| Figure 2|
Figure 2 plots output voltage versus load current tested at seven temperature points between 25°C and 250°C, illustrating very limited move with temperature of the load regulation curves. Under constant-load conditions, output-voltage stability has been shown to be better than 2% over the temperature range "55°C to 225°C.
Tests performed on this device at the NASA Glenn Research Laboratory have demonstrated a record operating temperature range of "200°C to +300°C, with voltage stability better than 3% throughout the entire 500°C temperature interval. Long-term reliability has been demonstrated by life testing for 15,000 hours at 250°C and 10,000 hours at 300°C.
High-temperature system design example
An example showing how high-temperature ICs can effectively be combined with external passive components to create a robust, high-temperature system can be seen in a reference design for a step-down DC-DC converter capable of achieving up to 85% power efficiency. The first generation of Cissoid's VOLCANO family of DC-DC converter reference designs, ETNA, combines individual high-temperature functions to enable a high-efficiency converter suitable for direct application in equipment such as aircraft actuators, automotive ECUs, or equipment for down-hole drilling or data-acquisition.
The converter operates in voltage mode, with constant-frequency PWM. The duty-cycle modulator is implemented using a high-temperature 555 timer, fed by clocking and error-amplifier signals. The regulator also benefits from a non-overlapping circuit that avoids short-circuit currents during switching, thereby improving efficiency. The input voltage range is 8V to 30V, for conversion to an output from 2.5V to 25V, and the load-current range is 10mA to 1A. The converter has an input feed-forward architecture, which maintains DC line regulation at 1mV/V and delivers faster response to input transients than can be achieved using a conventional control loop.
Tests performed on the converter show a low drift of output voltage with temperature, of 150V/K. Output voltage ripple below 50mV at 25°C increases to 100mV at 175°C. This rise is attributable to the lowering of the output capacitor and the increase in ESR of the output capacitor with temperature. Indeed, because high-temperature capacitors are very expensive, the choice of the output capacitor value is a trade-off between performance (voltage ripple), volume and cost.
The ETNA DC-DC Converter Reference Design is being integrated by CMAC MicroTechnology as a Multi-Chip Module (see Figure 3), which will further improve reliability at high temperature as well as reducing the form factor.
| Figure 3|
DC-DC converter roadmap
VESUVIO Buck (Step-down) Converter is the second generation of the VOLCANO family based on a new PWM controller chip named MAGMA, a new synchronous high-side and low-side driver named HYPERION and two 40V N-channel MOSFET's. This new chipset increases the integration of active functions and reduces the number of external passive components as shown on the high temperature board in Figure 4.
| Figure 4|
VESUVIO can improve power efficiency, in excess of 90% at 225°C (see Figure 5), and offers new functionalities such as soft-start capability, power-good indication, under-voltage lockout, external clock synchronization, and an Enable control allowing the converter to be put into a low-power standby mode. In addition, the speed of HYPERION allows higher switching frequencies, meaning smaller passive components. It is also capable to drive larger MOSFETS, increasing DC-DC converter output current and power, respectively up to 4A and 50W at 225°C.
| Figure 5|
New DC-DC converters designs are on the roadmap: EREBUS will increase the maximum input voltage of ETNA and VESUVIO from 30V up to 50V while STROMBOLI isolated DC-DC converter will extend this maximum input voltage up to 300V.
High-temperature IC's using Cissoid technology have been demonstrated to operate reliably in excess of 225°C, with record temperature range from "195°C to 375°C.
Electronic systems like DC-DC Converters based on these components have also showed high robustness at high temperature.
DC-DC Converters to be built as MCM modules for operation at high temperature have been announced, as well as a clear roadmap for further high-temperature devices including highly integrated ICs performing common functions and DC-DC converters delivering extended features and performance.
Pierre Delatte is chief technology officer at CISSOID, a high-temperature semiconductor developer based in Mont-Saint-Guibert, Belgium.