The nonsynchronous rectifying type, which has been in use for years, features a simple circuit for a switching regulator, with an efficiency barely topping 80%. Subsequently, the appearance of battery-powered applications drawing relatively large power and requiring high efficiency, such as notebook PCs, has led to the development of a succession of ICs for synchronous rectifying switching regulators capable of delivering superior efficiency. Aided by innovations that have facilitated the design of the synchronous rectifying type requiring complex control and circuitry, this type of switching regulator, offering a maximum efficiency close to 95%, has gradually become predominant.

Flyback Voltage and its Origin

A Flyback voltage or an Inductive Flyback is a voltage spike created by an inductor when its power supply is removed abruptly. The reason for this voltage spike is the fact that there cannot be an instant change to the current flowing through an Inductor.

Time Constant of the Inductor determines the rate at which the current can change through an inductor. This is similar to the time constant of a Capacitor, which determines the rate at which its voltage can change.

Time Constant of Inductor τ = L/R, where L is Inductance in Henries and R is series resistance in Ohms.

Similar to a capacitor, it takes nearly 5 time constants (5τ) to dissipate the current in an inductor.

Assume, in the above circuit, the inductor is 10H and the series resistor is 10Ω. So, when the switch is closed, maximum current flows through the inductor.

Now, let us see what happens when the switch is opened suddenly.

First, let us calculate the time constant. Using the formula of time constant and substituting the above assumed values, it is clear that the time constant is 1 second.

So, it will take approximately 5 seconds from the moment the switch is open, to completely stop the flow of current. This means that current flows in the circuit even after the switch is open (assuming it would take a few milli seconds to completely open the switch). How is this possible?

This can be understood from the inductors point of view. The switch gap, which is essentially air, is viewed as a huge resistor by the inductor and the resistance is in the order of few mega Ohms. This means that the circuit is still closed from inductor’s point of view with a huge resistor filling in the air gap.

Now that it is confirmed that the circuit is still closed, the inductor will try to dissipate the current and in order to do so, the inductor will drop voltage across the air gap resistance by reversing its polarity by using the energy stored in it in the form of magnetic field.

Now, the inductor tries to flow current as per its current dissipation curve. This could be problematic according to Ohm’s Law, V = I x R.

Even for a small current, when it is multiplied by the huge air resistance (few hundreds of Mega Ohms) will lead to a very high voltage across the air resistor. This is the origin of the Flyback Voltage or Voltage Spike.

Impact of Flyback Voltage on Switches

As there is no physical resistor when the switch is opened, sparks / arcs will occur between the switch and the other terminal, if a mechanical switch is used. All the energy from the arc is usually discharged across the contacts of the switch in the form of heat.

This could potentially damage the switches permanently or drastically reduce the lifetime of the switches. when speaking of switches, they can be mechanical switches or semiconductor switches like Transistors.

How Flyback Diode can prevent Voltage Spikes?

To protect the switch from being damaged due voltage spikes or inductive Flyback, a Flyback Diode or a Freewheeling Diode is used. The basic idea behind the use of a Flyback Diode is to provide an alternative path for the inductor for its current to flow.

The above image shows the same inductor circuit but with additionally a Flyback Diode. It is important to note that the diode is connected in reverse bias when the switch is closed.

As a result, the diode doesn’t impact the operation of the rest of the circuit when the switch is closed and maximum current flows through the inductor.

But when the switch is opened, the change in polarity of the inductor will make the diode to be placed in forward bias. Hence, the diode will allow current to flow at a rate determined by the time constant of the inductor.

The resistance of the diode, when it is forward biased, is very less and hence the voltage drop across the diode will be significantly less for the current to flow. This prevents arc at the switching device and as a result protects the switching device from damage.

eries Diode – By far the cheapest and easiest to install is a diode in series with the positive power lead. Anode toward the power supply and Cathode (banded end) going to the radio. For a few pennies a 1N400x series diode will handle many QRP rigs up to 1A and a 1N540x series will work up to 3A. Although simple and cheap it comes at a cost. The diode will drop about 0.6-0.9V which could result in lower transmit power and if the equipment uses voltage regulators this lower voltage may be under the regulator level.

Bridge Rectifier – The most foolproof for reverse polarity protection. For about a dollar or two, the bridge will correct the polarity no matter how you connect the power supply. With a bridge there are always 2 diodes conducting which can result in greater losses of round .8-1.5V.   This works good only when a single power supply is used with a single radio but should be avoided when multiple pieces of equipment are connected together via the grounds.

Fuse with crowbar diode – With most protection circuits there is usually some form of loss in the process, as seen in the diode examples above. However, the fuse with crowbar diode has no voltage drop loss. The crowbar works by putting a diode across the voltage lines with the Cathode toward the positive side. This in effect leaves the path through the diode open unless the polarity is reversed. When that happens the diode now becomes an almost short circuit causing the fuse to blow. The cost of course if that a hefty diode needs to be used to handle the large current flow until the fuse opens up, not to mention a new fuse each time it’s tested.

MOSFET¹ – Not only can MOFSET’s do a great job at switching, a P-channel MOSFET does an excellent job of reverse polarity protection. Depending on your needs the cost is from a few pennies to a dollar or two. Under normal circumstances the MOSFET is turned on with a small voltage drop across it, typically fractions less than the diode example above. When the polarity is reversed the MOSFET is forced off and creates an open circuit. See the sidebar on MOSFET selection for more information in selecting a P-channel MOSFET.

MOSFET RPP Rules:

1. VDS – Voltage Drain to Source – must be greater than the power supply voltage.
2. RDS(on) – Resistance of the Drain to Source when the MOSFET is turned on. This must be as low as possible. Usually the lower the Rds(on) the more expensive.
3. Vgss – Voltage from Gate to Source – The maximum voltage allowed from Gate to Source. This should also be greater than the power supply voltage. Many MOSFET’s will have a lower Vgss. In these cases the addition of a simple zener diode and resistor are required as follows:
   1. Resistor is not critical, almost any value between 10K and 100K is good.
   2. Zener voltage must be less than the MOSFET Vgss. Additionally it must be greater than the MOSFET Vgs(th) – Threshold Voltage from Gate to Source – This is the minimum voltage required to fully turn on the MOSFET.
4. There is a small capacitor in the Drain to Gate circuitry. This capacitor is optional but recommended and only used for static discharge protection.²