Radiation hardened DC-DC converters are designed to deliver an optimum DC output voltage and efficiency by withstanding different types of radiation effects. These converters are immune to a high level of ionizing radiation, radiation due to high energy electromagnetic fields from outer space, radiation from nuclear reactors, and also during warfare where the potentiality of a chemical explosion is sufficiently high to cause a change in electrical characteristics of the converter.

The radiation-hardened DC-DC converters are different from other converters because of the material used and manufacturing design which makes them less susceptible to radiation effects. These converters are tested against various radiation effects such as Total Ionizing Dose (TID), Enhanced Low Dose Rate Effects (ELDRs), Single Event Effects (SEE), and various others.

Solar Particle Events

Particles from the sun arrive with a large flux of high-energy ions, which also contain X-ray radiation. This radiation causes interference with the energy within the electronic components and therefore results in reduced performance and other radiation-induced damages.

A Van Allen radiation belt is a zone of highly charged particles trapped and kept around a planet by its magnetosphere. They mostly originated from the solar wind and pose problems for applications involving satellites due to their geo-location position.

Particles from the high-energy region of the electromagnetic spectrum can create ionizing radiation which includes cosmic radiation that can affect space stations and other high-altitude space systems, and nuclear warfare and explosion affecting military and civilian electronic devices.

Radioactive Alpha Decay in Chip Packaging Materials

Chip packaging materials also lead to soft errors in the manufactured DRAM chips. These soft errors are usually in the form of alpha particle emissions that results due to radioactive alpha decay in the nucleus. A positively charged alpha particle can travel through the traces of the semiconductor, collides with the neighboring electrons, and can likely cause disturbance in the distribution of electrons and electron flow. If this disturbance is sufficiently large, it can discharge a capacitor that is used to store the DRAM data bits and can result in a flipped digital logic bit (from 0 to 1 or vice-versa). Combinational logic circuits in particular suffer relatively significant damage due to the transient nature of the alpha particle emission. Hence, very low decay rates are required to maintain low soft errors.

Single Event Latchup (SEL) Effects

A single event latch-up is a type of short circuit method that happens during the creation of a low-impedance path between the power supply rails of MOSFET that triggers its parasitic structure (PNPN). It is usually seen in the form of heavy ions, solar flares, or protons from cosmic rays.

A parasitic structure is very similar to a thyristor where the PNPN structure acts as a PNP and an NPN transistor stacked adjacent to each other. When either of them is conducting during a latch-up, the other transistor also begins to conduct due to the low-impedance path between them. Both the transistors are kept in the saturation mode as long as the structure is forward biased, which usually indicates a power-down condition. 

This effect causes the rail voltage to exceed the diode voltage drop due and leads to a breakdown of the internal junction. However, SEL effects can be eliminated to a desirable level using radiation hardening manufacturing techniques.

Lattice Displacements 

Lattice displacements can change the arrangement of atoms in the crystal lattice structure and can deplete minority carriers affecting the electrical characteristics of the semiconductor. Such effects are more common in Bipolar transistors which depend on minority carriers in their base region. Electronics devices or converters certified as ELDR-free typically show no damage and have flux below 0.01 rad/s.

Ionization Effects

Ionization effects are transient effects that are caused by high-energy charged particles. They can destroy the converters if they trigger latch-up-like damage mechanisms. These effects can vary depending on the parameters including type of radiation, radiation flux, total dose, and device load. When the dose becomes high, the accumulation of holes in the MOSFET’s oxide layer can create charge carrier imbalances leading to deterioration in the performance, and hence the converters may not be able to achieve the optimum efficiency.

Total Ionizing Dose (TID) Effects

This effect refers to the cumulative damage of the crystal lattice of the semiconductor due to ionizing radiation over a defined time interval. In CMOS devices, this radiation creates electron-hole pairs in the gate-insulation layers causing photocurrents during their recombination. In this process, the holes get trapped in the lattice defects and lead to gate biasing thereby influencing the threshold voltage of the transistor. Therefore, the N-type MOSFET transistors will be relatively easier to switch ON than the P-type counterpart. In certain cases, this accumulated charge can be large such that the transistors are either in a permanently open or closed condition which leads to device failure.

Radiation Hardened Systems

Environments with high levels of ionizing radiation can affect the functioning of electronic devices. A single charged particle with high energy can collide with neighboring charged particles, knock them out of their lattice points, and set them loose thereby resulting in electronic interferences. These electronic interferences may lead to problems in satellite subsystems, military design systems, nuclear power stations, and other applications where an extremely high temperature of the surrounding environment may have adverse effects on the DC-DC converter. To mitigate such effects, appropriate design methodologies are employed in radiation hardening techniques to develop systems that are hence called radiation-hardened systems.

Radiation Hardening Techniques

Physical Manufacturing Techniques - Silicon-On-Sapphire (SOS)

Radiation hardened converters are often manufactured using insulating substrates rather than semiconductor wafers. The Silicon-on-Sapphire (SOS) is a hetero-epitaxial process for manufacturing MOS integrated circuits. It consists of a thin layer of silicon that is grown on a sapphire (Al₂O₃) substrate. Upon heating the sapphire substrates, the silicon is deposited by decomposing the silane gas on those heated substrates and it offers excellent electrical insulation properties. Therefore, radiation effects that cause high-energy ions or alpha particles followed by stray currents are prevented from spreading to nearby circuit elements. Due to sufficient isolation between n- and p-wells, the problem of latch-up is greatly reduced in power electronic devices.

Power electronic devices produced using this technique can survive ionization doses from 1000 to 3000 grays (100 to 300 krad) compared to devices manufactured on semiconductor wafers (50 to 100 grays). 1 gray is the unit used to provide a quantitative measure of radiation absorption. 1 gray equals 100 rads. Therefore such devices can be used in space, military, aerospace, step-down switching regulators, and high or low voltage regulation systems.

Shielding the package to reduce radioactive penetration improves tolerance and performance.

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