Pass-transistor

Another eonsideration is the type of pass unit to be used. From a headroom loss standpoint, it makes absolutely no differenee whether a bipolar power transistor or a power MOSFET is used. The differenee eomes in the drive eireuitry. If the headroom voltage is high, the eontroller (usually a ground-oriented eireuit) must pull eurrent from the input or output voltage to ground. For a single bipolar pass transistor this eurrent is... 

The power lost just in driving the bipolar pass transistor is... 

This drive loss ean beeome signifieant. A driver transistor ean be added to the pass transistor to inerease the effeetive gain of the pass unit and thus deerease the drive eurrent, or a power MOSFET ean be used as a pass unit that uses magnitudes less de drive eurrent than the bipolar power transistor. Unfortunately, the MOSFET requires up to 10 VDC to drive the gate. This ean drasti-eally inerease the dropout voltage. In the vast majority of linear regulator applieations, there is little differenee in operation between a buffered pass unit and a MOSFET insofar as effieieney is eoneerned. Bipolar transistors are mueh less expensive than power MOSFET and have less propensity to oseillate. 

This circuit is a bridge rectifier followed by a filter capacitor to produce a DC voltage with ripple at Vin. Connected to Vin is a linear regulator made from a Zener voltage reference and an NPN pass transistor. We will first run a Transient Analysis to see the operation of the circuit at room temperature (27°C). To set up a Transient Analysis, select PSpice and then New Simulation Profile from the Capture menus, enter a name for the profile and then click the Create button. By default the Time Domain (Transient) Analysis type is selected. Fill in the parameters as shown in the Time Domain dialog box below ... 

Fig. 10.23 shows a cross-sectional view of a typical circuit for a scanner, and shows the p-i-n sensor and the pass transistor. This particular circuit takes nine mask levels. The metals used are chromium and aluminum, the former for contacts to the TFTs and sensors and the latter for the interconnecting lines. The transparent conducting contact to the sensor is made with ITO and polyimide is used for passivation and isolation of the devices. 

Linear regulators, equivalently called series-pass regulators, or simply series regulators, also produce a regulated dc output rail from an input rail. But they do this by placing a transistor in series between the input and output. Further, this series-pass transistor ... 

Besides being step-down in principle, linear regulators have another limitation — poor efficiency. Let us understand why that is so. The instantaneous power dissipated in any device is by definition the cross-product V x I, where V is the instantaneous voltage drop across it and I the instantaneous current through it. In the case of the series-pass transistor, under steady application conditions, both V and I are actually constant with respect to... 

The component providing a signal b(t) that externally controls the pass transistor modelled as a switch is denoted as switch control. Its output signal b(t) is an input to the switched network and may be subject to disturbances. 

The op amp provides the base current that is then multiplied by the p of the pass transistor (so called because all of the load current is passed through it) to produce load current. The p of the transistor is... 

The designer must check the power dissipated by the pass transistor (the Darlington transistor) to determine the heat sinking requirements. 

Unlike discrete voltage regulators, IC voltage regulators routinely provide internal protection against overpower situations in the way of thermal shutdown. This means the device shuts off current from the internal pass transistor until it cools sufi ciently and resets. If the load attempts to draw too much current, thermal shutdown would occur, the device would eventually cool, operation would resume with the uncorrected load, and the cycle would repeat. 

Recall that with simple current limiting, discussed in Sec. 10.3.4, the current was held constant at some maximum value as the load resistance fell below Rnnn (the minimum value of load resistance for which regulation is provided). This is illustrated in Fig. 10.32(a). Although this offers some degree of protection for the pass transistor and diodes, it is stiU dissipating much power unnecessarily. This is especially undesirable because heat is usually responsible for variation and failure in voltage regulator circuits. 

Pass transistor A transistor used to boost the load current, so-called because all of the load current passes through it. 

The linear regulated supplies with power transformer, rectifiers, capacitor filtering, pass transistor, current sense resistor for current limitation either for constant current or with foldback characteristics and the error amplifier including the voltage reference source. 

The regulators are of the type LASIOOO or equivalent. As a pass transistor a npn-Dar1ington transistor is used (type TIPlOO or equivalent). 

Iv) 8-10% of the line voltage indicates a short circuit after the pass-transistor, and the current limitation is not working properly, therefore all output voltages have to be measured to locate the defective component. 

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