Pressure construction of power semiconductor devices (PSD) allows materials with different thermal expansion coefficients to move relative to one another without mechanical stresses, thus increasing resistance to thermal cycles. In contrast to brazed construction, in pressure construction, semiconductor elements with diameter of up to 150 mm can be incorporated into power devices, which in turn provides higher specific power of converters.
PSD sizes and construction have special requirements for their exploitation and operation. It is important to pay attention to special conditions of PSD mounting, control, and exploitation.
When mounting disk devices, it is necessary to know that such devices must be pressed with a definite force noted in corresponding controlling documents in order to provide appropriate electrical and thermal contact.
It is important to provide a uniform mounting force distribution on the contact surface of the device because unequal force distribution leads to destruction of silicon, local overheating, and as a result to device failure. It is inadmissible to press the PSD at an angle or use heat sinks which do not meet the requirements of contact surface flatness.
Besides the flatness requirement, PSD and heat sink contact surfaces must be without nicks and irregularities which exceed allowed levels so that suitable thermal contact is provided. In PSD mounting, electro-heat-conducting pastes must be used with special care because under conditions of insufficient electrical and heat contact the paste mat deteriorate. Over time, increased temperature may cause all the paste to dry and this can decrease the operation time of the PSD.
In PSD of stud type, the necessary heat and electrical contact of the semiconductor element with power lead and base plate is provided during production. For stud type devices it is important to provide sufficient heat contact with a heat sink. It is necessary to follow tightening torque requirements for the heat sink surface.
In PSD of module type, heat and electrical contact is provided during production on account of special springs which create the necessary compressive force for a semiconductor element. Galvanic isolation of the base plate and current conductor lines is realized with the use of alumina-nitride ceramics. Since power emitted in the PSD is generally removed through the cooper base plate, for such devices heat contact of the base plate with a heat sink is very important.
It is necessary to provide the follow requirements:
- flatness of heat sink surface;
- heat contact surface must be without dints, nicks and irregularities which are beyond the acceptable level;
- heat conducting paste must be spread on the contact surface with a layer of 60-90 mkm.
When mounting a module to the heat sink, the following requirements apply:
- clean the module base plate and heat sink surface with coarse calico;
- spread heat conducting paste proportionally on the module base plate;
- check thermal paste layer thickness using a comb;
- place a module in a correspondent position on the heat-sink and press it down while moving it side to side (to grind)
- fasten fixing bolts placed in diagonal with a half force from the nominal;
- fasten fixing bolts placed in the other diagonal with nominal force;
- fasten fixing bolts which were half-fastened up to the nominal fastening force;
- check the tightening force of fixing bolts an hour after finishing the installation.
In order to provide reliable and long-term operation of PSD, it is necessary to operate them without exceeding the maximum permissible values of the relavent operational parameters of the PSD.
- In projecting converters, it is necessary to make calculations of heat and electrical operating regimes, considering necessary reserves.
- The maximum PSD voltage during operation must not exceed 0.67 of the device class (in reasonable cases 0.8).
- The intervals between the DC impulses must be not more than 2 time values of a thyristors switching in this regime.
- An operating regime of current must be chosen considering that the nominal calculated temperature of the p-n junction must be 5-10 degrees lower than the maximal temperature specified in the special conditions (TU).
In the calculation of the heat in the regime of fast PSD operation, it is necessary to consider both static and dynamic losses in the PSD. It is also very important to use protective means against abnormal operating modes (snubber circuits, suge protectors, voltage suppressors, etc.).
Requirements for common thyristor applications
(In case of special appliance it is better to consult a specialist)
Gate current impulse consists of force impulse and feed current.
IGon – feed current rate
IGon = (3-5)•IGT;
IGT –enable gate continuous current ( the parameter is taken for the minimal operating temperature under which a thyristors will be used)
IGM – force impulse amplitude
diG/dt – rise rate or gate current
diG/dt ≥ 1 A/ms;
For the value diG/dt there is no upper limits
tpf – time of force gate impulse
tpf = (2 > 3)•tgd ≈ 10 ms - 20 ms;
where tgd = delay time
For low operating temperatures, it is recommended to choose the longer time for gate impulse
tpon –time of feed current impulse
tpon = (3 - 5)•tgt ≈ 50 ms - ∞ ;
where tgt is turn-on time depending on the device turn-off scheme.
Feed current time is defined by the force character and the scheme functioning conditions in which there is a thyristor. Feed current functions as a safety net for a thyristors if there is a possibility of anode current drop to the hold current value with its following growth.
Simultaneous presence of direct gate current and back voltage (anode-cathode) is not recommended.
For the minimal influence of gate conductors on the current rise speed, the floating voltage of a gate driver must be in the limits of 15-30 V.
To increase the level of interference immunity of a thyristor application, negative voltage to gate circuit must not go beyond the limit of 5 V.
The block driver is mounted as close to a thyristor as possible, laying gate wires among themselves. In order to avoid the influence of electromagnetic interference on the gate circuit, it is necessary to take measures to avoid having gate wires contacting a surface having high potential or having an influence of a fast changing electromagnetic field.
To provide reliable operation of a thyristor, the minimal anode current under which a thyristor turns on must be higher than the given value depending on the type of a device and may vary from several amperes to several dozens of amperes.
The observance of the requirements to thyristor control and recommendations in case of special thyristor usage is one of the most important terms of reliable thyristors operating. Observance of recommendations and requirements of PSD mounting and exploitation provides reliability and durability of the equipment as well increased performance of the devices produced.
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