As small volume power module, is moving toward low input voltage, high current output, and large power density direction development. However, high integration, high power density makes the unit volume on the influence of temperature more and more become system reliable work, the biggest obstacle to performance improvement. Statistics show that the electronic temperature rise every 2 ℃, a 10% drop in its reliability, the life in temperature 50 ℃ is 1/6 when the temperature 25 degree. So hot design purpose is to discharge heat in a timely, and make the temperature of the products at a reasonable level, ensure that components of the thermal stress in the worst environment temperature still will not exceed the specified value. For very valued the reliability of the power supply, heat treatment is necessary in the design of a ring.
Heat production
To investigate thermal design method, firstly must know how to produce power module supply temperature rise is. According to the law of conservation of energy, the power input should be equal to the total power output of the total power, also the energy conversion efficiency (eta) constant at 100%, but the actual situation is the conversion efficiency (eta = 1 - Ploss/Ptotal) is less than 100%, which means there will be a part of the energy is lost (Ploss).Then where is this part of the loss of energy consumption? In addition to a small part of into electromagnetic waves spread into the air, the rest were turned into heat energy, to promote its temperature increase.High temperature can make the power equipment internal invalidation, the reliability of the entire device is reduced.
Contact loss power module and the parameters of the heat is heat resistance (thermal hold), it is defined as the heating device to heat release around the "resistance", it is because of the existence of "resistance", makes the hot (hot points) and around a certain temperature, like electric current flows through the resistor creates a voltage drop.The thermal resistance of different material is different, the smaller the thermal resistance, heat dissipation is stronger, the unit for ℃ / W.
1 modeling analysis
From the above analysis we can get to calculate temperature rise of the first method: establish the loss power of the components of each part and thermal resistance model, and then according to the following formula calculating the temperature rise of the power device.
caculation of temperature rise of a basic expressions:
Δ Τ = RthJ - X • Р loss (1)
Among them, the Δ Τ = temperature difference value or temperature rise;RthJ - X = power devices from the node to the thermal resistance of X.
It can be seen: since components is the root cause of the heat loss of power, then find out the loss of power devices is the key to solve the heat treatment. Now with Jin Shengyang company a 12 w, efficiency as explained 91% of the products.For who drive synchronous rectifier excitation is based on PWM converter, general application circuit principle as shown in figure 1.
The loss of power devices is shown in figure 2.In figure 2, the Pt is the original edge transformer loss; The wastage of the Pl is the output filter inductance; The wastage of the Pmos transistor MosFET is;Pd1 is the wastage of the rectifier diode;Pd2 is fly-wheel diode loss; Pother is loss and other device.
Now, some of the semiconductor device manufacturers can give more detail about the depletion of the parameters, and the power supply r&d staff, also can calculate the power device in the actual engineering, the actual loss, in turn, constantly corrected these values, make the loss of these components can be very close to the real value. So request out of the power device in the actual temperature rise resulting from consumption of a certain power of, now the key to consider the thermal resistance.
But the thermal resistance value is usually affected by the following factor is very big, such as loss of power components of the air flow speed, direction, the level of disturbance, the influence of the neighboring power components, the direction of the PCB board, etc. So the general thermal measurement conditions is very strict. Now look at for a is used for natural air cooling, but around the seal and no fan power components of thermal test method.