The objective of this lab activity is to examine active rectifier circuits. Specifically one that combines an op amp, a low-threshold P-channel MOSFET, and feedback to synthesize a one way current valve or rectifier with less forward drop than a conventional PN junction diode.
Power supplies that use conventional diodes to rectify an AC voltage to obtain a DC voltage must deal with certain inherent inefficiencies. A standard diode or ultra-fast diode can have a 1 V forward voltage or higher at its rated current. This forward drop of the diode is in series with the AC power source which lowers the potential DC output voltage. In addition this drop in voltage times the current supplied through the diode can represent significant power dissipation and heat generation.
The lower forward voltage of Schottky diodes is an improvement over a standard diode. But Schottky diodes also have a built in fixed forward voltage. Higher efficiencies can be achieved by actively switching MOSFET devices synchronously with the input AC waveform to emulate diodes, taking advantage of the FET's lower conduction losses. Active rectification, also often referred to as Synchronous rectification, means switching the FET device on and off at an appropriate point in the AC waveform according to the polarity, so it acts as a rectifier conducting current only in the desired direction.
Unlike the situation with the junction diode, conduction losses depend upon ON resistance (RDS(ON)) and current. Choosing a large enough FET with a low RDS(ON) reduces the forward voltage drop to a fraction of what any diode can achieve. Hence, the synchronous rectifier will have a much lower loss than a diode, helping improve the overall efficiency.
Having to synchronize the gate signal used to turn on and off the FET complicates the circuit design over diode based rectifiers. This complexity is often a better alternative to the added complexity caused by having to remove the heat generated by the diodes. With ever increasing efficiency requirements, there is often no better alternative than to use synchronous rectification.
ADALM2000 Active Learning Module
1 - AD8541 CMOS op amp with rail-to-rail input/output
1 - ZVP2110A PMOS transistor ( or equivalent )
1 - 4.7 uF capacitor
1 - 220 uF capacitor
1 - 10 Ω resistor
1 - 2.2 KΩ resistor
1 - 47 KΩ resistor
1 - 1 MΩ resistor
Build the simple half wave rectifier circuit shown in figure 1 on your solder-less breadboard. An active gate drive circuit, such as shown in the figure, uses an op-amp (AD8541) to sense when the AC input waveform, from the output of the AWG, is more positive than the output voltage, VOUT, and turn on the PMOS transistor, M1. The circuit provides active rectification for AC voltages as low as the minimum power supply voltage for the op-amp ( 2.7 volts for the AD8541 ) or the gate threshold voltage of the PMOS device ( typically 1.5 V for the ZVP2110A ). At lower input voltages, the MOSFET's back-gate to drain diode takes over as an ordinary diode rectifier.
Figure 1 Active half wave rectifier with self-powered op amp
The op-amp will turn on the PMOS transistor when VIN is more positive than VOUT according to the following equation:
where: ( voltages with respect to ground )
VGATE is the voltage at the gate of M1
VIN is the AC input voltage
VOUT is the output voltage at C1and RL
You can relate the input and the output voltages to the PMOS's drain-to-source, VDS, and gate-to-source voltage, VGS, according to the following equation:
We can combine these equations to relate the MOSFET's gate drive to a function of the drain-to-source voltage:
If the value of R2 is 21 times larger in value than R1, ( 1MΩ / 47KΩ ), a +75 mV drop across the M1's drain-to-source voltage, VDS, is sufficient to turn on the PMOS trnasistor with a threshold voltage of -1.5 V. Larger ratios of R2 to R1 could be used to either lower the input to output voltage drop or to accommodate transistors with higher threshold voltages.
The op amp is powered from the output smoothing capacitor C1 so no additional power supply is necessary. There are certain requirements for the op-amp chosen for this circuit. The amplifier must have rail-to-rail inputs and outputs and demonstrate no phase inversion of the gain when operated near the power supply rails. The op amp's bandwidth limits the frequency response of the circuit. Low supply current op amps are often chosen for this application for efficiency which generally results in low bandwidths and slew rates. At higher AC input frequencies, perhaps higher than 500 Hz, the amplifier's gain will begin to decline. The AD8541 single supply CMOS op amp has all of these requirements and a small supply current of only 45 uA.
AWG1 is connected as VIN and should be configured as a sine wave with an amplitude greater than 6 volts, zero offset and a frequency of 100 Hz. The scope inputs are used to monitor various points around the circuit such as VIN, VOUT, the voltage across RS and thus the current through RS, and the gate of M1.
Figure 2 Active half wave rectifier with self-powered op amp Bread Board Circuit
Start by using the large value 220 uF capacitor for C1. The 220 uF and 4.7 uF capacitors are polarized so be sure to connect the positive and negative terminals to your circuit correctly.
Use the two scope inputs to monitor the input AC waveform at VIN and the DC output wave form at VOUT. VOUT should be very close to the peak value of VIN. Now replace the large 220 uF capacitor with the much smaller 4.7 uF capacitor. Observe the change in the waveform seen at VOUT. When is VOUT closest in value to VIN and compare that interval of the AC input cycle with the voltage at the gate of transistor M1.
Figure 3 Vout and Vin at 220uF capacitor
Figure 3 Vout and Vin at 4.7uF capacitor
With scope channel 2 connected across shunt the 10Ω resistor RS, use the Measure feature to obtain the peak and average value of the current. Compare the average value with the DC current in the 2.2 KΩ load resistor RL you calculate based on the voltage you measure at VOUT. Repeat this measurement for both the 220 uF and 4.7 uF capacitor values. How do the peak and average values compare between these two capacitor values?
Change the peak amplitude of the AC input waveform. What is the minimum peak value of the AC input where the PMOS transistor is still actively rectifying the input, i.e. the VGS is greater than the threshold voltage and the device is turned on? What determines this minimum voltage?
Try different frequencies for the AC input. How does the frequency effect the peak value and the width of the current pulse in RS?
Try different wave shapes for the AC input other than a sine wave, such triangle, square and trapezoid waves. How does this alter the peak and average current flowing through RS?
There are other potential uses for a circuit that essentially allows current flow in only one direction with very low voltage drop across the switch. In battery chargers, where the input power source might be intermittent such as a solar panel or wind turbine generator, it is necessary to prevent the battery from discharging when the input power source is not generating a high enough voltage to charge the battery. Generally a simple Schottky diode is used for this purpose but as was pointed out in the background section this can lead to losses in efficiency. If an op amp with sufficiently low operating supply current is employed this can often be lower than the reverse leakage current in a large Schottky diode.
For Further Reading:
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