TPT January 2007

P rogress in W elding T echnology, E quipment & C onsumables

MOSFET

MOSFET A

MOSFET B

MOSFET C

MOSFET D

800

800

800

800

V

(V)

DS

44

34

44

44

(A)

ID

FREDFET

FREDFET

CoolMOS

FREDFET

Technology

Rated Avalanche Yes

Yes

Yes

Yes

ISOTOP

SIMOPAC

ISOTOP

ISOTOP

CASE

› Table: Characteristics of selected Mosfet for testing with traditional and new test circuits

In this case, the inverter works below the resonance frequency level (capacitive mode). It is because of this situation that it is extremely important to identify the most suitable and robust Mosfet for an inverter. There are some noticeable differences between test conditions and inverter switching conditions. In real conditions, the dead time between driving pulses is approximately 200ns. To improve the conditions between test and real conditions, the device under testing (DUT) is turned ‘on’ while a direct current is flowing through the internal body diode. Approximately 200ns before the internal body diode switches ‘off’, the driving pulse at the same Mosfet is in the ‘off’ position. Figure 5 shows the new semiconductor test circuit and driving pulses to different Mosfet inverters. The dead time is also adjustable in this new circuit.

maximum voltage at the inductor, which corresponds to the zero crossing of the current. If a short-circuit occurs in the heating inductor, there is an abrupt increase in the output frequency of the resonant circuit. › Figure 5a (top) layout of new semiconductor test circuit; 5b (middle) new semiconductor test circuit; and 5c (above) gate signal at DUT and auxiliary Mosfet

› Figure 6a (top) new semiconductor test circuit with ‘fail parameter’ ‘DC voltage’, and 6b (above) layout of new semiconductor test circuit with external DC voltage sources connected in series

fi Figure 7: a) VDS and ID at test circuit and gate condition ‘resistance’ b) Diode failure at DC voltage 400 VDC c) VDS, VGS and ID at test circuit and gate condition ‘gate signal’ d) Diode failure at DC voltage 350 VDC

In Mosfet selection, the GH Group uses a V DC of 550V and requires 800Vdevices. The GH Group selected four types of Mosfet from different manufacturers with varying technical capabilities. All Mosfets are 800V and maximum current. In the research, the GH Group selected ‘failure DC voltage’ as a fail parameter. This parameter is DC voltage of the input source when Mosfet fails. Figure 6 shows the test circuit with a variable DC source and different

gate conditions (gate resistance or gate signal). The conclusion from this research led the GH Group to select Mosfet D at 500 VDC, due its leading robustness in capacitive switching. The company claims that failures never occur when using Mosfet D. Article supplied by Mr Jose Jorda and Mr Gemi Roy, GH Group/Universitat de Valencia, Spain GH Group – Spain › Figure 8: Failure voltage (V) with different Gate conditions, temperature 25ºC and 125ºC for Mosfet type A, B, C and D

b)

a)

Fax : +34 96 135 21 71 Email : ghgroup@ghe.es Website : www.ghe.es

d)

c)

90

J ANUARY /F EBRUARY 2007