Based on the safety spark test apparatus, the short-circuit spark discharge (SCSD) characteristics of the simple capacitive circuit and switching converter are studied. It is pointed out that their SCSD process can be divided into four stages, that is, dielectric-breakdown stage, spark-generated stage, spark-maintenance stage, and spark-extinguish stage; moreover, there is different equivalent spark resistance in each stage. For the simple capacitive circuit, its spark resistance is larger and maintaining voltage is almost unchanged in the spark-maintenance stage. For the switching converter, its output short-circuit characteristics depend strongly on the load resistance and its maintaining voltage reduces rapidly with the decrease of the load resistance. The circuit model is proposed, which can simulate the output SCSD process of the switching converter. By using the least-squares method, the relationship expressions between the discharge duration and capacitance in each time-stage are derived and the corresponding equivalent resistance is obtained. The mathematical models are established, and the expressions of the discharge current and voltage are deduced. Experiment and simulation results are positive in the analysis showing the feasibility of the proposed models.

Electronic equipment applied in flammable and explosive conditions must meet anti-explosive requirements. Intrinsic safety is the optimal means for anti-explosive requirements. Therefore, the electronic circuit and electrical equipment applied in the explosive and dangerous places are always designed to be intrinsic safety instead of others.

As for research on the intrinsic safety, the electronic circuit is usually divided into the simple capacitive circuit, the simple inductive circuit, and the complex circuit [

At present, there are a lot of researches on the discharge characteristics of the inductive circuit, and the obtained research results [

As for research on the intrinsic safety problem, the output of the switching converter can be regarded as a capacitive circuit [

To solve the above problem, the SCSD characteristics of the simple capacitive circuit and switching converter are studied in depth, and a circuit and mathematical models which can simulate the SCSD characteristics are presented in this paper. The influence of the capacitance on the discharge duration will be analyzed.

The principle circuit researching the SCSD characteristics of the simple capacitive circuit is shown in Figure

Short-circuit discharge principle circuit of a simple capacitive circuit.

Suppose the switch

The discharge curve of the capacitor is shown in Figure

Short-circuit discharge waveforms of the simple capacitive circuit with noncontact switch.

From Figure

Supposing the switch

Spark discharge experiment circuit of the capacitor.

In the safety spark experiment of the capacitive circuit, the spark test methods are adjusted according to the parameters of the test circuit. For example, in order to guarantee enough charge time, the ignition interval of the tested circuit should be suitably extended by reducing the number of tungsten wires. During the experiment, the short-circuit discharge waveforms are recorded by using TDS3020.

Short-circuit discharge waveforms of the capacitive circuit with contact switch (

Through the statistical analysis of a large number of waveforms, the typical discharge current and voltage waveforms are drawn in Figure

Waveforms of the short-circuit discharge in the capacitive circuit.

Stage I is dielectric-breakdown stage, from

Stage II is spark-generated stage, from

Stage III is spark-maintenance stage, from

Stage IV is spark-extinguish stage, from

According to the above analysis, the release of the short-circuit discharge energy in the capacitive circuit is strongly concentrated; therefore, it is easy to ignite flammable and explosive gas. For igniting explosive gas, the instantaneous power must be particularly considered besides the spark discharge energy; that is, the short-circuit spark discharge must have enough both large energy and power to ignite flammable and explosive gas

A switching converter belongs to a typical nonlinear system, which consists of inductor, capacitor, and nonlinear switches; moreover, there are various working states and operating modes. Because the output of a switching converter has a large filter capacitor, it can be regarded as a capacitive circuit from the output of the switching converter, as shown in Figure

Output equivalent circuit of the switching converter.

The short-circuit discharge characteristics of the switching converter are more complicated than a simple capacitive circuit, which will be discussed in detail in the following sections.

The explosive experiments are carried out for the output of the switching converter through the spark test apparatus based on IEC standards. Typical output short-circuit discharge current and voltage waveforms of buck converter are shown in Figure

Output short-circuit discharge current and voltage waveforms of buck converter.

