1/9 april 2004 AN554 application note choice of protection in automotive applications (classical topology) rev. d2a - 3584 introduction this paper describes a protection schematic based on discrete components, together with a general meth- od of choosing the components to suppress the surge effects on automotive modules. figure 1. general protection topology general protection schematic positive impulsive overvoltages this type of overvoltage is clamped by the protection component p at maximum voltage v cl . resistance rs limits the dissipated energy in the protection comp onent without compromising the clamping function. negative impulsive overvoltages there are two ways to limit these: ? without diode d: the protection component operates as a rectifier diode and clamps the voltage at the unit terminals to approximately 1v. ? with diode d: the diode is reverse-biased and therefore protects the unit. one important thing to take into account is the peak reverse voltage limit of d. v rrm = 400v seems a good compromise (see curve n 6 of the iso/tc22 standard). positive continuous overvoltages during this phase, the protection component must be in the stand- by phase (very low current passing through the component). negative continuous overvoltages this protection is achieved by diode d which is reverse-biased. impulsive voltage drop during this phase, the unit is fed by capacitor c while diode d prevents c from discharging into the battery circuit. io im electronic module p v bat c d + ? r s
AN554 application note 2/9 the choice of components diode (d) the following paramete rs will constitute the selection criteria: - the average current used by the electronic module. - the maximum repetitive peak reverse voltage v rrm - the maximum ambient temperature t amb . the following inequality mu st apply in all cases: t amb + r th p < t j max where p = v to i f (av) + rd i 2 f(r ms ) r th = thermal resistance (junction - ambient) for the device and mounting in use. resistance (rs) its presence allows a "size" (and thus cost ) reduction of the protection component. its value is a function of the following elements: v bat min: lowest battery voltage which is specified in the technical note issued by the manufacturer. v cc min: minimum voltage needed for the electronic unit in operation. i cc max: maximum supply current of the electronic module. the maximum value of r s will be: r s max = (v bat min - v cc min)/i cc max capacitor (c) its role is to make sure that the voltage at the terminals of the electronic unit is greater than or equal to vcc min while the starter circuit is active. its value depends on: v bat : voltage across the battery before the disturbance v cc min: see ?b: battery voltage?. t: length of the disturbance (130 ms: see application note 4.1, paragraph iii.4) the minimum value of c will be: cmin = (130 * 10 -3 /r eq )/ln (v cc min/v bat ) with r eq = equivalent resistance of the electronic unit r eq = v cc min/i cc max
3/9 AN554 application note protection component (p) - how it works: figure 2. transil behaviour a: disturbance b: voltage across the protection device c: current through the protection device the role of the protection device is to suppress the destructive effects of the surge (see figure 2a), the most agressive being the load dump impulse. to achieve this, the transil clamps the spike at a maximum value v cl (see figure 2b). a surge current flows through the suppressor during this phase (see figure 2c). the choice of the protection device parameters to take into account to choose the transil we have to know the surge parameters and the application requirements. surge parameters. the surge is defined by the peak value ip and the duration t p of the current wave flow- ing through the protection device during the clamping. as shown in the iso/tc22 standard the most ener getic impulsive disturbance is the load dump surge. most car manufacturers recommend the schaffner nsg 506 generator to synthesise this wave (see figure 3). tp t t t a b c v v p v bat v v cl v bat i i pp i pp /2
AN554 application note 4/9 figure 3. equivalent circuit of schaffner generator this circuit allows us to determine the parameters of the current wave seen by the transil. the peak current ip is equal to: i p = (v p - v cl ) / (r g + r s ) where: v p = peak voltage of the surge (+ 80v) v cl = clamping voltage of the transil r g = series resistance of the generator (2 ? ) r s = series resistance of the module to be protected (see ?resistance (rs)?) for example with v p = 80v, v cl = 40 v and r s = 0 ? , we have i p = 20a. figure 4. current pulse duration versus v p and v cl the curves of figure 4 give the duration tp of the current wave in the transil during clamping. this parameter depends on the peak voltage v p of the surge and on the clamping voltage v cl of the pro- tection device. for example with v p = 80v and v cl = 40v, t p = 27.5 ms. application requirements three values are necessary: ? the maximum operating voltage, which is the greatest battery potential. often the car?s electrical equipment has to withstand two battery voltages (due to starting aids: see iso/tc22 standard). these parameters define the minimum stand off voltage v rm of the transil. 1.5 ? 0.5 ? 3.5 ? 5f 47mf v p device or module to be tested v cl = 30v v cl = 35v v cl = 40v v cl = 45v v cl = 50v 35 30 25 20 15 10 80 70 100 90 60 vp (v) tp (ms)
