APPLICATION NOTE NEW HIGH VOLTAGE ULTRA-FAST DIODES : THE TURBOSWITCHTM A and B SERIES B. Rivet In today's power converter, the commutation speed of the transistor and the operating frequencies are higher and higher. Fast diodes used for freewheel, snubber, and rectifier functions become one of the main causes of the power losses. In the range of 600V-1200V, SGS THOMSON has developed a new family of ultrafast diodes. Taking into account these new constraints which are different from one application to another, SGS-THOMSON proposes two series : TURBOSWITCH "A" and TURBOSWITCH "B". The specific characteristics of these two series offer to the designer a double choice, allowing him to use the best diode in this application. Fig.1 : Approximation of the forward characteristic I. INTRODUCTION II.2. Turn on losses The choice of the optimum diode for a given application depends on the estimation of the power losses generated by the diode. This note explains how to calculate the different losses with information given in the datasheet and shows the difference between TURBOSWITCH"A" and TURBOSWITCH"B" and their respective advantages. When the diode turns ON, the voltage across the diode increases to VFP (Peak forward voltage) before it decreases to 1.1 VF at the time tFR (Forward recovery time) (Fig.2). II. LOSSES CALCULATION IF IF rd V to VF VF Fig.2 : Turn ON waveforms IF II.1 Conduction losses (dIF/dt)ON Conduction losses are estimated with the classical formula : t Pcon = Vto IF(AV) + rd I2F(RMS) Vto rd IF(AV) IF(RMS) : Threshold voltage (Fig.1) : Dynamical resistance (Fig.1) : Average current : RMS current VFP 1.1 V F VF t TM : TURBOSWITCH is a trademark of SGS-THOMSON MIcroelectronics AN601/0193 t FP 1/5 APPLICATION NOTE Turn ON losses can be approximated by the following formula : Fig.4 : Current and voltage waveforms of a freewheel diode at turn-OFF and the associated transistor at turn-ON Pon = 0.4 (VFP - VF ) x tFR x IF x f Where f is the operating frequency : VFP and tFR depend on (dIF/dt)ON and IF. Curves in the datasheet giving VFP and tFR versus (dIF/dt)ON allow the estimation of Pon for each application. Example : IF = 8A (dIF/dt)ON = 64A/s f = 100 kHz With an STTA806D (TURBOSWITCH A, 8 A / 600 V / TO220AC) in these conditions = 10V VFP(max) tFR(max) = 500ns Pon = 1.4 W V I L + I RM Vaa IL TRANSISTOR I S=tb/ta (dIF/dt) OFF II.3. Turn-on losses ta Turn-off losses are studied in the case of a freewheel function where the switch is a MOS transistor (Fig.3). tb DIODE Fig.3 : Basic circuit with freewheel diode I RM D IL Vaa IRM The turn-OFF losses in the diode can be calculated by : Vaa tr POFF = IL + IRM Vaa.IRM 2 .S.f 6 (dIF/dt )OFF II.4. Transistor losses due to the diode IL = Load current IRM = Maximum reverse recovery current of the freewheel diode D The typical waveform of the current and the voltage when the transistor switches ON and the diode switches OFF is shown in Fig.4 in the case where the stray inductance is low (< 50 nH ). 2/5 When the diode switches OFF, the recovery current flows in the transistor which induces turn-ON losses in the transistor. The turn-ON losses in the transistor due to the diode can be estimated by : PON (tr)= Vaa.IRM 2 (3+2S) f Vaa.IRM.IL (S+2) f + 6 (dIF/dt )OFF 2(dIF/dt ) OFF Turn-ON losses in the transistor are generally much higher than turn-OFF losses in the diode. These two formulas include IRM and S parameters which characterize the turn ON behaviour. These parameters depend on the (dIF/dt)OFF. APPLICATION NOTE In the datasheet, curves giving IRM and S versus (dIF/dt)OFF allow to calculate these losses for a given application. Example : Vaa = 400V f = 30 kHz IL = 12 A (dIF/dt)OFF = -500A/s Tj = 125C with a STTA12060 (TURBOSWITCH"A", 12 A / 600 V / TO220AC) we find : Poff = 0.43 W Pon(tr) = 9.5 W III. COMPARISON BETWEEN TURBOSWITCH "A" AND TURBOSWITCH "B" III.1. Difference between characteristics The design of a fast rectifier is known to be the result of a trade-off for a given reverse voltage, and the compromise can be explained in the fig.5. Fig.5 : Compromise between I RM and VF for a given reverse voltage Fig.6 : Main characteristics of a