Converter Waveforms and Acoustic Noise

CONVERTER WAVEFORMS and ACOUSTIC NOISE

In common with most textbooks, the waveforms shown in this section (and later in the guide) are what we would hope to see under ideal conditions. It makes sense to concentrate on these ideal waveforms from the point of view of gaining a basic understanding, but we ought to be warned that what we see on an oscilloscope may well look rather different! We have seen that the essence of power electronics is the switching process, so it shouldn't come as much of a surprise to learn that in practice the switching is seldom achieved in such a clear-cut fashion as we have assumed. Usually, there will be some sort of high-frequency oscillation or 'ringing 'evident, particularly on the voltage waveforms following each transition due to switching. This is due to the effects of stray capacitance and inductance: it should be anticipated at the design stage, and steps should be taken to minimize it by fitting 'snubbing ' circuits at the appropriate places in the converter. However complete suppression of all these transient phenomena is seldom economically worthwhile so the user shouldn't be too alarmed to see remnants of the transient phenomena in the output waveforms.

Acoustic noise is also a matter which can worry newcomers. Most power electronic converters emit whining or humming sounds at frequencies corresponding to the fundamental and harmonics of the switching frequency, though when the converter is used to feed a motor, the sound from the motor is usually a good deal louder than the sound from the converter itself. These sounds are very difficult to describe in words, but typically range from a high-pitched hum through a whine to a piercing whistle. They vary in intensity with the size of converter and the load, and to the trained ear can give a good indication of the health of the motor and converter.

COOLING OF POWER SWITCHING DEVICES

Thermal resistance

We have seen that by adopting a switching strategy the power loss in the switching devices is small in comparison with the power throughput, so the converter has a high efficiency. Nevertheless almost all the heat which is produced in the switching devices is released in the active region of the semiconductor, which is itself very small and will overheat and breakdown unless it's adequately cooled. It is therefore essential to ensure that even under the most onerous operating conditions, the temperature of the active junction inside the device does not exceed the safe value.

Consider what happens to the temperature of the junction region of the device when we start from cold (i.e. ambient) temperature and operate the device so that its average power dissipation remains conjunction temperature begins to rise, so some of the heat generated is conducted to the metal case, which stores some heat as its temperature rises. Heat then flows into the heatsink (if fitted), which begins to warm up, and heat begins to flow to the surrounding air, at ambient temperature. The temperatures of the junction, case and heat sink continue to rise until eventually an equilibrium is reached when the total rate of loss of heat to ambient temperature is equal to the power dissipation inside the device.

The final steady-state junction temperature thus depends on how difficult it's for the power loss to escape down the temperature gradient to ambient, or in other words on the total 'thermal resistance 'between the junction inside the device and the surrounding medium (usually air).

Thermal resistance is usually expressed in 8 C/W, which directly indicates how much temperature rise will occur in the steady state for every watt of dissipated power. It follows that for a given power dissipation, the higher the thermal resistance, the higher the temperature rise, so in order to minimize the temperature rise of the device, the total thermal resistance between it and the surrounding air must be made as small as possible.

The device designer aims to minimize the thermal resistance between the semiconductor junction and the case of the device, and provides a large and flat metal mounting surface to minimize the thermal resistance between the case and the heatsink. The converter designer must ensure good thermal contact between the device and the heatsink, usually by a bolted joint liberally smeared with heat-conducting com pound to fill any microscopic voids, and must design the heatsink to minimize the thermal resistance to air (or in some cases oil or water).

Heatsink design offers the only real scope for appreciably reducing the total resistance, and involves careful selection of the material, size, shape and orientation of the heatsink, and the associated air-moving system (see below).

One drawback of the good thermal path between the junction and case of the device is that the metal mounting surface (or surfaces in the case of the popular hockey puck package)can be electrically 'live '.This poses a difficulty for the converter designer, because mounting the device directly on the heatsink causes the latter to be dangerous. In addition, several separate isolated heatsinks may be required in order to avoid short-circuits. The alternative is for the devices to be electrically isolated from the heatsink using thin mica spacers, but then the thermal resistance is appreciably increased.

Increasingly devices come in packaged 'modules 'with an electrically isolated metal base to get round the 'live 'problem. The packages contain combinations of transistors, diodes or thyristors, from which various converter circuits can be built up. Several modules can be mounted on a single heatsink, which does not have to be isolated from the enclosure or cabinet. They are available in ratings suitable for converters up to hundreds of kilowatts, and the range is expanding. This development, coupled with a move to fan-assisted cooling of heatsinks has resulted in a dramatic reduction in the overall size of complete converters, so that a modern 20kW drive converter is perhaps only the size of a small briefcase.

Arrangement of heatsinks and forced air cooling

The principal factors which govern the thermal resistance of a heatsink are the total surface area, the condition of the surface and the air flow.

Most converters use extruded aluminum heatsinks, with multiple fins to increase the effective cooling surface area and lower the resistance, and with a machined face or faces for mounting the devices. Heatsinks are usually mounted vertically to improve natural air convection. Surface finish is important, with black anodized aluminum being typically 30% better than bright.

ill. 19 Layout of converter showing heatsink and cooling fans. A typical layout for a medium-power (say 200 kW) converter is shown in ill. 19.

ill. 20 Sketch showing the influence of air velocity on effective thermal resistance.

