0 Preface
In the field of power electronics engineering, it serves a variety of electronic control devices. Where thyristors are used, they are grouped into a functional circuit as intended by the designer. For example, various single-phase, three-phase, six-phase rectifier bridges, anti-parallel lines, and parallel, series application lines of multiple thyristors, and the like. Different applications have different lines, which can be described as ever-changing and numerous.
In such a power module with independent function, when it is working with a large current, its heat generation and heat dissipation are a very important contradiction. The application should understand its ins and outs and properly solve it. Otherwise it will have a major impact on the reliability of the whole machine. When a 2500A DC output three-phase full-wave rectifier bridge is working, the heat emitted by the unit itself can be as high as about 6KW. If the heat is not dissipated in time, the consequences are unimaginable.
As far as air cooling is concerned, the contents involved in heat dissipation include: radiators, fans, and ducts. The disciplines involved include fluid mechanics, heat transfer, materials science, and air duct structure design.
1 Thyristor heating (power consumption) principle
The thyristor's own power consumption includes power consumption due to forward current, switching loss, and reverse leakage current loss. The switching loss is very small under power frequency conditions, and the leakage current loss is relatively small, about one or twenty watts, so the latter two are not discussed in this paper.
1.1 Forward characteristics of thyristors:
Figure 1: Thyristor forward characteristic curve
The forward characteristic curve of the thyristor is not linear and can be approximated as two straight lines: before the voltage VT0 (ie, less than VT0), the thyristor is not actively turned on, and the current is extremely small; when the voltage is greater than VT0, the current is related to the voltage. Rising, can be regarded as a straight line, and there is a slope, expressed by the slope resistance rT0, in Ω (ohms).
The functional relationship of the curves in the figure is:
1.2 thyristor forward power consumption
Sine wave:
Formula 2 is substituted into 1,
The IFM is the peak current of a sine wave, and the same VFM can be expressed as the peak voltage of a sine wave.
Forward average current:
Forward average voltage:
Forward power consumption:
After calculating the simplification:
Where F is the waveform factor, which varies with the conduction angle.
For sinusoidal resistive loads:
Where IF·F=IF(RSM) and IF(RSM) are the forward current rms values. Therefore, the rms current of the thyristor can be directly used in the calculation.
Since the thyristor forward power consumption P is obtained by integrating the product of iF and vF from 0 to 180°, it is not linear. It is not correct to use the average current measured by the meter to multiply the average voltage to find the power consumption.
The VT0 and rT0 of each type of thyristor are listed on the parameter table of the thyristor product specification.
3 Radiator thermal resistance Rth (cA)
The thermal resistance of the heat sink can be divided into two types: steady state thermal resistance and transient thermal resistance.
3.1 Thermal resistance Rth concept:
The thermal resistance is the reciprocal of the thermal conductivity. The unit is: °C/W (°C/W). The heat at both ends of the thermal conductor with temperature difference is transmitted from the high temperature end to the low temperature end. For example, if the heating power at the end of the temperature is P, the following relationship is met:
P is the thermal power generated stably at the A terminal. The Rth hour TA - TB is also small, that is, the temperature difference is also small, and the temperature difference is also large. It can be seen that the heat conductor with small thermal resistance can quickly conduct heat from the hot end.
The steady state thermal resistance of the radiator can be approximated by an empirical formula:
Under self-cooling conditions:
Under air cooling conditions:
Where: l is the length of the radiator rib; b is the thickness of the rib; L is the length of the radiator; n is the number of ribs; A is the surface area; uS is the wind speed; the unit used: l: m, A: m 2, b: m, L : meter, uS: m / sec
The rib length is the length of the radiator fin
Where: a takes 2.5; KS (aluminum) = 140 kcal / hour · m · ° C; KS (copper) = 340 kcal / hour · m · ° C
In addition to the above empirical formula, the heat resistance of the radiator is generally provided by the production unit on the sample, and can also be determined experimentally (GB GB/T 8446.2-2004).
3.2 Steady-state thermal resistance
Steady-state thermal resistance is the thermal resistance when the heat generated by the system is equal to the amount of heat dissipated. At this point, the temperature at each point on the heat sink is constant and in equilibrium. The calculation formula described above applies to steady state thermal resistance.
3.3 Transient thermal resistance
The transient thermal resistance represents the process of changing the thermal resistance of the heat sink from the thermal shock to the establishment of the thermal equilibrium (ie, the temperature at each point is constant) before the thermal equilibrium is established. The thermal resistance value gradually changes from small to large.
