Diodes ZXLD1370 - Manual (Page 23)

ZXLD1370
Document number: DS32165 Rev. 5 - 2
23 of 39
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September 2012
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ZXLD1370
A Product Line of
Diodes Incorporated
Application Information
(cont.)
Matching the MOSFET with the controller is primarily based on the rise and fall time of the gate voltage. The best rise/fall time in the application
is based on many requirements, such as EMI (conducted and radiated), switching losses, lead/circuit inductance, switching frequency, etc. How
fast a MOSFET can be turned on and off is related to how fast the gate capacitance of the MOSFET can be charged and discharged. The
relationship between C (and the relative total gate charge Qg), turn-on/turn-off time and the MOSFET driver current rating can be written as:
I
Qg
I
C dV
dt =
⋅
=
where
dt = turn-on/turn-off time
dV = gate voltage
C = gate capacitance = Qg/V
I = drive current – constant current source (for the given voltage value)
Here the constant current source” I ” usually is approximated with the peak drive current at a given driver input voltage.
(Example 1)
Using the DMN6068 MOSFET (V
DS(MAX)
= 60V, I
D(MAX)
= 8.5A):
Æ Q
G
= 10.3nC at V
GS
= 10V
ZXLD1370 I
PEAK
= I
GATE
= 300mA
ns 35
mA 300
nC 3 . 10
I
Q
dt
PEAK
g
= = =
Assuming that cumulatively the rise time and fall time can account for a maximum of 10% of the period, the maximum frequency allowed in this
condition is:
t
PERIOD
= 20*dt Æ f = 1/ t
PERIOD
= 1.43MHz
This frequency is well above the max frequency the device can handle, therefore the DNM6068 can be used with the ZXLD1370 in the whole
spectrum of frequencies recommended for the device (from 300kHz to 1MHz).
(Example 2)
Using the ZXMN6A09K (V
DS(MAX)
= 60V, I
D(MAX)
= 12.2A):
Æ Q
G
= 29nC at V
GS
= 10V
ZXLD1370 I
PEAK
= 300mA
ns 97
mA 300
nC 29
I
Q
dt
PEAK
g
= = =
Assuming that cumulatively the rise time and fall time can account for a maximum of 10% of the period, the maximum frequency allowed in this
condition is:
t
PERIOD
= 20*dt Æ f = 1/ t
PERIOD
= 515kHz
This frequency is within the recommended frequency range the device can handle, therefore the ZXMN6A09K is recommended to be used with
the ZXLD1370 for frequencies from 300kHz to 500kHz).
The recommended total gate charge for the MOSFET used in conjunction with the ZXLD1370 is less than 30nC.