COOLING OF POWER DEVICES

Losses leads to heat generation Rise of temperature till the rate of heat dissipation matches the losses Junction Base Heat Sink Heat transfer mostly by air cooling-convection and radiation- metal finned heat sinks When level of heat dissipation is high – forced air cooling and water/liquid cooling

Heat Transfer T – Temperature - C or K P – Heat flow - W (Joules/sec) R – Resistance to heat flow (thermal resistance) - C /W or K/W T1 – T2 = P R (like Ohms Law) Temperature difference 1-2 Heat flow between 1-2 Thermal resistance 1-2

Typical Cooling Arrangements for Power Devices(Sec 1-10,Lander) Rja = Rjb + Rbh + Rha Tj - Ta = P Rja Tj = Ta + P Rja

Natural-air cooling

Air Coolers Conduction, radiation and convection Lower thermal resistance by increasing surface area Natural Cooling Sheet metal (1°C/W) Castings/Extruded (0.25 °C/W) Thermal resistance non linear Temperature, environment (reflections) Forced cooling lowers thermal resistance to 20% of natural cooling

Forced- air cooling

Forced Cooling

Liquid/water Cooling

Liquid Cooling Hollow liquid-carrying bus-bars Electrical insulated couplings Compact assembly Totally enclosed Typically 0.025°C/W for water at 0.07 l/sec

Transient Thermal Impedance Characteristics of (IRF530 N-Channel Power MOSFET)

Transient Thermal Impedance Characteristics

PROTECTION OF POWER DEVICES

FAULT CURRENT PROTECTION The power semiconductor device requires protection against excessive voltage, current, and certain rates of changes, so that the device is not damaged. Much stress has been given to the current and voltage levels experienced by the devices in given applications. FAULT CURRENT PROTECTION The device has to be protected against the fault current, where a device provides a short-circuit path to the supply source. Refer: Textbook-Power Electronics-CW Lander- Chapter 10  

Short-circuit fault via supply. (a) A.C. source. (b) D.C. source. Refer: Textbook-Power Electronics-CW Lander- Chapter 10  

FUSE The device used for current protection is the fuse. Characteristics of Fuse: It must carry continuously the device rated current. 2. Its thermal storage capacity must be less than that of the device being protected 3. The fuse voltage during arcing must be high enough to force the current down and dissipate the circuit energy. 4. After breaking the current, the fuse must be able to withstand any restriking voltage which appears across it. Refer: Textbook-Power Electronics-CW Lander- Chapter 10  

FUSE Fuselink is a strip with several narrow notches. The fuse material is usually silver enclosed in sand which absorbs the vaporization products of the arc. While carrying rated current, the heat generated in the notches is conducted to the wide sections and dissipated. At abnormal conditions, the notches melt and several arcs are struck in series. The arc is absorbed thus preventing the faulty current though the main circuit. Refer: Textbook-Power Electronics-CW Lander- Chapter 10  

FUSING STAGES At fault, the temperature rises, by time t1 the notches in the fuse melt The fault current decays At t2 the fault is cleared, the current decays to zero Refer: Textbook-Power Electronics-CW Lander- Chapter 10  

POSITION OF FUSE Positioning of fuses and added inductors. (a) Passive load. (b) Live (motor) load Refer: Textbook-Power Electronics-CW Lander- Chapter 10  

OVERVOLTAGE PROTECTION The origin of voltage transients having high dv/dt values may be from one of three sources:   Mains or supply source due to contactor switching, lightning, or other supply surges. 2. Load source, the voltage arising from the commutator arcing on a d.c. motor. 3. From within the converter itself, due to switching of other devices, commutation oscillations, or the fuse arc voltage when it is clearing a fault. Refer: Textbook-Power Electronics-CW Lander- Chapter 10  

SNUBBER CIRCUIT  To protect against all three sources of voltage transients, it is necessary to protect each device individually A capacitor C across the thyristor (or diode) means that any high dv/dt appearing at the thyristor terminals will set up an appropriate current (I = C dv/dt) in the capacitor. The inductance in the circuit will severely limit the magnitude of the current to the capacitor and hence limit di/dt. The RC combination placed in parallel to the device is often referred to as a Snubber network Refer: Textbook-Power Electronics-CW Lander- Chapter 10  

REDUCTION OF SWITCHING LOSSES USING SNUBBERS Switching devices that can be turned off by base or gate signals are subject to rapid rise of voltage across the device at turn-off, in addition to the possible rapid rise of current at turn-on Snubber networks which can reduce these rapid changes Refer: Textbook-Power Electronics-CW Lander- Chapter 10  

REDUCTION OF SWITCHING LOSSES USING SNUBBERS   At turn-off the decay of the current may be slower than the rate at which the voltage recovers, as shown in Fig. The addition of a capacitor C across the device will slow the rate of rise of voltage, reducing the stress on the device and reducing the switching loss by reduction in the vi values. At turn-on the capacitor will be discharged via the device and resistor R2; hence the stored energy in the capacitor is lost as heat in R2. Likewise at turn-off, the current in the snubber inductor L is diverted into the resistor R1 and its stored energy lost as heat in R1 The diodes D1 and D2 are necessary to prevent current flow in the resistors when the inductor is limiting the current rise rate and the capacitor is limiting the rate of voltage rise. Refer: Textbook-Power Electronics-CW Lander- Chapter 10