TPT September 2010

A rticle The working frequency is real time calculated to adapt transistor firing to the real operating conditions, thus avoiding the break of the device SOA. If the frequency is not correct, in fact, voltage spikes may occur, leading to a device failure. The supervising board not only manages the inverter control, but is also involved in fault decoding and power regulation. 3.4 Design validation The design validation is particularly related to the evaluation of the module losses, to verify the efficiency and the water cooling requirements. Input power has been directly measured (DC voltage and DC current). System losses have been estimated by measuring input and output water temperature and the water flow through the equation where the power [W] is obtained by using the flow in [l/sec] and 4186.8 [J/kg K] is the water specific heat. The trial has been carried out with a load having a natural resonance frequency of 400kHz, to take into account the maximum of the switching losses. The results are reported in table 1 for some testing points. It can be noted that in each condition the efficiency is higher than 90%, increasing at the increase of input power. The experimental results confirm the calculation made starting from the datasheet parameters and adopted in the design step.

V

[V]

50

66

82

DC

[A]

100

130

160

I

DC

P [kW]

5

8.6

13.1

[°C]

26

26.7

27.9

θ

in

[°C]

28.2

30

32.6

θ

out

[kW]

0.49

0.79

1.04

P

diss

0.9

0.91

0.92

η

Table 1 : Results of the calorimetric tests

An oscilloscope acquisition of the voltage across a basic switch is presented in figure 8. It can be noted that the frequency is around 330kHz and the amplitude of the spikes (located where the commutation happens) is limited, thus avoiding transistor failures. 3.5 Module Connection The module flexibility is related to having different connections according to the considered application. In particular the voltage may vary to get a proper level. If low output voltage (400V) is required, as in hardening, a single bridge configuration can be used, with a certain number of modules in parallel to achieve the power. If high output voltage (1,000V) is required, a double bridge configuration is used, thus doubling the output value. Again a proper number of couples of modules has to be put in parallel to achieve the power. If a further voltage boost is required, it can be obtained by using a specifically designed capacitive divider.

Figure 6 : Digital supervising board

Figure 8 : Waveform of the voltage across the switches

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S eptember 2010

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