Rectifier/transformer diode rectifiers

In order to improve the slope compensation circuitry that depends upon a resistor to ground all the time, I will split this resistor between the secondary winding of the current transformer and after the rectifiers. I will double the value of the two resistors (150 ohms each), so that when the diodes are conducting, the net value is the same. 

The load could be a utility distribution line coupled to the converter via a transformer. (Utilities would hkely require transformer coupling to provide d.c. a.c. isolation.) If that were the case, then the same topology could be used to charge the battery by opening and closing the P and N switch pairs at the appropriate times. In fact, if the switches were replaced by appropriately oriented diodes, the topology would be that of the familiar full-wave rectifier. 

The ER system makes use of a one-sided rectified current. This has the advantage that the neutral point of the transformer can be used as the cathode. In the case of a two-sided rectified current, there are problems if the minuses of the diode bridges are connected to one another. 

In the case of rectifying elements, the total installed cost of a thyristor system is usually less than that of one based on diodes. Operating costs also are lower. Maintenance costs, as we saw in Section 8.3.1.2B, are lower because of the use of static, solid-state control devices rather than the mechanical switchgear of a tap changer. Rectification efficiency might be expected to suffer from the loss of part of the sine wave before firing, but this loss usually is comparable to the inefficiencies of the regulating transformers and the saturable reactors in a diode system. 

The fuU-wave rectifier utilizes both halves of the input sinusoid one possible implementation is shown in Fig. 10.15(a). The fuU-wave rectifier consists of two half-wave rectifiers connected to a load R. The secondary transformer winding is center tapped to provide two equal voltages Vi for each half-wave rectifier. When the node A is at positive polarity V with respect to node B, D1 will be forward biased and D2 will be reverse biased. Therefore, diode D1 will conduct and the current will flow through R back to the center tap of the transformer. When the node B is at positive polarity Vi with respect to node A, D2 will be forward biased and diode D1 will be reverse biased. The current conducted byD2 will flow through R and back to the center tap. The current through R is always in the same direction... 

The full-wave rectifier just discussed requires a center-tapped transformer. An alternative implementation of the full-wave rectifier, as shown in Fig. 10.16 (a), is the bridge rectifier. This rectifier has four diodes arranged similar to Wheatstones bridge and does not require a center tapped transformer. 

At the end of the on time, when the power switch opens, the transformer primary and the secondary currents have reached their maximums. The current through inductor L1, which is the same as the transformer secondary current, starts to fall toward zero, and the energy stored in inductor L1 is transferred to the output. Assuming 1 V drops in the secondary circuit rectifier diodes, the change in inductor current during this time is described by the relationship... 

Some of the advantages of these converters are fixed-frequency ZCS or ZVS PWM operation with characteristics closer to the PWM converters, and operation in ZCS or ZVS for both the switches and rectifier diodes for wide load and line voltage variations. The leakage inductance of the HF transformer, however, must be minimized in the isolated version of the converters. Also, extra components and drive circuits are required. 

In addition to the d.c. voltages considered above, there are energy losses across the transformer and rectification equipment. All cell installations use a.c. power, which is rectified by silicon diodes in which the energy losses are minimized by operating at greater than 100 V. This voltage is achieved by operating at least 25 cells in series. 

The full-wave rectifiers may be bridge or centre-tap connection. The bridge connection is quite popular because of its flexibility, simplicity and use of a more economical transformer design. However, economics may dictate a choice between transformer cost and the total rectifier cost, the bridge connection requiring two additional diodes. 

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