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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 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/s VR = 30V - Tj = 25C DTV64D DTV82D DTV110D 135 ns 125 ns 115 ns time of 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 Ip2 = Rd 2 3
Where : Ip : peak current in the diode : duty cycle of the conduction time Vto : Threshold voltage of the damper diode Rd : dynamical resistance of the damper diode Example : With a DTV64D Vto (typ.) = 0.89V Rd (typ.) = 35m and Ip = 6A = 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/s. Fig.2 shows the current and voltage across the diode when it turns on, in the following conditions : Ip = 6A dIF/dt = 60A/s Tj = 100C with DTV64D, DTV82D and DTV110D.
Fig.1: Maximum reverse recovery 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
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APPLICATION NOTE
DTV64D
2A/D
0A 0V
DTV82D
2V/D 100ns/D
2A/D
0A 0V DTV110D
2V/D 100ns/D
2A/D
0A 0V
2V/D 100ns/D
Fig. 2: Oscillograms of VFP for DTV64D, DTV82D, DTV110D with Ip=6A dIF/dt = 60A/s Tj = 100C
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APPLICATION NOTE
The corresponding energy can be calculated by : Total losses in the damper diode : PT The reverse losses due to the leakage current are negligible and the switching OFF losses with ST damper diodes are also negligible. So total losses in the damper diodes are the sum of the conduction losses and the switching ON losses : PT = PON + Pcond Example : DTV64D Ip = 6A = 0.45 F = 64kHZ PT = 2.1W CONCLUSION The new damper diodes have been optimized for horizontal deflection circuits working at high frequencies. A new technology has been developed to reduce the peak forward voltage as much as possible. The compromise between trr and VF has been chosen to be sure that switching OFF losses are negligible. SGS THOMSON offers high frequencies damper diodes DTV64D, DTV82D, DTV110D for operation typically at 64, 82 and 110kHz. Obviously each diode can be used for higher frequencies : for example a DTV82D can be used at 110kHz, in this case the total losses will be higher than with a DTV110D.
W ON =
tFR o
v. i dt
by using this formula and the switching oscillogram of the DTV64D we find : WON = 11.3 J Switching ON losses are given by :
PON = Won x F
Example : With a DTV64D Ip = 6A dIF/dt = 60A/s Tj = 100C F = 64kHz We have
WON = 11.3 J
and PON = 0.73W
Information furnished is believed to be accurate and reliable. However, SGS-THOMSON Microelectronics assumes no responsability for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of SGS-THOMSON Microelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. SGS-THOMSON Microelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of SGS-THOMSON Microelectronics.
(c) 1996 SGS-THOMSON Microelectronics - Printed in Italy - All rights reserved. SGS-THOMSON Microelectronics GROUP OF COMPANIES Australia - Brazil - Canada - China - France - Germany - Hong Kong - Italy - Japan - Korea - Malaysia - Malta - Morocco The Netherlands - Singapore - Spain - Sweden - Switzerland - Taiwan - Thailand - United Kingdom - U.S.A. 3/3
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