Design of a DC/DC converter radiation warning unit for VDMOS device damage

The DC/DC converters used in aerospace work in the space radiation environment, and radiation damage is their main failure mechanism. The reliability of DC/DC converters is related to the reliability of the entire spacecraft. The failure mechanism, failure mode, and radiation screening and reinforcement measures of DC/DC converters have been widely studied at home and abroad. With the development of aerospace industry and the use of a large number of commercial devices in aerospace equipment, it is urgent to find new methods to ensure the reliability of DC/DC converters. Foreign researchers proposed to use the precursor unit technology for DC/DC converters. Without changing the topological structure of the DC/DC converter, a monitoring unit is added to monitor the parameters of vulnerable devices or DC/DC converters, and an alarm is issued before the DC/DC converter fails to ensure the reliability of the DC/DC converter. However, the use of the precursor unit technology in the study of the radiation resistance reliability of DC/DC converters has not been seen. In this paper, the radiation damage relationship between the radiation damage sensitive parameters of VDMOS devices and the key sensitive parameters of DC/DC converters is established by analyzing the relationship between radiation failure physics and DC/DC converter electrical transmission. The precursor unit is designed based on the radiation damage relationship to send out a warning signal before the DC/DC converter fails, and measures are taken as soon as possible to reduce the losses caused by radiation damage failure.

1. Design principle of the precursor unit

1.1 Technical principle of the precursor unit

The precursor unit of electronic products is a unit circuit, which is integrated with the working circuit (also called the host circuit) on the same chip. The precursor unit circuit and the host circuit work under the same stress and environmental conditions. The precursor unit circuit collects environmental information of the host circuit or monitors a parameter of the host circuit in real time, and issues a warning signal before the host circuit fails. The core idea of ​​the precursor unit technology is the reliability bathtub curve, as shown in Figure 1, which shows that the bathtub curve is divided into three areas. The first area is the early failure area with a high failure rate. The second stage is the accidental failure area with a low failure rate. The third stage is the wear failure area, where the failure rate increases significantly and the life of most products will end. The precursor unit makes a judgment before the device enters the wear area and issues an alarm to the user. Premonitory units can be divided into two types: one is a precursor unit that accelerates life; the other is a precursor unit that monitors parameters. The parameters that are sensitive and easy to monitor of the circuit or device are selected, and the change of the parameter is monitored online to achieve the warning effect.

1.2 Theoretical basis for the design of the precursor unit

This paper adopts a monitoring parameter precursor unit. The key to designing a monitoring parameter precursor unit is to determine the sensitive and easy-to-monitor parameters, and then establish the relationship between the monitoring parameters and the radiation damage of the DC/DC converter. The radiation failure modes of DC/DC converters mainly include output voltage drift, reduced conversion efficiency, increased output ripple, increased linear regulation rate and load regulation rate, etc. The ionizing radiation effect produces oxide layer trap charges and interface state charges in the MOS system. The total dose radiation effect failure modes of VDMOS devices include threshold voltage drift, transconductance degradation, increased leakage current, etc. The relationship between sensitive monitoring parameters and radiation damage can be obtained through subsequent circuit analysis.

FIG. 2 shows a typical topological structure diagram of a single-ended flyback converter, in which a pulse width modulator chip (PWM) is used to control the switch of a VDMOS device, and the VDMOS device is used as a switch in the converter.

(1) After radiation, the threshold voltage of the VDMOS device drifts negatively. When the threshold voltage of the VDMOS device is lower than the low level of the PWM output voltage, the VDMOS device cannot be turned off, the transformer cannot transmit energy, the output voltage drops sharply, and the converter fails completely; (2) The negative drift of the threshold voltage causes the conductive channel to open more, the drain current increases, and the power consumption of the DC/DC converter increases, resulting in failure. Similar reports have been reported at home and abroad. For the first failure mode, the threshold voltage of the VDMOS device and the output voltage of the DC/DC converter are selected as radiation sensitive parameters. After radiation, the threshold voltage of the VDMOS device is not lower than the low level of the PWM output voltage, and the output voltage remains stable. For the second failure mode, the threshold voltage and power consumption of the VDMOS device are selected as radiation sensitive parameters. The conduction loss caused by radiation damage Switching loss Total loss are shown in equations (1) to (3) respectively.

