Littelfuse’s Gate Driver Promises Fast Turn-on Speed in Solid-state Relays

September 22, 2021 by Abdulwaliy Oyekunle

Using load biasing, the new MOSFET gate driver has turn-on speeds in the order of tens of microseconds. It's also the so-called first of its kind that doesn't require an external power supply.

Fast turn-on switch speed is an essential feature when choosing a gate driver.

Littelfuse is aiming to address slow turn-on speeds with its newly-announced optically-isolated MOSFET gate driver, the CPC1596. This device employs a load-biasing technique, so designers don’t need to include an opto-gate driver auxiliary power supply in their designs. In addition, the product is housed in a small 8-pin dual-in-line and surface mount technology package.



Image of the CPC 1596 isolated load-biased MOSFET gate driver. Image used courtesy of Littelfuse


How might these two features—fast turn-on speed and no auxiliary power—help designers working with industrial controls, HVAC controls, medical devices, and IoT? 


Electromechanical Relays vs. Photovoltaic Gate Drivers

Solid-state relays employ components such as BJTs and MOSFETs for isolated switching applications by means of an optocoupler.

From bounce noise to mechanical aging, electromechanical relays have some limitations that prevent them from fulfilling efficient design demands. Photovoltaic gate drivers, on the other hand, are characterized by bounceless switching, high response speed, and low power consumption. What’s more, the external power supply to drive the gate of the MOSFET is eliminated.

Both components, however, provide isolations between high-power and low-power sides of electrical systems.


The Need for Photovoltaic Gate Drivers

An optocoupler makes use of a galvanic insulator to optically couple a photo transmitter with a photodetector that has a PN junction. When a photodetector such as a photodiode or a solar cell conducts current, electrons move to the n region and holes move to the p region to create an electron-hole pair. This creates a difference in potentials between the p and n regions.

Further, the photodetector generates a DC voltage that can be used to drive the gate of a MOSFET without a power supply.


DC voltage is generated to drive the gate of the MOSFET

When the array of photodetector detects light greater than its energy band gap, DC voltage is generated to drive the gate of the MOSFET. Image used courtesy of Panasonic


The high response speed in photovoltaic gate drivers is dependent on the on-resistance and the gate capacitance of the driven MOSFET. At the same time, the output current from the gate drivers in the microampere range causes low turn-on speed for larger gate charges of MOSFETS. This low turn-on speed makes it difficult to meet the demands of applications that require a faster response.

Typical photovoltaic isolators find applications in industrial controls, custom solid-state relays, and load distribution, among other applications.


Electrical Characteristics of the CPC1596

The new optically isolated MOSFET gate driver features an internal regulator circuit that regulates an input voltage up to 570 V to 12.2 V.  Its gate voltage ranges from 14.4 V to 16 V. Its maximum fast turn-on speed is 80 µs.

Block diagram of the CPC1596

Block diagram of the CPC1596 in a custom solid-state relay switching for an AC load. Image used courtesy of Littelfuse 


The CPC1596 is suitable for use in designs that require low-duty-cycle switching and low-frequency operation of external power MOSFETs with 4nF of gate capacitance.


Fast Turn-on Applications

The CPC1596 gate driver is useful in most designs that require the use of power MOSFETS. It also finds applications in solid-state relay solutions for both AC and DC loads. In power electronics applications, it is suitable for use in industrial and HVAC controls. What’s more, the product meets the fast turn-on speed requirements in medical devices.

Because fast turn-on speed is guaranteed to meet design demands, when designers use this product, they should include an external storage capacitor to efficiently drive the MOSFET.