AN 5951 Estimation of turn-off losses in a thyristor due to reverse recovery Application Note AN5951-3 January 2010 LN27002 Authors: Dinesh Chamund; Colin Rout Introduction: The total power losses in a thyristor are comprised of off-state losses, switching losses and conduction losses. The off-state losses are the steady state losses as a result of blocking voltage and current (leakage current). The switching losses are the dynamic losses encountered during the turn-on and the reverse recovery phases of the thyristor. The conduction losses are the steady state onstate losses during the conduction phase of the thyristor. In the majority of the phase control thyristor applications the conduction losses are the dominant power losses compared to others. Therefore it is often sufficient to design thermal circuit using just the conduction losses with some safety margin. To help towards this process Dynex i2 phase control thyristor datasheets give charts of power dissipation under the commonly encountered waveforms such as sine wave and the rectangular wave for different conduction angles. The switching power losses are the function of the repetition frequency and the commutating di/dt. Therefore these losses become significant at higher frequencies and for high di/dt. For high voltage applications the contribution made by the reverse recovery losses can no longer be ignored. The reverse recovery energy is given by: = × () (1) To calculate the energy loss as per equation (1), detailed knowledge of the reverse recovery current and voltage waveforms is required. This is usually acquired through actual measurements in the real circuit. However for initial design purposes and dimensioning of the device, a quick method of estimating the recovery losses is desirable. In this Application Note a method of estimating power losses due to reverse recovery is outlined. Fig. 1 Thyristor Turn-off waveforms Approximation of reverse recovery waveforms: Fig. 1 shows the current and voltage waveforms observed during the turn-off phase of a thyristor. The charge stored during the conduction phase is extracted as reverse Page 1 of 5 AN 5951 recovery current when a thyristor undergoes turn-off. The reverse recovery phase is characterised by the peak reverse recovery current IRR and the recovered charge QS. QS is given by the integral of the reverse recovery current. = (2) This is the shaded area in Fig. 1. For practical reason the datasheet value of QS is integrated for 150µs by which time the reverse recovery current is virtually zero. ≈ ∞ (5) where VR is assumed to be quasi constant and equal to applied peak reverse voltage VRpeak . And from (5), ≈ 0.5 × × (6) Also, ≈ 0.5 × × − (7) The charge QA can be approximated by the area of a triangle formed by IRR and tA. Thus = 0.5 × × (8) But = (9) / Thus = 2 0.5× / (10) Substituting in (7) we get, ≈ 0.5 × × − In Fig. 2 the total charge QS is divided into two regions, QA and QRR respectively; where = 0 ∞ (3) (4) The reverse voltage during the time interval tA is negligible (Fig. 1) and hence energy contribution during the period tA can be approximated to zero. Then from (1 and 4), Page 2 of 5 (11) Worked example: Fig. 2 Triangular Approximation = 2 0.5× For illustration purpose, thyristor part number DCR3030V42 is chosen and the charts of stored charge and reverse recovery current from the datasheets are reproduced in the Fig. 3 and Fig. 4 respectively. The calculation begins with known parameters of the circuit the VRM(line voltage), VRpeak (controlled by the snubber circuit) and the dI/dt. The dI/dt of the turn-off current is usually controlled by the commutation inductance LC. AN 5951 and QSmin = 1357.3 x (10)0.6271 = 5751µC. Similarly from chart of Fig. 4 the IRRmax = 275A and IRRmin =198A. VRpeak is 2500V. 18000 QSmax = 3397.4*(di/dt) 0.5061 16000 14000 Using the equation (11), Stored Charge, Q S - (uC) 12000 Erec(max) = 8.89J and Erec(min) = 4.74J per pulse. 10000 If the repetition frequency is say 50Hz, then the power losses are: 8000 QSmin = 1357.3*(di/dt) 0.6271 6000 Prec(max) = 8.89 x 50 = 444.5W and Conditions: Tj = 125oC VRpeak ~ 2500V VRM ~ 1700V snubber as appropriate to control reverse voltages. 