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Low-Cost Power Modulator Switch

IP.com Disclosure Number: IPCOM000047014D
Original Publication Date: 1983-Sep-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 2 page(s) / 37K

Publishing Venue

IBM

Related People

Black, AG: AUTHOR [+2]

Abstract

This article describes a circuit utilizing relatively slow and cheap bipolar transistors (instead of FETs) together with diodes, to perform as a switch to modulate a high frequency AC signal. A typical high power switching supply uses a pair of FET devices to control the flow of power into a power supply. This control is typically achieved by taking a DC voltage, chopping it at a relatively high frequency, such as 20 KHz, and then transformer coupling it to an output stage. The output voltage is then rectified and filtered. 20 KHz is typically chosen as the operating frequency to allow the supply to use small scale magnetics and to guarantee that the supply operates above the audio range. FETs are usually chosen as the switching device because they operate with very high efficiency at these frequencies.

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Low-Cost Power Modulator Switch

This article describes a circuit utilizing relatively slow and cheap bipolar transistors (instead of FETs) together with diodes, to perform as a switch to modulate a high frequency AC signal. A typical high power switching supply uses a pair of FET devices to control the flow of power into a power supply. This control is typically achieved by taking a DC voltage, chopping it at a relatively high frequency, such as 20 KHz, and then transformer coupling it to an output stage. The output voltage is then rectified and filtered. 20 KHz is typically chosen as the operating frequency to allow the supply to use small scale magnetics and to guarantee that the supply operates above the audio range. FETs are usually chosen as the switching device because they operate with very high efficiency at these frequencies. Bipolar transistors are typically not used in a high power design of this nature because of their slow switching speeds compared to FET devices. Their slow switching speeds cause them to consume an excessive amount of power during switching transitions, leading to early device failure. If bipolar devices are used in a high power supply, the drive stage for these devices is complex and costly. This is necessary in order to enable the devices to operate successfully at high frequencies. The power control stage shown in the figure achieves the cost savings of bipolar devices with the simplicity of control circuitry normally found only with FETs. Two elements of the...