1, Overview

Ultra-fast acting fuses can protect the junction of Gate Turn Off (GTO) thyristors under high current conditions.

At present, a large number of GTO inverters are protected in various fields. Because the I²t of the Insulated Gate Bipolar Transistor (IGBT) is so low,  the junction surface cannot be protected by conventional fast acting fuses. 

As with other semiconductor devices, a short-circuit high-current fault can cause a sharp increase in the energy inside the GTO and its components, causing IGBT to explode. However, a number of electrical tests have shown that IGBT's explosive I²t can be determined and that ultra-fast acting fuses are capable of preventing IGBT from exploding.

In addition, a large number of experiments have been conducted to test the influence of fast acting fuses on the inductance of the circuit and the current carrying capacity of fuses at high frequencies. It can be said that fuse technology and circuit design have a great impact on the total inductance of the circuit. (Ldi/ DT) It is necessary to select the proper fast acting fuse type with appropriate characteristic curve and data to protect the inverter.

2, Ultra fast acting fuse application in IGBT

Since the inductance is much larger than Inductance L, the current I_d and I_c ratio are negligible when the fast acting fuse is blown off.

Fast acting fuses are used in inverter circuits to prevent semiconductor devices from exploding and even to effectively protect the junction of semiconductor devices in case of short circuit. For the circuit shown in Figure 2, the fuse's primary function is to prevent capacitor discharges in the event that two series arms simultaneously conduct electricity and cause a short circuit. When a semiconductor element is triggered or destroyed by an error, a short circuit can occur between the two arms. 

Since L inductance is low, di/dt will be high, so the fast acting fuse will melt quickly. The short circuit current is the summary of the IC ( Capacitor discharge) and the ID from the supply power side.

3, Where ultra fast acting fuse is installed?

1), Fuse is placed on the bridge arm of the inverter (Figure 3) :

2), Ultra fast acting Fuse is placed on the DC circuit of the inverter (FIG. 4) : the current rating of high speed fuse is 1.732 times that of fuse on the bridge arm

3),  Location of fuse on DC feeder (FIG. 5) : Between capacitor (or other type of DC supply) and rectifier. It can also be installed in combination with Figures 5 and 3 or in combination with figures 5 and 4.

4, What's the main parameter for ultra fast acting fuse selection?

When an IGBT or GTO component is damaged, a short circuit will occur in the capacitor branch, and the ultra fast acting fuse as shown in Figure 6 to protect the circuit.

E- Voltage value of DC power supply

In- Stantaneous voltage of U capacitor

I_c- Capacitor output transient fault current

I_d- Instantaneous fault current of dc power output

L- Total inductance of capacitor discharge circuit (inductance between DC power supply and capacitance)

R- The resistance value of R capacitor discharge circuit

4.1. Conditions for selecting ultra fast acting fuse

•  inductance L

For such inverters, there is an inductance between the capacitance and the rectifier that provides the DC voltage. It's usually much bigger than L. In most cases, the ID from the rectifier for the first half cycle of a capacitor discharge is negligible. The following cases are sufficient to prove that the ID is negligible.

• Resistance R

The resistance value R (including the fuse resistance) is usually low enough to allow capacitor discharge oscillations. The oscillation condition is R=

In order to simplify the calculation of fuse selection, the condition of R is defined as follows: R =

Under the conditions of R and L, the oscillation frequency T can be calculated by the following simplified formula.Fault current I and voltage U:

Larger R resistance values are acceptable, but simplified formulas and the simple fuse selection method mentioned in this paper are no longer fully acceptable, as the oscillating waveform is no longer approximate to the sinusoidal wave. However, computers and simulation software can make accurate calculations for any short circuit.

•  The capacitor voltage U

Although the voltage U is oscillating, this does not mean that the fast acting fuse is operating at ac voltage. When the pre-arc state of the fuse ends (TP time), the fast fuse begins to arc inside. At the same time, the voltage at both ends of the capacitor is no longer oscillating (the fuse is no longer of low impedance). The arc in the fast acting fuse changed the circuit

Features. When the front arc state of the fuse ends, the voltage at both ends of the capacitor is:

Since the fuse arcs at dc voltage, it is necessary to specify the maximum voltage U_PM at which the fuse arcs. This voltage value is also a feature of the fuse and must meet the following conditions: 

U_P < U_PM

•  DC voltage E

After arc extinction, the power supply will generate overvoltage and discharge at both ends of the capacitor, which is a transient phenomenon.  The peak transient voltage is UTRANSIENT. The transient voltage is much higher than the initial voltage (E) (Figure 7). Assuming that the circuit has no resistance and the capacitance has been fully discharged, the maximum transient voltage can theoretically reach 2E.

