1. Parameters And Ratings (General Note)
Unless explicitly mentioned in conditions, all date given in the data sheets are valid over the entire
operating temperature of -40℃ to Tjm.
Non-repetitive peak off-state and reverse voltage VDSM and VRSM
Maximum allowable peak value of non-repetitive transient off-state and reverse voltages.
Repetitive peak off-state and reverse voltages VDRM and VRRM
Maximum allowable peak values of repetitive transient off-state and reverse voltages.
Mean on-state current ITAV (IFAV)
Absolute maximum value of continuous on-state (forward) current for the current waveforms,
temperatures and cooling conditions stated, with no margins for overload. At this current the maximum
allowable junction temperature will be reached. In practical applications, to allow for current or voltage
overloads, for degradation of cooling conditions, and for increases in the ambient temperature by,
for example, local heating due to adjacent components, it is recommended that only 80 % of maximum
mean on-state (forward) current as given is utilized.
RMS on-state current ITRMS
Absolute maximum value of continuous rms current for the chosen current conduction angle,
waveform, and cooling conditions.
Allowable overload currents
The overload currents which are permissible for short-term overloads, for intermittent duty or at
frequencies below 40 Hz may be calculated by means of the transient thermal impedance or the
thermal impedance under pulse conditions , so that the virtual junction temperature at no instant
exceeds the specified maximum value.
Surge on-state (forward) current ITSM (IFSM)
Maximum peak value of a single half sine wave current surge of 10 ms duration. After occasional current surges up to this limiting value the thyristor (diode) will withstand the peak reverse voltages stated.
i 2 t value
The i 2 t value for the thyristor or diode is given to assist in the selection of suitable fuses to protect
against damage due to short circuits. The i 2 t value of the fuse for the prospective short circuit current
and for the input voltage used must be less than the value for the thyristoror diode for t = 10 ms.
Since the i 2 t value of the fast operating fuses normally used for the protection falls more rapidly with
increasing temperature than the i 2 t value for the thyristor or diode, it is generally sufficient to
compare the i 2 t value of the thyristor or diode at 25℃with the i 2 t value of the (unloaded) fuse.
Gate trigger voltage VGT and trigger current IGT
These are the lowest values of gate voltage and current with a 100 μs pulse and anode voltage of
12V which will ensure firing.
Holding current IH
The minimum anode current which will still hold the thyristor in its on-state. If the thyristor is switched
on from cold at below 25 oC the value of holding current may initially be slightly higher.
Maximum gate non-trigger voltage VGD and non-trigger current IGD
These values of triggering voltage and current will not cause the thyristor to fire in the operating
temperature range. Interfering signals in triggering circuits must be restricted to amplitudes below
these values.
Critical rate of rise of on-state current di/dt
Immediately after the triggering of the thyristor the on-state current flows only in the region of the
gate connection, and to avoid excessive dissipation near this gate connection the rate of rise of this
current must be limited to below the critical value given. The specified critical values are valid under the
following conditions: Frequency 50 to 60 Hz; peak current value equal to the crest value of the
permissible on-state current for half sine waves; a gate triggering current at least five times the
minimum gate trigger current IGT having a rate of rise of at least 2 A/μs. The triggering pulse width
should be at least 10 μs. The critical rate of rise of on-state current falls as the frequency increases,
but rises as the peak on-state cur-rent is decreased. For these reasons at frequencies above 60 Hz
the peak on-state current must be reduced if a high di/dt is to be employed.
Critical rate of rise of off-state voltage dv/dt
The given values are valid for an exponential increase in off-state voltage to 2/3VDRM. If dv/dt is exceeded the thyristor may break over and self trigger.
Circuit commutated turn-off time tq
When a thyristor is switched off, tq is the time interval from the instant when the principal current has
decreased to zero to the earliest instant when the thyristor is capable of supporting a steeply rising reapplied off-state voltage without breaking over. With fast turn-off thyristors it is specified as a maximum value under the following conditions:
- On-state current ITM before switching equal to the value of the rated mean on-state current at 125℃ junction temperature,
- Rate of fall of on-state current -diT/dt = 10-40 A/μs,
- Rate of rise or reapplied off-state voltage dv/dt = 30 V/μs,
- Reverse bias VRM ≤100 V,
- Peak off-state voltage VDM = 2/3VDRM
For thyristor types which are intended for use in line commutated convertors and a. c. controllers, this parameter is not important, so only when a user needs, typical values are given which are not guaranteed.
Recovered charge Qrr
This is the total charge which flows through the anode circuit during the reverse recovery time trr. The stored charge is dependent upon the peak value of on-state current ITM flowing before turn-off, and up on the rate of decrease of that current -diT/dt.
Thermal resistance Rth
The internal thermal resistance from junction to case Rth(j-c) and the contact thermal resistance between the case and the heatsink Rth(c-h) are given, the latter being valid only if the assembly instructions stipulated below are followed.
At frequencies below 200 Hz, the thermal resistance Rth(j-c) junction-case of the thyristors depends on the current waveform and the conduction angle
2 Notes For Applications
Gate Drive Considerations
The main purpose of a gate drive for a thyristor is to provide a gate current of the right amplitude, at the right time, and of the right duration, this would seem simple, but the analysis of failed thyristors due to inadequate gate pulses leads to the conclusion that the proper design of a gate drive unit is not trivial.
A thyristor is a current-controlled bipolar semiconductor unlike MOSFFIs or IGBTs which are voltage controlled. Therefore a thyristor gate drive unite is primarily a current source. Supplying a specifically shaped current puls from gate to cathode.
In the following case, a high IGM, and a low tr i.e. a high diG/dt is particular importance.
-High turn-on di/dt.
-Series connected thyristors.
-Paralled connected thyristors.