STTA806D and a STTB806D TYPE (dIF/dt)OFF = 500 A/s IF = 8A Tj = 125C IF = 8A Tj = 125C (dIF/dt)ON = 64 A/s Tj = 25C IRM S VF VFP tFR typ typ max max max STTA806D 14 A 0.45 1.5 V 10 V 500 ns STTB806D 28 A 0.79 1.3 V 8V 500 ns Data in this table show that conduction losses and switch-ON losses will be lower in a TURBOSWITCH "B" while switch-OFF losses will be lower in a TURBOSWICH "A". The oscillogram in Fig.7 shows the current in a STTA806D and in a STTB806D when the diodes switch-OFF in the following conditions : VR = 350V (dIF/dt)OFF = -300 A/s Tj = 100C = 12 A IF Fig.7 : Switch-OFF oscillogram of STTA806D and STTB806D IRM SWIT CH ING L OS SES VF CO ND UCT ION L OS SES INCREASING THE SPEED OF A RECTIFIER For the diode of the family "A", the compromise VF - IRM has been chosen to reduce the total losses in both the diode and the companion transistor in a freewheel configuration. On the other hand, the compromise of the family "B" has been chosen to minimize the conduction losses. Table in Fig.6 summarizes the main characteristics of a STTA806D (TURBOSWITCH"A", 8 A / 600V / TO220AC) and a STTB806D (TURBOWSITCH"B", 8 A / 600V / TO220AC) This oscillogram shows that the IRM value is approximately two times lower with a STTA1206D, and that STTB1206D is a very soft diode. III.2. Application examples Example 1 : In this example, a comparison of the loss differences is done in a freewheel application where the current in the diode is rectangular. The main parameters are : Peak current = 12 A IM Vaa = 400 V 3/5 APPLICATION NOTE Duty cycle: = 0.6 (IF/dt)ON= 200 A/s (dIF/dt)OFF= 500 A/s Tj = 125C f = 30 kHz In these conditions the reverse recovery characteristics of the diodes are given in fig.7 : The losses of the table fig.8 are calculated by Fig.7 : Reverse recovery characteristics of STTA1206D and STTB1206D with the conditions of the example 1 TYPE IRM S STTA1206D 16 A 0.42 STTB1206D 30 A 0.90 using relations given in part 2. In this type of application, the TURBOSWITCH "A" Fig.8 : Comparison between STTB1206D and STTA1206D in a freewheel diode function TYPE STTA1206D Total Con- Switch Switch Transistor losses OFF duction ON losses losses losses losses 9W 0.1 W 0.43 W 9.5 W 19 W STTB1206D 7.8 W 0.07 W 3.2 W 29.8 W 40.9 W is obviouslythe better choice. Example 2 : In this example, the diode is used as a rectifier diode with the following conditions : IF = 12 A (dIF/dt)ON = (dIF/dt)OFF = 100 A/s Vaa = 350 V Tj = 125C = 0.8 f = 20 kHz The estimated losses are summarized in the table fig.9 Fig.9 : Comparison between STTA1206D an STTB1206D in a rectifier function TYPE Conduction losses Switch ON losses Switch OFF losses Total losses STTA1206D 14.4 W negligible 0.2 W 14.6 W STTB1206D 12.4 W negligible 1W 13.4 W In this application, we have to take into account the leakage inductance and the fact that a very soft diode is required to limit the overvoltage. The total losses are 10% lower with the STTB1206D, therefore the TURBOSWITCH "B" is the best choice. 4/5 IV. CONCLUSION This note shows how to calculate the different losses due to the diodes in basic power switching circuits. These calculations can be done by using the parameters given in the datasheet of the TURBOSWITCH "A" and the TURBOSWITCH "B". In most of cases, it is easy to choose between the "A" type and the "B" type. The "A" type is very efficient in freewheel diode applications with high frequencies (f > 10 kHz). The "B" type is better when conduction losses are predominant like in the case of the power factor corrector circuit in discontinuous mode (low (diF/dt)OFF), or for applications where very high soft recovery behaviour is required (commutation with series inductances, for example). APPLICATION NOTE 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 the express written approval of SGS-THOMSON Microelectronics. 1993 SGS-THOMSON Microelectronics - Printed in France - All rights reserved. SGS-THOMSON Microelectronics Group of Companies Australia - Brazil - France - Germany - Hong Kong - Italy - Japan - Korea - Malaysia - Malta - Morocco - The Netherlands Singapore - Spain - Sweden - Switzerland - Taiwan - United Kingdom - U.S.A. 5/5