Build a simple complementary-bracket-pulse generator
Gaps between pulses minimize power consumption.
Horst Koelzow, Global Thermoelectric, Calgary, AB, Canada; Edited by Martin Rowe and Fran Granville -- EDN, August 20, 2009
When building push-pull switching power converters or motor controllers, you often need alternating pulses with a small amount of dead time between them to minimize simultaneous conduction in output-switching devices. Switching controller ICs have this feature, but they usually operate within closed loops to minimize IC pin count. When optimizing switching output stages, you may need open-loop control. Figure 1 shows how you can build such a generator with just two common ICs. As a bonus, both the overlapping, P-channel drive and the nonoverlapping, N-channel drive are available simultaneously.
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The circuit’s input, Pin 10 of IC1, comes from clock generator IC2F. A slightly delayed and inverted version occurs at IC1’s Pin 9 from IC2A. IC1 then decodes the original and delayed inputs to form the desired outputs (Table 1). Because IC1 is an analog demultiplexer, you can set its outputs either active high or active low with pull-up or pull-down resistors. You determine the high or low inactive state by tying the X or Y pins to either the power-supply voltage or ground. Depending on the state of IC1A’s A and B inputs, internal switches in IC1 close between X and X0 to X and X3, as well as from Y and Y0 to Y and Y3. Buffers IC2B through IC2E buffer and invert the resulting outputs. You can use the remaining gate as a variable-frequency or variable-duty-cycle generator. You determine the dead time, which is independent of frequency or duty cycle, using the time constant of R1 and C1. Depending on output-device characteristics and switching frequency, output buffers may require an additional stage, or you can replace them with MOSFET-gate-driver devices. Supply voltage is not critical but should be high enough to guarantee that output devices fully turn on. In general, a higher supply voltage allows for higher-speed operation. The MC14xxx series of ICs is the same as the CD4xxx series. If you need higher-frequency operation at lower supply voltages, then use the 74HC4xxx-series devices. All of these ICs are available from a number of manufacturers, including Texas Instruments (Reference 1) and On Semiconductor (Reference 2).
References
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“CD4051B, CD4052B, CD4053B,” Texas Instruments, October 2003.
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“Semiconductor and Integrated Circuit Devices,” On Semiconductor.