When the threshold voltage drifts negatively to a certain value, the power consumption is too high and the efficiency is lower than the specified standard, and the DC/DC converter fails. From the analysis of the above two failure modes, it can be seen that when the threshold voltage drifts negatively to a certain value, the DC/DC converter fails. The omen unit designed to monitor the parameter is to monitor the threshold voltage of the VDMOS device and send an alarm signal when the threshold voltage has not reached the failure point.

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2 Premonition Unit Design

2.1 Selection of early warning scheme

For the above two failure modes, a suitable early warning scheme is selected, and a warning unit is added to the DC/DC converter. The structure of the DC/DC converter after adding the warning unit is shown in Figure 3. The alarm signal uses a jump from low potential to high potential. For this purpose, a reasonable warning unit circuit is designed. When the converter is about to fail, the output signal changes from low level to high level.

2.2 Unit Circuit Design

According to the technical principle of the precursor unit described above, the precursor unit circuit for monitoring parameters should include the following three parts. (1) VDMOS device damage monitoring circuit; (2) monitoring signal amplification circuit; (3) output circuit. The design process of each functional part is introduced below. (1) Radiation damage monitoring circuit. The monitoring device is given a fixed gate bias through R1 and R2. This gate bias is the alarm threshold point. When the threshold voltage of the monitoring device drifts negatively to the fixed gate bias, the monitoring device is turned on; (2) Monitoring signal amplification circuit. A load is added to the upper potential of the monitoring device. The load is a resistor or a p-type MOSFET active load. The load and the monitoring device form an amplifier. When the threshold voltage of the monitoring device drifts negatively to the alarm threshold point, Vsense is converted from a high level to a low level; (3) Output circuit. The output circuit is implemented using a radiation-hardened inverter. The radiation-hardened inverter, as shown in Figure 4, adds a pull-up p-type MOSFET (p1) and introduces an additional n-type MOSFET (n1) below the n-type MOSFET. When the input is high, p1MOSFET is turned off and n1MOSFET is turned on, and the original inverter operation is not affected. When the input is low, plMOSFET acts as a pull-up, so that the connection point of the drain terminals of p1MOSFET and n1MOSFET is at a high level, so that the n-type MOSFET can be turned off more effectively. The source-drain voltage of the n-type MOSFET is reduced, which reduces the leakage current. After radiation, the output can still be kept near Vcc. The overall circuit is shown in Figure 4. When the radiation damage of the VDMOS device reaches a certain level, the monitoring signal is amplified and the output circuit finally outputs a high-level signal.

3 Alarm Threshold Determination and Simulation

3.1 Determination of alarm threshold

The alarm threshold point is determined by the damage relationship between the VDMOS device and the DC/DC converter. The above two different failure modes correspond to different failure threshold points. The following method can solve the problem of different threshold points. For the failure mode in which the VDMOS device cannot be turned off, the threshold voltage drift amount △V1=Vth-V1 (V1 is the PWM output low level voltage) during failure. For the efficiency degradation failure mode, the threshold voltage drift amount is calculated to be △V2 according to formula (3) and national military standards. Comparing the negative drift of the threshold voltage of the two failure modes, the one with the smaller negative drift is selected as the failure negative drift. The failure threshold voltage negative drift is reduced by 20% as the monitoring threshold voltage negative drift. The monitoring point of the threshold voltage is given by formula (4)

3.2 Simulation Verification

According to the selected DC/DC converter circuit, the parameters are determined and substituted into formula (4) to calculate Vs=1.2V. The resistance values ​​of R1 and R2 are adjusted to make the monitoring voltage of the DUT device 1.2V. The simulation results under Pspice are shown in Figure 5. When the threshold voltage of the VDMOS device drifts negatively to 1.2V, the output voltage changes from low level to high level, and an alarm is realized. Since the circuit adopts anti-radiation reinforcement design, the radiation damage of other devices has little effect on the output, and the simulation results have not changed. The simulation results are no longer given here. The DC/DC converter for aerospace works in a wide temperature range. The simulation results of the temperature change of the precursor unit are shown in Figure 6. The output voltage is not affected, and the slight change of the jump point has little effect on the alarm. The simulation results prove that the designed precursor unit circuit and early warning scheme are reasonable and feasible.

4 Conclusion

In this paper, the warning and sound management (PHM) method is used to study the radiation resistance reliability of DC/DC converters, and the radiation damage relationship between VDMOS devices and DC/DC converters is studied. Combining the radiation damage relationship and the technical principle of the precursor unit, a DC/DC converter radiation precursor unit based on VDMOS device damage is designed. The simulation results show that the designed precursor unit can give an early warning of the failure of DC/DC converter radiation damage.