application note how the tde1897/8 behave in extreme overload conditions by u. moriconi the tde1897/8 is a monolithic intelligent power switch (ips) in high side configuration and bcd tecnology (see fig.1),dedicated to drive resistive and inductive load such as lamps, relays, elec- tro-valves, etc. an internal voltage clamping diode to +v s creates, in inductive load, a fast demag- netization path without external components. suitable for industrial application, it operates in the 18 to 35v supply range delivering output cur- rent up to 500ma. in typical application it can drive up to 1 - 1.5h load coil (48 to 60 w typical associated resistance). overload conditions to investigate the behaviour of tde1897/8 in ex- treme inductive overload conditions, that may oc- cur when too big a load is connected to the de- vice output, tests were performed, in bias conditions that lead the device to function out of the datasheetoperatives and rated limits. AN453/0392 the circuit designer may be interested and get some insight on how the tde1897/8 behave, if extreme overload conditions are forced on to them. although the conditions may range outside the limits of the datasheet guarantieed performances, erroneous connections during an installa- tion phase may occur and momentarily create such conditions. the performed tests confirm the extreme ruggedness of these devices and their ability to survive the accidental overload. figure 1: block diagram 1/6
test conditions (referred to the circuit of fig. 2) v s = +24v; i o = internal limited; t amb =25 c; l = 1.4h (non saturating); r l =12 w ;vi=2v (vih)(#); t j = from lim-hy to lim and above (*) (#) the input signal asks for a permanent oono state. (*) lim & hy = thresholds of intervention and histeresis of the internal thermal protection circuit. overload operation due to the internal limitation (i sc ), the output cur- rent (i o ) is not limited by the load (v s /r l = 2a; i sc 1.5a) but by the device itself. as soon as the current reaches i sc , the i.p.s. goes out of the minimum resistance state and increases its volt- age drop so that i o =i cs . the silicon temperature of the d.u.t. increases rapidly up to the thermal protection threshold value (lim) and such pro- tection tries to cut-off the output dmos. the turn- off of the output forces the demagnetization cycle, that discharges the energy of the inductive load (to v s ) through the device. the higher clamped current value (i sc ) will pro- duce, during the demagnetization, more stress conditions because of both: - the higher energy in the magnetic load - the higher peak power (1) during the oono state the power (p don ) on the d.u.t (see the 225msec. interval in fig.3) is de- fined by the i o (i sc ) and r l values. the chip tem- perature rapidly increases and reaches the upper thermal protection threshold value (lim); at that moment the protection is triggered on, inducing the attempt of switch-off, the associated demag- netization phase (some 50msec. after the 225msec. interval), and finally the switch-off. the d.u.t. starts then to cool down staying in the off-state, until the chip temperature goes down to lower thermal threshold value (lim-hy). when lower limit (lim-hy) value is underpassed, the thermal protection circuit withdraws itself, the chip resumes its normal functions and restarts another cycle. in facts its input has been connected per- manently to a voltage level of more than 2v, meaning a continuos request for conduction. a new overload cycle is so started, and a periodic repetition of: load charging current limitation overtemperature and demagnetization cooling down in the off state . it can be noted that, for given thermal parameters (z th , thermal protection levels and hysteresis), differences in p don affect only the ot on o and ot off o duration and ratio of such periodic repeti- tion. the minidip device (odpo suffix) suffers heavier stress conditions than the sip9 option (ospo suf- fix) because of the package differences (minidip vs. sip9 involves higher thermal gradients). note(1 ) during the demagnetization phase , the power dissipated inside the i.p.s. chip is: i o (t) * v cl -i o (t) decays to zero from i sc . -v cl is set by the i.p.s. itself to about 50v figure 2: inductive load equivalent circuit and demagnetization cycle waveforms io application note 2/6
some measurements and calculations for a typical tde1897 sample in minidip package (see fig. 3) in othermalo periodic repetition, the current (self-limited region) is limited to 1.1a and the voltage across the d.u.t. is = 10.8v for 225msec. oono time. the energy dissipated on the d.u.t. in the demagnetization cycle is = 1.28 j (**) the repetition cycle rate is = 0.27hz(t = 3.7sec.). p don (average) = 1.1a ? 10.8v ? 0.225sec/3.7s= 0.72w p dem . (average) = 1.28j ? 0.27cycles/s = 0.346w adding the small power dissipated for operating quiescent current and for i o (t)^2*r on in load- charging region, the total power p (tot) = 1.1w is a realistic value. minidip (on the test-socket) r thj-amb is about 85 c/w that leads the average temperature in the hot region of the chip) to 115-120 c (the chip isn't homogeneous in temperature. higher tem- peratures are reached, during dissipation, in the area of the output dmos). figure 4: tde1897 in minidip package output current and temperature in the test point, vs. time. figure 3: tde1897 in minidip package output voltage (ch2) and output current (ch1) vs. time in ther- mal periodic repetition. 225ms ch1 = 200ma/div ch2 = 10v/div t = 50ms/div ch1 = 200ma/div ch2 = 50 c/div t = 50ms/div tmin @ 100 c ? tmax @ 165 c application note 3/6
for a typical tde1898 sample in sip9 package (see fig. 5) in othermalo periodic repetition, the current (self limited region) is limited to 1.15a and the voltage across the d.u.t. is = 10.2v for 300msec. oono time. the energy dissipated on the d.u.t. in the demagnetization cycle is = 1.38j (**). the repetition cycle rate = 0.52hz (t = 1.92sec.). p don (average) = 1.15a ? 10.2v ? 0.3s/1.92s= 1.83w p dem (average) = 1.38j ? 0.522 cycles/s = 0.72w the total power = 2.6w the r th j-amb for sip9 oon socketo is about 50 c/w that leads the average temperature on the hot region of the chip to 150 c . note(**) the formula to use is : w=v cl ? l/r l *{i o -[(v cl -v s )/r l ] ? log[1+(i o* r l )/(v cl -v s )]} it is also interesting to see (fig. 4 and 6) the tem- perature versus time (mesaured monitoring the forward voltage drop of an internal diode placed 1.5mm from the center of the power dmos) in a region of the chip at lower average temperature. on the ohoto region, the estimated temperature is quite higher (up to + 60 c. on the peak tempera- ture, during the demagnetization phase) however no failure could be observed on the cheked devices also reducing the r l value down to 8 w , on some minidip samples. figure 5: tde1898 in sip9 package output voltage (ch2) and output current (ch1) vs time in ther- mal periodic repetition ch1 = 200ma/div ch2 = 10v/div t = 100ms/div 300ms figure 6: tde1898 in sip9 package output current and test point temperature vs. time ch1 = 200ma/div ch2 = 50 c/div t = 100ms/div tmin @ 120 c ? tmax @ 165 c application note 4/6
conclusion the complex protection sistem of tde1897/8 proves effective also in extreme overload condi- tions. althougth the behaviour of such devices in those conditions cannot be guaranteed due to the high temperatures that accelerate the intrinsic ageing mechanism, the test performed show that there is a lot of margin beyond the guaranteed limits of the device datasheet. these test also show that it is very likely that such devices will survive to non permanent overloads like the ones possible in practice during the installation or modification of an industrial control system. application note 5/6
information furnished is believed to be accurate and reliable. however, sgs-thomson microelectronics assumes no responsibility 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 men- tioned 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 ex- press written approval of sgs-thomson microelectronics. ? 1995 sgs-thomson microelectronics - 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 - thaliand - united kingdom - u.s.a. application note 6/6
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