Start-up circuits

The start-up and bias supply eireuit provides the operating voltage for the eontrol IC and power switeh drive seetions. This eireuit is sometimes ealled the bootstrap start-up circuit. Sinee all of the power drawn and delivered by this eireuit is eonsidered a loss, it is important to keep its overall funetion as effi-eient as possible. 

Figure 3-47 The high-voltage linear regulator bootstrap start-up circuit (used only at startup and foldback periods).

The first circuit that can burn up a significant amount of power is the startup circuit. Here dc current is taken from the input voltage so that the control IC and driver circuits have enough power to start the power supply. If the type of start-up circuit does not cutoff its current flow after a successful start-up has been done, then up to 3 W can be continuously dissipated within the circuit depending upon the input voltage. 

I will be using the current limited linear regulator type of start-up circuit. 

The magnetic field rotates at a synchronous speed, so it should also rotate the rotor. But this is not so in an induction motor. During start-up, the rate of cutting of llux is the maximum and so is the induced e.m.f. in the rotor circuit. It diminishes with motor speed due to the reduced relative speed between the rotor and the stator flux. At a synchronous speed, there is no linkage of flux and thus no induced e.m.f. in the rotor circuit, consequently the torque developed is zero. 

From a short-circuit test draw the start-up current at an angle ( ). ... 

To contain the start-up inrush current, as a result of low start-up impedance and to control the same as needed through external resistance in the rotor circuit. 

During start-up since only the outer cage is in the circuit with a very high current, the motor is heated up quickly by every start and may not be suitable for frequent starts and reversals. 

For similar human reasons, we should document every single oscilloscope plot carefully the moment we capture it (assuming it seems meaningful of course). Write down the input and output voltages, currents, specific applied conditions (i.e., power-up into short circuit), the state of the other pins, and so on. Don t forget to keep close track of what each channel represented (or later, just watch yourself suffer Hey look, by moving the pole-zero pair apart, I now have negligible overshoot at start-up. Oops, that must have been the Enable... 

The Transient Analysis setup is the same as in the example without the start-up clear circuit ... 

Always preset or clear all flip-flops before running a simulation. Use the digital setup dialog box or a start-up clear circuit to set the initial state of the flip-flops. 

A minimum propagation delay is required for the start-up of this circuit. Make sure the data sheet or SCD of this part has a minimum listed propagation delay. 

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