competitiveness.

To discover more of Infineon’s broad cutting-edge portfolio of game-changing solutions and find the best fit for your needs, please browse our related web contents.

References

[1] Hancock J., Stueckler F., Vecino E., “CoolMOS™ C7: Mastering the art of quickness, A technology description, and design guide,” Application Note AN2013-04 V1.0, Infineon Technologies, April 2013

[2] Infineon Technologies, “Linear mode operation with high-voltage superjunction MOSFETs: Challenges with CoolMOS™ generation 7 devices in linear mode,” Application Note AN_2002_PL52_2005_172726, June 2020; Click to open

[3] Schoiswohl J., “Linear mode operation and safe operating diagram of power-MOSFETs,” Application Note AP99007, Infineon Technologies, May 2017

 


References

[1] Hancock J., Stueckler F., Vecino E., “CoolMOS™ C7: Mastering the art of quickness, A technology description, and design guide,” Application Note AN2013-04 V1.0, Infineon Technologies, April 2013

[2] Infineon Technologies, “Linear mode operation with high-voltage superjunction MOSFETs: Challenges with CoolMOS™ generation 7 devices in linear mode,” Application Note AN_2002_PL52_2005_172726, June 2020; Click to open

[3] Schoiswohl J., “Linear mode operation and safe operating diagram of power-MOSFETs,” Application Note AP99007, Infineon Technologies, May 2017


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