[introduction]Thermal resistance (Θ ) it is to point to medicinal powder of the temperature rise on hot method and the quantity of heat that had shed same way than. Will describe with method length bounds, exempleBe likeSemiconductor devicenode arrives crust (Θ J-c) or node arrive periphery environment (the thermal resistance between Θ J-a) . Be in individualSemiconductor deviceIn, all quantity of heat are nodded in the some with identical temperature centrally (Semiconductor chip) dissipation. Quantity of heat is passed from this bits enclose (or crust) , pass in surroundings next, of the quantity of heat of difference in temperature and generation comparing is node comes to crust or node the thermal resistance of periphery.

What is thermal resistance

Thermal resistance (Θ ) it is to point to medicinal powder of the temperature rise on hot method and the quantity of heat that had shed same way than. Will describe with method length bounds, for example the node of semiconductor device arrives crust (Θ J-c) or node arrive periphery environment (the thermal resistance between Θ J-a) . In individual semiconductor device, all quantity of heat are nodded in the some with identical temperature centrally (semiconductor chip) dissipation. Quantity of heat is passed from this bits enclose (or crust) , pass in surroundings next, of the quantity of heat of difference in temperature and generation comparing is node comes to crust or node the thermal resistance of periphery.

Open posture converter

In open-shelf type converter, did not arrive from source of single hear rate of external environment onefold medicinal powder hot method, so the concept of thermal resistance cannot completely applicable. Many parts of an apparatus are contained in converter of an open-shelf type and have different temperature, partial quantity of heat conducts PCB, partial quantity of heat is enclosed in conducting air directly. This but by described as method of many Θ C-a mixes.

In addition, yuan the proportion that the power that parts of an apparatus consumes and whole module spend power changes as the change of working requirement. For instance, should input when voltage is taller, the power that dissipation inputs a part in converter can be gotten accordingly big, but in low input voltage circumstance falls, the power that the input consumes partly is lesser, the scale in inputting the power that consumes partly to consume power in whole module right now is lesser, and the power of other part is used up more remarkable. DifferentLoad currentAlso have similar effect. Below light laden circumstance, the power of magnetism core is used up much is occupied in always be used up, and fall in serious laden case, the power that switch field effect provides is used up bigger.

Substrate type or sealed type converter

To having field power supply (Yottapwr.com) substrate type or sealed type converter, the temperature of parts of an apparatus is more consistent, because glue of in-house heat conduction makes,the difference in temperature of parts of an apparatus maintains inside a very little scope. Next the temperature close of whole converter, and onefold difference in temperature is had between motherboard and external environment. Below this kind of circumstance, housing arrives the outside (the thermal resistance of Θ C-a) is significant. Normally same

The converter of dimension is in given have below air velocity of flow very identical identical perhaps Θ C-a.  

Example: In the graph 1 in, converter job is in low input voltage, be fully loaded with and below the requirement with external environment low temperature, the hottest parts of an apparatus is segregation MOSFET

The load of 25% , input MOSFET temperature is highest, poor value is 7 ℃

Graph 1 Job of open-shelf type PQ60015QGA40 is in 25 ℃ and the heat that small negative charge runs below 25% load are video

The load of 50% , output MOSFET temperature is highest, poor value is 1 ℃

Graph 2 Same a heat that converter issues in 50% load is video

The load of 100% , output MOSFET temperature is highest, poor value is 26 ℃

Graph 3 Same a heat that converter issues in 100% load is video

In the graph 1, graph 2 with the graph 3 in, the parts of an apparatus that hot video graph makes clear temperature is highest can work from the site changes to be nodded to another job. Below this kind of circumstance, besidesLoad currentAll conditions are identical outside. Nod change and the hottest parts of an apparatus that are put in very big difference in temperature to be able to use estimation thermal resistance along with different job.

The advantage of open-shelf type converter depends on it can consuming power the many parts of an apparatus that go up to converter PCB dispersedly. Different job is nodded and the power of each parts of an apparatus below laden condition is used up different. Be not as a result of the quantity of heat that conducts external come from at individual parts of an apparatus, because this is onefold,the concept of thermal resistance does not apply to open-shelf type module.

Substrate type and the contrast that open posture converter

The two example place below is shown be nodded for same job and identical medicinal powder two kinds of below hot condition same pattern are different enclosed converter is heated up like the graph, if the graph is shown 4 times,open posture, substrate type is shown 5 times like the graph. The hot character that can see them has very big different, and the design of substrate type makes whole face is in same below temperature. Converter of type of this specification open-shelf cannot come with singular thermal resistance accurate description.

Graph 4 The job is in be fully loaded with, the open-shelf type converter that 25 ° C and condition of wind speed 200lfm issue outside ambient temperature

Graph 5 The job is in with the graph 4 show the substrate type converter below same condition

In this example thermal resistance is applicable, because substrate type converter has a temperature only. Because the energy of dissipation is 13W, fall for 25 ℃ condition in external temperature, motherboard / module temperature is 74 ℃ , so the Θ C-a of this example falls for 200lfm in wind speed for 49 ℃ / ℃ of 13W = 3.8 / W. Wind speed is higher, temperature rise is smaller, Θ C-a is smaller also.

Summary

To open-shelf type converter, using fixed thermal resistance is inapplicable, use up temperature of parts of an apparatus because of internal power abhorrent, and the hottest parts of an apparatus follows metabolic circuit the job orders with laden condition change. To sealed type converter, using fixed thermal resistance to be worth to external environment from housing is reasonable, because substrate has identical temperature.

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