(The thermal resistance in still air is taken as 100 %.)

The fans are positioned either at the top or bottom of the heatsink, and draw external air upwards, assisting natural convection. The value of even a modest air-flow is shown by the sketch in ill. 20.With an air velocity of only 2 m/s, for example, the thermal resistance is halved as compared with the naturally cooled setup, which means that for a given temperature rise the heatsink can be half the size of the naturally cooled one. Only a little of this saving in space is taken up by the fans, as shown in ill. 19.Large increases in the air velocity bring diminishing returns, as shown in ill. 20,and also introduce additional noise which is generally undesirable.

Cooling fans

Cooling fans have integral hub-mounted inside-out motors, i.e. the rotor is outside the stator and carries the blades of the fan. The rotor diameter/length ratio is much higher than for most conventional motors in order to give a slim-line shape to the fan assembly, which is well-suited for mounting at the end of an extruded heatsink (ill. 19).The rotor inertia is thus relatively high, but this is unimportant because the total inertia is dominated by the impeller, and there is no need for high accelerations.

Mains voltage 50 or 60 Hz fans have external rotor single-phase shaded-pole motors, which normally run at a fixed speed of around 2700RPM, and have input powers typically between 10 and 50 W (see Section 6).The torque required in a fan is roughly proportional to the cube of the speed, so the starting torque requirement is low and the motor can be designed to have a high running efficiency (see Section 6).

Slower-speed (but less efficient) versions are used where acoustic noise is a problem.

Low-voltage (5, 12 or 24 V)DC fans employ brushless motors with Hall effect rotor position detection (see Section 10).The absence of sparking from a conventional commutator is important to limit interference with adjacent sensitive equipment. These fans are generally of lower power than their AC counterparts, typically from as little as 1W up to about 10W, and with running speeds of typically between 3000 and 5000 rev/min. They are mainly used for cooling circuit boards directly, and have the advantage that the speed can be controlled by voltage variation, thereby permitting a trade-off between noise and volume flow.

REVIEW QUESTIONS

1) In the circuit of ill. Q.1, both voltage sources and the diodes can be treated as ideal, and the load is a resistor.(Note: this question is specifically aimed at reinforcing the understanding of how diodes behave: it's not representative of any practical circuit.)

ill. Q.1

Sketch the voltage across the load under the following conditions:

(a) V1 is a sinusoid of amplitude 20 V, and 2 is a constant voltage of +10V (b) V1 is a sinusoid of amplitude 20 V, and V 2 is a constant voltage of 10 V (c)The same as (a),except that the diode D1 is placed below, rather above, V1.

2) What is the maximum DC voltage available from a fully controlled bridge converter supplying a motor and operating from low impedance 230V mains?

3) Estimate the firing angle required to produce a mean output voltage of 300V from a fully controlled 3-phase converter supplied from stiff 415 V, 50Hz mains. Assume that the load current is continuous. How would the firing angle have to change if the supply frequency was 60 Hz rather than 50 Hz?

4) Sketch the waveform of voltage across one of the thyristors in a fully controlled single-phase converter with a firing angle delay of 60 deg. Assume that the DC load current is continuous. ill. 9 may prove helpful.

5) Sketch the current waveform in the AC supply when a single-phase fully controlled converter with alpha = 45 deg. is supplying a highly inductive load which draws a smooth current of 25 A. If the AC supply is 240 V, 50 Hz, and losses in the devices are neglected, calculate the peak and average supply power per cycle.

6) A DC chopper circuit's often said to be like an AC transformer.

Explain what this means by considering the input and output power relationships for a chopper-fed inductive motor load supplied with an average voltage of 20 V from a 100 V battery. Assume that the motor current remains constant throughout at 5 A.

7) A 5kHz step-down transistor chopper operating from a 150 V battery supplies an R /L load which draws an almost-constant cur rent of 5 A. The resistance of the load is 8V.

Treating all devices as ideal, estimate:

(a) the mark: space ratio of the chopper; (b) the average power in the load; and (c)the average power from the source.

8) This question relates to the switching circuit of ill. Q.8, and in particular to the function of the diode.

Some possible answers to the question 'what is the purpose of the diode 'are given below:

(a) to prevent reverse current in the switch;

(b) to protect the inductor from high voltages;

(c) to limit the rate of change of current in the supply;

(d) to limit the voltage across the MOSFET;

(e) to dissipate the stored energy in the inductance;

Discuss these answers and identify which one(s) are correct.

9) In the circuit of ill. Q.8, assume that the supply voltage is 100 V, and that the forward volt-drop across the diode is 0.7V. Some common answers to the question 'when the current is freewheeling, what is the voltage across the MOSFET 'are given below:

(a) 99.3 V;

(b) 0.7 V;

(c) 0 V;

(d) depends on the inductance;

(e) 100.7 V.

Discuss these answers and identify which one is correct.