1. University Federico II Dept of Electronics and Telecommunications Paolo Spirito Power Semiconductor Devices 1 Review of basic of power amplifiers for analog electronics In typical analog circuits (as in operational amplifiers and audio systems) the power amplifier that drives the load must pay attention to the following aspects: Power circuit input signal drivers load P L Supply P S Pi Power conversion efficiency  = P L / P S defined as the ratio between the power given to the load and the one taken by the power supply (always less than 100%) Power dissipation on the active device Linearity of the output signal delivered to the load Frequency range of the output signal the power gain between input and output signals is not the main goal for the power stage (except for RF power amplifiers)

2. University Federico II Dept of Electronics and Telecommunications Paolo Spirito Power Semiconductor Devices 2 Classes of operation of power amplifiers (and power circuits) The operation of the power amplifiers is defined in different classes according to the way the active devices in the circuit are operating during the period of input signal. For analog amplifiers we define 2 main classes of operation: Class A where the active device is conducting during the entire time period of input signal waveform Class B, where the device is conducting for about one/half the time period of the input signal waveform (two devices are required to obtain a good output linearity) For the switching circuits we can define a Class D operation where the device is made to commutate between full conduction (on) and interdiction (off) states (we will discuss it later on)

3. University Federico II Dept of Electronics and Telecommunications Paolo Spirito Power Semiconductor Devices 3 Class A operation In class A operation the device (here it is assumed a BJT) is biased at the middle point – Q(Io, Vo) - of the load line, and the operating point is driven by the input signal along the load line to a max current less or equal than Imax and min current larger ore equal than 0. The output power is max when the operating point reaches Imax (ideally, when the Vcesat is neglected) and 0. Class-A BJT output stage in emitter follower configuration

4. University Federico II Dept of Electronics and Telecommunications Paolo Spirito Power Semiconductor Devices 4 Class A power efficiency Max power efficiency: assuming a linear operation up to the limit values one has: The max power efficiency is then: The power absorbed by the supply is always constant and equal to Ps, so the efficiency is linearly dependent on output power P L.

5. University Federico II Dept of Electronics and Telecommunications Paolo Spirito Power Semiconductor Devices 5 Power balance in class A amplifiers The power balance of the circuit is:where: The max power efficiency is: The max power dissipation on the device is obtained for zero a.c. power on the load, then: Let’s consider the meaning of these results: To obtain a (controlled ) power output of 50 W one need a supply power of at least 200 W the device must dissipate 100 W in the steady state to transfer a max power of 50 W to the load!

6. University Federico II Dept of Electronics and Telecommunications Paolo Spirito Power Semiconductor Devices 6 Class B operation In class B operation the device (here it is assumed a BJT) is biased at zero current point – Q(0, V CC ) - of the load line. As a result the power dissipation in the quiescent state is zero. Two devices (and two power supplies) are needed to obtain an output signal analog to the input one. The NPN device operate as an emitter follower for positive signal swing, while the PNP device operates as an emitter follower for negative signal swing. Class-B output stage with 2 BJT in push-pull configuration

7. University Federico II Dept of Electronics and Telecommunications Paolo Spirito Power Semiconductor Devices 7 Class B power efficiency Max power efficiency: assuming again a linear operation up to the limit values, and sine signals, one has: The max power efficiency is then: Q1Q1 Q2Q2 I MAX t T T/2 Q1Q1 For the power Ps absorbed from the supply, assuming the following plot for the currents given by the two supplies, one has: Then, the max output power on the load is:

8. University Federico II Dept of Electronics and Telecommunications Paolo Spirito Power Semiconductor Devices 8 Power balance in class B amplifiers The power balance of the class B circuit is: where: This is a second order function in I P, and the max is located somewhere between 0 and I MAX. It can be found as: The power dissipation on the devices is null for for zero power on the load. To evaluate the power dissipation (on both devices) as a function of the output signal, one has: Substituting that value of I P * in (a) one has: and we obtain the following ratio between P DMAX and P LMAX

9. University Federico II Dept of Electronics and Telecommunications Paolo Spirito Power Semiconductor Devices 9 Let’s consider the meaning of these results: To obtain with class-B a power output of 100 W one need a supply power of at least 130 W To transfer a max power of 100 W to the load, each device must be able to dissipate 20 W (at the I P * rated) Power balance in class B amplifiers From the previous results on power efficiency and power dissipation it comes out that: The max power conversion from power supply to load is 78% the total power dissipation (on both device) is 40% of the max output power: then each device must dissipate 20% of the max output power Conclusion: Class B is better than class A in power conversion, (we pay this with some degradation in linearity), but this is still not sufficient if we need power conversion above several kW. For a 10 kW output power we need a power dissipation on each device of more than 2kW and this is not feasible with usual power packages, as we will see later on.

