AN874 Datasheet, PDF (1/3 Pages) Maxim Integrated Products – Using the MAX6950 and the MAX6951 LED Display Drivers with Multiplex Dual-Digit Displays

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APPLICATION NOTE
HIGH FREQUENCIES DAMPER DIODES
B. RIVET
INTRODUCTION
The trend in new monitors is for ever increasing
switching frequencies of the horizontal deflection
stage : 64kHz ---> 110kHz.
SGS THOMSON has developed new 1500V
Damper diodes (DTV64D-DTV82D-DTV110D)
using a new silicon structure and a suitable lifetime
reduction process both optimized in order to
reduce the peak forward voltage (VFP) .
For high switching frequencies, the key
parameters optimization of the damper diodes
becomes more and more critical. This application
note describes these key parameters and the
associated power losses.
KEY PARAMETERS OF THE DAMPER DIODE
Voltage drop : VF
This parameter fixes the value of the conduction
losses (Pcond) in the diode. This losses can be
estimated by :
Pcond
=
Vto
Ip
2
δ
=
Rd
Ip²
3
δ
Where :
Ip : peak current in the diode
δ : duty cycle of the conduction time
Vto : Threshold voltage of the damper diode
Rd : dynamical resistance of the
damper diode
The key parameters of a damper diode are the
peak forward voltage (VFP), the forward voltage
(VF ) and the recovery time (trr).
Reverse recovery time : trr
The table in fig.1 gives the maximum reverse
recovery time for the three high frequency damper
diodes.
trr max
IF =1A - dIF/dt =50 A/µs
VR = 30V - Tj = 25°C
DTV64D
135 ns
DTV82D
125 ns
DTV110D
115 ns
Fig.1: Maximum reverse recovery time of
DTV64D, DTV82D and DTV110D.
The application note "CHOICE OF DAMPER
DIODE FOR A HORIZONTAL DEFLECTION"
explains in detail the very particular mechanism of
the switching OFF losses (Poff) in the damper
diode. The maximum value of trr has been chosen
to be sure that the switching OFF losses in the
damper diode will be negligible.
AN874/0996 Ed : 2
Example : With a DTV64D
Vto (typ.) = 0.89V
Rd (typ.) = 35mΩ
and Ip = 6A
δ = 0.45
We find
Pcond =1.4W
Peak forward voltage : VFP
This parameter has to be as low as possible in
order to reduce switching ON losses in the diode.
The peak forward voltage depends mainly on the
dIF/dt. (VFP increases with dIF/dt). For this
application the dIF/dt is typically equal to 60A/µs.
Fig.2 shows the current and voltage across the
diode when it turns on, in the following conditions :
Ip = 6A dIF/dt = 60A/µs Tj = 100°C with DTV64D,
DTV82D and DTV110D.
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