Gate to cathode capacitor, impact on dV/dt immunity

AN4030

6/15

Doc ID 022635 Rev 1

Figure 5.

SCR dV/dt increase versus added gate to cathode capacitor

1.3

Behavior of Triacs regarding dV/dt

A Triac silicon structure differs from an SCR structure. First, Triacs can conduct current in

both directions. A Triac is equivalent to 2 SCRs back-to-back with a common gate. The real

gate area of the reverse SCR is on the opposite side of the gate terminal connection (see

Figure 6).

Figure 6.

Triac simplified silicon structure

This device can turn on with dV/dt slope in direct or in reverse. When a positive dV/dt static

is applied on A2, the device SCR1 can improve its immunity with a gate to cathode capacitor

or gate to cathode resistor. With this polarity the device is equivalent to an SCR. It’s really

external gate to cathode capacitor or gate to cathode resistor is then in parallel and

improves immunity. When the positive dV/dt slope is applied on A1, there is no possibility to

reinforce this immunity. The gate electrode is on opposite side and there is no possibility to

dV/dt [CGK] / dV/dt [RGK = 220 ]

Ω

0.0

2.5

5.0

7.5

10.0

12.5

15.0

0

20

100

160

200

VD = 0.67 x VDRM

RGK=220 Ω

40

60

80

120

140

180

N3

N1

P2

N2

P1

N4

A2

A1

G

P2

N2

P1

G

K

A

SCR1

A

K

SCR2