It is not primarily in their power handling capacities.
While power management IC's in mobile sets working on Power Electronic principles are
meant to handle only a few milliwatts, large linear audio amplifiers are rated at a few thousand
The utilisation of the Bipolar junction transistor, Fig. 1.2 in the two types of amplifiers best
symbolises the difference. In Power Electronics all devices are operated in the switching mode -
either 'FULLY-ON' or 'FULLY-OFF' states. The linear amplifier concentrates on fidelity in
signal amplification, requiring transistors to operate strictly in the linear (active) zone, Fig 1.3.
Saturation and cutoff zones in the VCE - IC plane are avoided. In a Power electronic switching
amplifier, only those areas in the VCE - IC plane which have been skirted above, are suitable. Onstate
dissipation is minimum if the device is in saturation (or quasi-saturation for optimising
other losses). In the off-state also, losses are minimum if the BJT is reverse biased. A BJT switch
will try to traverse the active zone as fast as possible to minimise switching losses.
An example illustrating the linear and switching solutions to a power supply specification will
emphasise the difference.
The linear solution, Fig. 1.4 (a), to this quite common specification would first step down the
supply voltage to 12-0-12 V through a power frequency transformer. The output would be
rectified using Power frequency diodes, electrolytic capacitor filter and then series regulated
using a chip or a audio power transistor. The tantalum capacitor filter would follow. The balance
of the voltage between the output of the rectifier and the output drops across the regulator device
which also carries the full load current. The power loss is therefore considerable. Also, the stepdown
iron-core transformer is both heavy, and lossy. However, only twice-line-frequency ripples
appear at the output and material cost and technical know-how required is low.
In the switching solution Fig. 1.4 (b) using a MOSFET driven flyback converter, first the line
voltage is rectified and then isolated, stepped-down and regulated. A ferrite-core high-frequency
(HF) transformer is used. Losses are negligible compared to the first solution and the converter is
extremely light. However significant high frequency (related to the switching frequency) noise
appear at the output which can only be minimised through the use of costly 'grass' capacitors.
Version 2 EE IIT, Kharagpur
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