5 Proposed cascaded ideal diode configuration

The proposed solution uses two ideal diode controllers with respective MOSFETs Q1 and Q2 connected in series configuration, as shown in Figure 6. The clamping circuits of each controller not only ensure keeping CATHODE to ANODE below 75V but also act as an equalizing network to share equal voltage between Q1 and Q2 during fault events. Let us consider how the circuit works for two common fault scenarios

Case 1: During startup, with the output (VOUT) powered at 54V and input VIN at 0V, the mid-point voltage VMID stays at 0V. The second LM74700D-Q1 controller keeps GATE2 shut-down due to reverse current blocking scenario with VOUT > VMID and Q2 blocks 54V. In this case, the user applies reverse voltage of 54V at VIN, and the first LM74700D-Q1 controller keeps GATE1 in OFF state due to a reverse polarity scenario with ANODE < 0V and Q1 blocks 54V.

Case 2: In this scenario, VIN starts in fault state (at -54V, for example) and then the system is powered up with VOUT = 54V. The mid-point voltage VMID stays at 0V as the first LM74700D-Q1 controller keeps GATE1 in OFF state to block reverse voltage at VMID. Similarly, the second LM74700D-Q1 controller keeps GATE2 shut-down due to reverse current blocking condition. Both MOSFETs Q1 and Q2 incur voltage stress of 54V. Because the voltage across MOSFETs is below 60V during the fault cases, the solution provides flexibility for the customer to select legacy 60V-rated FETs that are easy to multi-source.

As seen in Figure 6, the solution also incorporates a transient clamping network (DC, Q3, RB and DB) in the ground path to handle switching transient voltages beyond the absolute max rating of the LM74700D-Q1. In normal operation, the potential difference between device ground and system ground is just VBE of Q3, but whenever VIN exceeds the breakdown voltage (VBR-DC) of diode DC, the transistor Q3 drops voltage across it and lifts up the device ground potential. This helps to limit the ANODE-GND voltage of the LM74700D-Q1 close to the breakdown voltage of DC, facilitating a scalable transient handling solution. The purpose of diode DB is to block the reverse current path under input supply reverse condition.