6.2 Influence of CPE on rated current of ultra fast acting fuse

When the main current is at high frequency, the correction factor CPE is shown in Table 1.

The data shown in Table 1 is not accurate because it does not take into account how many fusion cores are connected in parallel, nor does it take into account the effect of distance D, but it is sufficient for fuse selection.

However, for most applications, only the harmonics are the high frequency components and the fast acting fuse current is: I = i0+ i1+ i2 +..+ iK

I₀ is the main part of the current: In most cases, DC current or 50Hz current..;

I1, i2,....IK is a significant harmonic. The effective current value of the ultra fast acting fuse is:

The calculation of rated current of fuse can be equivalent to:

(That is, the total value of C_PE is:)

The C_PEK of each harmonic is a function of each harmonic frequency shown in the table above.

The rated current I_N of the ultra fast acting fuse can be calculated using I_EQ and all classic correction parameters:

(A1 is the temperature coefficient, C1 is the connection mode coefficient, and A '2 is the aging coefficient under the condition of alternating current)

Although harmonics are often uncertain, it is necessary to estimate C_PE based on the switching frequency of IGBT. According to Table 2, it can be estimated:

Note: This coefficient can be applied to the estimation of the RMS of the total current, i.e. :

Thus, the fuse rating I_N can be calculated using I_EQ and all the classic correction parameters:

6.3. Influence of IGBT switching frequency 

IGBT elements cannot operate at rated current at any frequency. In fact, IGBT elements with high voltage and current ratings are more sensitive to frequency than products with smaller ratings, and the current grade drops faster. A comparison of IGBT and fast acting fuses shows that fuses are more severely affected at low frequencies and then ultra fast acting fuses perform better at high frequencies, especially those higher than 10kHz.

 7, 75A IGBT exploring

Test different IGBT components with or without fuses.§7.1.., §7.2. And §7.3 show the test results using 1200 V 75 A IGBT element.

7.1. Test without fuse

The test results of IGBT at 1200 V 75 A were as follows: the melting time at the junction was T₁ = 35 s, and the corresponding I²T was 12,700 A s

The occurrence time of the maximum peak current is t₂ = 55 s, and the corresponding i²T is 35 000 A s

The time of explosion is t₃ = 66 s, and the corresponding i²T is 48,800 A s

7.2. Test for IGBT series 315A ultra fast acting fuses

The test results showed that the total I²T of the ultra fast acting fuse was 27,500 A S, and IGBT could not observe the external damage.

7.3. Test of SERIES 400A fast acting fuses for IBGT

The test results showed that the total I²T of the fuse was 37000 A S, and some damage (deformation, cracking) could be seen on the shell of IGBT.

Conclusion : All the above tests show that the ultra fast acting fuse can protect IGBT of 1200 V 75 A.The value of shell burst I²T for fuse selection can be 30 000 A S.

8. PWM inverter protection example of IGBT is adopted

8.1 specification

• Circuit parameters:

E = 600V Max

R = 10^-3 (including internal resistance of the selected ultra fast acting fuse).

L = 2.2 10^-7 H

C = 2 10^-3 F

   = 10^-4 H

Two ultra fast acting fuses in the DC circuit (FIG. 4) : The current through the fuses is 130 AMP.

•  IGBT data:

Rated current at 5kHz (effective value) : 75 A

FERRAZ SHAWMUT's actual explosion I²t (see §7) : 30 000 A S (but not the fuse I²t at the junction = 12700 A S)

Blocking voltage: 1200V

Inverter operating conditions: IGBT operating frequency: 5 KHz-75 A effective value, environmental temperature: 50°C, natural air cooling.

Circuit characteristics of capacitance discharges without fuses:

Period T = 132 s, maximum peak current I_max = 53,200a, the time of the first maximum peak current t_m = 32 s 66 s I²T of the first half wave: 93,300 A s, far greater than the I2T of the fuse at the IGBT connection.

The fuses of the PSC 690 V URD series are optional because E_M = 900V (see fuse data in Figures 12, 13, 14 and 15)

Remark: Ferraz Shawmut PSC 690V fuse cross reference to Bussmann PN: 170M30xx, 170M31xx, 170M32xx......

8.2. Selection of rated current of fuse: 

The correction factor used is:

=.894 C1 =0,85;

A3 = 0 ,80;

C_PE = 0, 9

The rated current I_N of the ultra fast acting fuse should be 238AMP.

8.3. Operation time of fast acting fuse and calculation of I²T

The pre-arc time T_P can be calculated by the maximum d_i/ D_T when the fault occurs:

The result is multiplied by the coefficient G for the different fuse rated currents in Figure 12.

Take the fuse in 315A as an example: The graph of 0 V in Figure 13 shows the functional relationship between the pre-arc time T_P and G d_i/d_t.

Therefore, the ante-arc time is:

t_p = 14  10-6 

T_p<T/6  can be verified, because T/6 is equal to 22, 10-6s.

When T_P = 14 s, the calculation result of U_P is:

PM = 600V, conditions U_P < U_PM can thus be validated.

The two ultra fast acting fuses will operate in series.

The arc front time is T_P = 14 10-6 s, and it can be confirmed that the voltage is evenly Shared between the two fuses.

Therefore, the total I²t can be calculated when the fuse operates at 471/2 = 236 V.

The total I²t is equal to the front I²t given in the list (Figure 12) times the correction factor k (figure 14).

The curve shows that K = 1.4 when Up = 236V and I T in front of 250A fuse is 5800 A S.

The total I²T of the fuse is 5800 * 1.4 = 8120 A S (much lower than the IGBT's allowable of 30,000 A2s)

I²t is equal to 8120 A S

The graph shown in Figure 15 shows U_m = 630V.

This value is lower than the blocking voltage of IGBT.

The total arc extinguishing time of the fuse t_t can pass through FIG. 13

Curve estimation in.

When G D_I /d_t = 3.57 1010, t_t = 27 s at 380V as shown in the graph.

But the actual voltage is 236V.

Through the line

Sex interpolation method, between 0- 380V, 196V: so t_t = 22 10-6s

The peak permissible current I_C of the fast acting fuse is:

9, The conclusion

Even under very poor working conditions, it is possible to find a suitable fast acting fuse to prevent IGBT from exploding.

The total I²t of the ultra fast actingfuse can be calculated from the related graph to ensure that it can provide protection.

The proximity effect coefficient CPE presented in this paper cannot take into account all possible cases, but they can deal with the problems encountered in the conventional design, and also enable the preliminary correct selection of the face fast fuses to be realized.

Thanks to Xilian Fuse Institute.