**8. Conclusions**

128 Power Quality Harmonics Analysis and Real Measurements Data

b) ua 200V/div; ub 200V/div; х=5µs/div

b) ua 200V/div; ub 200V/div; х=5µs/div

Fig. 12. Oscillograms illustrating the boundary operation mode of the converter

Fig. 11. Oscillograms illustrating the main operation mode of the converter

c) i0 5А/div; ub 200V/div; х=5µs/div

c) i0 (5 А/div); ub (200 V/div) х=5µs/div

Fig. 10. Experimental output characteristics of the converter.

a) ua 200 V/div; i 5 А/div; х=5µs/div

a) ua 200V/div; i 5А/div; х=5µs/div

The operation of an LCC transistor resonant DC/DC converter with a capacitive output filter and working above the resonant frequency has been investigated, taking into account the influence of snubber capacitors and a matching transformer. The particular operation modes of the converter have been considered, and the conditions under which they are obtained have been described. The output characteristics for all operation modes of the converter have been built including at regulation by means of changing the operating frequency. The boundary curves between the different operation modes of the converter as well as the area of natural commutation of the controllable switches have been shown in the plane of the output characteristics. Results from investigations carried out by means of a laboratory prototype of the converter have been obtained and these results confirm the ones from the analysis.

The theoretical investigations show that the conditions for ZVS can be kept the same for high-Ohm loads and the converter can stay fit for work even at a no-load mode. For the purpose, it is necessary to have the natural capacity of the matching transformer bigger than the one of the snubber capacitors.

The output characteristics show that in the zone of small loads the value of the normalized output voltage increases to reach a value higher than unit what is characteristic for

**5** 

Adrian Plesca

*Romania* 

**Thermal Analysis of Power** 

 **Semiconductor Converters** 

Quantity Symbol Measure

unit

I A

R

C F

Q As

1/m

Power devices may fail catastrophically if the junction temperature becomes high enough to cause thermal runaway and melting. A much lower functional limit is set by temperature increases that result in changes in device characteristics, such as forward breakover voltage

Heat generation occurs primarily within the volume of the semiconductor pellet. This heat must be removed as efficiently as possible by some form of thermal exchange with the

Heat loss to the case and heat-sink is primarily by conduction. Heat loss by radiation accounts for only 1-2% of the total and can be ignored in most situations. Finally, loss from the heat-sink to the air is primarily by convection. When liquid cooling is used, the heat loss is by conduction to the liquid medium through the walls of the heat exchanger. Heat transfer by conduction is conveniently described by means of an electrical analogy, as it

THERMAL ELECTRICAL

current

0C Voltage U V

resistance

capacity

charge

conductivity

unit

P W Electric

Rth 0C/W Electrical

Cth J/0C Electrical

W/m0C Electrical

Heat Q J Electrical

or the recovery time, and failure to meet device specifications.

ambient, by the processes of conduction, convection or radiation.

Quantity Symbol Measure

**1. Introduction** 

shows in Table 1.

Loss power

Temperature variation

> Thermal resistance

Thermal capacity

Thermal conductivity

Table 1. Thermal and electrical analogy

*Gheorghe Asachi Technical University of Iasi* 

converters with controllable rectifying. This can be explained by the similar mechanism of the rectifier operation in the investigated converter.

The results from the investigation can be used for more precise designing of LCC converters used as power supplies for electric arc welding aggregates, powerful lasers, luminescent lamps etc.

#### **9. References**

