Observations

AN2626

10/13

3

Observations

Choosing to use fast diode technology MOSFETs is a good choice. STMicroelectronics has

developed the new Fast diode MD mesh generation that shows excellent performance in

these types of topologies. These device families guarantee operation in safe conditions, so

far from the triggering on of the parasitic bipolar, so that they are very suitable for the full

bridge phase-shift ZVS topologies.

In order to understand these advantages we have compared a standard MOSFET versus a

FD MOSFET.

The FD technology device has a lower Qrr than the standard device; meaning that when the

diode is reverse biased by the drop voltage on the channel MOSFET, very low in low load

condition, it will be faster to complete the recovery and so when it will highly reverse biased

by the turn on of the switch in the its same leg it will work in a much more safe condition in

term of dv/dt stress (see Figure 8).

Figure 8.

Comparison between standard and fast diode technology

As you can see the ST fast diode technology MOSFET works better, keeping the overall

system in a more safe condition.

Table 1.

Measured dynamic electrical parameters of the devices compared

Device

Irm

Trr

Qrr

(Di/Dt=100A µs, Isd=20 A, Vdd=100 V, Tj=25°C)

(A)

(nS)

(µC)

Standard STW20NM60

25

390

5

Fast diode

STW20NM60FD

16

240

1.8