Ideal Full Wave Bridge (FWB1):

A full wave bridge circuit consists of four interconnected diodes. Now it might seem possible and desirable to use 4 ideal diodes to realize the device, as shown in Figure 1 following, and this is possible but this creates simulation problems. That realization would have too many loops possible where ideal sources are interconnected. Moreover, it is overly complex.

Figure 1
Possible Full Bridge realization

A better and simpler realization is to use logical expressions to implement the device. Because the bridge is ideal, the output device should have a voltage output which is the absolute value of the input voltage. However, such a device would NOT be conservative. That is, the output loading would not be reflected to the input. Consequently, we must be a little careful in our implementation. The approach chosen is shown in Figure 2 following:

Figure 2
Full Bridge realization

In Figure 2 there are two implementations of the B2 device equation shown, one for version 4, and the second for version 5.1.5+. The B1 device is suitable for use in either case. Operation of the circuit is straightforward. Generator B1 applies the absolute value of the input AC voltage applied between IN1 and IN2 terminals to the ammeter AM1 and the load.

Resistor R1 is provided to allow a circuit ground to be applied at either the IN2 or the Von output negative terminals. The B2 generator applies an appropriate current to flow at the input terminals. If the voltage from IN1 to IN2 is positive, then the current measured by the ammeter is positive and this value is used. If the input voltage is of the opposite sense, the negative of the ammeter current is applied at the input terminals.

An ideal diode U1 is shown in series with the output terminals. This is required to prevent the output from discharging through the generator.

A test circuit for this realization is shown in Figure 3 following:

Figure 3
Full Bridge test circuit #1

Devices V2 and V3 are provided for test purposes to prevent nodes IN2 and Von being renamed should a ground be present at the V1 input of the Von terminals before it is turned into a subcircuit. Without them the internal nodes would be renamed necessitating some changes in the device equations. V1 is a 10V, 1KHz sine wave. C1 is supplied with a 40 milliohms ESR for a little realism. NOTE that the integration method used is GEAR, which should always be used if sharp switching diodes are present or are simulated.

Below the circuit is an identical circuit where the Full Bridge device has been turned into a part.

A graph of the output of Figure 3 is shown in Figure 4 following:

Figure 4
Full Bridge test circuit #1 graph

Here we see that the conduction angle is short. Because of the ideal components, there is a sharp pulse of current at the start of each conduction cycle. Because of sampling effects, the GRAPHED values will vary a bit, and could be checked by using a smaller value of step ceiling in the test, but the other values of interest are correctly shown. After this pulse, the current will decrease as the charge on the output capacitor is replaced until the voltage present at the output exceeds the rectified drive to the load.

The portion of the circuit with the newly created PWL1 part is graphed at the bottom of Figure 4, and it behaves as does the top portion of the circuit it is based on.

Conclusions:

A half wave diode bridge circuit has been created. This adds more behavioral models to our available parts for selection in modeling. This model should be capable of addition to any SPICE3 product. If the product DOES directly support behavioral expressions, these could be easily modified to incorporate these devices.