-Thyristors operated in Electrically Noise Environments.
So we recommended values for IGM,dIG/dt, tr and tp(IGM) for all thyristors are:
IGM=2-5A (<10A) diG/dt≥2A/μs
tr≤1μs tp=5-20μs
Parallel connection of thyristors
One must ensure that equal current sharing occurs under both firing and steady state conditions. The triggering pulses must have steep leading edges and good safety factors on drive levels (see section on critical rate of rise of on-state current), also important is the equal division of inductances and resistances in the main circuits. It is recommended that only 70 to 80 % of the current rating for single thyristors is utilized so that margins are allowed for slight inequalities in the current sharing. In special
cases current balancing reactors must be used to restrict the rate of rise of on-state current.
Parallel connection of rectifier diodes
When parallel connection of diodes is necessary to achieve the required forward current, care must be taken in the construction of the assembly to ensure that equal current sharing occurs. In particular the connecting busbars must have nearly identical impedances. If the average forward currents of the diodes are limited to 80 % of their maximum value, then no special selection of diodes for equal forward voltage drop is necessary.
Series connection of thyristors
When the blocking voltages of a single thyristor are insufficient for the circuit requirement two or more devices may be connected in series. Equal division of the reverse voltage is necessary both under transient and steady state conditions. To equalize transient voltages an RC network is connected across each thyristor, for steady state equalization a parallel resistor is used. Also it is most important to use high and steeply rising trigger pulses so that all the series connected thyristors fire at the same instant since, if one device fires late, for a short time it will experience the total voltage. The blocking voltages allowed for each thyristor in series connection should be at least 10 % less than the maximum rating for a single device.
Series connection of rectifier diodes
When diodes are connected in series to provide a higher reverse voltage rating parallel resistances should be connected across each diode to ensure equal division of the reverse voltage.
If steeply increasing reverse voltages are expected then an additional parallel capacitor of the same value as recommended for the RC snubber network should be added. In any case the reverse voltage per diode must be limited to 90 % of the single diode rating.
3 Protection
Thyristors and diodes must be reliably protected against too high currents and voltages as well as against interference pulses in the gate circuit.
Protection against over-voltage
Over-voltages may occur in a power System, for instance due to
- switching operations in the main supply
- atmospheric effects
- switching of a transformer at no-load
- switching of inductive loads
- delay characteristics of power semiconductors.
Since thyristors and diodes can be damaged by such voltage transients of a few microseconds only a careful selection of over-voltage protection is essential. When designing a suitable snubber circuit, the blocking capability and the critical rate of rise of off-state voltage dv/dt have to be taken into consideration.
Protection against over-current and short circuits
Thyristors have a high current carrying capability in continuous operation but they are very sensitive against over-currents due to their low thermal capacity and, therefore, require matched protective measures.
The choice of suitable protective measures is dependent on the type of the occuring over-current. Fundamentally it is distinguished between short term protection and long term protection.
Fast operating fuses should be used to protect against short circuits whereas magnetic or thermally actuated circuit breakers may be employed to protect against slowly increasing overloads. In particular thermally operated circuit breakers should be used when forced cooling is employed to prevent damage to the thyristors in event of a failure of the fan. Another way of protecting a thyristor is to inhibit triggering when overload occurs.
4 Mounting
Assembly instructions
The mounting surface of the heatsink must be flat and clean, and thermal compound must be coated on all mating surfaces between the base of the thyristor and the heatsink. Care must be taken neither to exceed nor to fall short of the specified mounting torque (M). A torque spanner should be used. The heatsinks should be mounted so that their cooling fins are parallel to the flow of cooling air, and they should be mounted near to the air inlet so that the air is not preheated by other components. If forced cooling is used, care must be taken in the design of ducting that the whole flow of air passes through the heatsink fins and also that the air flow is equally distributed. With natural cooling, equipment cabinets should be constructed to produce a "chimney effect" giving a strong air flow from the bottom to the top of the cabinet. The ceramic seals for the cathode and gate connections should not be subjected to mechanical stresses during assembly. In particular, with the small thyristors the connections to the terminals should be made by soldered flexible wires the screw connections should not be used. Special mounting instructions for capsule thyristors and diodes
In order to obtain the maximum possible current from a capsule thyristor, double-sided cooling is normally used. In this case it is clamped between two identical heatsinks. It is also permissible to have single-sided cooling only. In the case of double-sided cooling the thermal resistance figures relate to both heatsinks together. The mounting instructions given for double-sided cooling may also be applied in the case of single-sided cooling with the appropriate modifications. In order to guarantee good electrical and thermal contact, the contact areas of the heatsinks must be clean and metallic bright. The unevenness remaining after grinding these areas should be less than 10 μm, the roughness less than 10μm. The contact areas should be coated by a thin layer of thermal compound. The clamp must be shaped so that one of the two heatsinks may move freely during mounting. It is therefore recommended that one of the clamp plates should form a pivoting support together with the heatsink (contact between a spherical and a flat surface). The other plate should be shaped so that the heatsink does not deform when The heatsink which moves freely must not be used the screws are tightened. for supporting the unit in the mechanical structure of the stack or assembly. The electrical connection must be made by a flexible lead.
Note
Capsule thyristors and diodes contain thin layers of precious metal between the silicon chip and the hard metal contacts. When the capsule is pressed these layers are deformed and provide intimate contacts with the brittle surfaces. If the thyristor is too often assembled and disassembled (and eventually shaked when loose) there is the danger that the pieceparts move and the contacts deteriorate. For this reason any tests or measurements should be done with the lower specified value of the mounting force F. For blocking voltage test 80 % of this lower specified value are sufficient. Two times disassembling and reassembling are harmless provided the loose thyristor or diode is not shaked.