10. University Federico II Dept of Electronics and Telecommunications Paolo Spirito Power Semiconductor Devices 10 Example : Do a SWCAD simulation of a simple Class B amplifier using MOS devices Compute Power efficiency

11. University Federico II Dept of Electronics and Telecommunications Paolo Spirito Power Semiconductor Devices 11 Class D operation To increase the power available at the output of a power circuit, one must decrease the power dissipation of the active devices, that is limited by the package heat dissipation (we will come back on that point later on). The best way of reducing the power dissipation on the device is to let it operate in two limit operating points: a) OFF state, where the power dissipation is zero because the device current is null. b) ON state, at the minimum voltage drop allowed by the operation of the device (often indicated as saturation voltage) This is the Class-D operation: the device operates as a switch, that is either open (OFF state) or closed (ON state). In this way, the device, driven by input pulses capable to bring it either in ON or OFF state, can operate at a power much less than the available output power, thus increasing both the power output and the power efficiency.

12. University Federico II Dept of Electronics and Telecommunications Paolo Spirito Power Semiconductor Devices 12 Class D operation With reference to a BJT device, the operating load line can trepass the max power locus of the power dissipation, because in the ON state (point B) the dissipated power is much less than the maximum power dissipation P DMAX, and in the OFF state (point A) is almost zero (assuming negligible the leakage current)

13. University Federico II Dept of Electronics and Telecommunications Paolo Spirito Power Semiconductor Devices 13 However, one must pay attention on the time required by the device to switch between ON and OFF states: we can define an average steady-state power dissipation P DS and an average dynamic power dissipation P Dd : TT ON TT Class D operation P DS : the average power dissipation in the ON state (assuming negligible the one in the OFF state) P Dd : the average power dissipation during the switching transition between ON and OFF states The power dissipation P D is the sum of the two components P DS and P Dd indicated above.

14. University Federico II Dept of Electronics and Telecommunications Paolo Spirito Power Semiconductor Devices 14 Class D amplifier In class D amplifier, the information content of the signal can not modify the amplitude of the pulses, because these latter are of constant amplitude, but it can be transferred to the output by a modulation of the width of the pulses. In other words, we need a Pulse Width Modulation (PWM) to drive the device and to transfer this information to the (amplified) output, i.e. to the load. The simplest PWM modulation technique is done by using a signal comparator to compare the analog signal with a triangular waveform. The output will be made of a pulse train having an amplitude equal to the supply voltage of the comparator, and ON (OFF) duration defined by the time interval where the triangular waveform is lower (higher) than the one of the modulation signal.

15. University Federico II Dept of Electronics and Telecommunications Paolo Spirito Power Semiconductor Devices 15 PWM Modulation of Class D amplifiers An example of PWM modulation, made by a sinusoidal signal f S using a signal comparator and a triangular waveform f M, is reported in the following plot.

16. University Federico II Dept of Electronics and Telecommunications Paolo Spirito Power Semiconductor Devices 16 Class D power block Power supply Class D circuit signal demodulation (filtering) load PWM modulation signal frequency f S carrier frequency f M To reconstruct the output signal after the class D operation we need to demodulate the signal by a low pass filter, that will cut off the carrier frequency f M, while leaving unaltered the signal frequency f S. The filter must be realized with only L, C components to minimize the power losses.

17. University Federico II Dept of Electronics and Telecommunications Paolo Spirito Power Semiconductor Devices 17 The filter cut-off frequency f F must be: f F <<f M to suppress effectively the carrier frequency of the waveform f S <<f F to leave unaltered the signal frequency (up to the max frequency contained in the signal waveform) Then f S <<f M - This basic need to push up the operating frequency f M will require power devices with high operating frequency and low switching times. PWM Modulation Low-pass LC filter with a slope of 40 db/octave RLRL L C 40 dB/dec fFfF f fSfS fMfM R L increas.

18. University Federico II Dept of Electronics and Telecommunications Paolo Spirito Power Semiconductor Devices 18 Circuit simulation of a class D power Amplifier A SWCAD analysis of a push-pull power amplifier operated in class D with a PWM modulation with a voltage comparator and an LC filter at the output, and two complementary Power MOS is done using the followingschematics:

19. University Federico II Dept of Electronics and Telecommunications Paolo Spirito Power Semiconductor Devices 19 Power Circuits DC Power supply DC/DC converter load unregulated DC regulated DC control The basic power circuits are: DC/DC converters, that control the d.c. power on the load, by variable control signals DC Power supply DC/AC converter load unregulated DC regulated AC control AC line AC/AC converter load unregulated AC regulated AC control DC/AC converters (Inverters), that generate a regulated a.c power from a d.c. power supply, and control the a.c. power delivered AC/AC converters, that generate a controlled a.c power (both in frequency and amplitude) from the line a.c. power supply

20. University Federico II Dept of Electronics and Telecommunications Paolo Spirito Power Semiconductor Devices 20 DC/DC converters Basic applications for these circuits are: Regulated DC power supply DC Motor drive

21. University Federico II Dept of Electronics and Telecommunications Paolo Spirito Power Semiconductor Devices 21 DC/DC converters The DC/DC converters act as controlled d.c. voltage trasformers; the basic versions are: Step-down (or Buck) converter that gives an output voltage lower than the input one Step-up (or Boost) converter that gives an output voltage higher than the input one There are also some combination of the two (like Buck-boost or Cuk) that allow an output voltage both higher and lower than the input one, according to the control signal A most general scheme is the bridge converter that allows both d.c. and a.c. output power conversion. The DC/DC converters usually have the DC input voltage generated from the a.c. line supply through a rectifier circuit made by diodes connected in a full-wave bridge configuration