![]() ![]() This is one of the primary reasons that over-clocking a computer’s CPU usually requires greater measures to be taken to cool the chip. Thus as the speed increases, the ratio of steady-state to transition-state decreases and more power is lost to heat. When the clock speed of a digital chip is increased, the time spent in either off or on states decreases but the transition time remains the same. During this transition time, the device will be dissipating power in the form of heat. Once again we run into the issue that these devices are not perfect for a given CMOS transistor there is a minimum transition time based on the dimensions and parasitic capacitances of the physical device which limits how fast it can switch between off and on. In digital applications of semiconductors, the goal is to switch between on and off states at the clock frequency, spending no time in between and thus dissipating no power. In this case the amplifier dissipates the least amount of power as heat in the zero input signal condition, making Class – B amplifiers more thermally efficient than Class – A. A Class – B output (in which each output device operates for exactly half of the wave cycle) will generate much less heat, as it spends half its time in the fully off state to which it is biased. This is important to note, because the power dissipated during the zero input condition is what will be used to determine the size of heatsink needed to keep the transistors from overheating. Thus a Class – A amp will dissipate the most power with zero signal applied, because at this point the transistor will be exactly halfway between the off and on states. In a power handling device such as an audio power amplifier, transistors will amplify sinusoidal waveforms to be delivered into a low impedance load, usually a speaker.Ī properly biased Class – A output (one that is operating for the entire 360 degree wave cycle) will have this output transistor constantly transitioning between off and on, never quite reaching either, in a sinusoidal pattern mimicking that of the input. For most cases these values are low enough that the power dissipated is very low it is the state between off and on that dissipates the most power. However in real life we are not using ideal components: BJT’s have a VCE_Sat usually around 0.2 Volts when fully on and even MOSFETs have a small resistance RDS_On when fully on. ![]() Thus, no power is dissipated by the switch in either the closed or open cases. ![]() presenting no resistance so our equation for power dissipated by the switch becomes We know that an ideal closed switch is a perfect conductor, i.e. Now that there is current through our switch, let’s look at how much power it is dissipating. Now in case 2, the switch has been closed and now there is 1mA of current flowing through the switch and resistor. In case 1, the switch is an open circuit and no current may flow through the switch or the resistor because we have from Ohm’s law that: To illustrate this we can use the classic example of an ideal switch: On to output transistors- in an ideal world, transistors do not dissipate power when they are fully on or fully off. What happened to the other 7 watts? They have been burned off in the form of heat we will come back to this example when talking about dissipating this heat. With regulators, the power lost to heat is pretty easy to conceptualize: if you have a 12 Volt supply and wish do generate an output of 5 Volts using a regulator, then with a circuit drawing one Amp of output current, the regulator has 12 Watts going into the device, and 5 Watts coming out. In analog applications of semiconductors, heat will most often be encountered in voltage regulators and output transistors. However for some of these devices heat is a real issue that needs to be addressed- in particular the heat generated by semiconductors. In many of these components the heating is trivial unless you are a power engineer, you probably will not have to deal with keeping copper wire cooled with mineral oil. Heat is the effect caused by current traveling through a resistance in which power is lost to the surrounding media in the form of a temperature increase. Heat in circuitry is caused by resistance, a property found in every part of a circuit: wire, capacitors, resistors, semiconductors, batteries, solder. ![]() With regard to circuits, the broad answer to this question is – all of them. ![]()
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