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Active rectifier for waterfuelcells

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The losses in a bridge rectifier can easily become significant when low voltages are being rectified. The voltage drop across the bridge is a good 1.5 V, which is a hefty 25% with an input voltage of 6V. The loss can be reduced by around 50% by using Schottky diodes, but it would naturally be even nicer to reduce it to practically zero. Thats possible with a synchronous rectifier. What that means is using an active switching system instead of a passive bridge rectifier.The principle is simple: whenever the instantaneous value of the input AC voltage is greater than the rectified output voltage, a MOSFET is switched on to allow current to flow from the input to the output. As we want to have a full-wave rectifier, we need four FETs instead of four diodes, just as in a bridge rectifier. R1 R4 form a voltage divider for the rectified voltage, and R5 R8 do the same for the AC input voltage. As soon as the input voltage is a bit higher than the rectified voltage, IC1d switches on MOSFET T3. Just as in a normal bridge rectifier, the MOSFET diagonally opposite T3 must also be switched on at the same time. Thats taken care of by IC1b. The polarity of the AC voltage is reversed during the next half-wave, so IC1c and IC1a switch on T4 and T1, respectively. As you can see, the voltage dividers are not fully symmetrical. The input voltage is reduced slightly to cause a slight delay in switching on the FETs. That is better than switching them on too soon, which would increase the losses. Be sure to use 1% resistors for the dividers, or (if you can get them) even 0.1% resistors. The control circuit around the TL084 is powered from the rectified voltage, so an auxiliary supply is not necessary. Naturally, that raises the question of how that can work. At the beginning, there wont be any voltage, so the rectifier wont work and there never will be any voltage... Fortunately, we have a bit of luck here. Due to their internal structures, all FETs have internal diodes, which are shown in dashed outline here for clarity.They allow the circuit to start up (with losses). Theres not much that has to be said about the choice of FETs its not critical. You can use whatever you can put your hands on, but bear in mind that the loss depends on the internal resistance. Nowadays, a value of 20 to 50 mW is quite common. Such FETs can handle currents on the order of 50 A. That sounds like a lot, but an average current of 5 A can easily result in peak currents of 50 A in the FETs.The IRFZ48N (55 V @ 64 A, 16 mW) specified by the author is no longer made, but you might still be able to buy it, or you can use a different type. For instance, the IRF4905 can handle 55 V @ 74 A and has an internal resistance of 20 mR. At voltages above 6 V, it is recommended to increase the value of the 8.2-kR resistors, for example to 15 kR for 9V or 22 kR for 12 V.


Here another one.... 8)
Rectifier Bridge Has No 2Vf drop! 
The venerable full-wave rectifier bridge (Fig. 1) is a common, familiar circuit for converting an AC input voltage to a DC output voltage.  It is also useful for translating a DC input of arbitrary polarity into a DC output of known polarity, as is commonly required in electronic telephones or other telephony devices, and has application in protecting against battery reversal in battery-powered circuits.
 Fig. 1  A drawback of the classic four-diode rectifier bridge is the unavoidable forward voltage drop (Vf) of two diodes when current is flowing.  With conventional silicon diodes, this could typically amount to 1.5 volts or more.  The result of this is wasted power and reduced efficiency in power supply applications, or loss of working voltage in telephony or battery-powered applications.
In telephony applications in particular, it is possible for a device to have as little as 4 volts available to it under worst case conditions of loop current and line length.  Since most integrated circuits, telephony or otherwise, are decidedly unfriendly about power supply reversals, it is common practice for the line-powered electronics to be surrounded by a full-wave rectifier bridge in order to guarantee power supply polarity.  But with only 4 volts of line voltage, a 1.5-volt drop in the rectifier would leave only 2.5 volts for the electronics!
Similarly, in battery powered circuits, it is often the case that the loss of efficiency caused by  series diodes to protect against inadvertent battery reversal is unacceptable.
The circuit shown in Fig. 2 eliminates these drawback by replacing the diodes with MOSFETS.  The four MOSFETs are connected in such a way as to conduct in opposing pairs.  Which pair conducts is a function of the polarity of the applied voltage.  The conducting pair is such as to steer the applied voltage to the appropriate output terminals so as to always maintain the same polarity at the output.  In other words, the circuit rectifies.
 (http://www.thetaeng.com/images/PeetersBridge.gif)Fig. 2  Interestingly, if one looks at the intrinsic drain-to-source body diodes of the MOSFETs, ignoring the MOSFETs themselves, they form the conventional rectifier bridge configuration.  Indeed, when voltage is first applied, the circuit acts the same as a conventional rectifier bridge in that the forward voltage drop of two diodes (2Vf) appears between the input and the output.  But as soon as the applied voltage exceeds the turn-on threshold of two MOSFETs (or more precisely, the sum of an N-channel threshold and a P-channel threshold), the appropriate pair of MOSFETs turns on, effectively bypassing the pair of diodes that is conducting.  The voltage-drop performance of the bridge is now a function of drain-to-source resistance (RDS(on)), which, with modern MOSFETs, is pretty darn good!  In telephone line applications, a voltage drop in the millivolt range can easily be achieved.  Also, with low-threshold MOSFETs achieving thresholds in the 1-volt range these days, it is possible to construct a bridge where the MOSFET turn-on occurs not long after the diode turn-on as the applied voltage ramps up.
A limitation of the circuit, as shown, is that the applied voltage cannot exceed the gate-to-source voltage (VGS) rating of the MOSFETs.  Typically, this is 20 volts.  For higher voltage applications, it is possible to put a resistor in series with each gate and use a zener clamp between the gate and source of each MOSFET to limit the VGS experienced by any individual MOSFET, as shown in Fig. 3.  With such a provision, the primary limitation on applied voltage then becomes the drain-to-source breakdown (BVDS) rating of the MOSFETs.  (http://www.thetaeng.com/images/ZenerBridge.GIF)Fig. 3  One caveat of the FET bridge circuit: do not use it as the rectifier in front of a capacitor-input power supply!  In a conventional rectifier bridge, the diodes prevent the backflow of current from the power supply input capacitor as the applied voltage drops below the voltage on the capacitor.  With this design, the MOSFETs act like switches rather than one-way valves for current flow.  They don’t care which way current flows, hence the input capacitor of the power supply will be discharged to near zero volts with each half-cycle of the applied AC power!  This limits the power supply applications for this circuit to inductive- or resistive-input designs.
However, it would be possible to use this circuit with a polarized capacitor in power-factor correction applications.  Correction of an inductive power factor would normally require a non-polarized capacitor directly across the AC line.  By putting the FET bridge circuit in front of the capacitor, a polarized capacitor could be used instead which may be advantageous in terms of size and cost.  I haven't tried this particular application, so I can't vouch for it, but if you have success with the idea, please let me know.

