Toshiba Unveils Photovoltaic-Output Optocoupler For Solid State Relays
The new electrically isolated device features the high open voltage needed to drive power MOSFETS
Toshiba’s TLP3910 features a minimum open voltage of 14 volts, double that of Toshiba’s present device, the TLP3906.
The TLP3910. Image courtesy of Toshiba
Toshiba TLP3910 Photocoupler is composed of two Aluminium Gallium Arsenide (GaAlAs) LED packaged together and optically coupled with a high-speed photodetector. The photodetectors are connected in series, enabling the TLP3910 to drive MOSFETs.
Solid State Relays
A mechanical relay is basically an electromagnet that causes an electrical switch to close. Solid-state relays, on the other hand, consist largely of a solid-state device, here a MOSFET, that is driven to saturation to conduct power.
The operation is illustrated below, although the TLP3910 features two LEDs, not one as illustrated here.
Operation of a solid-state relay. Image courtesy of Toshiba
When the LED(S) are energized, voltage is generated in the photodetectors, which in turn drive. the MOSFET.
Among the many advantages of SSRs over electromechanical relays are speed and reliability. In addition, SSRs don’t generate sparks, a vital concern in many use cases, especially in industrial environments where volatile gasses are present.
A Possible Source of Confusion about Driving MOSFETS
MOSFETs are voltage-driven devices, so what is the purpose of the multi-watt MOSFET gate drivers that the readers of these pages have become familiar with? The answer is speed.
The input of a MOSFET presents significant capacitance, and it takes current to overcome those capacitances in the time frames in which MOSFETs operate when they are part of devices like switched-mode power supplies (SMPS). The TLP3910 and similar devices put out far less current than do the MOSFET gate drivers employed in those types of applications.
SSRs operate on far slower time scales than SMPS do, so while the relatively tiny amount of power that the TLP3910 provides takes longer to drive a MOSFET, it’s still fast enough for a myriad of purposes. Toshiba offers a tutorial on the subject.
Electrical Specifications for the TLP3910
The device provides a minimum output current of 12 µA, with the maximum spec’d at 70 µA. These are relatively tiny amounts, but enough for the relatively slow speeds at which many SSRs operate.
Switching characteristics are:
- Turn-off time is 0.1 ms typical, 0.5 ms maximum
- Turn-on time is 0.3 ms typical and maximum time is 1.0 ms
Input (to the driver LED) is 30 mA, and from 100 to 125℃ the derating is -0.8mA/℃. At 10 mA, the LED’s input forward voltage ranges from 3 to 3.6 volts
- Input capacitance is 23 pF (typ)
- Total input to output capacitance is 0.8 pF
- MOSFET gate drivers
- Programmable Logic Controllers (PLC)
- Measurement devices
- Factory Automation
- The TLP3906 is available in a 10 x 3.84 x 2.1 mm SOL6 package
- The unit operates over a temperature range of -40 to +125 ℃
Regulatory and Safety Concerns
- The unit provides a 5000Vrms (minimum) isolation voltage
- UL-recognized: UL 1577, File No.E67349
- VDE-approved: EN 60747-5-5
- cUL-recognized: CSA Component Acceptance Service No.5A File No. E67349
- The TLP3906 is RoHS complaint