? semiconductor components industries, llc, 2001 march, 2001 rev. 0 1 publication order number: and8057 and8057 options for lowering the capacitance in tspd devices prepared by: alfredo ochoa, alex lara, and gabriel gonzalez bpd applications engineers introduction in some of the telecom applications, the capacitance is a critical factor because it can cause disturbances and bad resolution in the telecom systems. the gas discharge tube (gdt) is one of the most traditional devices used for telecom protection because the typical capacitance values are from 1 to 1.25 pf. however, as known, they are relatively slow, and unfortunately, some gas tubes' electrodes burn out after a few hundred hits making them inefficient and unreliable. different from the gdts, the thyristor surge protective device (tspd) offers a very reliable operation and consistency in terms of switching. the tspd is capable of draining a surge current pulse to ground whenever a transient voltage appears between its two terminals (this occurs when a specific maximum voltage, delimited by the maximum breakover voltage of the device, is reached). nevertheless, the capacitance of the tspds is higher than the capacitance of the gdts since the tspds are semiconductor devices. a certain capacitance value will be always be present, and it will depend on the structure and die size of each semiconductor device. therefore, the main purpose of this note is to describe some solutions for lowering the capacitance in the tspd devices manufactured by on semiconductor. industrial standard bellcore gr1089core and gr974core according to the industrial standard, bellcore gr1089core, the levels of protection to be covered by the tspds are classified in three main categories: ? primary protection (central offices, switchboards) ? secondary protection (modems, internet hardware, coax, and hfc) ? station protection (modems, home telephone sets, voicemail) this industrial standard also establishes that each of the previous categories have different lightning surge ratings depending on the telephone lines' density and the kind of applications. these surge ratings are described in the following tables: table 1. primary protection standard number peak surge voltage (v) voltage waveform ( s) current waveform ( s) rating ipp (a) bellcore gr1089core first level 2500 1000 2 x 10 10 x 1000 2 x 10 10 x 1000 500 100 bellcore gr1089core second level 5000 2 x 10 2 x 10 500 table 2. secondary protection standard number peak surge voltage (v) voltage waveform ( s) current waveform ( s) rating ipp (a) bellcore gr1089core first level 2500 1000 2 x 10 10 x 1000 2 x 10 10 x 1000 150 50 bellcore gr1089core second level 5000 2 x 10 2 x 10 150 http://onsemi.com application note
and8057 http://onsemi.com 2 table 3. station protection standard number peak surge voltage (v) voltage waveform ( s) current waveform ( s) rating ipp (a) bellcore gr1089core 1000 10 x 1000 10 x 1000 300 bellcore gr1089core 2000 1.2 x 50 8 x 20 1000 additionally, the industrial standard bellcore gr974core establishes that the capacitance of the telecommunications line protector units (tlpus) between terminals and for each terminal to ground, shall be a maximum of 200 pf when measured at 1 mhz with a 1 volt test signal voltage and a 0 volt dc bias. the actual tspds manufactured by on semiconductor (MMT05B230, 260, 310t3, and mmt10b230, 260, and 310t3 series) offer a capacitance value lower than 200 pf, their typical values are shown below: table 4. secondary protection 1 vrms @ 1 mhz 15 mvrms @ 1 mhz device n mber 0 v 1.5 v 2 v 0 v 1.5 v 2 v device number (pf) (pf) (pf) (pf) (pf) (pf) MMT05B230t3 41 36 35 51 40 38 mmt05b260t3 39 33 32 48 37 36 mmt05b310t3 36 31 30 45 35 34 table 5. primary protection 1 vrms @ 1 mhz 15 mvrms @ 1 mhz device n mber 0 v 1.5 v 2 v 0 v 1.5 v 2 v device number (pf) (pf) (pf) (pf) (pf) (pf) mmt10b230t3 149 128 124 185 139 66 mmt10b260t3 141 122 118 168 126 60 mmt10b310t3 130 111 108 159 120 55 however, there are some applications in which the capacitance is required to be much lower than 200 pf. for those applications, it has been validated that the concepts shown in the figures below are good options to follow: option 1 mur120 tspd option 1 this option is only considering one arrangement of two diodes, mur120, connected in inverseparallel in series with the tspd. the total capacitance value of the complete arrangement, including the tspd, could be theoretically calculated by using the following formulas: ? capacitance in parallel: ct = c1 + c2 + . + cn ? capacitance in series: 1/ct = (1/c1) + (1/c2) + . + (1/cn) based on these formulas and knowing that the typical capacitance value of a diode, mur120, measured at 15m vrms @ 1 mhz (0 vdc bias) is 46 pf, the total capacitance for the arrangement could be calculated as: diodes || capacitance = 46 pf + 46 pf = 92 pf mmt10b230t3 (tspd) = 185 pf therefore, the total capacitance = (1/ct) = (1/92) + (1/185) = 61.4440 pf.