@Donaldwfc: Thanks  ;D
Here is something more
A major cause of losses in a conventional power supply using a 50/60-Hz transformer is the bridge rectifier. This article shows how to build a “greener” rectifier, substantially reducing losses by eliminating the diodes in the bridge rectifier and substituting modern low-RDS(ON) power MOSFETs. The MOSFETs used are typically employed in high-frequency switch-mode power supplies. Aside from the power MOSFETs, the circuit uses only two comparators and a few inexpensive transistors, diodes, capacitors, and resistors.
Four IRF2804 n-channel power MOSFETs, T1-T4, replace the bridge diodes (Fig. 1). The remaining components are needed to steer the gates of the MOSFETs. The power MOSFETs’ body diodes (shown by dashed lines) make up a diode bridge rectifier in the usual way.
During the first half cycle after power-up, this “parasitic” bridge rectifier charges load capacitor C3. When VOUT becomes higher than 2.7 V, comparators U1 and U2 get into the act. In addition, driver stages T9-T12 on the right side, which are also powered by VOUT, now have enough supply voltage to switch on the gates of the T3 and T4.
After the second half cycle, the two boost capacitors on the left side, C1 and C2, have charged to the peak value of the input voltage, and supply driver stages T5-T8 for the power MOSFETs T1 and T2. The voltages across C1 (VB1) and C2 (VB2) are always positive with respect to the source connections of T1 and T2, respectively.
Comparator U2’s inputs are connected to T4’s source and drain connections, so it also senses the voltage polarity of this transistor’s body diode. Whenever the polarity across T4 becomes negativethat is, when a forward current could flow through T4’s body diodethe power MOSFET is switched on via U2’s output and the driver stage T11/T12. (The gate voltage, VG, is shown as R_A in Fig. 2.) The drain-source voltage VDS (the blue trace, Ch. 2, in Fig. 2) now becomes very small, since VDS = ID × RDS(ON), and the transistor’s RDS(ON) is only 2 m?.
Virtually all the current now flows from source to drain and almost no current is flowing through the body diode. Notice that VDS remains negative, so the comparator can keep T4’s gate high. At the same time, T1 is also switched on, with the help of T14 (trace R_B in Fig. 2) and driver stage T5/T6.
Later in the cycle, when the current through T1 and T4 drops to zero (that is, when the transformer output voltage dives below VOUT), T4’s VDS also becomes zero, and the comparator cuts off both T4 and its leftside partner, T1. While T1 is conducting, boost capacitor C2 amasses charge that’s needed one-half period later for dumping into T2’s gate.
After that one-half period, similar things happen to the other power MOSFET pair. Comparator U1 senses T3’s VDS and switches on this transistor and its cousin T2 on the left side just at the moment before a current begins to flow through the respective body diodes.
The values of C1 and C2 must be high enough to ensure that the gate-source voltage at the end of the gate-charging process is high enough to switch on the respective power MOSFET completely. For a gate charge of QG = 160 nC (the data-sheet value for the IRF2804) and an allowed voltage drop of, say, ?UG = 100 mV, the minimum capacitance would be CMIN = QG/? UG = 1.6 F. Therefore, 10 ?F is high enough. Multilayer chip capacitors can be used, but beware of the voltage dependency of dielectrics like Y5V.
The two comparators are LT1716 low-power devices in small SOT-23 packages. They are particularly suitable for this application because they can cope with negative voltages on their inputs, even when running from a single supply. That’s important because the drains of T3 and T4 become negative with respect to ground.
Another advantage of this comparator is its wide operating voltage rangefrom 2.7 V to 44 V. Unfortunately, the device’s output drive is too low to drive T3 and T4 directly. That’s why the need arises for driver stages T9-T12. They are small p- and n-channel MOSFETs that put a maximum voltage swing on the gates of the power MOSFETs.