4000 2000 Prec(min) = 4.74 x 50 = 237W. 0 0 10 20 30 Rate of decay of on-state current, di/dt - (A/us) Fig. 3 Stored Charge 600 IRRmax = 48.236*(di/dt) 0.7553 Reverse recovery current, IRR - (A) 500 It should be noted that the minimum recovery losses correspond to the maximum conduction losses and vice a versa. Ideally both the conditions should be calculated and the worst case value should be used to design the thermal circuit (heat sink etc). Using both the maximum conduction losses and maximum recovery losses will lead to over dimensioning of the heatsink. Measurement Method: 400 In this method the reverse recovery energy is determined by the measurement of the reverse recovery current and voltage using stored charge test equipment. The thyristor part tested was DCR2400B85. Fig.5 shows the oscillogram of the measured waveforms. 300 IRRmin = 29.853*(di/dt) 0.8222 200 Conditions: Tj=125oC VRpeak ~ 2500V VRM ~ 1700V snubber as approriate to control reverse voltages 100 0 0 10 20 30 Rate of decay of on-state current, di/dt - (A/us) Fig. 4 Reverse Recovery Current Thus dI/dt = VRM/LC If we assume dI/dt =10A/us, the value of Qs is given by using the equation on the chart of Fig. 3; QSmax = 3397.4 x (10)0.5061 = 10895µC Fig.5 Reverse recovery waveforms Page 3 of 5 AN 5951 Conclusion: Reverse recovery voltage (V) Fig. 6 shows the digitised reverse recovery current and voltage waveforms plotted in an Excel chart. The Excel spreadsheet is used to multiply the digitised voltage and current waveforms to obtain the instantaneous power waveform as shown in Fig.7. Finally integrating this power waveform gives the energy per pulse. Again numerical integration was performed within the spreadsheet using the trapezium rule. The result of this integration gave the measured value of the reverse recovery energy: Emeas = 17.8 J. Using the approximation method (Eqn. 11) for the test results thus: Erec = 0.5x3030x(15610-(0.5x(225.1)2/5.5)) = 16.7 J. The approximation result is within 10% of the measured value. Using the datasheet curves for DCR2400B85, the maximum and minimum values of recovery energy per pulse are 21.7 J and 15.9 J respectively. 0 -500 -50 -1000 -100 -1500 -150 -2000 -200 -2500 -250 -3000 -300 Voltage Current -3500 -350 0.0E+00 1.0E-04 2.0E-04 3.0E-04 4.0E-04 Time (s) Fig. 6 Reverse recovery current and voltage 400000 350000 Reverse recovery power (W) Test equipment readings: QS = 15610µC integrated over 500µs Irr = 225.1A dI/dt = 5.5 A/µs 0 Reverse recovery current (A) The test conditions are: Tj = 125°C VRpeak = 3030V Snubber setting: 14Ω and 12µF 300000 250000 200000 150000 100000 50000 0 A method for estimating reverse recovery losses in a thyristor using datasheet curves is presented and verified with actual measurement. The approximated value lies within 10% of the measured value. Page 4 of 5 0.0E+00 2.0E-04 4.0E-04 Time (s) Fig. 7 Reverse recovery instantaneous power AN 5951 HEADQUARTERS OPERATIONS CUSTOMER SERVICE DYNEX SEMICONDUCTOR LIMITED Phone: +(0) 1522 502753 / 502901 Doddington Road, Lincoln, Lincolnshire, LN6 3LF Fax: +(0) 1522 500020 United Kingdom. e-mail: [email protected] Phone: +44 (0) 1522 500500 © Dynex Semiconductor 2003 TECHNICAL DOCUMENTATION-NOT FOR RESALE. Fax: +44 (0) 1522 500550 PRODUCED IN UNITED KINGDOM Web: http://www.dynexsemi.com This publication is issued to provide information only which (unless agreed by the Company in writing) may not be used, applied or reproduced for any purpose nor form part of any order or contract nor to be regarded as a representation relating to the products or services concerned. No warranty or guarantee express or implied is made regarding the capability, performance or suitability of any product or service. The Company reserves the right to alter without prior notice the specification, design or price of any product or service. Information concerning possible methods of use is provided as a guide only and does not constitute any guarantee that such methods of use will be satisfactory in a specific piece of equipment. It is the user’s responsibility to fully determine the performance and suitability of any equipment using such information and to ensure that any publication or data used is up to date and has not been superseded. These products are not suitable for use in any medical products whose failure to perform may result in significant injury or death to the user. All products and materials are sold and services provided subject to the Company’s conditions of sale, which are available on request. All brand names and product names used in this publication are trademarks, registered trademarks or trade names of their respective owners. Page 5 of 5

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