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Dielectric Voltage-Withstand (Hipot) Testing and Insulation Resistance Testing of Rechargeable Batteries

What is the key component in a high-voltage, bidirectional constant current circuit?

In this, my third article, I will discuss an experience I had when developing a bespoke device for a client. My brief was to modify a TOS series electrical safety tester to create a low-cost, dedicated hipot and insulation resistance tester that used direct current only. Notably, this tester would be used to test rechargeable batteries. Unlike regular appliance testing, when testing rechargeable batteries, you need to assume that the electrical energy stored in the battery will flow back to the tester via the connecting cables. Depending on the amount of energy involved, this can cause the tester to malfunction or be damaged.

Protecting against short-circuiting of the device under test

When testing rechargeable batteries, if they are not connected and configured correctly, excess current flowing from the device under test (DUT) to the tester can cause the DUT to short. (Figure 1)

Figure 1. The device under test short circuited by the low-side cable of the modified TOS and the ground

The trick is incorporate a protection circuit that will limit current flow when it senses excess current. (Figure 2)

Figure 2. The modified TOS with a protection circuit incorporated on the low side

Creating a bidirectional, constant current circuit that can withstand high voltages

I realized that my mission was to design a protection circuit, in other words a bidirectional constant current circuit, that would operate at voltages in the range of several kilovolts, at a low cost. I decided to explore a constant current circuit that used MOSFETs (metal-oxide semiconductor field-effect transistors). Upon assembling and testing the circuit, I found that while it worked fine up to the MOSFET’s breakdown voltage, as soon as the applied voltage reached approximately double the breakdown voltage, the MOSFETs failed, and the circuit was no longer able to maintain constant current. (Figure 3)

Figure 3. Protection circuit comprising only a voltage divider, constructed using resistors

Dealing with unevenly-distributed voltage

One also needs to be careful when arranging several MOSFETs in series to create a high voltage circuit, because if all the MOSFETs do not switch simultaneously, the voltage across the respective MOSFET elements may become unbalanced, causing some MOSFETs to be exposed to excessive voltages. In other words, the greatest challenge when connecting MOSFETs in series is the avoidance of unevenly distributed voltage. I had not yet realize this, however.

While my prototype was able to keep current constant, it did not resolve the hipot issues. I attempted to increase the voltage my circuit was able to withstand by increasing the number of levels in the series, but it was ultimately unable to resolve the issue of uneven voltage distribution, and the MOSFETs obviously continued to fail. I didn’t know what to do.

The best approach in these situations is to simply seek help. When I consulted the more senior members of my team, they explained MOSFET parasitic capacitance to me, and I learned that parasitic capacitance could simply be ignored. Rethinking the problem, I proceeded to add capacitors to the circuit, such as I would be able to ignore parasitic capacitance. (Figure 4)

Figure 4. The circuit with voltage balancing capacitors added between the MOSFETs’ gate and drain terminals

Capacitors are the key to balancing voltage

A MOSFET’s parasitic capacitance varies according to its temperature and applied voltage. In my initial MOSFET constant current circuit (Figure 3), uneven parasitic capacitances caused the voltages across the respective stages to also become uneven, with the result that an excessively high voltage was formed across one MOSFET in the group, which then failed. However, in the revised circuit (Figure 4), by adding capacitors to evenly distribute the voltage, I was able to eliminate the fluctuations in capacitance attributable to the parasitic capacitance generated when voltage was applied. The MOSFETs is no longer failed, even at high voltages. My simple bidirectional constant current circuit was now complete. This was also an economical solution, costing only around a quarter of a conventional circuit built with high-voltage components, and was achievable without major modifications.

Reconsidering the basic properties of the components in the circuit

This experience made me realize that when you have a setback, it is important to stay calm and re-check the specifications, performance and characteristics of the components in your circuit. If I had done this at the beginning, I might have been able to solve the problem more quickly. (The change in parasitic capacitance of MOSFETs is a basic property of field-effect transistors.) While I was able to recognize, and ultimately solve, the issue thanks to advice from the more senior members of my team, the experience made me keenly aware that I had reached a stage in my career where I needed to be able to solve these kind of problems myself. I wanted to be able to take any problem, consider it calmly, deduce the cause, and implement a solution.

While the development of this tester went slightly over schedule, my boss congratulated me on building a low-cost circuit that had all the required functions. It is my hope that I will continue to accumulate knowledge and experience in this way as I grow as an engineer.

Masaki Miyata
Solutions Development Division, Solutions Development Section

[Major achievements in product development]
Bespoke AC ripple power supply systems
Bespoke power supply systems, bespoke electronic load device systems

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