and8057 http://onsemi.com 3 in addition to the theoretical calculations, this option can be validated by measuring the arrangement through an lcr meter (hp 4284a), which would measure similar readings of capacitance (refer to the values in table 6 and table 7 below). table 6. secondary protection 1 vrms @ 1 mhz (option 1) 15m vrms @ 1 mhz (option 1) device n mber 0 v 1.5 v 2 v 0 v 1.5 v 2 v device number (pf) (pf) (pf) (pf) (pf) (pf) MMT05B230t3 32 28 27 32 28 27 mmt05b260t3 30 26 25 30 26 25 mmt05b310t3 28 24 23 28 24 23 table 7. primary protection 1 vrms @ 1 mhz (option 1) 15m vrms @ 1 mhz (option 1) device n mber 0 v 1.5 v 2 v 0 v 1.5 v 2 v device number (pf) (pf) (pf) (pf) (pf) (pf) mmt10b230t3 102 85 82 62 60 58 mmt10b260t3 97 81 78 59 57 55 mmt10b310t3 89 74 71 54 52 50 based on the previous capacitance measurements, it is possible to observe that the theoretical calculations made for mmt10b230t3, connected in series with two diodes, mur120, replicates with the real capacitance measurements done in the same arrangement through the lcr meter. the theory resulted in a capacitance value of 61.44 pf, while the real measure resulted in 62 pf, which represents almost the same value of capacitance. tspd mur120 mur120 option 2 option 2 in the same way than in the option 1, the option 2 shows how it could be possible to reduce even more the capacitance value of a tspd and it is basically based in the same principal than in the option 1, the only difference is that this option contemplates to connect two arrangements of diodes mur120 in series with the tspd thereby the capacitance of the complete arrangement would be significantly reduced. in a similar way than for the option 1, the capacitance value of the option 2 could be theoretically calculated by using the same formulas previously mentioned: ? capacitance in parallel: ct = c1 + c2 + . + cn ? capacitance in series: 1/ct = (1/c1) + (1/c2) + . + (1/cn) let's take the same example than for the option 1: diodes || capacitance = 46 pf + 46 pf = 92 pf (arrangement 1) diodes || capacitance = 46 pf + 46 pf = 92 pf (arrangement 2) mmt10b230t3 (tspd) = 185 pf therefore, the total capacitance = (1/ct) = (1/92) + (1/92) + (1/185) = 36.83 pf.
