As electronic sub-systems play a significant role in the reliable and safe operation of the overall product, adding an overvoltage protection circuit around the DC/DC step-down regulator may be advisable to prevent damage to the latest digital logic processors considering their absolute maximum voltage (<2V on some rails) is significantly less than intermediate bus voltages. (For further insight into what operating environments and conditions warrant such protection refer to “μModule Regulator Powers and Protects Low Voltage μProcessors, ASICs and FPGAs from Intermediate Bus Voltage Surges”) A critical portion of the overvoltage protection circuit is the crowbar, a component which when triggered shorts the output and ground connections of the step-down regulator to relieve the voltage stress on the load.
Two of the most common circuit components utilized as a crowbar are the MOSFET and silicon controlled rectifier (SCR) also known as a thyristor. We compared the capability of both devices to protect a 1.0V output rail typical of modern digital logic cores using the LTM4641, a 38VIN, 10A step-down regulator as our test platform (Figure 1). This µModule regulator has an integrated output overvoltage detection circuit and crowbar driver. When an output overvoltage condition is sensed at the output, a built-in driver at the crowbar pin goes high within 500ns and the MOSFET at MSP disconnects the input supply, VINH from the DC/DC converter.
For our tests, we assumed a 1.0V nominal output voltage representative of the core voltage of modern logic devices including FPGAs, ASICs, and microprocessors. A quick overvoltage response time is imperative for protecting low voltage logic whose absolute maximum voltage rating typically ranges from 110% to 150% of nominal. This fact is particularly important when the upstream rail is an intermediate bus voltage such as 12V, 24V, & 28V. In our tests, the input voltage was set at 38V and the adjustable output overvoltage protection trigger threshold was left at the default value of 110% of the nominal output voltage.
First up is the MOSFET. An NXP PH2625L 3mΩ, 1.5VTH, 100A MOSFET in a 5 × 6mm Power SO-8 was installed as the crowbar with the gate tied to the CROWBAR driver pin. Under a direct short from VINH to SW, the excursion never exceeded 1.16V or 116% of nominal (Figure 2).
Next, we replaced the MOSFET with a silicon controlled rectifier (SCR) from Littelfuse and connected the CROWBAR driver pin to the gate of the SCR. The Littelfuse S6012DRP is rated at 100A peak surge current, 1.6V peak on-state voltage, and 1.5V trigger threshold in a 6.6 × 11.5mm TO-252 package. After protection engaged from an OVP event, the output voltage remained relatively constant at 1.6V, 60% over the nominal regulated voltage coinciding with the peak on-state voltage of the Littelfuse SCR (Figure 3). A probe added at VINH shows us that even though the input supply had already decreased to near zero, VOUT still remained high. Clearly, the Littelfuse SCR is unable to provide effective overvoltage protection.
For our next test, the Littelfuse SCR diode was exchanged for a Vishay SCR with a lower on-state voltage and trigger threshold. The Vishay 10TTS08S is rated at 110A peak surge current, 1.15V peak on-state voltage, and 1.0V trigger threshold in a 10 × 15mm TO-263 package. At first glance, the OVP protection with the Vishay SCR seems like an improvement over the Littelfuse SCR, however the output voltage peaked higher at 1.7V or 70% over the nominal regulated voltage (Figure 4). In this case, there appears to be no correlation between the OVP peak value and the on-state voltage of the SCR. In both cases with the SCR in place, the output voltage peaked between 12-14µs after the overvoltage event starts which indicates a response time delay by the SCRs.
This bench data supports our claim that silicon controlled rectifiers (thyristors) react too slowly and have too high an on-state voltage to be an effective crowbar for today’s most advanced FPGAs, ASICs and microprocessors. Furthermore, the SCRs require more PCB area than the MOSFET which outperformed them. A MOSFET such as the NXP PH2625L in conjunction with the LTM4641 is a more reliable and effective method of powering and protecting the latest digital logic devices.