5/9 AN554 application note ? the minimum destructive voltage, which is the voltage value over which the device will be destroyed. this limit determines the maximum clamping voltage v cl of the protection device. ? the maximum ambient temperatur e tamb that would decrease t he power dissipatio n capability of the transil. choice of the protection device the choice of component is made with the help of the parameters t p , p p in the curve p p = f(t p ) from the "protection devices" data book. figure 5. peak pulse power versus exponential pulse duration (1.5ke, 10v < v br < 250 v) if the operating point defined by t p and p p = v cl * i p is on or below the curve, the transil can operate in the application at 25c of ambient temperature. the ambient temperature effect component characteristics are given at an ambient te mperature of 25c (die temperature before clamping action). the following chart shows the effect of junction temperature on the power suppre ssion capability. figure 6. allowable power dissipa tion versus junction temperature 10 6 10 5 10 4 10 3 10 2 10 0.001 0.01 0.1 1 10 100 t amb = 25?c tp (ms) expo p p (w) p p (tj) / p p (tj = 25?c) 100% 50% 0% 0 50 100 150 200 tj initial (?c)
AN554 application note 6/9 this curve gives the derating to be ap plied to the peak power capability of the protection device according to junction temperature. the second temperature effect is the shift of v br . v br (at t) = v br (at 25c) * (1 + t (t-25)) where t is the temperature coefficient of v br . calculation of clamping voltage v cl the clamping voltage v cl can be estimated as follows: v cl = v br max + (r d i p ) where r d is the dynamic resistance of the transil table 1. typical r d for wave of t p = 30 ms at 25c example a: disturbances the load dump is the most agressive b: battery voltage the electronic unit will have to func tion with battery voltage of 11 v. c: ambient temperature t amb = 85c d: electrical characteristics of the module table 2. module characteristics e: analysis e1: calculation of r s max r s max = (v bat - v cc min)/i cc max r s max = (11-8)/0.6 = 5 ? bzw04 p23 p6ke 30p 1.5ke 30p bzw50-22 ldp24as rd typ ( ? ) 1.2 0.75 0.35 0.15 0.12 pa r a m e t e r s v cc i cc description supply voltage supply current min 8 ? typ 12 400 max 32 600 unit volts ma
7/9 AN554 application note e2: diagram figure 7. allowable power dissipa tion versus junction temperature e3: peak current i pp = (v g -v cl )/(r g +r s ) = (80-32)/7 = 6.9a e4: peak power p p = v cl * i pp = 32 * 6.9 = 221 w e5: conduction time t p = 30 ms e6: choice of the transil table 3. module characteristics conclusion diode bzw50-22 is an efficient protection device within the 85c temperature range, and the v cl max is given as follows: v br (85c) = v br (25c) * (1 + t(85-25)) = 29.8 * (1 + 9.6 * 10-4 * 60) = 31.5v v cl (85 c) = v br (85 c) + r d ip = 31.5 + (0.15 * 6.9) = 32.5 v pa r a m e t e r s v cc i cc description supply voltage supply current min 8 ? typ 12 400 max 32 600 unit volts ma r g = 2 ? r s = 5 ? v g = 80v module
AN554 application note 8/9 revision history table 4. revision history date revision description of changes march-1993 1 first issue 1-apr-2004 2 stylesheet updat e. no content change.
9/9 AN554 application note information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsibility for the co nsequences 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 stmicroelectronics. specifications mentioned in this publicati on are subject to change without notice. this publication supersedes and replaces all information previously supplied. stmicroelectronics prod ucts are not authorized for use as critical components in life support devices or systems without express written approval of stmicroelectro nics. the st logo is a registered trademark of stmicroelectronics. all other names are the property of their respective owners ? 2004 stmicroelectronics - all rights reserved stmicroelectronics group of companies australia - belgium - brazil - canada - china - czech republic - finland - france - germany - hong kong - india - israel - ital y - japan - malaysia - malta - morocco - singapore - spain - sweden - switzerland - united kingdom - united states www.st.com
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