and8057 http://onsemi.com 4 this option can also be validated by measuring the arrangement through an lcr meter (hp 4284a), which would measure similar readings of capacitance (refer to the values shown in table 8 and table 9 below. table 8. secondary protection 1 vrms @ 1 mhz (option 2) 15 mvrms @ 1 mhz (option 2) device n mber 0 v 1.5 v 2 v 0 v 1.5 v 2 v device number (pf) (pf) (pf) (pf) (pf) (pf) MMT05B230t3 25 22 21 25 23 22 mmt05b260t3 24 20 19 24 21 20 mmt05b310t3 22 19 18 22 20 19 table 9. primary protection 1 vrms @ 1 mhz (option 1) 15 mvrms @ 1 mhz (option 2) device n mber 0 v 1.5 v 2 v 0 v 1.5 v 2 v device number (pf) (pf) (pf) (pf) (pf) (pf) mmt10b230t3 53 46 45 38 37 36 mmt10b260t3 50 44 43 36 35 34 mmt10b310t3 46 40 39 33 32 31 based on the previous capacitance measurements, it is possible to observe that the theoretical calculations made for this option (mmt10b230t3 connected in series with two diodes, mur120) replicate with the real capacitance measurements done in the same arrangement through the lcr meter. the theory resulted in a capacitance value of 36.83 pf, while the real measure resulted in 38 pf, which represents almost the same value of capacitance. in addition to the previous capacitance measurements, it is also very important to consider and validate the surge capability of the diodes, mur120, under the 10 x 1000 m sec waveform to establish whether the diodes are able to cover the ratings for secondary and primary protection or not. based in this, a surge of 10 x 1000 m sec characterization was done in a sample of ten diodes, mur120, to check their behavior. the surge stand point and the results are shown in the table below: table 10. surge capability of the diodes surge current 10 x 1000 sec 50 a 75 a 100 a 125 a 150 a 175 a 200 a 225 a waveform fail/sample fail/sample fail/sample fail/sample fail/sample fail/sample fail/sample fail/sample 10 x 1000 m sec 0/10 0/10 0/10 0/10 0/10 2/10 6/10 9/10 based on these results, it could be established that the diodes, mur series, would be able to cover the surge ratings needed for the two series of tspd devices that on semiconductor manufactures (MMT05B230t3 and mmt10b230t3 series). therefore, as a reference, the most common tspd configuration used to protect a telecom system of two wires (tip and ring) is shown below: ? if it is used, MMT05B230t3 devices, the capacitance between each terminal to ground would be in the order of 42 pf when measured at 1 vrms @ 1 mhz, 0 vdc. ? if it is used, mmt10b230t3 devices, the capacitance between each terminal to ground would be in the order of 149 pf when measured at 1 vrms @ 1 mhz, 0 vdc. tip outside plant ring gnd tspd tspd telecom equipment
and8057 http://onsemi.com 5 for this configuration, the capacitance of the telecommunications line protector units (tlpus) for each terminal to ground is lower than 200 pf when measured at 1 mhz with a 1 volt test signal voltage and a 0 volt dc bias (which is within the range established by the industrial standard, bellcore gr974core). however, to lower the capacitance value, follow either the option 1 or option 2: option 1 ? if it is used, MMT05B230t3 devices, the capacitance be- tween each terminal to ground would be in the order of 32 pf when measured at 1 vrms @ 1 mhz, 0 vdc which repre- sents a lower value than the one from the original configu- ration. ? if it is used, mmt10b230t3 devices, the capacitance be- tween each terminal to ground would be in the order of 102 pf when measured at 1 vrms @ 1 mhz, 0 vdc which also represents a lower value than the one from the original configuration. tspd tspd gnd mur120 mur120 tip outside plant ring telecom equipment option 2 ? if it is used, MMT05B230t3 devices, the capacitance be- tween each terminal to ground would be in the order of 25 pf when measured at 1 vrms @ 1 mhz, 0 vdc which repre- sents a lower value than the one from the configuration of option 1. ? if it is used, mmt10b230t3 devices, the capacitance be- tween each terminal to ground would be in the order of 53 pf when measured at 1 vrms @ 1 mhz, 0 vdc which repre- sents a lower value than the one from the configuration of option 1. mur120 tspd mur120 gnd mur120 tspd mur120 ring outside plant tip telecom equipment
and8057 http://onsemi.com 6 the two options show how the capacitance value of each tspd is lowered by connecting diodes in series with them. the functionality of the circuit is similar in both options since the tspd devices will be acting as open circuits whenever the signal voltage in the tip and ring lines are lower than their vbo. typically, the voltage in the tip and ring lines is between 50 v and 140 v, depending on the kind of application. if a transient voltage occurs in any of the two telecom lines (tip or ring), the corresponding tspd device will be triggered draining the surge current to ground and protecting the telecom equipment. as soon as the surge current drops below the ih value of the tspd, it will to to the offstate and will then be ready to act again if another transient voltage occurs. it is important to mention that the diodes would be driving the same surge current as the tspd devices. the tspd will be operating anytime the vbo is reached, and will not get damaged if the maximum surge current rating is not exceeded. in summary, there are two options to lower the capacitance of the tlpus in applications in which the capacitance value is critical. it is important to mention that designers should take into account that extreme environment temperatures could affect the surge performance of the diodes. it is a fact that they can operate between 15 c to 40 c, so to use any of these concepts out of this temperature range, it would be necessary to do additional surge characterizations.
and8057 http://onsemi.com 7 notes
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