With a 5-A load, the circuit worked with transformer voltages of 2.8 V rms to 14 V rms. The lower limit is determined by the gate threshold voltages of the MOSFETs, and the upper limit is determined by the maximum allowed gate voltages. If the circuit must run at higher transformer voltages, the supply voltages for the driver stages should be limited by resistors/Zeners or voltage regulators.
The circuit’s efficiency is quite good. At a 10-A dc output (7 V rms ac input), none of the components require a heatsink. The power MOSFET case temperatures stay well below 50°C.
Due to a lack of equipment, I could not test the circuit at higher currents. But beyond 10 A, it may be worthwhile to connect two MOSFETs in parallel to reduce RDS(ON) even further. But pay attention to the resistances of the PCB traces, since they could be higher than the MOSFETs’ RDS(ON)!
The circuit was compared to a popular KBU8B silicon diode rectifier. At an input voltage of 5 V rms at 50 Hz and a constant load of 5 A dc, the KBU8B’s output was 4.45 V dc, average, measured across C3 (15,000 ?F). Under the same conditions, the “greener” rectifier produced an output of 5.9 V dc, average.
Another comparison that may be even more meaningful involves determining what rectifier input voltage is needed for a given dc output voltage. For this measurement, a transformer with several output windings (Ultron ULT2) was connected to the mains via a Variac. The desired output was 5 V dc, average. Measurements were done at two constant load currents: 5 A and 10 A.
For the KBU8B rectifier and a 5-A load, the transformer’s 6-V output winding was used. The Variac had to be adjusted for a transformer output (secondary) voltage of VSEC = 5.55 V rms, which had to be corrected slightly to 5.48 V when the rectifier got hot. The measured input power was 47 W. With a 10-A load, which is already beyond the specs of the KBU8B, the 8-V output winding had to be used. The Variac was adjusted to 5.97 V rms (5.87 V rms when hot). Under these conditions, the real input power of the Variac was 88 W.
Using the “greener” rectifier with a load current of 5 A, the Variac had to be tuned back to a transformer output voltage of VSEC = 4.34 V rms (off the 6-V winding). The Variac’s real input power was only 36 W. At 10 A, the 6-V winding could still be used, with the Variac tuned to 4.82 V rms. The real input power was 69 W. Thus, the power MOSFET rectifier circuit saved roughly 10 W at 5 A and 20 W at 10 A.
At high currents and low voltages, and especially when the output ripple voltage increases, the two power MOSFETs on the right get a little warmer than those on the left. The reason is because the driver stages on the left have their own filter capacitors (C1 and C2) that provide a smooth dc voltage, while the driver stages on the right are directly supplied from the high-ripple output voltage. Unfortunately, right at the moment when the gates of the right-side MOSFETs should be taken high, the available output voltage is rather low, since output capacitor C3 has discharged to its minimum value (traces CH1 and R_A in Fig. 2).
The cure for this problem is simple. Add a diode and a capacitor to supply the right half of the circuit (Fig. 3).
The whole circuit fits into roughly the same volume as a conventional bridge rectifier. Considering that there’s usually no need for heatsinks, the circuit should pay off quickly. Also, in many cases, a smaller and cheaper transformer can be employed.

Totally useless to do , normally you wouldnt use low level signals anyways for the stuff we do here  , only low level signal processing needs active rectifiers usually .

The losses will never be significant enough for this to be worth it .

Theres a 15 HP motor at school that we work with , it just has big diodes on a heatsink , it does the job .

I am sure thats the case for all companies , when bigger motor need bigger heatsinks and diodes , its that simple .


Do you agree that the resistance of two diodes in parallel is the same as 1 single diode? Do you agree that the heat losses will be the same having one or two or three diodes in parallel?

There active rectifiers makes the difference! Reduce Losses!!!

If you connect two mosfets or igbts in parallel you half the losses, if you connect 4 you have 4 times less losses in comparison to use only one. 

you see?

Gud luck


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