MV1012SC

14

Additional explanation of resonant period in quasi-resonant operation

Example of gate charge characteristics

Figure 8 Adjusting the delay

Ideally, the MOSFET should be turned on at a valley of resonance as shown in Figure 8. The corresponding condition is

expressed by the following formula:

Coss: Output capacitance of MOSFET

Cr:

Capacitance of D-S capacitor

Cj:

Junction capacitance of fly-wheel diode

actual use, MOSFETs do not need to be switched on precisely at a valley. If the on-timing is far from the valley, it can be

adjusted as follows:

(1) The left side is large:

(2) The right side is large:

 Delay the on-timing.

 Lengthen the resonant period.

• Increase C8 or R4.

• Add or increase Cr.

toff2

1/4 of resonant

period

Tdon

Vth

Vi-Vo

Vb

Qg(Vth)/IG_source

t

t

Qg (Vth): Accumulated charge until the

voltage reaches the gate threshold

Ig

)

Vth

(

Qg

Td

)

Cj

Coss

Cr

(

L

2

4

1













Tdon: Delay in response

CAT.No.1A0602-1E