From what I now read, synchronous rectification is another term for swich mode voltage regulation, which is already sorted (not part of this circuit). This circuit must a) step the AC down to AC below 20V and b) rectify it to DC as efficiently as possible.
Hi Andrew,
As has been stated, 'switch mode' and 'synchronous rectification' are two independent and separate techniques.
(1) Switch Mode
If you have a supply voltage, say from a rectifier and reservoir capacitor, of say 100V and you want to generate a 5V stabilised supply, you have to regulate. Note that the voltage from the rectifier will not only vary with changes in the alternator speed, but also with temperature. On top of that there will be a ripple voltage saw-tooth which will be inversely proportional to the size of reservoir capacitor and alternator speed, and directly proportional to current drawn from the reservoir capacitor. In short, the voltage on the reservoir capacitor will be all over the place. To get a stabilised 5V supply you have three options:
(1.1) Series linear regulator
Here you have a control element in series with the target output voltage. Its job is to waste the voltage difference between the reservoir capacitor and output voltage. it does this dynamically so that responds to any changes, including ripple voltage, on the res cap.
This is by far the best and simplest way of doing the job, with one reservation- it is dreadfully inefficient and simply wastes the power necessary to maintain a constant output voltage.
Take the example: 100V in 5V out. Say that the 5V supply is providing the full USB current of 1A. That means that the series regulator must waste Vin-Vou *I= 95 Watts and there is the problem. (of course in the case of the cycle alternator this would not happen but don't worry about that for the explanation)
(1.2) Shunt linear regulator
The shunt regulator does the same job but it would dissipate 95W all the time, 1A load or 0A load.
(1.3) Switch mode regulator
The switch mode regulator overcomes this massive power loss and but operates in a much more complex way. You have 100V and you want to generate 5V at from 0A to 1A say.
The Switch Mode Power Supply (SMPS) first charges up an inductor.
VL * T = IL. The energy in an inductor = (IL ↑ 2)L
When you put a voltage across an Inductor the current through the inductor will increase literary like a ramp.
At a time defined by design the, inductor is then connected to a capacitor across the 5V supply and some of the energy stored in the inductor is transferred to that capacitor. As a consequence the capacitor voltage rises. When it reaches 5V, the control circuit switches the inductor back to the res cap. This switching takes place typically from 1K Hz to 4MHz, depending on many design parameters
The net result is that the 5V supply is stabilised and unlike the linear regulators no power is wasted.
Of course, in the real word some power will be lost but not much. SMPS are typically 80 to 95% efficient. Assuming 90% efficiency, this means that only 5*1*0.1 =500mW would be dissipated, a bit different from 95W of the two linear regulators!
Note that I have greatly exaggerated and simplified to illustrate the principle.
(2) Synchronous Rectification
Before the widespread use of SMPs, synchronous rectification (detection) was mostly used for high-speed radio frequency detectors, simply because it can be made faster than a simple diode detector (much simplified).
Before describing the operation of synchronous detectors in SMPs, it is best to recall what is going on with a conventional mains transformer,diode, capacitor (TDC) rectifier.
(2.1) Transformer, Diode, Capacitor Rectifier
The first point to note is that it is amazing that the transformer capacitor diode rectifier works at all, let alone reliably. The only reason it is used so widely is because it is cheap small and easy to design. It only works so well because the manufactures have done a fantastic job of making diodes that will stand the abuse they are exposed to every cycle of the mains supply.
In a conventional rectifier circuit the diode only conducts for a few milliseconds at the peak of the AC sine wave from the transformer secondary. This means it has to pass a massive current for a short time. Consequently the instantaneous dissipation is very high, not to mention the other stresses the diode suffers.
(2.2) Rectifying Element
Taking a humble 1A 1N4001 rectifier diode, the data sheet shows that it can pass 1A average current, but its peak repetitive current is 30A. At 1A the diode drops 1V1 and at 30A, 2V. 30A would not be a very high peak current in a rectifier circuit but, all the same, the 1N4001 would have an instantaneous dissipation of 30W. Not only does this mess up the stability of the DC rectified voltage and stress the diode, but it is also a total loss and even though it only lasts for a few milliseconds, the average energy loss is significant. This is one of the reasons why the voltage of a TDC rectifier sags under load.
A very basic model of a normal silicon rec diode is a 600mv battery (band gap) in series with a small resistor. Shockey diodes improves on this with a bandgap of 400mV.
A fully turned on MOFET is different. Between its drain and source it is a resistance, there is no band gap. A high-power MOSFET would typically have a drain source resistance (Rds) of 15m Ohms at 25 deg C. That would rise to around 30m Ohms at 100 deg C, a typical junction temp for a rectifier. Taking the 1N4001 example with an instantaneous peak current of 30A, that would be a voltage drop of 30m Ohms * 30A= 900mV, quite a bit lower than the 2V of the 1N4001.
You could not simply replace a diode with a MOSFET. Instead you must turn it on just before the peak of the Tx secondary sine wave, and off just after the peak. In fact it is synchronised with the mains frequency, Hence the term synchronous rectification'. '
The other advantage of synchronous rectification is speed. The turn on and turn off can be made faster than a diode so switching losses are reduced, but that is a whole story in itself!
It does not comply with the customers specification. With a 100V output from the alternators the zener diodes, to give them their full name, will catch fire and explode (100v-24 = 76V. 76*1A= 76Watts). The circuit may work well and optimise the use of the energy from the cycle alternator, but that is only engineering.
