10. Output Stages and Power Supplies. 10. Output Stages and Power Supplies TLT-8016 Basic Analog Circuits 2005/2006 1

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1 10. Output Stages and Power Supplies 10. Output Stages and Power Supplies TLT-8016 Basic Analog Circuits 2005/2006 1

Heat sinks are mounted together with power devices to cool them. Figure 10.1 Typical heat sink for a power device.

2 10.1 Thermal Considerations Considerable power is dissipated as heat in power devices. Unless provisions are for made for this heat to flow flow in the air, the device can be destroyed by overheating. Heat sinks are mounted together with power devices to cool them. Figure 10.1 Typical heat sink for a power device. Figure 10.4 Clip-on heat sinks are suitable for power dissipations of a few watts. 10. Output Stages and Power Supplies TLT-8016 Basic Analog Circuits 2005/2006 2

3 Thermal Resistance Most important is the temperature of the pn-junction. The temperature difference is proportional to the thermal power: T J T A = θ JA P D (10.1) T J is the temperature of the junction, T A is the temperature of the ambient P D is the power dissipated in the device θ JC is the called thermal resistance from junction to ambient. For the case of the device T J T C is the temperature of the case θ JC is the called thermal resistance from junction to case JA T C JC = θ JC CS P D θ θ + θ + θ SA (10.2) = (10.3) 10. Output Stages and Power Supplies TLT-8016 Basic Analog Circuits 2005/2006 3

4 Power BJT s 10.2 Power Devices Table Comparison of the parameters of a small signal BJT with those of a power BJT Parameters Maximum power dissipation P Dmax Maximum collector current I Cmax V CEmax f t Output capacitance C obo ( C µ ) Smallsignal BJT (2N2222) 1.8W 0.8A 40V β min V EBmax 6V 7V 300MHz 8pF Power BJT (2N3055A) 115W 15A 60V 0.8MHz pF 1. Power BJT have smaller values of current gain β. 2. The device capacitances of power BJT are higher because of higher junction area. 3. The transition frequency tends to be much lower for power BJT. 4. The reverse leakage current of collector-base junction is larger for power BJT. 10. Output Stages and Power Supplies TLT-8016 Basic Analog Circuits 2005/2006 4

5 The Effect of Temperature on BJT Parameters As the temperature increases: 1. β increases. For temperature variation from -55 C to +150 C β can triple in value. 2. The leakage current I CB0 increases. It doubles for each 10 C increase in temperature. 3. Base-emitter voltage V BE decreases by 2.5mV per 1 C. 10. Output Stages and Power Supplies TLT-8016 Basic Analog Circuits 2005/2006 5

6 Maximum Ratings and Safe Operating Area Maximum power dissipation limit I V = P (10.4) C CE D max Figure 10.6 Maximum power dissipation limit. Figure 10.7 Safe operating area for a typical 10-A, 100-V, 50-W power BJT. Notice the logarithmic scales for V CE and I C. 10. Output Stages and Power Supplies TLT-8016 Basic Analog Circuits 2005/2006 6

7 Other limits: 1. Maximum collector current I Cmax. If it is exceeded the bond-wires melt. 2. Maximum collector-emitter voltage V CE max. Defined from avalanche breakdown or punchthrough. 3. Second breakdown limit. At high V CE the current becomes concentrated in a small area of the junction, in which an overheating appears. Figure 10.7 Safe operating area for a typical 10-A, 100-V, 50-W power BJT. Notice the logarithmic scales for V CE and I C. 10. Output Stages and Power Supplies TLT-8016 Basic Analog Circuits 2005/2006 7

8 Power MOSFETS Figure 10.8a Double-diffused power MOSFET. When a positive voltage is applied to the gate, a channel of n-type semiconductor appears in the p+ area below the gate, which connects the drain and the source. The drain current enters from the drain electrode (at the bottom of the picture), reaches the upper surface, where laterally flows to the source. Since the channel is short, its resistance is small, which permits a large current. This device is more appropriate for operating as electronic switch. 10. Output Stages and Power Supplies TLT-8016 Basic Analog Circuits 2005/2006 8

9 10.3 Class - A Output Stages The Class - A Emitter - Follower Output Stages Figure Emitter-follower output stage. Efficiency P η = o 100% (10.18) For class A power amplifiers η P max = 25% supplies 10. Output Stages and Power Supplies TLT-8016 Basic Analog Circuits 2005/2006 9

10 Direct coupled circuit having complementary symmetry. During the positive half-wave conducts Q 1 forming the positive half-wave at the output. During the negative half-wave conducts Q 2 forming the negative half wave at the output Class-B Amplifiers Review of Crossover Distortion In fact Q 1 is conducting when v s > 0.6V. Q 2 is on when v s < -0.6V. For 0.6 < v s < 0.6 no BJT conduct. In this way large cross-over distortion are presenting. Figure Basic class-b amplifier that exhibits serious crossover distortion. Figure 9.10 Waveforms for the circuit of Figure Output Stages and Power Supplies TLT-8016 Basic Analog Circuits 2005/

11 Biasing Circuits Figure Adding the bias voltage sources reduces crossover distortion, but unless V bias is reduced with temperature, thermal runaway can occur. Figure By using diodes in the bias network, automatic adjustment of the bias voltage with temperature is provided. 10. Output Stages and Power Supplies TLT-8016 Basic Analog Circuits 2005/

12 Power Calculations in Class B Amplifiers Since only one BJT is conducting during one half period, the efficiency of class B amplifier is better. Vmπ η = 100% (10.18) 4V CC η max = 78% at V m = V CC. Figure Output power, device power, and efficiency versus peak output amplitude. 10. Output Stages and Power Supplies TLT-8016 Basic Analog Circuits 2005/

13 10.5 Linear Voltage Regulators Overview of Power - Supply Design Ripples of the output voltage of power supplies. Due to Variation of the input ac voltage; Non-ideal rectifying; Variation of the load current. Voltage regulator: circuit that automatically adjusts the output voltage to maintain a nearly constant value, regardless the input voltage and the load current. Linear voltage regulators: transistors are operating in active region. Switching regulators: transistors operate as switches. Figure Line-operated power-supply block diagram. 10. Output Stages and Power Supplies TLT-8016 Basic Analog Circuits 2005/

14 Linear Voltage Regulators The Voltage Regulators as a Negative - Feedback System Figure Linear series regulator. 10. Output Stages and Power Supplies TLT-8016 Basic Analog Circuits 2005/

15 Series versus Shunt Regulators Series regulators: the output voltage is regulating by the output of an amplifier, which is in series with the load. Shunt regulators: regulating element is in parallel the the load. Figure Simple shunt regulator circuit. The Zener diode operates in reverse breakdown region. The breakdown voltage is stable and depends very small from the current through the diode. 10. Output Stages and Power Supplies TLT-8016 Basic Analog Circuits 2005/

16 Integrated - Circuit Voltage Regulators LM78Lxx IC regulator for various positive voltages depending on the digits xx (e.g for 5V; 7809 for 9V; 7812 for 12V; etc.). LM79Lxx IC regulator for various negative voltages depending on the digits xx. 10. Output Stages and Power Supplies TLT-8016 Basic Analog Circuits 2005/

ECE:3410 Electronic Circuits

ECE:3410 Electronic Circuits Output Stages and Power Amplifiers Sections of Chapter 8 A. Kruger Power + Output Stages1 Power Amplifiers, Power FETS & BJTs Audio (stereo) MP3 Players Motor controllers Servo