So the actual data： U_transient = 1.75 E

At this peak, the ultra fast acting fuse will arc again. Therefore, the initial supply voltage E must be less than or equal to the maximum EM.

EM is another feature of fuses. The following conditions must be checked when selecting the fast acting fuse: E ≤ EM

•  Fore-arc time T_p

Since the Utransient depends on the voltage drop at both ends of capacitance in the fast acting fuse's pre-arc time, the PRE-arc time T_P should not be too long. When E = EM, the recommended value is: T_P =T / 6

Note 1: When T_P = T / 6, the voltage at both ends of the capacitor is: U_p =E / 2; Then, the maximum instantaneous peak voltage at both ends of the maximum capacitance is approximately 1.6e.

Note 2: Obviously, these conditions are not important when the DC voltage E is much less than the SELECTED fuse E_M.

•  Oscillation period T:

As a reference condition, T is designed to limit the time for the voltage at both ends of the capacitor to reach its maximum value. If this condition is not met, cutting off the fault would be almost like cutting off the direct current supplied by the battery, and all calculations would be different. The expected value of T should be:

T =10 ms.

4.2. Necessary information for ultra fast acting fuses

4.1 All notes and conditions are explained. Fuse manufacturers must determine these corresponding parameter values for various fuse products, such as E_M and U_PM, as well as specific curves, allowing the user to calculate T_P, total I²T, and arc voltage U_m equivalence. 

In the table, the parameters G and are related and can be used to calculate the time before arc and the total action time. The example in § 8 illustrates how to use these data and graphs.

5. Inductance of fuse

The value of the inductance L depends mainly on the shape and length of the line. When a fuse is applied to a circuit, it changes the shape and length of the circuit. 

Thus, the inductance L of the whole circuit becomes L + L. (figure 8)

L is not a constant inductor located inside the fuse.

D_i/Dt was tested after the circuit closure of various fuses was measured in combination with the circuit shown in Figure 11.

Without fuse d_i/d_t = E/L

With fuse d_i/d_t = E/(L+ L)

The fuse replaces the copper bus during the measurement to ensure maximum line length invariability. When the fuse shape is flat, the L is very small (see Figure 9).

Example: L = 10NH-50mm flat fuse.

L = 35NH-300mm flat fuse.

Summary : IGBT is a large power element, and its explosion will directly affect the safety of human and property.  In order to protect the power IGBT element from explosion, the ultra fast acting fuse must be able to withstand a maximum of 7.2 kV, and its fusing limit I²T must be smaller than IGBT's explosion I²T.  Therefore, the ultra-fast fuse is an important element to protect IGBT power conversion equipment.

6. High-frequency influence

6.1. Impact on ultra fast acting fuses

At high frequencies, currents produce two phenomenas: Surface effects and proximity effects.

A. Surface effect:

The fast acting fuse plate produces surface effects, although its thickness is less than 0.5mm, because the current density is unevenly distributed in the core zone (FIG. 10); There is no surface effect in the depth of the melt, and the surface effect is 0.95mm at 5kHz. The surface effect mainly occurs at the conductive contact.

B. Proximity effect:

Current equalization is related to frequency and distance D between fuse and other conductors ( The shorter the distance D, the better the current equalization).

When d >200M;  Also with a frequency of less than 20, 000 hz, the proximity effect inside the fuse is negligible. Proximity effect has a greater effect on fast acting fuses than surface effect.

• Surface effect is much smaller, but when more than 1000 hz frequency, its influence can't be ignored

• Other of the problems of high frequency and overheating problems caused by the hysteresis losses of magnetic components. That's why when the frequency is over 1000HZ,  the PROTISTOR fuse does not use magnetic components.

(Not finished, Remaining factors in this article:

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