The gate-driver current consumption is calculated based on the following assumptions:

• Digital isolator primary side voltage = 3.3 V
• Digital Isolator secondary side voltage = 5 V
• Gate drive PNP transistor power supply voltage = 8.5 V
• The SSR MOSFET gate current consumption during switching is neglected.
• The AC current of digital isolators are neglected, as the SSRs are expected to be switching at a very low rate and the digital isolator AC current is negligible.
• The on-state voltage drop across PNP transistor Q2 is neglected

ISO7760 secondary current (Vi = Vcc), I_{CC2_ISO7760} = 3.3 mA.

ISO7740 secondary current (Vi = Vcc), I_{CC2_ISO7740} = 2.2 mA.

Gate current of single SSR, with 10-kΩ resistor across MOSFET gate and source, I_G = (8.5 – 0.7) V / 10 kΩ = 780 µA.

PNP transistor biasing current, I_BIAS = 8.5 V / (10 kΩ + 100 kΩ) = 77.3 µA.

Secondary side total gate drive current (approximate) with all SSR on, I_{GD_ON} = 3.3 mA + 2.2 mA + (10 × (780 + 77.3) µA) = 14.07 mA.

Secondary side total gate drive current (approximate) with all SSR off, I_{GD_OFF} = 3.3 mA + 2.2 mA = 5.5 mA.

The current consumption can be reduced by increasing the resistance across the gate-to-source terminals of the MOSFET, at the expense of increased SSR turn off time.