Comparing Figure

In the spark-maintenance stage, the reducing speed of the maintaining voltage becomes faster with the decrease of the load resistance. It is mainly caused by a parallel connection of the output capacitor and the load resistance. The spark discharge characteristics may be regarded as an equivalent spark resistance when the spark is produced. From the test waveforms in Figure

In the spark-maintenance, the minimum spark-maintenance voltage also becomes smaller due to the load resistance.

In the spark-extinguish stage, the maximum current decreases because the minimum spark-maintenance voltage becomes smaller.

Therefore, when the output of the switching converter is short-circuit, the output short-circuit current and the minimum maintaining voltage reduce with the decrease of the load resistance. The SCSD characteristics depend strongly on the load resistance.

As shown in Figure

To get the model circuit simulating the short-circuit discharge process, using

For

According to the above analysis, a model circuit, which can simulate the output SCSD process of the switching converter, is proposed, as shown in the dotted box in Figure

Model circuit of short-circuit discharge process.

Each current-flowing loop in Figure

The above analysis shows that the proposed model circuit may simulate the whole output short-circuit discharge process of the switching converter.

In the

In the

In the

In order to obtain the mathematic model of the output short-circuit discharge of the switching converter, the expressions of the duration (

There are many factors to affect the discharge duration, where the capacitance and the initial capacitor-voltage are the most important factors. Only the impact of these two factors on the discharge duration is discussed in the following section.

Discharge duration versus initial capacitor-voltage.

From Figure

Discharge duration versus capacitance.

In (

Through using the least-squares method of the curve fitting, suppose the sum of error’s square is

In (

Letting

In (

Note that the discharge duration should be zero in

Similarly, the relationship expression between

Through testing and simulating, the fitting curve is very close to the experimental curves, as shown in Figure

From the above analysis, the whole output short-circuit discharge characteristics of the switching converter can be described through mathematical expressions. As shown in Figure

Combined with (

In (

According to (

In the

In this stage, the change of the capacitor-voltage is in the range of

In (

According to (

To verify the obtained mathematical model, the main parameters are

Using the established mathematical model to simulate the output short-circuit discharge characteristics of the switching converter, the output short-circuit discharge curve can be obtained, as shown in Figure

The simulation waveforms of mathematical model.

Additionally, the output short-circuit discharge experiments of the switching converter are done by using the safety spark test apparatus based on IEC standard. The experimental waveforms of voltage and current are shown in Figure

From Figures

Comparing with the noncontact capacitive circuit, the short-circuit discharge process of the capacitive circuit with contact switch is more complicated, where the SCSD can be produced.

As for a simple capacitive circuit with contact switch and the switching converter, the short-circuit discharge process can be divided into four stages: dielectric-breakdown stage, spark-generated stage, spark-maintenance stage, and spark-extinguish stage.

In the spark-maintenance stage, the spark discharge current is smaller, and there is a larger equivalent spark resistance. As for a simple capacitive circuit, the maintaining voltage is almost constant. However, the maintaining voltage reduces rapidly with the decrease of the load resistance for the switching converter. At the same time, the smaller the load resistance, the lower the minimum maintaining voltage.

The spark discharge energy during the dielectric breakdown stage and sparks production stage is larger and is also the main energy to ignite the flammable and dangerous mixture.

The output short-circuit discharge process can be divided into 3 time-stages in the establishing of mathematical analysis model. The discharge duration becomes longer with the increase of the capacitance in each time-stage, but it is unrelated to the initial capacitor-voltage.

The proposed model circuit can simulate the output short-circuit discharge process of the switching converter. Equivalent discharge resistance is obtained in each time-stage. Moreover, equivalent resistance is smaller in the

The proposed mathematical models can simulate the short-circuit discharge process of the switching converter. The analytic expressions of the current and voltage are obtained. The simulation and experimental results show that the established model can simulate the output short-circuit discharge characteristics of the switching converter.

The obtained conclusions can provide theoretical guidance for designing the intrinsic safety switching converter.

The authors declare that there is no conflict of interests regarding the publication of this paper.

This work is supported by the National Natural Science Foundation of China (50977077, 51277149).