February/March 2021

24 POWER SEMICONDUCTORS www.nexperia.com/RETs Issue 1 2021 Power Electronics Europe www.power-mag.com on, even at high load currents. Also, the residual voltage across this switch and the on status is lower compared to a standard transistor, so overall they achieve a much better power efficiency. Dies size can also be reduced. In use it is important that as little heat is generated in the load path transistor, so again, it is useful if the transistor has a good current amplification (h FE ), which does not degrade and improves with temperature over the load current. As an example, Figure 4 shows the performance of PBRN113ZT 600 mA, 40 V R1/RT2 1 k Ω /10 k Ω NPN performance-based RETs. Another benefit of the technology is the relative tight tolerance of the R1/R2 ratio of the integrated resistors for RETs with current values above 100 mA, which at +/-10% are three times more accurate than other products. This enables the input voltage safety margin to be reduced. Design considerations While RETs offer many significant benefits, some important design considerations should be noted. Figure 5 shows the switching characteristic of a RET plotting V CE versus V i . V i(off) is the input voltage the RET turns off. The condition required for an off- state is that the collector leakage current must be 100 µA at a collector-emitter voltage of 5 V. For safe turn-off, V (ioff)max must not be exceeded. When determining the on-state, V i(on)min must be considered. The circuit controlling the RET has to provide at least this voltage for a safe turn. On-state is defined as a collector current of 10 mA while the collector-emitter voltage Figure 2: Resistance ‘meander’ created using a standard device processing step Below Figure 3: Cross section of a 100 mA automotive RET is 0.3 V. The datasheet V i rating is valid for the test condition defined only. If a bigger collector current shall be switched the RET requires more base drive voltage Vi(on): V i < V I(off)max - a RET is in off-state for every device shipped V i < V i(off)typ - a typical RET is in off-state V i > V i(on)typ - a typical RET is in on-state V i > V i(on)min - a RET is in on-state for every device shipped. It is also important to consider the same diagram at different temperature ranges. BJT switches become more effective as the temperature goes up, so they will safely stay on if temperature becomes higher. That’s fine, for example, in an autonomous vehicle application in a hot climate; however, if you drive in a cold climate you have to be careful that the BJT stays in on-state, because at low temperatures the BJT gain goes down and the BJT current gain goes down and the voltage drop V BE increases, so a higher turn-on voltage is required. Table 1 shows the dependency of the on- and off-state input voltages on the resistor divider configuration in RETs from NHDTC series RETs. The selection of a proper resistor divider option is also important to ensure that the control voltage window of the RET matches the driving stage. New 80 V automotive RETs BJTs are preferred in many automotive applications due to their robustness. Figure 4: Performance of PBRN113ZT 600 mA, 40 V R1/RT2 1 k Ω /10 k Ω NPN RET