As electronic sub-systems play a significant role in the reliable and safe operation of the overall product, adding an overvoltage protection (OVP) circuit around the DC/DC step-down regulator may be advisable. The intent is to prevent damage to logic or microprocessors (μP). Note that in the latest μP versions, their absolute maximum voltage is significantly less than intermediate bus voltages. Typical supply voltages are <2V on some rails.
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" from the LT Journal of Analog Innovation.
A critical portion of the OVP circuit is the crowbar, a component which, when triggered, clamps or shorts the output and ground (circuit common) connections of the step-down regulator to relieve the voltage stress on the load. The name comes from the idea that a large metal bar is dropped across the power supply's output terminals when trouble occurs.
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 MSP MOSFET disconnects the input supply, VINH from the DC/DC converter.
Test circuit schematic.
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 percent to 150 percent of nominal. This fact is particularly important when the upstream rail is an intermediate bus voltage such as 12V, 24V, and 28V. In our tests, the input voltage was set at 38V and the adjustable output OVP trigger threshold was left at the default value of 110 percent of the nominal output voltage.
First up is the MOSFET. An NXP PH2625L 3mΩ, 1.5VTH, 100A MOSFET in a 5x6mm 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).
Overvoltage response with NXP PH2625L MOSFET crowbar.
VOUT never exceeds 1.16V in the case of a direct short from VINH to SW.
VOUT: 200mV/DIV; CROWBAR: 5V/DIV
SCR No. 1:
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 x 11.5mm TO-252 package. After protection engaged from an OVP event, the output voltage remained relatively constant at 1.6V, 60 percent 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 OVP.
Overvoltage response with Littelfuse S6012DRP SCR crowbar.
The S6012DRP SCR was unable to pull VOUT below 1.6V (60% above VOUT-NOM).
V INH: 20V/DIV; VOUT: 200mV/DIV, 1V @ 0DIV (1VDC offset); CROWBAR: 2V/DIV
SCR No. 2:
For our next test, the Littelfuse SCR 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 x 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 percent 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.
Overvoltage response with Vishay 10TTS08S SCR crowbar.
The output voltage peaked at 1.7V (70% above VOUT-NOM) before decreasing.
VINH: 20V/DIV; VOUT: 200mV/DIV, 1V @ 0DIV (1VDC offset); CROWBAR: 2V/DIV
This bench data supports our claim that SCRs 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 that 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.
Jason Sekanina, design engineer, contributed to this blog by providing assistance fabricating the test fixtures, running the tests, and collecting the scope screen-shots. The blog would not have been possible without his help.